Resilience to seasonal heat wave episodes in a Mediterranean pine forest

Tree Growth, Contraction and Recovery: Disentangling Soil and Atmospheric Drought Effects

We investigate how soil and atmospheric droughts jointly impact tree growth and recovery dynamics in a semi-arid pine forest, leveraging high-resolution stem diameter variation data and an irrigation experiment. The irrigated plot, where soil drought was mitigated, served as a benchmark to isolate the effects of atmospheric drought and distinguish them from the compound drought conditions experienced by control trees. Using a suite of tools based only on stem diameter variation, we identified growth modes that vary in accordance with soil water availability. Control trees showed negligible growth during the dry season but rapidly recovered with the onset of the wet season, matching the baseline growth rates of the irrigated trees, suggesting minimal compromise in hydraulic functioning. Our main finding is that heatwaves consistently depress stem-expansion rates, regardless of treatment. However, during the dry season, this negative impact diverges sharply between the treatments. Because irrigated trees benefit from a hydraulic buffer supplied by ample soil water and thus retain a positive growth baseline, the depression merely slows their expansion, whereas control trees already near zero are driven into net contraction. These findings offer new understanding of how trees balance growth, contraction, and recovery under varying drought conditions, revealing the pivotal role of soil water in shaping drought responses across seasons. As climate change intensifies the frequency and severity of drought events, this knowledge is critical for anticipating shifts in tree growth and resilience.

Identifying climatic drivers of forage quantity and quality in Mediterranean rangelands using remote sensing

Rangelands are dynamic ecosystems shaped by fluctuations in precipitation, temperature, and grazing intensity. Accurate assessment of forage availability is critical for optimizing land use, preventing overgrazing, and mitigating degradation, especially under future climate change scenarios. This study employed a multi-scale approach to monitor pasture using Sentinel-2 satellite imagery, calibrated with ground truth measurements, and enhanced with drone-derived vegetation cover estimates. The research was conducted in a Mediterranean grassland ecosystem under varying grazing intensities in northeastern Israel. A time series of Sentinel-2 images from 2018 to 2023 was analyzed to monitor five vegetation growth cycles. We evaluated how climate variables, such as temperature, rainfall, and vapor pressure deficit (VPD), impacted forage availability and nutritional quality. Over the past three decades, significant climatological trends showed increasing temperatures and more concentrated winter rainfall. Correlation analysis between field data and Sentinel-2 imagery demonstrated strong agreement (R2 = 0.73 for biomass and 0.72 for forage quality), validating the effectiveness of this integrated remote sensing approach. Moderate grazing reduced forage quantity but improved its quality, while extreme weather events, including drought and heatwaves, negatively impacted forage biomass and quality. Key meteorological indices, including the Standardized Precipitation-Evapotranspiration Index (SPEI) and the Simple Daily Intensity Index (SDII), were found to influence forage quantity, indicating that drought stress and concentrated rainfall events reduced biomass production. Higher values of daily temperature range and vapor pressure deficit adversely impacted forage quality. This study highlights the importance of adaptive management strategies to mitigate the projected reductions in forage quantity and quality due to climate change. The methodologies developed offer novel insights into improving ecological monitoring for sustainable rangeland management under changing climatic conditions.

Pre-drought effects on northern temperate trees and vine invasion in forest gaps hindering regeneration

Northern temperate coniferous forests serve as crucial connectors between boreal and temperate forests, yet they are vulnerable to various stressors such as climate change and human activities. Severe drought poses a significant threat to plant species within these forests, prompting recent research into its impacts. However, many studies lack explicit definitions of post-disturbance vegetation processes and fail to identify potential interactions with disturbance factors, necessitating comprehensive discussions. This study examines the effects of drought on tree growth patterns of the main dominant species in northern temperate regions: Abies nephrolepis and Picea jezoensis, along with two commonly associated Betula ermanii, and Quercus mongolica. Additionally, new disturbance factors in forests inhabited by these species (A. nephrolepis and P. jezoensis) were evaluated based on community classification. The study sites were located in the Mt. Baekdu (Changbai) and South Korea regions, which are positioned at the southern limit of the phytogeographical patterns of target species. Results indicate that A. nephrolepis and P. jezoensis exhibit high levels of recovery and resilience, while B. ermanii and Q. mongolica demonstrate high resistance. Species-specific responses align with drought intensity, with resistance, recovery, and resilience decreasing notably with increasing pre-drought radial growth. South Korean forests, the invasion of the vine species Tripterygium regelii after the death of A. nephrolepis in the overstory vegetation threatens the regeneration of new trees. However, certain environmental factors, such as high rock exposure and dense overstory canopy, limit vine invasion. Based on the results, pre-drought radial growth emerges as a key determinant in how trees respond to drought. Additionally, the results suggest the potential for new disturbances to emerge in forest gaps due to overstory vegetation mortality induced by global warming. These findings contribute to a deeper understanding of increasing drought stress, aid in identifying climate refugia, and inform conservation priorities based on habitat characteristics.

