Publications

Drought stress causes multiple feedback responses in plants. These responses span from stomata closure and enzymatic downregulation of photosynthetic activity to structural adjustments of xylem biomass and leaf area. Some of these processes are not easily reversible and may persist long after the stress has ended. Despite a multitude of hydraulic model approaches, simulation models still widely lack an integrative mechanistic description of how this sequence of physiological to structural tree responses may be realized that is also simple enough to be generally applicable. Here, we suggest an integrative, sequential approach to simulate drought stress responses. First, decreasing plant water potential triggers stomatal closure alongside a downregulation of photosynthetic performance, thereby effectively slowing down further desiccation. A second protective mechanism is introduced by increasing the soil-root resistance, represented by a disconnection of fine roots after a threshold soil water potential has been reached. Further decreases in plant water potential due to residual transpiration and loss of internal stem water storage consistently lead to a loss of hydraulic functioning, which is reflected in sapwood loss and foliage senescence. This new model functionality has been used to investigate the responses of tree hydraulics, carbon uptake, and transpiration to soil and atmospheric drought in an extremely dry Aleppo pine (Pinus halepensis Mill.) plantation. Using the hypothesis of a sequential triggering of stress-mitigating responses, the model was able to reflect carbon uptake and transpiration patterns under varying soil water supply and atmospheric demand conditions - especially during summer - and respond realistically regarding medium-term responses, such as leaf and sapwood senescence. We could show that the observed avoidance strategy was only achieved when the model accounted for very early photosynthesis downregulation, and the relatively high measured plant water potentials were well reproduced with a root-soil disconnection strategy that started before major xylem conductance losses occurred. Residual canopy conductance was found to be pivotal in explaining dehydration and transpiration patterns during summer, but it also disclosed the fact that explaining the water balance in the driest periods requires water supply from stem water and deep soil layers. In agreement with the high drought resistance observed at the site, our model indicated little loss of hydraulic functioning in Aleppo pine, despite the intensive seasonal summer drought.

Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (gbr), transpiration rate (E) and net photosynthesis (Anet) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and gbr to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the gbr and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.Under high evaporative demand conditions, mature Aleppo pine trees demonstrated increased transpiration and photosynthesis in response to supplementary irrigation. Stomatal conductance exhibited hypo-sensitivity to increasing water demand (high vapour pressure deficit) but was sensitive to a decrease in water supply, expressed as soil water content or plant water potential. Our findings highlight that water supply, rather than water demand, is the primary limiting factor for transpiration in Aleppo pine trees.

Suppression of carbon emissions through photovoltaic (PV) energy and carbon sequestration through afforestation provides complementary climate change mitigation (CCM) strategies. However, a quantification of the "break-even time"(BET) required to offset the warming impacts of the reduced surface reflectivity of incoming solar radiation (albedo effect) is needed, though seldom accounted for in CCM strategies. Here, we quantify the CCM potential of PV fields and afforestation, considering atmospheric carbon reductions, solar panel life cycle analysis (LCA), surface energy balance, and land area required across different climatic zones, with a focus on drylands, which offer the main remaining land area reserves for forestation aiming climate change mitigation (Rohatyn S, Yakir D, Rotenberg E, Carmel Y. Limited climate change mitigation potential through forestation of the vast dryland regions. 2022. Science 377:1436-1439). Results indicate a BET of PV fields of -2.5 years but >50× longer for dryland afforestation, even though the latter is more efficient at surface heat dissipation and local surface cooling. Furthermore, PV is -100× more efficient in atmospheric carbon mitigation. While the relative efficiency of afforestation compared with PV fields significantly increases in more mesic climates, PV field BET is still -20× faster than in afforestation, and land area required greatly exceeds availability for tree planting in a sufficient scale. Although this analysis focusing purely on the climatic radiative forcing perspective quantified an unambiguous advantage for the PV strategy over afforestation, both approaches must be combined and complementary, depending on climate zone, since forests provide crucial ecosystem, climate regulation, and even social services.

Drought stress is imposing multiple feedback responses in plants. These responses span from stomata closure and enzymatic downregulation of photosynthetic activity to structural adjustments in leaf area. Some of these processes are not easily reversible and may persist long after the stress ended. Unfortunately, simulation models widely lack an integrative mechanistic description on how this sequence of tree physiological to structural responses occur.Here, we suggest an integrative approach to simulate drought stress responses. Firstly, a decreasing plant water potential triggers stomatal closure alongside a downregulation of photosynthetic performance. This is followed by a disconnection of roots and soil and the reliance on internal stem water storage or water uptake from deep soil layers. Consistently, loss in hydraulic functioning is reflected in sapwood loss of functionality and foliage senescence. This new model functionality has been used to investigate responses of tree hydraulics, carbon uptake and transpiration to soil- and atmospheric drought in an extremely dry Aleppo pine (Pinus halepensis L.) plantation.Using the hypothesis of a sequential triggering of stress-mitigating responses, the model was able to reflect the carbon uptake and transpiration patterns under varying soil water supply and atmospheric demand especially during summer and responded realistically regarding medium-term responses such as leaf and sapwood senescence. In agreement with the high drought resistance observed at the site our model indicated little loss of hydraulic functioning in Aleppo pine, despite the intensive seasonal summer drought.

Forestation of the vast global drylands has been considered a promising climate change mitigation strategy. However, its actual climatic benefits are uncertain because the forests reduced albedo can produce large warming effects. Using high-resolution spatial analysis of global drylands, we found 448 million hectares suitable for afforestation. This areas carbon sequestration potential until 2100 is 32.3 billion tons of carbon (Gt C), but 22.6 Gt C of that is required to balance albedo effects. The net carbon equivalent would offset ~1% of projected medium-emissions and business-as-usual scenarios over the same period. Focusing forestation only on areas with net cooling effects would use half the area and double the emissions offset. Although such smart forestation is clearly important, its limited climatic benefits reinforce the need to reduce emissions rapidly.
Just a little help
Forestation of the global drylands has been suggested to be a way to decrease global warming, but how much promise does it actually have? Rohatyn
et al. found that the climatic benefits are minor. Although drylands have considerable carbon sequestration potential, which could be used to lower the amount of carbon dioxide in the atmosphere and thereby slow warming, the reduction of albedo caused by forestation would counteract most of that effect. So, although forestation is clearly important, it cannot substitute for reducing emissions. HJS Dryland forestation would do little to slow global warming.

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as dryland mechanisms. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.

Global warming and drying trends, as well as the increase in frequency and intensity of droughts, may have unprecedented impacts on various forest ecosystems. We assessed the role of internal water storage (WS) in drought resistance of mature pine trees in the semi-arid Yatir Forest. Transpiration (T), soil moisture and sap flow (SF) were measured continuously, accompanied by periodical measurements of leaf and branch water potential (ψleaf) and water content (WC). The data were used to parameterize a tree hydraulics model to examine the impact of WS capacitance on the tree water relations. The results of the continuous measurements showed a 5-h time lag between T and SF in the dry season, which peaked in the early morning and early afternoon, respectively. A good fit between model results and observations was only obtained when the empirically estimated WS capacitance was included in the model. Without WS during the dry season, ψleaf would drop below a threshold known to cause hydraulic failure and cessation of gas exchange in the studied tree species. Our results indicate that tree WS capacitance is a key drought resistance trait that could enhance tree survival in a drying climate, contributing up to 45% of the total daily transpiration during the dry season.

Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, highprecision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remotesensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c.2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg.A novel dualreference method was developed to increase the accuracy of infrared needleleaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates.We accurately captured surface temperature and leaftoair temperature differences of needleleaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ±0.23°C and ±0.28°C, respectively.Routine highprecision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leafscale processes and their dynamics.

Drylands cover more than 40% of Earth's land surface and occur at the margin of forest distributions due to the limited availability of water for tree growth. Recent elevated temperature and low precipitation have driven greater forest declines and pulses of tree mortality on dryland sites compared to humid sites, particularly in temperate Eurasia and North America. Afforestation of dryland areas has been widely implemented and is expected to increase in many drylands globally to enhance carbon sequestration and benefits to the human environment, but the interplay of sometimes conflicting afforestation outcomes has not been formally evaluated yet. Most previous studies point to conflicts between additional forest area and water consumption, in particular water yield and soil conservation/desalinization in drylands, but were generally confined to local and regional scales. Our global synthesis demonstrates that additional tree cover can amplify water consumption through a nonlinear increase in evapotranspiration-depending on tree species, age, and structure-which will be further intensified by future climate change. In this review we identify substantial knowledge gaps in addressing the dryland afforestation dilemma, where there are trade-offs with planted forests between increased availability of some resources and benefits to human habitats versus the depletion of other resources that are required for sustainable development of drylands. Here we propose a method of addressing comprehensive vegetation carrying capacity, based on regulating the distribution and structure of forest plantations to better deal with these trade-offs in forest multifunctionality. We also recommend new priority research topics for dryland afforestation, including: responses and feedbacks of dryland forests to climate change; shifts in the ratio of ecosystem ET to tree cover; assessing the role of scale of afforestation in influencing the trade-offs of dryland afforestation; and comprehensive modeling of the multifunctionality of dryland forests, including both ecophysiological and socioeconomic aspects, under a changing climate.

