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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 area’s 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.

Soil carbon turnover time (tSOC), the ratio between soil organic carbon stocks (SOC), and soil heterotrophic respiration (Rh), is a critical factor in determining soil carbon storage and a key parameter in terrestrial carbon models. While tSOC is generally expected to increase with drying conditions, its interactions with the carbon fluxes and soil moisture are still poorly constrained. Our study centered on a five-year manipulation experiment in the Yatir semi-arid pine forest in Israel, where supplement irrigation eliminated the summer drought. Soil CO2 fluxes (Fs) and soil organic carbon (SOC) stocks were measured under trees and in open areas in a "control" forest plot (CTRL) and an 0.1 ha “irrigated” plot (IRRI). During the dry period (May-November), daily average Fs in the open areas was near zero in the CTRL but significant in the IRRI plots (0.06 and 2.02 µmol CO2 m-2 s-1 respectively, with a similar trend under the trees). Annual-scale fluxes in the open areas were 82 and 321 g C m-2 yr-1 in the CTRL and IRRI plots, respectively (with similar trends under trees). Using published results from the same site enabled us to partition Fs and estimate Rh, which indicated that under the drought conditions (CTRL) tSOC was x5 longer in the open area (and x2 longer under trees) compared with the non-droughted (IRRI) plot. However, no significant changes in the SOC stock down to 40 cm (the typical soil depth at this site) were observed. Furthermore, there were no differences between treatments in regard to the ratio between the stable mineral-associated organic carbon fraction and the particulate organic carbon fraction. The stability of SOC stocks, despite the large changes in tSOC suggests that carbon inputs must have increased proportionally to match the changes in carbon outputs and provided the main source for the increased Fs. The results indicate that changes in the intensity of the seasonal drought can result in large changes in fluxes and tSOC values with little impact on soil carbon storage and its stability.

Efficient heat dissipation under high radiation load is critical to plant functioning. It includes processes that are clearly observed in dry ecosystems but likely extend to other environments where it will be further enhanced by climate change. We observed that despite near-zero evaporation during the seasonal drought in a semi-arid pine forest, leaf temperatures were within the physiological range at about 35C. At the same time, exposed soil at the same site reached temperatures up to 70C. These leaf temperatures were also similar to that in an irrigated plot where evapotranspiration (ET) was enhanced by x10. A detailed energy budget demonstrates that heat dissipation under drought relies on a large sensible heat flux (H) that must depend, in turn, on reducing aerodynamic resistance to heat transfer. At the canopy scale, a “convector effect” of the high-roughness dry canopies generates a massive H that increases the depth of the planetary boundary layer and induces secondary circulations. Model simulation at larger scales indicated that such a process could modify local and regional climatic conditions. Assessing the global FLUXNET data from different environments further indicates that relying on H as a major heat dissipation process is not limited to dry ecosystems and is not dictated solely by the radiation load.

Climate change is predicted to reduce many regions' annual precipitation (P). Combined with the rise in population, this will impose substantial pressure on the water supply for vegetation, food production, and human consumption. Based on global measurements of water and energy fluxes and meteorological conditions, we find that ecosystems are efficient and conservative users of rainfall irrespective of biomes’ type, location, and climate. It results in annual ET remaining at ~450±200 mm, except in dry conditions where ET»P. As a result, changes in the residual ecosystem water yield (WYe=P-ET) are greatly amplified relative to changes in precipitation. For example, at a site with a mean annual P of 700 mm, a decrease to 500 mm in annual P (28% decrease, well within expected climate change scenarios) would result in WY dropping from 200 to zero (100% decrease). Such changes could drive local ecosystems to their limit of sustainability and eliminate residual water for runoff and consumption. The causality underlying the conservative nature of ET is not entirely clear at present. But we can nevertheless conclude that the unsustainable state of WY

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.

The modulation of the leaf energy budget components to maintain optimal leaf temperature are fundamental aspects of plant functioning and survival. Better understanding these aspects becomes increasingly important under a drying and warming climate when cooling through evapotranspiration (ET) is suppressed. Combining novel measurements and theoretical estimates, we obtained unusually comprehensive twig-scale leaf energy budgets under extreme field conditions in droughted (suppressed ET) and non-droughted (enhanced ET) plots of a semi-arid pine forest. Under the same high mid-summer radiative load, leaf cooling shifted from relying on nearly equal contributions of sensible ( H) and latent ( LE) energy fluxes in non-droughted trees to relying almost exclusively on H in droughted ones, with no change in leaf temperature. Relying on our detailed leaf energy budget, we could demonstrate that this is due to a 2× reduction in leaf aerodynamic resistance. This capability for LE-to-H shift in leaves of mature Aleppo pine trees under droughted field conditions without increasing leaf temperature is likely a critical factor in the resilience and relatively high productivity of this important Mediterranean tree species under drying conditions.