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

Physiological and phenological adjustments in water and carbon fluxes of Aleppo pine forests under contrasting climates in the Eastern Mediterranean

The ability of plants to adjust to the adverse effects of climate change is important for their survival and for their contribution to the global carbon cycle. This is particularly true in the Mediterranean region, which is among the regions that are most vulnerable to climate change. Here, we carried out a 2-year comparative ecophysiological study of ecosystem function in two similar Eastern Mediterranean forests of the same tree species (Pinus halepensis Mill.) under mild (Sani, Greece) and extreme (Yatir, Israel) climatic conditions. The partial effects of key environmental variables, including radiation, vapor pressure deficit, air temperature and soil moisture (Rg, D, T and soil water content (SWC), respectively), on the ecosystems' CO2 and water vapor fluxes were estimated using generalized additive models (GAMs). The results showed a large adjustment between sites in the seasonal patterns of both carbon and water fluxes and in the time and duration of the optimal period (defined here as the time when fluxes were within 85% of the seasonal maximum). The GAM analysis indicated that the main factor influencing the seasonal patterns was SWC, while T and D had significant but milder effects. During the respective optimal periods, the two ecosystems showed strong similarities in the fluxes' responses to the measured environmental variables, indicating similarity in their underlying physiological characteristics. The results indicate that Aleppo pine forests have a strong phenotypic adjustment potential to cope with increasing environmental stresses. This, in turn, will help their survival and their continued contribution to the terrestrial carbon sink in the face of climate change in this region.

The complexity of heatwaves impact on terrestrial ecosystem carbon fluxes: Factors, mechanisms and a multi-stage analytical approach

Extreme heatwaves have become more frequent and severe in recent decades, and are expected to significantly influence carbon fluxes at regional scales across global terrestrial ecosystems. Nevertheless, accurate prediction of future heatwave impacts remains challenging due to a lack of a consistent comprehension of intrinsic and extrinsic mechanisms. We approached this knowledge gap by analyzing the complexity factors in heatwave studies, including the methodology for determining heatwave events, divergent responses of individual ecosystem components at multiple ecological and temporal scales, and vegetation status and hydrothermal environment, among other factors. We found that heatwaves essentially are continuously changing compound environmental stress that can unfold into multiple chronological stages, and plant physiology and carbon flux responses differs in each of these stages. This approach offers a holistic perspective, recognizing that the impacts of heatwaves on ecosystems can be better understood when evaluated over time. These stages include instantaneous, post-heatwave, legacy, and cumulative effects, each contributing uniquely to the overall impact on the ecosystem carbon cycle. Next, we investigated the importance of the timing of heatwaves and the possible divergent consequences caused by different annual heatwave patterns. Finally, a conceptual framework is proposed to establish a united foundation for the study and comprehension of the consequences of heatwaves on ecosystem carbon cycle. This instrumental framework will assist in guiding regional assessments of heatwave impacts, shedding light on the underlying mechanisms responsible for the varied responses of terrestrial ecosystems to specific heatwave events, which are imperative for devising efficient adaptation and mitigation approaches.

Vapour pressure deficit was not a primary limiting factor for gas exchange in an irrigated, mature dryland Aleppo pine forest

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.

Long-term changes in forest response to extreme atmospheric dryness

Atmospheric dryness, as indicated by vapor pressure deficit (VPD), has a strong influence on forest greenhouse gas exchange with the atmosphere. In this study, we used long-term (10-30 years) net ecosystem productivity (NEP) measurements from 60 forest sites across the world (1003 site-years) to quantify long-term changes in forest NEP resistance and NEP recovery in response to extreme atmospheric dryness. We tested two hypotheses: first, across sites differences in NEP resistance and NEP recovery of forests will depend on both the biophysical characteristics (i.e., leaf area index [LAI] and forest type) of the forest as well as on the local meteorological conditions of the site (i.e., mean VPD of the site), and second, forests experiencing an increasing trend in frequency and intensity of extreme dryness will show an increasing trend in NEP resistance and NEP recovery over time due to emergence of long-term ecological stress memory. We used a data-driven statistical learning approach to quantify NEP resistance and NEP recovery over multiple years. Our results showed that forest types, LAI, and median local VPD conditions explained over 50% of variance in both NEP resistance and NEP recovery, with drier sites showing higher NEP resistance and NEP recovery compared to sites with less atmospheric dryness. The impact of extreme atmospheric dryness events on NEP lasted for up to 3 days following most severe extreme events in most forests, indicated by an NEP recovery of less than 100%. We rejected our second hypothesis as we found no consistent relationship between trends of extreme VPD with trends in NEP resistance and NEP recovery across different forest sites, thus an increase in atmospheric dryness as it is predicted might not increase the resistance or recovery of forests in terms of NEP.