The leaf economics spectrum^1,2 and the global spectrum of plant forms and functions³ revealed fundamental axes of variation in plant traits, which represent different ecological strategies that are shaped by the evolutionary development of plant species². Ecosystem functions depend on environmental conditions and the traits of species that comprise the ecological communities⁴. However, the axes of variation of ecosystem functions are largely unknown, which limits our understanding of how ecosystems respond as a whole to anthropogenic drivers, climate and environmental variability^4,5. Here we derive a set of ecosystem functions⁶ from a dataset of surface gas exchange measurements across major terrestrial biomes. We find that most of the variability within ecosystem functions (71.8%) is captured by three key axes. The first axis reflects maximum ecosystem productivity and is mostly explained by vegetation structure. The second axis reflects ecosystem water-use strategies and is jointly explained by variation in vegetation height and climate. The third axis, which represents ecosystem carbon-use efficiency, features a gradient related to aridity, and is explained primarily by variation in vegetation structure. We show that two state-of-the-art land surface models reproduce the first and most important axis of ecosystem functions. However, the models tend to simulate more strongly correlated functions than those observed, which limits their ability to accurately predict the full range of responses to environmental changes in carbon, water and energy cycling in terrestrial ecosystems^7,8.

The chapter describes a long-term (20002019) perspective of carbon and energy fluxes from the leaf to the whole ecosystem scale in a semi-arid Aleppo pine forest plantation (the Yatir Forest) in the northern edge of the Negev desert (Israel), using the eddy covariance approach combined with meteorological and supplemental small-scale measurements. The site is characterized by a long dry season (over 7 months) with mean annual precipitation of 285 mm. The forest was a carbon sink during the wet season from December to April, reaching a maximum Net Ecosystem Production (NEP) rate of 4 gC m−2 d−1 in March, and a carbon source in JuneOctober, with a mean summer value of ~ −1 gC m−2 d−1. The carbon inventory showed that during 20012016 Yatir Forest accumulated 145 ± 26 gC m−2 year−1, ~71% of which was stored in the soil. By sequestering carbon, the forest has a cooling effect on the earths surface. The solar radiation burden over the forest was high, with an annual average flux of 240 Wm−2, while the forest albedo (the reflected solar radiation) was 0.12 and significantly lower than that of the surrounding desert, which was 0.25. Heat exchange with the atmosphere was characterized by high sensible heat flux (H) representing 5090% of the net absorbed radiation flux (Rn). The net absorbed Rn by the forest ecosystem was 67% higher than in the surrounding desert; the additional absorbed radiation has a warming effect on the surface. Our campaign-based measurements in another Aleppo pine forest in northern Israel with 755 mm annual precipitation (the Birya Forest, 190 Km north of Yatir) showed that this forest was a carbon sink both in the wet and dry seasons with mean NEP ~5 and ~3 gC m−2 d−1, respectively. The vegetation in the ecosystems around Birya Forest was much more developed than the desert vegetation in Yatir, thus the albedo effect and the thermal radiation suppression were lower. The net radiation load difference between Birya Forest and its surroundings was one-third lower than between Yatir and its surrounding desert. This case study show that pine forests can adjust to conditions at the dry, semi-arid, timberline and store significant amounts of carbon below ground with long residence time. Its large effects on the surface energy budget can result with local warming, but this can change considerably across relatively small geographical scale.

Climate change will impact forest productivity worldwide. Forecasting the magnitude of such impact, with multiple environmental stressors changing simultaneously, is only possible with the help of process-based models. In order to assess their performance, such models require careful evaluation against measurements. However, direct comparison of model outputs against observational data is often not reliable, as models may provide the right answers due to the wrong reasons. This would severely hinder forecasting abilities under unprecedented climate conditions. Here, we present a methodology for model assessment, which supplements the traditional output-to-observation model validation. It evaluates model performance through its ability to reproduce observed seasonal changes of the most limiting environmental driver (MLED) for a given process, here daily gross primary productivity (GPP). We analyzed seasonal changes of the MLED for GPP in two contrasting pine forests, the Mediterranean Pinus halepensis Mill. Yatir (Israel) and the boreal Pinus sylvestris L. Hyytiälä (Finland) from three years of eddy-covariance flux data. Then, we simulated the same period with a state-of-the-art process-based simulation model (LandscapeDNDC). Finally, we assessed if the model was able to reproduce both GPP observations and MLED seasonality. We found that the model reproduced the seasonality of GPP in both stands, but it was slightly overestimated without site-specific fine-tuning. Interestingly, although LandscapeDNDC properly captured the main MLED in Hyytiälä (temperature) and in Yatir (soil water availability), it failed to reproduce high-temperature and high-vapor pressure limitations of GPP in Yatir during spring and summer. We deduced that the most likely reason for this divergence is an incomplete description of stomatal behavior. In summary, this study validates the MLED approach as a model evaluation tool, and opens up new possibilities for model improvement.

The rate of change in atmospheric CO2 is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55-year-old 30 km2 pine forest growing at the semi-arid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semi-arid lands, which cover ~ 18% of the global land area. The forest accumulated 145-160 g C m-2 y-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m-2 y-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g carbon m-2 and 372 g nitrogen m-2 . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 y). Regardless of un-expected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semi-arid soils and forest plantations, and imply that afforestation of even 10% of semi-arid land area under conditions similar to that of the study site, could sequester ~ 0.4 Pg C y-1 over several decades.

The future of forests and their productivity in dry environments will depend on both water availability through precipitation and ecosystem and plant water use characteristics. It is increasingly recognized that better understanding water use patterns and their response to change depends on our ability to partition evapotranspiration (ET). Here, we use chamber-based direct measurements of soil evaporation (Es) in a semi-arid Pinus halepensis forest to partition ET to Es and tree transpiration (Et), to assess the daily and seasonal changes and to compare annual-scale values with measurements carried out at the same site ten years earlier. The ecosystem is characterized by a high annual Es/ET ratio of 0.26, and an Et/ET of 0.63. Es diminished in the long dry season, but as much as 74 ± 5% of the residual flux was due to the re-evaporation of nighttime moisture adsorption, which may provide critical protection from soil drying. Over the 10 years observation period concurrent increase in the transpiration ratio (TR=Et/ET; +29%) and in leaf area index (LAI; +44%) were observed, with the ratio of TR/LAI remaining constant at ~0.31, and with persistently closed hydrological balance (ET/P of 0.941.07). The observed Et/ET values are similar to the estimated global mean values, but are attained at a much higher aridity index (5.5) than the mean one, demonstrating the potential for expanding forestation into dry regions.

Photoprotection strategies in a Pinus halepensis Mill. forest at the dry timberline that shows sustained photosynthetic activity during 6-7 month summer drought were characterized and quantified under field conditions. Measurements of chlorophyll fluorescence, leaf-level gas exchange and pigment concentrations were made in both control and summer-irrigated plots, providing the opportunity to separate the effects of atmospheric from soil water stress on the photoprotection responses. The proportion of light energy incident on the leaf surface ultimately being used for carbon assimilation was 18% under stress-free conditions (irrigated, winter), declining to 4% under maximal stress (control, summer). Allocation of absorbed light energy to photochemistry decreased from 25 to 15% (control) and from 50% to 30% (irrigated) between winter and summer, highlighting the important role of pigment-mediated energy dissipation processes. Photorespiration or other non-assimilatory electron flow accounted for 15-20% and ~10% of incident light energy during periods of high and low carbon fixation, respectively, representing a proportional increase in photochemical energy going to photorespiration in summer but a decrease in the absolute amount of photorespiratory CO loss. Resilience of the leaf photochemical apparatus was expressed in the complete recovery of photosystem II (PSII) efficiency (φPSII) and relaxation of the xanthophyll de-epoxidation state on the diurnal cycle throughout the year, and no seasonal decrease in pre-dawn maximal PSII efficiency (Fv/Fm). The response of CO assimilation and photoprotection strategies to stomatal conductance and leaf water potential appeared independent of whether stress was due to atmospheric or soil water deficits across seasons and treatments. The range of protection characteristics identified provides insights into the relatively high carbon economy under these dry conditions, conditions that are predicted for extended areas in the Mediterranean and other regions due to global climate change.

Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented. (C) 2019 Optical Society of America

Efficiency and energetics of the turbulent transport in the canopy sublayer of a semi-arid pine forest and the atmospheric surface layer of a sparse desert-like shrubland are investigated from ultrasonic anemometer measurements during the summer dry season. The results show that an increased sensible heat flux over the forest canopy is generated by more energetic turbulence at small and large scales, but the transport process itself is equally efficient compared to the neighbouring shrubland. In contrast, the turbulent momentum flux over the forest canopy is caused by more efficient and energetic momentum transport at large scales. The more energetic turbulence appears to reduce the aerodynamic resistance to heat transfer of the semi-arid forest, which enables the increased sensible heat flux at a lower surface temperature. The results help explain the observed differences in diurnal variation of statistical moments of turbulent quantities that are caused by the interaction between radiative forcing, the background wind and the different turbulence production regimes of the forest and the shrubland. Lastly, the results also explain the observed cooler nighttime temperatures and quicker formation of a residual layer at the forest site.