Climate change is often associated with increasing vapor pressure deficit (VPD) and decreasing soil moisture (SM). While atmospheric and soil drying often co-occurs, their differential effects on plant functioning and productivity remain uncertain. We aimed to elaborate on 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-atmosphere 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 (g ), the rates of transpiration (E) and net photosynthesis (A ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g to the massive rise in VPD (to a maximum of 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 that plant water potential is a good predictor of the g and E response to VPD. These results demonstrate that the release from drought on the supply-side, in combination with plant hydraulic regulation, eliminates the effect of atmospheric demand (VPD) as a stressor and on canopy gas exchange in mature, drought-adapted pine trees.

The modulation of the leaf energy budget and the balance between its sensible heat (H) and latent heat (LE) fluxes is vital for vegetation functioning and survival, as it is linked to maintaining leaf temperature below the thermal threshold, an increasingly important mechanism under a drying and warming climate, when evaporative cooling is suppressed.Combining measurements and theoretical estimates using a new methodology, we obtained rare and comprehensive energy budgets of leaves on twigs under field conditions in droughted and non-droughted plots of a semi-arid pine forest with low and high evapotranspiration rates, respectively.An examination of all components of the needle-leaf energy budget indicated that under the same radiative load, leaf cooling shifts from nearly equal contributions to H and LE in non-droughted trees to almost exclusively H in droughted ones while maintaining a similar leaf temperature.This LE-to-H shift in leaves of droughted trees highlights the efficiency of the ‘air cooling’ mechanism in maintaining temperature, which can enhance the resilience of trees to drying conditions. Additionally, leaf energy budgets are a fundamental tool to help understand leaf cooling and aerodynamic resistance under field conditions, and to improve modelling of ecosystem activity and its effect on the climate system.

Efficient leaf-scale temperature control is instrumental to vegetation functioning and ecosystem-scale resilience to a drying and warming climate. When evaporative cooling is suppressed during drought, leaf thermal regulation requires modulation of the leaf energy budget and the balance between sensible heat (H) and latent heat (LE) fluxes.We obtained rare leaf energy budgets under field conditions by combining measurements using a new methodology and theoretical estimates in naturally droughted and artificially irrigated plots of a dry Mediterranean pine forest, with low and high evapotranspiration rates, respectively.The measurements revealed that under the same radiative load, leaf cooling shifted from equal contributions to heat dissipation of H and LE in irrigated trees to almost exclusively through H in droughted ones while maintaining comparable leaf-to-air temperature differences.The results demonstrate that an assessment of the leaf energy budget in the field provides the means to identify effective leaf temperature control in pine trees under drought, enhancing their resilience to current drying trends. The shift from LE to H provides an ‘air cooling’ mechanism that equals the efficiency of evaporative cooling. It also provides a leaf-scale basis for the large ecosystem-scale ‘convector effect’ identified in semi-arid forests.

Remote sensing (RS) techniques have great potentials for land surface monitoring. Nevertheless, for most low to moderate resolution satellites, the problem of mixed pixels with information from the vegetation of interest and the background surfaces can cause significant biases in the signals and their interpretations. This is especially so in low-density forests and semi-arid ecosystems.This work was motivated by the observed mismatch between satellite data (Landsat 8; nadir view) and tower-based Skye (90° angle of view) radiometer, in a low-density semi-arid pine forest (the Yatir forest in southern Israel) during 2013-2019. The two records showed opposite seasonal cycles in canopy NIR reflectance. We hypothesized that the different contributions of the surface components in the footprint areas of the two sensors could explain these observations and that accounting for this effect can help resolve the actual canopy NDVI values. An image classification algorithm was derived from Unmanned Aerial Vehicle (UAV) multispectral images to estimate the fraction and reflectance of the three main surface components: canopy, shaded areas, and bare soil. The results showed 30% and 95% canopy fractions in the Landsat 8 and Skye footprints, respectively. Therefore, the Landsat 8 signal was strongly influenced by soil reflectance, which is, in turn, sensitive to soil moisture level. The Skye mainly reflected canopy properties, including pigment content and canopy structure.Based on these results, we developed an approach to correct the sunlit and shaded soil contributions to the mixed Landsat 8-pixel NDVI, and retrieve the canopy NDVI. This approach relied on canopy fraction, sun elevation angle and the pre-determined NDVI values of the non-canopy components derived at the tower area. The retrieved canopy NDVI values were consistent with those of the high-resolution UAV-based canopy NDVI and independent of variations in the observed satellite NDVI values. These results demonstrated a new approach for improving the use of satellite NDVI to monitor the activities of forest canopies in sparse ecosystems, as well as the need for its application.