Contrasting stem water uptake and storage dynamics of water-saver and water-spender species during drought and recovery

Drought is projected to occur more frequently and intensely in the coming decades, and the extent to which it will affect forest functioning will depend on species-specific responses to water stress. Aiming to understand the hydraulic traits and water dynamics behind water-saver and water-spender strategies in response to drought and recovery, we conducted a pot experiment with two species with contrasting physiological strategies, Scots pine (Pinus sylvestris L.) and Portuguese oak (Quercus faginea L.). We applied two cycles of soil drying and recovery and irrigated with isotopically different water to track fast changes in soil and stem water pools, while continuously measuring physiological status and xylem water content from twigs. Our results provide evidence for a tight link between the leaf-level response and the water uptake and storage patterns in the stem. The water-saver strategy of pines prevented stem dehydration by rapidly closing stomata which limited their water uptake during the early stages of drought and recovery. Conversely, oaks showed a less conservative strategy, maintaining transpiration and physiological activity under dry soil conditions, and consequently becoming more dehydrated at the stem level. We interpreted this dehydration as the release of water from elastic storage tissues as no major loss of hydraulic conductance occurred for this species. After soil rewetting, pines recovered pre-drought leaf water potential rapidly, but it took longer to replace the water from conductive tissues (slower labeling speed). In contrast, water-spender oaks were able to quickly replace xylem water during recovery (fast labeling speed), but it took longer to refill stem storage tissues, and hence to recover pre-drought leaf water potential. These different patterns in sap flow rates, speed and duration of the labeling reflected a combination of water-use and storage traits, linked to the leaf-level strategies in response to drought and recovery.

Extreme temperature events reduced carbon uptake of a boreal forest ecosystem in Northeast China: Evidence from an 11-year eddy covariance observation

Boreal forests, the second continental biome on Earth, are known for their massive carbon storage capacity and important role in the global carbon cycle. Comprehending the temporal dynamics and controlling factors of net ecosystem CO₂ exchange (NEE) is critical for predicting how the carbon exchange in boreal forests will change in response to climate change. Therefore, based on long-term eddy covariance observations from 2008 to 2018, we evaluated the diurnal, seasonal, and interannual variations in the boreal forest ecosystem NEE in Northeast China and explored its environmental regulation. It was found that the boreal forest was a minor CO₂ sink with an annual average NEE of -64.01 (± 24.23) g CO₂ m^-2 yr^-1. The diurnal variation in the NEE of boreal forest during the growing season was considerably larger than that during the non-growing season, and carbon uptake peaked between 8:30 and 9:30 in the morning. The seasonal variation in NEE demonstrated a "U" shaped curve, and the carbon uptake peaked in July. On a half-hourly scale, photosynthetically active radiation and vapor pressure deficit had larger impacts on daytime NEE during the growing season. However, temperature had major control on NEE during the growing season at night and during the non-growing season. On a daily scale, temperature was the dominant factor controlling seasonal variation in NEE. Occurrence of extreme temperature days, especially extreme temperature events, would reduce boreal forest carbon uptake; interannual variation in NEE was substantially associated with the maximum CO₂ uptake rate during the growing season. This study deepens our understanding of environmental controls on NEE at multiple timescales and provides a data basis for evaluating the global carbon budget.

Cultivation Using Coir Substrate and P or K Enriched Fertilizer Provides Higher Resistance to Drought in Ecologically Diverse Quercus Species

Nursery cultivation practices can be modified to increase resistance to water stress in forest seedlings following field establishment, which may be increasingly important under climate change. We evaluated the morphological (survival, growth) and physiological (chlorophyll fluorescence, leaf water potential) responses to water stress for three ecologically diverse Quercus species (Q. robur, Q. pubescens, and Q. ilex) with varying traits resulting from the combination of growing media (peat, coir) and fertilization (standard, P-enriched, K-enriched). For all species under water stress, seedlings grown in coir had generally higher growth than those grown in peat. Seedlings fertilized with P performed better, particularly for survival; conversely, K fertilization resulted in inconsistent findings. Such results could be explained by a combination of factors. P fertilization resulted in higher P accumulation in seedlings, while no K accumulation was observed in K fertilized seedlings. As expected, the more drought-sensitive species, Q. robur, showed the worst response, while Q. pubescens had a drought resistance equal or better to Q. ilex despite being classified as intermediate in drought resistance in Mediterranean environments.