We investigate the effects of an isolated meso-γ-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: 28km2, forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (600Wm-2 in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.

Carbonyl sulfide (COS) is a tracer of ecosystem photosynthesis that can advance carbon cycle research from leaf to global scales; however, a range of newly reported caveats related to sink/source strength of various ecosystem components hinder its application. Using comprehensive eddy-covariance and chamber measurements, we systematically measure ecosystem contributions from leaf, stem, soil, and litter and were able to close the ecosystem COS budget. The relative contributions of nonphotosynthetic components to the overall canopy-scale flux are relatively small (~4% during peak activity season) and can be independently estimated based on their responses to temperature and humidity. Converting COS to photosynthetic CO₂ fluxes based on the leaf relative uptake of COS/CO₂, faces challenges due to observed daily and seasonal changes. Yet, this ratio converges around a constant value (~1.6), and the variations, dominated by light intensity, were found unimportant on a flux-weighted daily time-scale, indicating a mean ratio of daytime gross-to-net primary productivity of ~2 in our ecosystem. The seasonal changes in the leaf relative uptake ratio may indicate a reduction in mesophyll conductance in winter, and COS-derived canopy conductance permitted canopy temperature estimate consistent with radiative skin temperature. These results support the feasibility of using COS as a powerful and much-needed means of assessing ecosystem function and its response to change.

The role of secondary circulations has recently been studied in the context of well-defined surface heterogeneity in a semiarid ecosystem where it was found that energy balance closure over a desert-forest system and the structure of the boundary layer was impacted by advection and flux divergence. As a part of the CliFF ("Climate feedbacks and benefits of semi-arid forests", a collaboration between KIT, Germany, and the Weizmann Institute, Israel) campaign, we studied the boundary layer dynamics and turbulent transport of energy corresponding to this effect in Yatir Forest situated in the Negev Desert in Israel. The forest surrounded by small shrubs presents a distinct feature of surface heterogeneity, allowing us to study the differences between their interactions with the atmosphere above by conducting measurements with two eddy covariance (EC) stations and two Doppler lidars. As expected, the turbulence intensity and vertical fluxes of momentum and sensible heat are found to be higher above the forest compared to the shrubland. Turbulent statistics indicative of nonlocal motions are also found to differ over the forest and shrubland and also display a strong diurnal cycle. The production of turbulent kinetic energy (TKE) over the forest is strongly mechanical, while buoyancy effects generate most of the TKE over the shrubland. Overall TKE production is much higher above the forest compared to the shrubland. The forest is also found to be more efficient in dissipating TKE. The TKE budget appears to be balanced on average both for the forest and shrubland, although the imbalance of the TKE budget, which includes the role of TKE transport, is found to be quite different in terms of diurnal cycles for the forest and shrubland. The difference in turbulent quantities and the relationships between the components of TKE budget are used to infer the characteristics of the turbulent transport of energy between the desert and the forest.

Aims: This study investigated the impact of canopy cover and seasonality on litter decay in Mediterranean pine forests to enhance climate predictions. Methods: We conducted litterbag experiments in plots of different tree densities in two Mediterranean pine forests differing in precipitation amounts. In each plot, local litter was placed in forest gaps and under tree canopies for 613 days, starting in the dry season. Results: Litter mass loss was greater in forest gaps than under tree canopies across forests and tree densities. Similarly, a reduction in tree density tended to increase mass loss. Additionally, while the decay rate slowed down from the first to the second wet season, the decay rate remained constant during the first and the second dry season, and the dry seasons contributed 30% to the overall mass loss. Conclusions: Reduction in canopy cover enhances litter decay, and the stability and magnitude of the dry season contribution to annual mass loss have the potential to control litter mass loss when accounting also for the dry periods in the wet season. Combined, the ongoing tree mortality and the predicted prolongation of dry periods due to climate change may enhance litter decay, possibly reducing ecosystem carbon stocks in drylands.

Estimations of ecosystem-level evapotranspiration (ET) and CO 2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach was previously proposed for ecosystem-level assessment relying on vegetation index and meteorological data (RS-Met) in temperate Mediterranean ecosystems. However, these RS-Met models have not been tested yet in extreme high-energy water-limited ecosystems that suffer from continuous stress conditions. Owing to the lack of ET and CO 2 flux estimations in the Eastern Mediterranean, we examined the RS-Met approach using a newly developed mobile lab system and the single active Fluxnet station operating in this region, in seven forest and non-forest sites across a climatic transect in Israel (280-770 mm y −1 ). The RS-Met models were used with and without the addition of a seasonal drought stress factor (f DS ), which was based on daily rainfall, temperature and radiation data.Results show that the RS-Met models with the inclusion of the f DS were significantly improved compared to the non-f DS models (r=0.64-0.91 compared to 0.05-0.80; P=0.06 and r=0.72-0.92 compared to r=0.56-0.90; P

Current climate models disagree on how much carbon dioxide land ecosystems take up for photosynthesis. Tracking the stronger carbonyl sulfide signal could help.

Climate and topography have both strong effects on forest water and carbon cycles. However, little is known about their combined effects on the long-term water use and productivity of forests that have different water-use strategies. Here, we used structural equation modelling (SEM) to test direct and indirect influences of climate and topography on the long-term (mean over 2000-2014) evapotranspiration (ET) and gross ecosystem productivity (GEP), as assessed from satellite-based models, across two major co-occurring Mediterranean forest types with different water-use strategies (oak woodlands and pine forests). The estimated GEP and ET were higher by c. 6% and 15%, respectively, in the oak woodlands (Quercus calliprinos) than in pine forests (Pinus halepensis). As a result, the water use efficiency was higher in the pine forests (by 9%), consistent with P. halepensis conservative behaviour. Using the SEM, we found that the mean annual surface skin temperatures had the largest influence on the productivity and ET, with a similar net adverse effect across both forest types. In contrast, the mean annual precipitation was not related to GEP across both forest types but positively affected the ET in the oak woodlands. Slope and aspect had both significant but secondary influence on the forests fluxes, with higher GEP and ET found on the steeper slopes across the oak woodlands and higher ET found on the steeper slopes across the pine forests, associated with north-facing aspects. Applying the SEMs for the pine and oak forests, we predicted reductions of 16% and 31% in the productivity of both forests for projected increases in temperatures of 1(circle)C and 2(circle)C, respectively. Our results suggest that projected warming may have a strong impact on the productivity of Mediterranean forests, severely decreasing the CO2 uptake of the trees, independent of their water -use strategy. (C) 2016 Elsevier B.V. All rights reserved.

Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (g s), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and g s of P. halepensis were threefold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 23-fold higher A and g s but with much more variability between seasons, especially for Q. ithaburensis with A and g s that decreased 1030-fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 58-fold higher in Birya than in Yatir in summer (P

The Yatir forest in Israel is a solitary forest at the dry timberline that is surrounded by semi-arid shrubland. Due to its low albedo and its high surface roughness, the forest has a strong impact on the surface energy budget, and is supposed to induce a secondary circulation, which was assessed using eddy-covariance (EC) and Doppler lidar measurements and large-eddy simulation (LES). The buoyancy fluxes were 220-290Wm^-2 higher above the forest, and the scale of the forest relative to the boundary-layer height is ideal for generating a secondary circulation, as confirmed by a LES run without background wind. However, usually a relatively high background wind (6ms^-1) prevails at the site. Thus, with the Doppler lidar a persistent updraft above the forest was detected only on 5 of the 16 measurement days. Nevertheless, the secondary circulation and convective coherent structures caused low-frequency flux contributions in the mixed-layer w spectra, the surface layer u and w spectra and in the surface-layer momentum fluxes. According to the ogive functions from the tower data and a control volume approach using the LES, such low-frequency contributions with timescales >30min were a major reason the non-closure of the energy balance at the desert site. In the roughness sublayer above the forest, these large structures were broken up into smaller eddies and the energy balance was closed.

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ¹⁸O and tritium content, dissolved inorganic carbon (DIC) and its δ¹³C () and Δ¹⁴C () content, and δ¹³C () and Δ¹⁴C () in the solid sediment. The profiles of moisture and δ¹⁸O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr¹). The source of carbon in the DIC is CO₂ released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ¹³C () profiles show clearly the chemical transition from dissolved CO₂ (δ¹³C = 22) to bicarbonate (δ¹³C = 14). At greater depth (11.3), the δ¹³C becomes similar to the δ¹³C in the aquifer below (12.5). The effect of secondary processes is evident in the profile of Δ¹⁴C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10 yr¹. Precipitation of carbon from the DIC on the sediment is 1.1 mg C L_sed¹ yr¹, negligible compared to the 28 g C in 1 L_sed. In the solid sediment, there is a gradient in Δ¹⁴C_carb at the top meter. The net precipitation of ¹⁴C from the DIC on the sediment (0.25 to 1.1 yr¹), corrected for decay, cannot be observed in the deeper sediment. The presence of ¹⁴C in the top 1 m of the sediment is explained by two possible processes: accumulation of ¹⁴C-tagged dust (~0.05 mm yr¹) and/or long-term cumulative precipitation from the DIC.

Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water-limited ecosystems. However, the dynamics in soil water availability for tree transpiration (T_t) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because T_t is influenced by soil water potential (Ψ_s) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth-dependent soil water retention curves, we quantified the 'transpirable soil water content' (tSWC) and its seasonal and inter-annual variations in a semi-arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season T_t was 57% of rain and 72% of the infiltrated SWC. In early winter, T_t was exclusively supported by soil moisture at the top 10cm (tSWC=11mm), whereas in spring (tSWC>18mm) and throughout the dry season, source water for T_t shifted to 20-40cm, where the maximum fine root density occurs. Simulation with the soil-plant-atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20-40cm soil layer critically depended on limited water infiltration below 40cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC>0) in this semi-arid ecosystem.

Laboratory studies indicate that plant respiratory efficiency may decrease in response to low nutrient availability due to increased partitioning of electrons to the energy-wasteful alternative oxidase (AOX); however, field confirmation of this hypothesis is lacking. We therefore investigated plant respiratory changes associated with succession and retrogression in soils aged from 10 to 120000 years along the Franz Josef soil chronosequence, New Zealand. Respiration rates and electron partitioning were determined based on oxygen isotopic fractionation. Leaf structural traits, foliar nutrient status, carbohydrates and species composition were measured as explanatory variables. Although soil nutrient levels and species composition varied by site along the chronosequence, foliar respiration across all sites and species corresponded strongly with leaf nitrogen concentration (r²=0.8). In contrast, electron partitioning declined with increasing nitrogen/phosphorus (r²=0.23) and AOX activity correlated with phosphorus (r²=0.64). Independently, total respiration was further associated with foliar Cu, possibly linked to its effect on AOX. Independent control of AOX and cytochrome pathway activities is also discussed. These responses of plant terminal respiratory oxidases - and therefore respiratory carbon efficiency - to multiple nutrient deficiencies demonstrate that modulation of respiratory metabolism may play an important role in plant responses to nutrient gradients.

Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO₂ concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO₂ in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO ₂ with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions. Key PointsCarbonyl sulfide can help falsify carbon cycle modelsCarbonyl sulfide can aid separation of NPP into GPP and RespThe oceanic COS source is probably much larger than currently thought

A study of CO₂ in soil gas was conducted in a bare plot in the unsaturated zone (USZ) of Yatir Forest, northern Negev, Israel. In 2006, 6 tubes for sampling of soil gas were inserted into the USZ to depths of 30, 60, 90, 120, 200, and 240 cm. Profiles of soil gas in the USZ were collected from the tubes 5 times between October 2007 and September 2008. Measurements of the collected profiles of soil gas were of CO₂ (ppm), δ¹³C (), and Δ¹⁴C (). At all times, the concentration of CO₂ in the soil gas was higher than in the air at the surface (CO₂ ~ 400 ppm; δ¹³C ~ -9). The main source of the CO₂ in soil gas is from biotic activity released through roots of trees and of seasonal plants close to the surface. In the winter, the CO₂ concentrations were lowest (6000 ppm) and the δ¹³C was -20. In the spring and through the summer, the CO₂ concentration increased. It was estimated that the major source of CO₂ is at ~240 cm depth (δ¹³C ~ -22; CO₂ ~ 9000 ppm) or below. Above this level, the concentrations decrease and the δ¹³C () become more positive. The ¹⁴C values in the measured profile are all less than atmospheric and biotic ¹⁴C. It was deduced that biotic CO₂ dissolves in porewater to form carbonic acid, which then dissolves secondary carbonate (δ¹³C ~ -8; ¹⁴C ~ -900) from the sediments of the USZ. With the ¹⁴C data, the subsequent release of CO₂ into the soil gas was then estimated. The ¹⁴C data, supported by the ¹³C and CO₂ data, also indicate a biotic source at the root zone, at about 90 cm depth.

Plants can alter rates of electron transport through the alternative oxidase (AOX) pathway in response to environmental cues, thus modulating respiratory efficiency, but the ¹⁸O discrimination method necessary for measuring electron partitioning in vivo has been restricted to laboratory settings. To overcome this limitation, we developed a field-compatible analytical method. Series of plant tissue subsamples were incubated in 12mL septum-capped vials for 0.5-4h before aliquots of incubation air were injected into 3.7mL evacuated storage vials. Vials were stored for up to 10 months before analysis by mass spectrometry. Measurements were corrected for unavoidable contamination. Additional mathematical tools were developed for detecting and addressing non-linearity (whether intrinsic or due to contamination) in the data used to estimate discrimination values. Initial contamination in the storage vials was 0.03±0.01atm; storing the gas samples at -17°C eliminated further contamination effects over 10 months. Discrimination values obtained using our offline incubation and computation method replicated previously reported results over a range of 10-31, with precision generally better than ±0.5. Our method enables large-scale investigations of plant alternative respiration along natural environmental gradients under field conditions.

Urbanization is accelerating across the globe, elevating the importance of studying urban ecology. Urban environments exhibit several factors affecting plant growth and function, including high temperatures (particularly at night), CO2 concentrations and atmospheric nitrogen deposition. We investigated the effects of urban environments on growth in Quercus rubra L. seedlings. We grew seedlings from acorns for one season at four sites along an urban-rural transect from Central Park in New York City to the Catskill Mountains in upstate New York (difference in average maximum temperatures of 2.4°C; difference in minimum temperatures of 4.6°C). In addition, we grew Q. rubra seedlings in growth cabinets (GCs) mimicking the seasonal differential between the city and rural sites (based on a 5-year average). In the field experiment, we found an eightfold increase in biomass in urban-grown seedlings relative to those grown at rural sites. This difference was primarily related to changes in growth allocation. Urban-grown seedlings and seedlings grown at urban temperatures in the GCs exhibited a lower root: shoot ratio (urban ∼0.8, rural/remote ∼1.5), reducing below-ground carbon costs associated with construction and maintenance. These urban seedlings instead allocated more growth to leaves than did rural-grown seedlings, resulting in 10-fold greater photosynthetic area but no difference in photosynthetic capacity of foliage per unit area. Seedlings grown at urban temperatures in both the field and GC experiments had higher leaf nitrogen concentrations per unit area than those grown at cooler temperatures (increases of 23 in field, 32 in GC). Lastly, we measured threefold greater ¹³C enrichment of respired CO2 (relative to substrate) in urban-grown leaves than at other sites, which may suggest greater allocation of respiratory function to growth over maintenance. It also shows that lack of differences in total R flux in response to environmental conditions may mask dramatic shifts in respiratory functioning. Overall, our findings indicating greater seedling growth and establishment at a critical regeneration phase of forest development may have important implications for the ecology of urban forests as well as the predicted growth of the terrestrial biosphere in temperate regions in response to climate change.

Nitrogen (N) and water availability are important factors affecting ecosystem productivity that can be influenced by land-use change. We hypothesized that the observed increase in carbon (C) sequestration associated with afforestation of semi-arid sparse shrubland must also be associated with an increase in N input. We tested this hypothesis by reconstructing the ecosystem N budget of two ecosystems, a semi-arid shrubland and a nearby planted pine forest, using measurements augmented with literature-based estimates. Our findings demonstrate that, contrary to our hypothesis, massive C sequestration by the pine forest could be accounted for without a change in the net N budget (i. e., neither elevated N inputs nor reduced N losses). However, in comparison to the shrubland, the forest showed an almost tripling in aboveground N use efficiency (NUE; 235 vs. 83 kg dry mass kg ^-1 N) and a doubling in ecosystem level C/N ratio (16 vs. 8, for the forest and shrubland, respectively). Nitrogen cycling slowed in the forest compared to the shrubland: net N mineralization rates in soils decreased by approximately 50%, decomposition rates decreased by approximately 20%, and NO _x loss decreased by approximately 64%. These adjustments in N cycling provide a possible basis for increased NUE and subsequent C sequestration without net change in the overall N budget, which should be addressed in future investigations.

Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W _i) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose ¹³C and ¹⁸O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280-700 mm) in Israel. The results show a clear trend of increasing W _i in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W _i trends were better correlated with the increase in atmospheric CO ₂ concentration, C _a, than with the local increase in temperature (~0.04°C year ^-1), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W _i from EW and BAI were dependent on the interannual variations in P. From reconstructed C _i values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W _i sensitivity to C _a between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C _i/C _a towards a homeostatic set-point observed at the most mesic site, with a decrease in the W _i response to C _i with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W _i driven, and there was an increase in the W _i sensitivity to C _a with aridity and a decreasing sensitivity of C _i to C _a. The apparent C _a-driven increases in W _i can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.