The leaf economics spectrum1,2 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 variability4,5. 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 ecosystems7,8., Three key axes of variation of ecosystem functional changes and their underlying causes are identified from a dataset of surface gas exchange measurements across major terrestrial biomes and climate zones.

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 semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145-160 g C m-2 year-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 year-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m2 and 372 g N/m2 . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.

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.

Climate change will have a considerable impact on 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.

Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high precision 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 non-contact remote sensing methods such as thermal infrared measurements provides an ideal solution, but their accuracy has been low (in the order of ~2 °C) due to necessity for corrections for material emissivity and fluctuations in background radiation (Lbg).A novel ‘dual-reference’ method was developed to increase the accuracy of infrared needle-leaf 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 leaf-to-air temperature differences of needle-leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ±0.23 and ±0.25 °C, respectively.Routine high precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf-scale processes and its dynamics.

Drier climates predicted for many regions can result in reduced evaporative cooling leading to leaf heat stress and enhanced mortality. To what extent non-evaporative cooling can contribute to plant resilience to the increasingly stressful conditions is poorly known at present.Using a novel, high accuracy infrared system for continuous measurements of leaf temperature in mature trees under field conditions, we assessed leaf-to-air temperature differences ΔTleaf−air of pine needles during drought.On mid-summer days, ΔTleaf−air remained H. ΔTleaf−air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but highly dependent on canopy structure and within-canopy turbulence that enhanced the sensible heat flux H.Non-evaporative cooling is demonstrated as an effective cooling mechanism in needle-leaf trees, which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy-scale ‘convector effect’.

Dry Mediterranean forests are characterized by a short rainy season followed by a long dry period with high temperatures and radiation levels. These ecosystem are also exposed to large interannual variations in precipitation. Taking advantage of contrasting rainfall years, we investigated the opportunistic nature of pine trees in this region. In our study site (the Yatir forest) mean annual precipitation is 288 mm, but it was 220 and 420 mm in the hydrological years 2018/19 and 2019/20, respectively. We used fluorescence measurements at the leaf, tower, and satellite scales, together with reflectance indices and eddy covariance measurements to assess the physiological response in the dry stressful season in these contrasting years.The results showed that following a low rainfall season, soil moisture contes (SWC) reaches the 16% threshold of no traspirable water in spring, followed by larg decrease in carbon uptake and quantum yield of photosynthesis and the activation of protection mechanisms, such as decrease in chlorophyll content, large NPQ, and drop in the chlorophyll to cartenoid ratio (CCI index, obtained from canopy reflectance). Following the high rainfall year, the active season is extended (as indicated also by the satellite data), but even after the SWC threshold is reached, and mid-day VPD reaches ~5 KPa), carbon uptake continues, the amount of energy allocated to photochemistry remains high (high Fv/Fm and Y(II) levels), without the onset of protective mechanisms: No decrease in leaf chlorophyll and in NPQ, or decrease in CCI. We hypothesize that the opportunistic response of the dry-land forest must rely on yet unidentified water storage outside the root zone (e.g. deep soil pockets within the bedrock, or within the plants), which allow the plant to maintain high, with the only apparent adjustment reflected in shift of activity to early morning hours, when VPD is still low but the PAR levels are sufficiently high.