Disentangling Soil, Shade, and Tree Canopy Contributions to Mixed Satellite Vegetation Indices in a Sparse Dry Forest

Remote sensing (RS) for vegetation monitoring can involve mixed pixels with contributions from vegetation and background surfaces, causing biases in signals and their interpretations, especially in low-density forests. In a case study in the semi-arid Yatir forest in Israel, we observed a mismatch between satellite (Landsat 8 surface product) and tower-based (Skye sensor) multispectral data and contrasting seasonal cycles in near-infrared (NIR) reflectance. We tested the hypothesis that this mismatch was due to the different fractional contributions of the various surface components and their unique reflectance. Employing an unmanned aerial vehicle (UAV), we obtained high-resolution multispectral images over selected forest plots and estimated the fraction, reflectance, and seasonal cycle of the three main surface components (canopy, shade, and sunlit soil). We determined that the Landsat 8 data were dominated by soil signals (70%), while the tower-based data were dominated by canopy signals (95%). We then developed a procedure to resolve the canopy (i.e., tree foliage) normalized difference vegetation index (NDVI) from the mixed satellite data. The retrieved and corrected canopy-only data resolved the original mismatch and indicated that the spatial variations in Landsat 8 NDVI were due to differences in stand density, while the canopy-only NDVI was spatially uniform, providing confidence in the local flux tower measurements.

In situ, direct observation of seasonal embolism dynamics in Aleppo pine trees growing on the dry edge of their distribution

Xylem embolism impairs hydraulic conductivity in trees and drives drought-induced mortality. While embolism has been monitored in vivo in potted plants, and research has revealed evidence of embolism in field-grown trees, continuous in situ monitoring of cavitation in forests is lacking. Seasonal patterns of embolism were monitored in branchlets of Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest. Optical visualization (OV) sensors were installed on terminal branches, in addition to monthly sampling for micro computed tomography scans. We detected 208 cavitation events among four trees, which represented an embolism increase from zero to c. 12% along the dry season. Virtually all the cavitation events occurred during daytime hours, with 77% occurring between 10:00 and 17:00 h. The probability for cavitation in a given hour increased as vapor pressure deficit (VPD) increased, up to a probability of 42% for cavitation when VPD > 5 kPa. The findings uniquely reveal the instantaneous environmental conditions that lead to cavitation. The increased likelihood of cavitation events under high VPD in water-stressed pines is the first empirical support for this long hypothesized relationship. Our observations suggest that low levels of embolism are common in Aleppo pine trees at the dry edge of their distribution.

Compound climate events increase tree drought mortality across European forests

Climate change can lead to the simultaneous occurrence of extreme droughts and heat waves increasing the frequency of compound events with unknown impacts on forests. Here we use two independent datasets, a compiled database of tree drought mortality events and the ICP-Forest level I plots, to study the impacts of the simultaneous occurrence of hot summers, with elevated vapour pressure deficit (VPD), and dry years on forest defoliation and mortality across Europe. We focused on tree drought mortality and background mortality rates, and we studied their co-occurrence with compound events of hot summers and dry years. In total, 143 out of 310 mortality events across Europe, i.e. 46% of cases, corresponded with rare compound events characterized by hot summers and dry years. Over the past decades, summer temperature increased in most sites and severe droughts resulted in compound events not observed before the 1980s. From the ICP-Forest plots we identified 291 (1718 trees) and 61 plots (128 trees) where severe defoliation and mortality, respectively, were caused by drought. The analyses of these events showed that 34% and 27% of the defoliation and mortality cases corresponded with rare compound climate events, respectively. Background mortality rates across Europe in the period 1993-2013 presented higher values in regions where summer temperature and VPD more steeply rose, where drought frequency increased. The steady increase in summer temperatures and VPD in Southern and Eastern Europe may favor the occurrence of compound events of hot summers and dry conditions. Giving that both, local and intense tree drought mortality events and background forest mortality rates, are linked to such compound events we can expect an increase in forest drought mortality in these European regions over the next decades.

The importance of tree internal water storage under drought conditions

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.