The ¹³C concentration in atmospheric CO₂ has been declining over the past 150 years as large quantities of ¹³C-depleted CO₂ from fossil fuel burning are added to the atmosphere. Deforestation and other land use changes have also contributed to the trend. Looking at the ¹³C variations in the atmosphere and in annual growth rings of trees allows us to estimate CO₂ uptake by land plants and the ocean, and assess the response of plants to climate. Here I show that the effects of the declining ¹³C trend in atmospheric CO₂ are recorded in the isotopic composition of paper used in the printing industry, which provides a well-organized archive and integrated material derived from trees' cellulose. ¹³C analyses of paper from two European and two American publications showed, on average, a - 1.65 1.00 trend between 1880 and 2000, compared with - 1.45 and - 1.57 for air and tree-ring analyses, respectively. The greater decrease in plant-derived ¹³C in the paper we tested than in the air is consistent with predicted global-scale increases in plant intrinsic water-use efficiency over the 20th century. Distinct deviations from the atmospheric trend were observed in both European and American publications immediately following the World War II period.

Effective leaf area index (LAI_e) in the semi-arid Pinus halepensis plantation, located between arid and semi-arid climatic zones at the edge of the Negev and Judean deserts, was measured bi-annually during four years (2001-2004) and more intensively (monthly) during the following two years (2004-2006) by a number of non-contact optical devices. The measurements showed a gradual increase in LAI_e from ∼1 (±0.25) to ∼1.8 (±0.11) during these years. All instruments, when used properly, gave similar results that were also comparable with actual leaf area index measured by litter collection and destructive sampling and allometric estimates. Because of the constraint of clear sky conditions, which limited the use of the fisheye type sensors to times of twilight, the fisheye techniques were less useful. The tracing radiation and architecture of canopies system, which includes specific treatment of two levels of clumpiness of the sparse forest stand, was used successfully for the intensive monitoring. The mean clumpiness index, 0.61, is considered representative for the specific environment. Finally, the LAI_e measurements at the start of each season were used to constrain phenology-based estimates of annual LAI_e development, resulting in a continuous course of LAI_e in the forest over the five-year period. Intra-seasonal LAI_e variation in the order of 10% of total LAI_e predicted by the model was also observed in the intensive TRAC measurements, giving confidence in the TRAC system and indicating its sensitivity and applicability in woodlands even with low LAI_e values. The results can be important for forest management decision support as well as for use in evaluation of remote sensing techniques for forests at the lowest range of LAI_e values.

The distribution of precipitation inputs into different hydrological components of water-limited forest ecosystems determines water availability to trees and consequently forest productivity. We constructed a complete hydrological budget of a semi-arid pine forest (285 mm annual precipitation) by directly measuring its main components: precipitation (P), soil water content, evapotranspiration (ET, eddy covariance), tree transpiration (sap flux), soil evaporation (soil chambers), and intercepted precipitation (calculated). Our results indicated that on average for the 4-year study period, ET accounted for 94% of P, varying between 100% when P 300 mm (with indications for losses to subsurface flow and soil moisture storage in wetter years). Direct measurements of the components of the ET flux demonstrated that both transpiration and soil evaporation were significant in this dry forest (45% and 36% of ET, respectively). Comparison between ecosystem ET (eddy covariance measurements) and the sum of its measured components showed good agreement on annual scales, but up to 30% discrepancies (in both directions) on shorter timescales. The pulsed storm pattern, characteristics of semi-arid climates, was sufficient to maintain the topsoil layer wet during the whole wet season. Only less often and intensive storms resulted in infiltration to the root zone, increasing water availability for uptake by deeper roots. Our results indicate that climate change predictions that link reduced precipitation with increased storm intensity may have a smaller effect on water availability to forest ecosystems than reduced precipitation alone, which could help forests' survival and maintain productivity even under drier conditions.

While there is currently intense effort to examine the ¹³C signal of CO₂ evolved in the dark, less is known on the isotope composition of day-respired CO₂. This lack of knowledge stems from technical difficulties to measure the pure respiratory isotopic signal: day respiration is mixed up with photorespiration, and there is no obvious way to separate photosynthetic fractionation (pure c_i/c_a effect) from respiratory effect (production of CO₂ with a different δ¹³C value from that of net-fixed CO₂) at the ecosystem level. Here, we took advantage of new simple equations, and applied them to sunflower canopies grown under low and high [CO₂]. We show that whole mesocosm-respired CO₂ is slightly ¹³C depleted in the light at the mesocosm level (by 0.2-0.8), while it is slightly ¹³C enriched in darkness (by 1.5-3.2). The turnover of the respiratory carbon pool after labelling appears similar in the light and in the dark, and accordingly, a hierarchical clustering analysis shows a close correlation between the ¹³C abundance in day- and night-evolved CO₂. We conclude that the carbon source for respiration is similar in the dark and in the light, but the metabolic pathways associated with CO₂ production may change, thereby explaining the different ¹²C/¹³C respiratory fractionations in the light and in the dark.

Forests both take up CO₂ and enhance absorption of solar radiation, with contrasting effects on global temperature. Based on a 9-year study in the forests' dry timberline, we show that substantial carbon sequestration (cooling effect) is maintained in the large dry transition zone (precipitation from 200 to 600 millimeters) by shifts in peak photosynthetic activities from summer to early spring, and this is counteracted by longwave radiation (L) suppression (warming effect), doubling the forestation shortwave (S) albedo effect. Several decades of carbon accumulation are required to balance the twofold S + L effect. Desertification over the past several decades, however, contributed negative forcing at Earth's surface equivalent to ∼20% of the global anthropogenic CO₂ effect over the same period, moderating warming trends.

The flux (R(s)) and carbon isotopic composition (delta(13)C (Rs)) of soil respired CO (2) was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the delta(13)C composition of leaf, root and soil organic matter (delta(13)C (SOM)) and metabolites. Simulations of R(s) and delta(13)C (Rs) were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of delta(13)C (Rs) over the study period were within the range of root metabolite and average delta(13)C (SOM) values, but enriched in (13)C relative to the bulk delta(13)C of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO (2) fluxes and the underlying features of delta(13)C (Rs). Observed diel variations of 0.5 per thousand in delta(13)C (Rs) were predicted by the model in response to temperature-related shifts in production rates along a approximately 3 per thousand gradient observed in the profile of delta(13)C (SOM). However, observed delta(13)C (Rs) varied by over 2 per thousand, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of delta(13)C (Rs).

This paper presents a study of the isotopic composition of dissolved inorganic carbon (DIC) stable and, radioactive, in pore water of the unsaturated zone (USZ) above the coastal aquifer of Israel The carbon. content and its isotopic composition in the gas and solid phases of the USZ are also presented. In the soil gas, large quantities of CO2 with quite modern C-14 activity were measured along the section (0.15 to 2.7% and 97 to 109 pMC respectively). In the inorganic fraction of the sediments, the C-14 activity between 2.5 and 13 m was 33 to 1.5 pMC. In the organic fraction the 14C activity between 7 and 9 m was 40 to 33 pMC. In the pore water, the high values of tritium and C-14 at depths of similar to 15-18 m have been attributed to the thermonuclear contamination of the 1960s. A significant decrease with depth in the DIC of pore water, from 23 to 4 mmol C/L, along with a decrease with depth in dissolved inorganic C-14, from 100 pMC near the surface to 72 pMC at depth of 20 m were observed. The delta C-13 values in the DIC are similar to the values in the inorganic sediment (similar to -10 parts per thousand). A first order reaction was applied to estimate the yearly rates of DIC loss by net precipitation (3.2%/year) and of C-14 activity (dpm/L) loss from the DIC by gross precipitation from the DIC on the sediment, (4.4%/year). From the gradient of dissolved inorganic C-14, the initial level at the bottom of the USZ, which is the groundwater table of the coastal aquifer of Israel, was estimated to be 0.54 of contemporaneous atmospheric value of C-14 when the rain fell on the ground. This value can be used for improved dating of groundwater with C-14 in the coastal aquifer of Israel. (C) 2009 Elsevier B.V. All rights reserved.