The reduced availability of evaporative cooling resulting from a hotter and drier climate can lead to high leaf temperatures resulting in overheating. This can affect a variety of biophysical and biochemical processes that could enhance mortality. Plant resilience to these increasingly stressful conditions could rely on non-evaporative cooling. However, to what extent this plays a role is poorly known at present.In order to assess heat dissipation under the long summer drought conditions, we measured leaf-to-air temperature differences ΔTleaf-air of pine needles in semi-arid conditions in a drought-exposed and in an experimentally irrigated plot. For this purpose, we developed a novel, high accuracy system based on an infrared camera capable of continuous measurements of leaf temperature under field conditions. Both drought-exposed and irrigated trees, which had a 10x higher transpiration rate, exhibited a similar ΔTleaf-air that remained mostly below 3.5°C. Variations in mean wind speed did not strongly affect ΔTleaf-air, but it depended highly on within-canopy turbulence. This suggests a non-evaporative cooling mechanism that relies on the low leaf resistance to heat transfer, thus generating a large sensible heat flux. The ~30% reduction in resistance between leaves of drought-exposed and irrigated trees in the same species must be a result of changes in leaf characteristics and differences in canopy structure influencing wind penetration into the canopy. This reduction in resistance is required to generate the sufficiently larger sensible heat flux of nearly 100 W m-2 observed between both treatments under high radiation.Non-evaporative cooling was demonstrated to be an effective leaf- and leaf-branch-scale cooling mechanism in trees with small leaves, which can be a critical factor in forest resistance to drying climates. The generation of a leaf-scale sensible heat flux is considered as a possible mechanism leading to the development of the previously identified canopy-scale ‘convector effect’.

Remote sensing (RS) techniques have great potentials for earth surface monitoring. Nevertheless, for most low to moderate resolution satellites, the problem of mixed pixels with information from the vegetation of interest and the background surfaces can cause large biases in signals and also in their interpretations. This is especially so in low-density forests and semi-arid ecosystems. Ground-level multispectral instruments reduce these effects by measuring at close range to the canopy. However, little work has been published on partitioning the contributions from vegetation and the background elements for both approaches.This work was motivated by the observed mismatch between data for the same ecosystem from Landsat 8 satellite and Skye radiometer installed on a flux tower in a low-density semi-arid pine forest from 2013-2019. Data from both sources showed similar seasonal patterns in NDVI, but large differences in the reflectance bands. This was most prominent in the NIR reflectance, which showed an opposite seasonal cycle in the two sensors. Thus, similar changes in NDVI were produced by different signals. We hypothesized that the different contributions of the surface components (canopy, shaded areas, and exposed soil) in the footprint areas of the two sensors can explain, and can help correct, these differences. Multispectral images with a spatial resolution of 5 cm were captured monthly using an Unmanned Aerial Vehicle (UAV) from April 2018 to November 2019. Reflectance-based algorithms were developed to identify and estimate the fraction and reflectance from the canopy, shaded areas, and open soil. This information was, in turn, applied in the equivalent nadir-viewing satellite pixel. For the tower-based Skye footprint, the same quantities were calculated from its 90° angle of view and the 3D canopy data.The results showed a canopy fraction of 45% and 95% in the Landsat 8 and Skye footprints. The remaining soil fraction showed a similar seasonal cycle in NDVI as the canopy, but different in the NIR reflectance. The partition between exposed and shaded soil was related to the sun angle, with the exposed soil having a NIR seasonal cycle opposite to that of the vegetation (correlating with soil moisture), and shaded soil having a weak NIR signal variably diluting the overall pixel NIR signal. Differences in the red reflectance were smaller with less effects on the seasonal NDVI cycles.The results demonstrated firstly, that accounting for the fractional contributions of the surface components can reconcile differences between satellite and ground-based RS. Secondly, vegetation indices such as NDVI obtained by satellite RS in low-density forests can provide misleading information, despite its apparent correlation with certain vegetation variables.

Infrared gas analyzers (IRGAs) are commonly used in Eddy Covariance (EC) system and are used for, in particular, the ecosystem water cycle. However, they suffer from a measurement drift of absolute concentrations with time, leading to the increasing bias of readings. It is recommended in the manuals to do a factory calibration once every 1-2 years (e.g., LI-6262) or user calibration when considerable drift occurs (e.g., LI-7000). However, our experience shows that a significant drift can occur within a few days already. At our semi-arid EC site of Yatir Forest (31˚20'N, 35˚03'E, Israel), we are measuring a vertical air humidity profile (absolute humidity, Cw in mmol×mol-1, and relative, RH, %), to study the VPD regime within the canopy and to analyze dew formation events, which requires highly accurate RH measurements, however accurate RH measurements are difficult to achieve.Air humidity in Yatir is measured by three different instruments: (1) LI-7000 close-pass IRGA above the canopy for EC flux calculations; (2) LI-6262 close-pass IRGA with inlets in 4 different heights from above the ground up to the sonic height, used for humidity profile measurements; (3) Rotronic HC2S3 air humidity (RH) and temperature (T) sensor above the canopy. Both IRGAs are placed within a temperature-controlled box, and calibrated for zero and span with N2, dew point generator and laboratory standard gases every 1-2 weeks. The Rotronic sensor has very low drift and does not require calibration, but is assumed to be less accurate, especially under high and low RH.To achieve highly accurate measurements on daily time scale we propose a correction routine that rely on the stability of the RH probe, and the accuracy of the IRGAs after calibration. Every time the IRGA is calibrated, a correction-1 to the RH probe is produced. Between calibrations, the trends in the drifting IRGAs data are corrected (correction-2) to the interpolated stable RH probe data.For the flux measurements, the mean absolute Cw error before correction was 1.0 mmol×mol-1, which translates under average temperature of 25°C and RH of 50% to errors of RH, VPD and dew point of 3.0%, 93.5 Pa and 0.9°C, respectively. Following our correction procedure, reduced the error to 0.5 mmol×mol-1, which decreased the errors in RH, VPD and dew point under the same conditions to 1.5%, 47 Pa and 0.4°C, respectively. For the humidity profile, Cw error after correction decreased from 1.9 mmol×mol-1 to 0.5 mmol×mol-1, which decreased the errors in RH, VPD and dew point under the same conditions by 4.1%, 131 Pa and 1.2°C, respectively.We will describe the method in more detail and demonstrate its application to our field measurements.