Warming and drying is predicted for most of the Mediterranean and other regions, and knowledge of this effect on forest carbon dynamics cannot be easily extrapolated from temperate climates. Instead, we provide quantitative information from a 6-year study in a 40-year old pine forest at the dry timberline (280 mm annual rainfall) on soil CO₂ efflux (F _s) and some of its controlling factors. Annual F_s was relatively low (405.9 ± 23.8 g C m^-2 a^-1), but within one standard deviation of a global nonlinear relationship we derived between mean annual precipitation and F_s. in forests. Seasonal variations in F_s were dominated by soil temperature (with Q ₁₀ = 2.45) in the wet season, and by soil moisture in the water-limited seasons, but not by pulse responses to precipitation No temperature sensitivity was observed in the dry season, but there was clear evidence for down regulation of sensitivity to Q₁₀ = 1.18 when soil moisture was kept high by a supplement summer irrigation treatment. Interannual variations in F₅ correlated linearly with cumulative rainwater availability, indicating the combined importance of both precipitation amount and its temporal distribution between the wet (and cool) season and the transitional periods characterized by high evaporative demand. Low rates of soil carbon loss combined with high belowground carbon allocation (41% of canopy CO₂ uptake) might explain the high soil organic carbon accumulation and net ecosystem productivity in this dry forest. Our results indicate that F_S in pine forests may adjust to dry conditions with better carbon economy than estimated from response to episodic drought in more temperate climate.

Drought is the major factor limiting wheat productivity worldwide. The gene pool of wild emmer wheat, Triticum turgidum ssp. dicoccoides, harbours a rich allelic repertoire for morpho-physiological traits conferring drought resistance. The genetic and physiological bases of drought responses were studied here in a tetraploid wheat population of 152 recombinant inbreed lines (RILs), derived from a cross between durum wheat (cv. Langdon) and wild emmer (acc# G18-16), under contrasting water availabilities. Wide genetic variation was found among RILs for all studied traits. A total of 110 quantitative trait loci (QTLs) were mapped for 11 traits, with LOD score range of 3.0-35.4. Several QTLs showed environmental specificity, accounting for productivity and related traits under water-limited (20 QTLs) or well-watered conditions (15 QTLs), and in terms of drought susceptibility index (22 QTLs). Major genomic regions controlling productivity and related traits were identified on chromosomes 2B, 4A, 5A and 7B. QTLs for productivity were associated with QTLs for drought-adaptive traits, suggesting the involvement of several strategies in wheat adaptation to drought stress. Fifteen pairs of QTLs for the same trait were mapped to seemingly homoeologous positions, reflecting synteny between the A and B genomes. The identified QTLs may facilitate the use of wild alleles for improvement of drought resistance in elite wheat cultivars.

Isotopic measurements of leaf water have provided insights into a range of ecophysiological and biogeochemical processes, but require an extraction step which often constitutes the major analytical bottleneck in large-scale studies. Current standard procedures for leaf water analysis are based on cryogenic vacuum or azeotrophic distillation, and are laborious, require sophisticated distillation lines and the use of toxic materials. We report a rapid technique based on centrifugation/filtration of leaf samples pulverised in their original sampling tubes, using a specifically adapted, simple apparatus. The leaf water extracts produced are suitable for isotopic analysis via pyrolysis gas chromatography isotope ratio mass spectrometry (PYR/GC/IRMS). The new method was validated against cryogenic vacuum distillation and showed an overall accuracy of ±0.5 (nine grouped comparisons, n = 110) over a range of 21. Effects due to the presence of soluble carbohydrates were near the detection limits for most samples analysed, and these effects could be corrected for (the extracted soluble organics could also be used for isotopic analysis). The extraction time for a routine eight-sample subset was reduced from 4 h (cryogenic distillation) to 45 min, limited only by the size of the centrifuge(s) used. This method provides a rapid, low-cost and reliable alternative to conventional vacuum and other distillation methods that can alleviate current restrictions on ecosystem- and global-scale studies that require high-throughput leaf water isotopic analysis.

Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO₂ fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO ₂ deposition between ĝ\u20ac"11.31 and ĝ\u20ac"17.56 mmol m⁻² per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO₂ deposition by BSC was calculated to compensate 120%, ĝ\u20ac"26%, and less than 3% of the concurrent soil CO₂ efflux from Novemberĝ\u20ac"January, Februaryĝ\u20ac"May and Novemberĝ\u20ac"May, respectively. Thus, BSC effectively compensated soil CO₂ effluxes when CO₂ uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO₂ deposition within the period Novemberĝ\u20ac"January by 11ĝ\u20ac"123% and on average by 27%. The analysis of CO₂ fluxes and water inputs from the various sources showed that the bulk of BSC-related CO₂ deposition occurs during periods with frequent rain events and subsequent condensation from water accumulated in the upper soil layers. Significant BSC activity on days without detectable atmospheric water supply emphasized the importance of high soil moisture contents as additional water source for soil-dwelling BSC, whereas activity upon dew formation at low soil water contents was not of major importance for BSC-related CO₂ deposition. However, dew may still be important in attaining a pre-activated status during the transition from a long "summer" anabiosis towards the first winter rain.

This study explored possible advantages conferred by the phase shift between leaf phenology and photosynthesis seasonality in a semi-arid Pinus halepensis forest system, not seen in temperate sites. Leaf-scale measurements of gas exchange, nitrogen and phenology were used on daily, seasonal and annual time-scales. Peak photosynthesis was in late winter, when high soil moisture, mild temperatures and low leaf vapour pressure deficit (D_L) allowed high rates associated with high water- and nitrogen-use efficiencies. Self-sustained new needle growth through the dry and hot summer maximized photosynthesis in the following wet season, without straining carbon storage. Low rates of water loss were associated with increasing sensitivity of stomatal conductance (g_s) to soil moisture below a relative extractable water (REW) of 0.4, and decreased g_s sensitivity to D_L below REW of approx. 0.2. This response was captured by the modified Ball-Berry (Leuning) model. While most physiological parameters and responses measured were typical of temperate pines, the photosynthesis- phenological phasing contributed to high productivity under warm-dry conditions. This contrasts with reported effects of short-term periodical droughts and could lead to different predictions of the effect of warming and drying climate on pine forest productivity.

To identify factors that influence the relatively high productivity of a semi-arid pine afforestation system in southern Israel, we investigated inorganic nitrogen deposition and mineralization for more than 2 years. To this end, we measured bulk and dry deposition, in situ N-mineralization over the seasonal cycle, and the potential activity of nitrifying microorganisms by soil slurry incubations. There was a small increase in bulk N deposition in the forest, compared with shrubland, but no change in dry deposition. An unexpected rapid increase in nitrite concentration in the forest soil was observed after soil rewetting by the first winter rains, which could not be explained by deposition. This was accompanied by a decrease in ammonium and only a slight increase in nitrate concentrations. Only a small increase in nitrite and a rapid increase in nitrate concentration in the mineral soil were observed in the surrounding shrubland. Soil slurry incubations from the forest sites exhibited significant delay in nitrite, compared with nitrate accumulation (up to 50 h under lab conditions) in samples taken in the dry season, but not in the wet season. This indicated different rates of ammonium and nitrite oxidation that are most likely linked to differential activation of different microbial populations after the summer stress. The initial oxidation process of ammonia to nitrate, upon soil rewetting in semi-arid environments, appears to occur as a partially uncoupled two-step process, as opposed to a rapid continuous one in wetter environments. This may have implications for the synchronization of nitrate availability to plants and therefore for high forest productivity and nitrogen use efficiency. Forest productivity in the semi-arid regions, in turn, is becoming increasingly more important with persistent predictions of warming and drying trends over the entire Mediterranean basin and other regions.

An approach is presented for determining leaf area index (LAI) of a forest located at the desert fringe by using high spatial resolution imagery and by implementing values from a moderate spatial but high temporal resolution sensor. A 4-m spatial resolution multi-spectral IKONOS image was acquired under clear sky conditions on March 25, 2004. Normalized differences vegetation index (NDVI) and a linear mixture model were applied to calculate fractional vegetation cover (FVC). LAI was calculated using a non-linear relationship to FVC and then compared with ground truth measurements made in ten 1000 m² plots using the tracing radiation and architecture of canopies (TRAC) canopy analyzer under bright and clear sky conditions during March and April, 2004. Calculated LAI, corrected with a measured clumping index, was highly correlated with measured LAI (R² = 0.79, p

Vacuum distillation is shown to be useful for the quantitative extraction of dissolved inorganic carbon (as CO₂) and water from sediments of the unsaturated zone in the Coastal Aquifer of Israel. Several tests of vacuum extractions from tap water and sediments are presented, including standard addition, which show that the distillation procedure is quantitative, with minimal or no carbon isotope fractionation. The optimal temperature of the sediment during the extraction was also defined. Examples of vacuum extractions of sediments are shown.

Ecosystems in dry regions are generally low in productivity and carbon (C) storage. We report, however, large increases in C sequestration following afforestation of a semi-arid shrubland with Pinus halepensis trees. Using C and nitrogen (N) inventories, based in part on site-specific allometric equations, we measured an increase in the standing ecosystem C stock from 2380 g Cm ^-2 in the shrubland to 5840 g Cm^-2 in the forest after 35 years, with no significant change in N stocks. Carbon sequestration following afforestation was associated with increased N use efficiency as reflected by an overall increase in C/N ratio from 7.6 in the shrubland to 16.6 in the forest. The C accumulation rate in the forest was particularly high for soil organic C (SOC; increase of 1760 g Cm^-2 or 50 g Cm^-2 yr ^-1), which was associated with the following factors: 1) Analysis of a small ¹³C signal within this pure C₃ system combined with size fractionation of soil organic matter indicated a significant addition of new SOC derived from forest vegetation (68% of total forest SOC) and a considerable portion of the old original shrubland SOC (53%) still remaining in the forest. 2) A large part of both new and old SOC appeared to be protected from decomposition as about 60% of SOC under both land-use types were in mineral-associated fractions. 3) A short-term decomposition study indicated decreased decomposition of lower-quality litter and SOC in the forest, based on reduced decay rates of up to 90% for forest compared to shrubland litter. 4) Forest soil included a significant component of live and dead roots (12% of total SOC). Our results suggest a role for increased N use efficiency, enhanced SOC protection and reduced decomposition rates in the large C sequestration potential following afforestation in semi-arid regions. These results are particularly relevant in light of persistent predictions of drying trends in the Mediterranean and other regions.

The isotopic composition of atmospheric O₂ depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on δ¹⁷O and δ¹⁸O of ocean and leaf water. While most of the factors affecting δ¹⁷O and δ¹⁸O of air O₂ have been studied extensively in recent years, δ¹⁷O of leaf water-the substrate for all terrestrial photosynthesis-remained unknown. In order to understand the isotopic composition of atmospheric O₂ at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in δ¹⁷O and δ¹⁸O between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln(δ¹⁷O + 1) vs. ln(δ¹⁸O + 1) plots are characterized by very high precision (∼0.001) despite of relatively large differences between duplicates in both δ¹⁷O and δ¹⁸O (0.02-0.05). This is so because the errors in δ¹⁸O and δ¹⁷O are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 ± 0.0013 and 0.5204 ± 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity (h) as 0.522-0.008 × h, for h in the range 0.3-1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves.

The aim of this research was to study the relationships between the biological soil crusts (BSC), spectral reflectance and photosynthetic activity. Twenty field campaigns, each lasting several days, were conducted during the 2002-2003 rainy season at sand dune and loess environments in the north-western Negev desert of Israel. Simultaneous measurements of CO₂ net exchange and spectral reflectance were carried out for several types of BSC. The Normalized Difference Vegetation Index (NDVI) was derived from the BSC reflectance and correlated with their CO₂ exchange data. The relationship between NDVI and CO₂ exchange is discussed in detail with respect to environmental factors, such as soil water content, air temperature, and light intensity. Fairly good correlations were found in the rainy season. The NDVI was useful in indicating the potential magnitude and capacity of the BSC assimilation activity. Furthermore, the index corresponded well with different rates of photosynthetic activity of the different types of microphytes. The results demonstrate that spectral reflectances of the BSC can be related to photosynthetic activities and posseses the potential to assess the amount of carbon sequestration by these microphytes on an areal scale using satellite images.

Eddy covariance technique to measure CO₂, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more than 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. In this paper a new standardized set of corrections is introduced and the uncertainties associated with these corrections are assessed for eight different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented. Through a factorial analysis we find that generally, uncertainties by different corrections are additive without interactions and that the heuristic u*-correction introduces the largest uncertainty. The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning interannual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements.

The use of the ¹³C : ¹²C isotopic ratio (δ¹³C) of leaf-respired CO₂ to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in δ¹³C of leaf dark-respired CO₂ (δ¹³C_r) under field conditions. Here, we explored variability in δ¹³C_r and its relationship to key respiratory substrates from collections of leaf dark-respired CO ₂, carbohydrate extractions and gas exchange measurements over 24-h periods in two Quercus canopies. Throughout both canopies, δ¹³C_r became progressively ¹³C-enriched during the photoperiod, by up to 7, then ¹³C-depleted at night relative to the photoperiod. This cycle could not be reconciled with δ¹³C of soluble sugars (δ¹³C_ss), starch (δ¹³C_st), lipids (δ¹³C _l), cellulose (δ¹³C_c) or with calculated photosynthetic discrimination (Δ). However, photoperiod progressive enrichment in δ¹³C_r was correlated with cumulative carbon assimilation (r² = 0.91). We concluded that there is considerable short-term variation in δ¹³C_r in forest canopies, that it is consistent with current hypotheses for ¹³C fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for δ¹³C_r, and that diel changes in leaf carbohydrate status could be used to predict changes in δ ¹³C_r empirically.

Associations between delta C-13 values and leaf gas exchanges and tree-ring or needle growth, used in ecophysiological compositions, can be complex depending on the relative timing of CO2 uptake and subsequent redistribution and allocation of carbon to needle and stem components. For palaeoenvironmental and dendroecological studies it is often interpreted in terms of a simple model of delta C-13 fractionation in C-3 plants. However, in spite of potential complicating factors, few studies have actually examined these relationships in mature trees over inter- and intra-annual time-scales. Here, we present results from a 4 years study that investigated the links between variations in leaf gas-exchange properties, growth, and dated delta C-13 values along the needles and across tree rings of Aleppo pine trees growing in a semi-arid region under natural conditions or with supplemental summer irrigation. Sub-sections of tissue across annual rings and along needles, for which time of formation was resolved from growth rate analyses, showed rapid growth and 6 C responses to changing environmental conditions. Seasonal cycles of growth and delta C-13 (Up to similar to 4%.) significantly correlated (P

A widespread conversion of sagebrush-dominated plant communities to post-fire communities dominated by herbaceous annual species has occurred in the Great Basin of North America's Intermountain West during the last century, mainly driven by invasions of alien species and a subsequent increase in fire frequency. Little experimental data, however, are available on the potential consequences of wildfire and post-fire plant community alteration on key ecosystem functions such as ecosystem hydrology. The objectives of our study were to (1) quantify the effects of this vegetation transformation on the spatial and temporal distribution of soil water in the rooting zone; (2) quantify the effects of wild-fire on surface soil water distribution (upper 20 cm) to infer about potential consequences on re-establishment of the native plant community; and (3) examine which processes (e.g., soil water recharge or replenishment, plant water uptake, and soil evaporation) may be responsible for observed changes soil water distribution. Continuous measurement of soil water content using a segmented time domain reflectrometry system in sixteen locations in an intact sagebrush ecosystem (eight shrub and eight inter-shrub locations) and in eight locations in an adjacent post-fire ecosystem from March 2001 to October 2002 indicated that soil water contents in the upper 75 cm of the soil were similar and very low during summers, but that substantially lower water recharge in post-fire ecosystems after large snowfalls - possibly due to decreased snow deposition and higher sublimation - caused large differences in water storage between the two ecosystems in winter. These differences declined with the onset of the vegetation period in March, probably due to higher plant water uptake in the more densely vegetated intact sagebrush ecosystem. The presence of patchiness of near-surface soil water (0-20 cm), quantified among 80 points of nested grid systems placed in each ecosystem, appears to benefit native perennial shrub re-generation in the intact ecosystem. Taken together, the results of our study indicate that vegetation transition from shrublands to post-fire successional plant communities may (1) decrease water recharge in winters leading to lower water storage in winter and spring and potentially reduce groundwater recharge; (2) decrease the amount of plant-available soil water in the root zone during phenologically important times of the years; and (3) reduce the lateral patchiness in surface soil water, which may impede the re-establishment of sagebrush or other patch-dependent native perennials.

Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest-field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3kg m^-2 lower than C stocks in original forest soils. This suggests that soil C losses from land-use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60-year-old grassland showing net ecosystem C gain of 2.1 kg m^-2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land-use change were smaller than reductions in C stocks (34% or 0.31 kg m^-2), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high-C forest-floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m^-2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land-use change. However, by selecting relatively C-poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.

The ¹⁸O content of atmospheric O₂ is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against ¹⁸O associated with the various O₂ consumption processes. Here we evaluated, for the first time, the in situ ¹⁸O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O₂] and δ¹⁸O of O₂ in the soil-air. The discriminations that were found are 10.1 ± 1.5, 17.8 ± 1.0, and 22.5 ± 3.6, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5 for Mediterranean woodland, and 15.4 ± 1:6 for tropical shrub land. Current understanding of the isotopic composition of atmospheric O₂ is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (∼18). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O₂ concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (∼27 . The intermediate discrimination (∼18) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies.

Accurate knowledge of surface emissivity is essential for applications in remote sensing (remote temperature measurement), radiative transport, and modeling of environmental energy balances. Direct measurements of surface emissivity are difficult when there is considerable background radiation at the same wavelength as the emitted radiation. This occurs, for example, when objects at temperatures near room temperature are measured in a terrestrial environment by use of the infrared 814-μmband. This problem is usually treated by assumption of a perfectly diffuse surface or of diffuse background radiation. However, real surfaces and actual background radiation are not diffuse; therefore there will be a systematic measurement error. It is demonstrated that, in some cases, the deviations from a diffuse behavior lead to large errors in the measured emissivity. Past measurements made with simplifying assumptions should therefore be reevaluated and corrected. Recommendations are presented for improving experimental procedures in emissivity measurement.