The chapter describes a long-term (2000–2019) 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 June–October, with a mean summer value of ~ −1 gC m−2 d−1. The carbon inventory showed that during 2001–2016 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 earth’s 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 50–90% 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.

Assessment of the plant-climatic interactions in the land biosphere requires a combined perspective of both the biogeochemical effects (BGC; such as the carbon sink), and the biogeophysical effects (BGP; such as the vegetation albedo and radiative balance), which can often have contrasting consequences for ecosystem functioning and climate. Aiming to increase our knowledge on semi-arid ecosystems that are insufficiently represented in global studies, we examine the variations in key BGP features among different vegetation types in a dry Mediterranean region in southern Israel.The study included planted pine forest (pinus halepensis), natural broad-leaf oak maquis (Quercus calliprinos), wheat field and a managed grassland, located in close proximity (within 2 km) under the same climatic conditions (mean annual temperature = 20.8C, annual mean precipitation, P= 403 mm, aridity index = 0.4). Using a state-of-the-art mobile laboratory, we carried out measurement campaigns of eddy covariance fluxes of CO2, sensible, H, and latent, LE, heat fluxes, and the radiation balance (incoming and outgoing short- and long-wave radiations) between the ecosystems and the atmosphere in different seasons during 2016-2018.The results showed significant differences in net radiation and in albedo among the ecosystem, with net radiation values of ~666, ~582, ~443 and 456 W m-2 and albedo values of ~0.13, ~0.16, ~0.19 and ~0.20 for pines, maquis, wheat and grassland, respectively. The lowest albedo of the pine stand was associated with the largest H (a ‘convector effect’) of ~583 W m-2 compared to ~313, ~198 and ~176 W m-2 in the maquis, wheat and grassland ecosystems (midday means of peak activity season). The pine stand was also more adjusted to stress conditions than the oak maquis ecosystem through ‘avoidance’ of high activities during extreme conditions of heat and drought (reducing canopy conductance and associated fluxes). It is likely that the observed differences between the pine and oak maquis stand help explain the greater expansion of pine stands into the semi-arid regions, even to areas with mean annual P of 290 mm (aridity index = 0.2) where oak maquis cannot be found.

High radiation, low albedo, and limited evaporative cooling greatly affect canopy temperature in many semi-arid ecosystems. This makes the dissipation of excess energy essential to tree survival. Remote sensing has the potential to optimise management and better understanding of tree survival mechanisms in this zone. Stand density is thought to affect canopy and soil temperature through shading, change in overall albedo, evapotranspiration, and its effect on convective cooling through wind penetration into the canopy layer. Our objective was to assess the effect of stand density on the canopy and the mean plot temperature as a basis to optimize energy management of a severely water-limited forest.We used a drone equipped with RGB, thermal (FLIR) and multispectral cameras (Parrot Sequoia, bands: 550nm, 660nm, 735nm & 790nm) alongside independent Lidar measurements in a set of five replicate plots of three different stand density treatments (100, 200 & 300 trees/ha) alongside ground-based measurements. Drone flights were performed during midday throughout the peak of the summer drought (lasting ~8 months) in our semi-arid Aleppo Pine forest research site in southern Israel (sun near NADIR & midday solar radiation >800 W·m-2, air temperature >30°C). Finally, a set of techniques were developed to automatically identify and extract data of individual tree canopies from the aerial images.Initial results highlight the importance of partitioning the forest into exposed and shaded soil and tree canopy: The canopy-to-air and exposed soil-to-air temperature differences reached up to 5°C and 35°C, respectively, while shaded soils were in the same temperature range as canopies. Ground-based measurements of DBH and photosynthetic activity increased with decreasing stand density. This is in spite of up to 30% more longwave radiation reaching the canopies through exposure to the hot soil and lack of shading from neighbouring trees in the lower density plots. Unexpectedly, there was a lack of significant canopy temperature differences among density plots, indicating that trees in all treatments dissipated the excess energy equally efficiently. Therefore, mean plot-scale forest surface (skin) temperatures (including both soil and canopy) were affected by the fraction of canopy cover rather than canopy temperature differences among different stand density plots. The results highlight the limitation of interpreting low-resolution satellite data in open canopy forests. Our results will allow us to assess the stand density effects on the balance between carbon sequestration (biogeochemical effects) and surface energy balance (biogeophysical effects).

The production of electricity from solar radiation should replace power production by burning fossil fuel and help reduce atmospheric concentrations of CO2. However, large photovoltaic (PV) fields can also influence the climate in more direct ways. The albedo of solar panels is low to allow efficient light absorption, but actual conversion efficiency is below 20%. The remaining 80% of the energy is reflected, re-emitted as thermal radiation or dissipated as sensible heat (H). These effects can heat the surface, influence local air circulations, and lead to the formation of “heat-islands”. Such effects are particularly significant in desert areas with high radiation load and high background albedo. The ultimate objective of this study will be to estimate the cost (in number of years) of CO2 emission suppression of a PV power generation (a “cooling effect”) associated with the albedo radiative forcing and the surface "warming effects" and the partitioning to its components. We used a state-of-the-art field laboratory to carry out eddy covariance flux measurements of sensible and latent heat, and the radiative balance of incoming and outgoing short- and long-wave radiations. A research drone equipped with a thermal and a multi-spectral camera was used to estimate the spatial average reflected and emitted radiation from the solar panels field. Measurements were carried out on campaign basis during 2018-2019, both inside and outside a PV field in the Arava desert in southern Israel. The preliminary results indicated that summer noon incoming solar radiation (S) is ~1000 Wm-2 and the desert surface albedo is on average 0.40. The mean solar panel field albedo is 0.23 (with panels projected area about 1/3rd of the PV field area), which is translated to ~170 Wm-2 higher S absorption by the PV field. A large fraction of the energy is converted to sensible heat flux with mid-day H values of 450 Wm-2, compared with 250 Wm-2 in the desert, or about 200 Wm-2 of extra heating above the PV field. A first approximation of the summer daily carbon suppression (assuming 12h daily average sunlight of ~500 Wm-2, PV efficiency of 0.2, and conventional power efficiency of ~200 gC/KWh) indicated ~0.08 Kg C per day per m-2 PV area. These preliminary results are being extended to include thermal emission effects and the annual scale perspective to assess the “PV forest” radiative forcing effect. But it is evident that the land use change examined here has a large impact on the surface energy budget and its surrounding environments.

A large terrestrial carbon sink significantly influences the rate of change in atmospheric CO2 concentrations, but uncertainties associated with its estimate are considerable. Here 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. Annual integrated carbon accumulation was 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. This carbon uptake reflect high carbon use efficiency NEP/GPP of 29 compared to ~21 in temperate forests, leading to the current ecosystem stocks of 7943 ± 323 g carbon m-2 and 372 g nitrogen m-2. In addition, carbon is mostly stored in the soil (~71 % of the current ecosystem C stock), with a long C turnover time of 59 ± 4 y (compared to mean value of 18 years in temperate forests). It is also estimated that soil carbon at the study site constitutes only ~25 % of the estimated soil saturation capacity. Irrespective of un-expected disturbances beyond those observed at the study site, the results support considerable C 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.

Warming, drying, and intensified water stress is expected in many ecosystems over the next century. In dry environments, evaporative cooling becomes increasingly limited and must be replaced with alternative means of heat dissipation if canopy and leaf temperature are to be maintained within the physiological range and mortality avoided. We have shown that in dry environments when latent heat flux is minimal, net radiation is high, and thermal radiation emission is suppressed, pine forest canopies can efficiently cool through a massive sensible heat flux, facilitated by the low aerodynamic resistance of the open canopy (a so-called ‘Convector Effect’). Using novel methodology, we also show that this phenomenon may originate at the leaf-scale, associated with needle properties, changes in heat transport characteristics across the canopy profile, and propagating across scales can ultimately influence the boundary layer, the local atmospheric dynamics, and potentially regional climate.