The oxygen-18 (O-18) content of atmospheric carbon dioxide (CO2) is an important indicator of CO2 uptake on land. It has generally been assumed that during photosynthesis, oxygen in CO2 reaches isotopic equilibrium with oxygen in O-18-enriched water in leaves. We show, however, large differences in the activity of carbonic anhydrase (which catalyzes CO2 hydration and O-18 exchange in leaves) among major plant groups that cause variations in the extent of O-18 equilibrium (theta (eq)). A clear distinction in theta (eq) between C-3 trees and shrubs, and C-4 grasses makes atmospheric (COO)-O-18 a potentially sensitive indicator to changes in C-3 and C-4 productivity. We estimate a global mean theta (eq) value of similar to0.8, which reasonably reconciles inconsistencies between O-18 budgets of atmospheric O-2 (Dole effect) and CO2.

Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO₂ and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO₂, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant-atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of 'Keeling plots.' This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO₂ recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO₂ exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limited arsenal of ecosystem-scale research tools.

Measurements of ¹⁸O in atmospheric CO₂ can be used to trace gross photosynthetic and respiratory CO₂ fluxes between the atmosphere and the terrestrial biosphere. However, this requires knowledge of the ¹⁸O signatures attributable to the fluxes from soil and leaves. Newly developed methods were employed to measure the ¹⁸O of soil-respired CO₂ and depth profiles of near-surface soil CO₂, in order to evaluate the factors influencing isotopic soil-atmosphere CO₂ exchange. The ¹⁸O of soil-respired CO₂ varied predominantly as a function of the ¹⁸O of soil water which, in turn, changed with soil drying and with seasonal variations in source water. The ¹⁸O of soil-respired CO₂ corresponds to full isotopic equilibrium with soil water at a depth ranging between 5 and 15 cm. The ¹⁸O of respired CO₂, in reality, results from a weighted average of partial equilibria over a range of depths. Soil water isotopic enrichment of up to 10 in the top 5 cm did not appear to strongly influence the isotopic composition of the respired CO₂. We demonstrate that during measurements 'invasion' of atmospheric CO₂ (the diffusion of ambient CO₂ into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the ¹⁸O of the soil-respired CO₂. The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO₂ diffusion out of the soil to be 7.2 ± 0.3. High-resolution (1 cm) depth profiles of ¹⁸O of near-surface (top 10 cm) soil CO₂ were carried out by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). This novel technique allowed us to observe the competitive diffusion-equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO₂¹⁸O content.

CO₂ profiles obtained along a 30-m-thick unsaturated zone under land irrigated with sewage effluents show two production regions: a seasonal one in the root zone and another, at steady state, near the water table (29 m). On an annual basis the CO₂ flux from the deep source toward the atmosphere (6.3 g C m^-2 yr^-1) is balanced by a similar influx of soluble organic carbon (SOC) from sewage effluents. The δ¹³C values of soil CO₂ indicate that CO₂ is produced from plant material in the root zone and from biodegradation of total sedimentary organic carbon in the capillary fringe. High CO₂ concentration in the capillary fringe (up to 2%) is likely to reflect a decrease in diffusivity relative to the unsaturated zone due to increase in both water content and tortuosity induced by the capillary fringe structure. The long residence time of SOC and CO₂ in the unsaturated zone (29 years at the study site) suggests that the unsaturated zone of deep aquifers may have a significant storage capacity for carbon and may act as a temporary carbon sink.

The ¹⁸O content of leaf water strongly influences the ¹⁸O contents of atmospheric CO2 and O². The ¹⁸O signatures of these atmospheric gases, in turn, emerge as important indicators of large-scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The ¹⁸O enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the ¹⁸O content of leaf water are influenced by nonclimatic, species-specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9 in the δ¹⁸O values of stem water, δ_S, and of about 14 in the mid-day δ¹⁸O enrichment of bulk leaf water, δ_LW - δ_s- Differences between δ¹⁸O values predicted by a conventional evaporation model, δ_M, and δ_LW ranged between - 3.3 and + 11.8. The δ¹⁸O values of water in the chloroplasts (δ_ch) in leaves of 10 selected plants were estimated from on-line CO₂ discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that δ_ch can significantly deviate from δ_M in species with high leaf peclet number. The δ¹⁸O values of bulk leaf water significantly correlated with δ¹⁸O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in δ¹⁸O and δ¹³C values of cellulose and in their peclet numbers, and from deciduous woodland species in their δ¹⁸O and δ¹³C values of cellulose. The results indicated that predictions of the δ¹⁸O values of leaf water (δ_LW, δ_M and δ_ch) could be improved by considering plant species-specific characteristics.

The ¹³C/¹²C and ¹⁸O/¹⁶O ratios of stem cellulose of Tamarix jordanis (a tree common in wadis of arid regions) increased with decreasing relative humidity (RH) in individual trees growing along a climatic gradient in Israel. The response to RH observed in the δ¹⁸O of the wood cellulose was strongly similar to that observed in leaf water over a diurnal cycle. Most of the data for δ¹³C and all of the data for δ¹⁸O could be fitted to two independent linear equations that, combined, allowed the reconstruction of RH and the δ¹⁸O of source water from the isotopic composition of ancient T. jordanis wood previously reported from the ancient fortress of Masada. Since the Roman period, RH at Masada decreased by about 17%, while the δ¹⁸O value of local groundwater remained similar to present-day values, suggesting that changing atmospheric circulation has played a role in climate change in the Middle East over the past two millennia.

THE atmospheric budget of carbon compounds can be balanced only by invoking a significant 'missing sink' for carbon dioxide^1-3. Identifying this sink requires a knowledge of CO₂ fluxes at global and local scales. The former can be estimated from global averages of CO₂ concentration and isotope composition^4-9; local-scale measurements have been made by analysing individual eddies of air^10,11. In both cases, the net CO₂ exchange is the sum of two opposing fluxes: uptake by gross primary productivity and release by respiration. Here we show that these two components can be estimated separately at the local scale from small vertical gradients in ¹³C and ¹⁸O in atmospheric CO₂ above vegetation. By also analysing the ¹⁸O content of moisture in the air samples, we can estimate evapotranspiration rates, providing information on water exchange between the biosphere and atmosphere¹². We suggest that this approach can be extended to the regional scale.

The contribution of the cyanide-resistant, alternative pathway to plant mitochondrial electron transport has been studied using a modified aqueous phase on-line mass spectrometry-gas chromatography system. This technique permits direct measurement of the partitioning of electrons between the cytochrome and alternative pathways in the absence of added inhibitors. We demonstrate that in mitochondria isolated from soybean (Glycine max L. cv Ransom) cotyledons, the alternative pathway contributes significantly to oxygen uptake under state 4 conditions, when succinate is used as a substrate. However, when NADH is the substrate, addition of pyruvate, an allosteric activator of the alternative pathway, is required to achieve the same level of alternative pathway activity. Under state 3 conditions, when the reduction state of the ubiquinone pool is low, the addition of pyruvate allows the alternative pathway to compete with the cytochrome pathway for electrons from the ubiquinone pool when the cytochrome pathway is not saturated. These results provide direct experimental verification of the kinetics consequences of pyruvate addition on the partitioning of electron flow between the two respiratory pathways. This distribution of electrons between the two unsaturated pathways could not be measured using conventional oxygen electrode methods and illustrates a clear advantage of the mass spectrometry technique. These results have significant ramifications for studies of plant respiration using the oxygen electrode, particularly those studies involving intact tissues.

THE level of the Dead Sea, the lowest (about -400 m) and one of the most salty (salinity about 340 g l(-1))lakes on earth, is lowering at a rate of approximately 0.5 m annually owing to extensive exploitation of its main perennial tributary (the Jordan River) and the extreme aridity of the region (annual precipitation is about 60 mm)(1). Consequently, new hypersaline sea shores are exposed, forming a unique, originally sterile ecosystem. The first plants invade these newly exposed shores after several years while soil water salinity is still extremely high, Here we use stable isotopes of oxygen and hydrogen to show that a variety of such perennial pioneer plants are able to make use of occasional floodwater which is distinct from the bulk of the hypersaline soil water found in their root zone. Our results provide new insight into the ways in which plants can invade extremely hostile environments and extend their ecological limits of distribution.

Two direct but independent approaches were developed to identify the average δ¹⁸O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the δ¹⁸O value of CO₂ in isotopic equilibrium with leaf water to reconstruct the δ¹⁸O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO₂ and H₂O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the noncatalysed exchange in other leaf water fractions. In the second approach, we measured the δ¹⁸O value of O₂ from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O₂ is produced from chloroplast water irreversibly and without discrimination, the δ¹⁸O value of the O₂ should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average δ¹⁸O value of chloroplasts water was displaced by 310 % (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig & Gordon model. Furthermore, this δ¹⁸O value was always lower than the δ¹⁸O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the δ¹⁸O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the δ¹⁸O values of atmospheric CO₂ and O₂, as well as in influencing the δ¹⁸O of plant organic matter.