Semi-arid forests represent some of the most sensitive ecosystems to climate change. Identifying adjustments to extreme conditions can indicate their resilience, and that of forests undergoing increasing aridity trends. We used eddy covariance and close-range sensing measurements over four years in a semi-arid pine forest to identify canopy-scale adjustments to the short active season and long seasonal drought. Peaks in light use efficiency (LUE), leaf chlorophyll content (LCC), and increasing absorbed photosynthetic active radiation (APAR; based on canopy absorption coefficient in the green range), all converged to support an early peak (March) in gross primary productivity (GPP), exploiting the narrow optimum between PARin, temperature and the rapidly decreasing soil moisture in spring. In contrast, during the long dry period (>200 days), while PARin increased, LCC and LUE decreased, offering physiological photoprotection as GPP sharply declined under the stressful conditions. The strong negative correlation between ρNIR and PARin indicated canopy biophysical adjustments that enhance light absorption under low radiation and eliminate photodamage under excessive radiation. The results provide clear indications of canopy-scale adjustments underlying the high productivity of the forest and its resistance to the harsh conditions, which may soon apply to forests in currently milder climatic regions.

Dry forests are expected to heat up considerably more than adjacent shrubland areas due to lower albedo and reduced latent heat flux. Paradoxically, the forest surface at our research site was observed to be cooler than the non-forested neighbouring areas during drought. This reflected the control over canopy temperature through the sensible heat flux, i.e. a 'convector effect'. Our objective was to examine how the efficient non-evaporative energy management, critical to protect the biological functioning of dryland ecosystems, develops at the small, leaf scale. We developed a novel system to continuously measure the energy balance and heat dissipation mechanisms on a leaf scale under field conditions. It allows the measurement of emitted leaf and background longwave radiation, and estimating the incoming, absorbed and reflected shortwave radiation using PAR measurements and full spectrum models. Latent heat exchange and photosynthetic activity were measured with branch chambers. The system was deployed during the long summer drought (>8 months) in drought-exposed and irrigated plots in our semi-arid research Aleppo Pine forest site in southern Israel (mean daytime temperature of >30°C).Preliminary results showed that in spite of a x10 higher transpiration rate in the irrigated plot compared with the control plots, leaf temperature remains within 1-2°C of air temperature on average in both plots during direct exposure to sunlight at midday. These results suggest an effective leaf to air heat transfer which prevents overheating independent of the latent heat flux. Under the high radiation load, the midday summer value of incoming shortwave radiation was >800 W·m-2 (mostly absorbed by the low albedo leaves), and background longwave radiation was >500 W·m-2. In turn, the energy dissipation in the drought-exposed trees was dominated by sensible heat flux of >500 W·m-2, while the long-wave radiation balance was near neutral (~50 W m-2), and the residual latent heat flux was -2. We demonstrated a system that provided new insights to leaf and canopy energy management under drought, which is a basis for the evolution of the convector effect.

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.

We investigate the effects of an isolated meso- $$\gamma $$ -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: $$28~\text{ km }^2$$ , 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 ( $$600~\text {W~m}^{-2}$$ in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.

Estimations of ecosystem-level evapotranspiration (ET) and CO2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach using remote sensing (RS) and meteorological data (RS–Met) is adjusted to extreme high-energy water-limited Mediterranean ecosystems that suffer from continuous stress conditions to provide daily estimations of ET and CO2 uptake (measured as gross primary production, GPP) at a spatial resolution of 250 m. The RS–Met was adjusted using a seasonal water deficit factor (fWD) based on daily rainfall, temperature and radiation data. We validated our adjusted RS–Met with eddy covariance flux measurements using a newly developed mobile lab system and the single active FLUXNET station operating in this region (Yatir pine forest station) at a total of seven forest and non-forest sites across a climatic transect in Israel (280–770 mm yr−1). RS–Met was also compared to the satellite-borne MODIS-based ET and GPP products (MOD16 and MOD17, respectively) at these sites.Results show that the inclusion of the fWD significantly improved the model, with R = 0.64–0.91 for the ET-adjusted model (compared to 0.05–0.80 for the unadjusted model) and R = 0.72–0.92 for the adjusted GPP model (compared to R = 0.56–0.90 of the non-adjusted model). The RS–Met (with the fWD) successfully tracked observed changes in ET and GPP between dry and wet seasons across the sites. ET and GPP estimates from the adjusted RS–Met also agreed well with eddy covariance estimates on an annual timescale at the FLUXNET station of Yatir (266 ± 61 vs. 257 ± 58 mm yr−1 and 765 ± 112 vs. 748 ± 124 gC m−2 yr−1 for ET and GPP, respectively). Comparison with MODIS products showed consistently lower estimates from the MODIS-based models, particularly at the forest sites. Using the adjusted RS–Met, we show that afforestation significantly increased the water use efficiency (the ratio of carbon uptake to ET) in this region, with the positive effect decreasing when moving from dry to more humid environments, strengthening the importance of drylands afforestation. This simple yet robust biophysical approach shows promise for reliable ecosystem-level estimations of ET and CO2 uptake in extreme high-energy water-limited environments.

Partitioning carbon fluxes is key to understanding the process underlying ecosystem response to change. This study used soil and canopy fluxes with stable isotopes (13C) and radiocarbon (14C) measurements of a 50-year-old dry (i.e., 287 mm of annual precipitation) pine forest to partition the ecosystem’s CO2 flux into gross primary productivity (GPP) and ecosystem respiration (Re) and soil respiration flux into autotrophic (Rsa), heterotrophic (Rh), and inorganic (Ri) components. On an annual scale, GPP and Re were 655 and 488 g C m−2, respectively, with a net primary productivity (NPP) of 276 g C m−2 and carbon-use efficiency (CUE = NPP / GPP) of 0.42. Soil respiration (Rs) made up 60 % of the total ecosystem respiration and was comprised of 24 ± 4 %, 23 ± 4 %, and 13 ± 1 % Rsa, Rh, and Ri, respectively. The contribution of root and microbial respiration to Re increased during high productivity periods, and inorganic sources were more significant components when soil water content was low. Compared to the mean values for 2001–2006 at the same site; (Grünzweig et al., 2009), annual Rs decreased by 27 % to the mean 2016 rates of 0.8 ± 0.1 µmol m−2 s−1). This was associated with decrease in the respiration Q10 values across the same observation by 36 % and 9 % in the wet and dry periods, respectively. Low rates of soil carbon loss combined with relatively high below ground carbon allocation (i.e., 40 % of canopy CO2 uptake) help explain the high soil organic carbon accumulation and the relatively high ecosystem CUE of the dry forest. This was indicative of the higher resilience of the pine forest to climate change and the significant potential for carbon sequestration in these regions.

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.

The temporal dynamics of water transport and storage in plants have major implications for plant functioning and survival. In trees, stress on the conductive tissue can be moderated by water storage. Yet, trees can survive high percent loss of conductivity (PLC, up to 80%), suggesting efficient recovery. We assess the role of tree water storage and PLC recovery based on simultaneous measurements of leaf transpiration, branch hydraulic conductivity, and stem sap-flow from different seasons in three study years in mature Pinus halepensis (Miller) trees in a semi-arid forest. During the wet season the rates of transpiration (T) and sap flow (SF) peaked at high morning and through the mid-day. During the dry season T peaked at ~9:00 and then decreased, whereas SF lagged T and fully compensated for it only in the evening, resulting in a mid-day water deficit of ~5 kg tree-1, and with up to 33% of daily T derived from storage. PLC of 30-40% developed during mid-day and subsequently recovered to near zero within 2-3 hr in the dry season (May, June, and September), but not in the wet season (January). The observed temporal decoupling between leaf water loss and soil water recharge is consistent with optimization of the trees’ water and gas exchange economy, while apparently facilitating their survival in the semi-arid conditions.

Drought‐related tree mortality had become a widespread phenomenon in forests around the globe. This process leading to these events and its complexity is not fully understood. Trees in the dry timberline are exposed to ongoing drought, and the available water for transpiration in the soil can determine their survival chances. Recent drought years led to 5%–10% mortality in the semi‐arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low pre‐dawn leaf water potential at the dry season (−2.8 MPa vs. −2.3 MPa in non‐stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg/g dw). Trends in tree‐ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15–20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site’s long‐term hydrological data supported the idea that these differences could result in higher soil water concentration (m³/m³) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity may therefore critical to assessing stand‐level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends. A plain language summary is available for this article.

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 0.94–1.07). 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.

The return-to-isotropy of turbulence across scales is studied for different thermal stratification and surface roughness regimes from measurements in the atmospheric boundary layer and it is compared with predictions of a homogenous model.