Annual Carbon Sequestration Patterns in Trees: A Case Study from Scots Pine Monospecific Stands and Mixed Stands with Sessile Oak in Central Poland

The need to understand the carbon sequestration ability of trees under current and future climatic scenarios is fundamental to predict the role of forest in counterbalancing the global warming. In this study, we investigated the carbon sequestration ability of Pinus sylvestris L. in a setting of pure and mixed forests with Quercus petraea (Matt.) Liebl. in Central Poland. Beside the traditional growth measures, i.e., Ring Width, Basal Area Increment, and wood density, we utilized also a new Index called BAIden, which combines Basal Area Increment and mean ring wood density to depict the carbon sequestration ability of trees. Pinus sylvestris showed different sensitivity to climatic variability depending on tree admixture, while the Basal Area Increment and wood density presented few differences between pure and mixed forests. According to the BAIden index, carbon accumulation in P. sylvestris showed similar sensitivity to climatic variability in pure and mixed forests. The new index was also informative on the main climatic drivers of carbon sequestration. Considering future climatic scenarios, the carbon sequestration ability of P. sylvestris will be facilitated by rising temperatures in late winter-early spring and reduced by decreasing precipitation and rising temperatures during summer. Finally, we discussed the perspective and applicability of BAIden for further studies on carbon sequestration ability under climate change.

Eucalyptus obliqua tall forest in cool, temperate Tasmania becomes a carbon source during a protracted warm spell in November 2017

Tasmania experienced a protracted warm spell in November 2017. Temperatures were lower than those usually characterising heatwaves. Nonetheless the warm spell represented an extreme anomaly based on the historical local climate. Eddy covariance measurements of fluxes in a Eucalyptus obliqua tall forest at Warra, southern Tasmania during the warm spell were compared with measurements in the same period of the previous year when temperatures were closer to average. Compared with previous year, the warm spell resulted in 31% lower gross primary productivity (GPP), 58% higher ecosystem respiration (ER) and the forest switching from a carbon sink to a source. Significantly higher net radiation received during the warm spell was dissipated by increased latent heat flux, while canopy conductance was comparable with the previous year. Stomatal regulation to limit water loss was therefore unlikely as the reason for the lower GPP during the warm spell. Temperatures during the warm spell were supra-optimal for GPP for 75% of the daylight hours. The decline in GPP at Warra during the warm spell was therefore most likely due to temperatures exceeding the optimum for GPP. All else being equal, these forests will be weaker carbon sinks if, as predicted, warming events become more common.

Thermal safety margins of plant leaves across biomes under a heatwave

Climate change has great impacts on forest ecosystems, especially with the increasing frequency of heatwaves. Thermal safety margin (TSM) calculated by the difference between body temperature and thermotolerance threshold is useful to predict thermal safety of organisms. It has been widely used for animals, whereas has rarely been reported for plants. Besides, most of the previous studies used only thermotolerance to estimate thermal safety or used thermotolerance and air temperature (Ta) to calculate TSM. However, leaf temperature (Tl) is the real "body" temperature of plant leaves. Tl decoupling from Ta might induce large error in TSM. Here, we investigated TSM of photosystem II (thermotolerance of PSII - the maximum Tl) of dominant canopy plants in four forests from tropical to temperate biomes during a heatwave, and compared the TSMs calculated by Tl (TSM.Tl) and Ta (TSM.Ta) respectively. Also, thermal related leaf traits were investigated. The results showed that both TSM. Tl and TSM.Ta decreased from the cool forests to the hot forests. TSM.Tl was highly correlated with the maximum leaf temperature (Tlmax), while had an opposite trend with thermotolerance across biomes. Thus, Tlmax instead of thermotolerance can be used to evaluate TSM. The maximum Ta (Tamax), Tlmax and leaf traits explained 68% of the variance of thermotolerance in a random forest model, where Tamax and Tlmax explained 62%. TSM.Ta could not distinguish thermal safety differences between co-occurring species. The overestimation of TSM by TSM.Ta increased from the tropical to the temperate forest, and increased with Tl within biome. Therefore, it is not recommended to use TSM.Ta in cold forests. The present study enriches the dataset of photosynthetic TSMs across biomes, proposes using Tlmax to estimate TSMs of leaves, and highlights the risk of hot dry forest during heatwaves.

'Dual-reference' method for high-precision infrared measurement of leaf surface temperature under field conditions

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 noncontact remote-sensing 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 L_bg . 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 L_bg 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°C and ± 0.28°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 their dynamics.

Evidence for efficient nonevaporative leaf-to-air heat dissipation in a pine forest under drought conditions

The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present. Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leaf-to-air temperature differences (ΔT_leaf-air ) of pine needles during drought. On mid-summer days, ΔT_leaf-air remained < 3°C, both in trees exposed to summer drought and in those provided with supplemental irrigation, which had a more than 10-fold higher transpiration rate. The nonevaporative cooling in the drought-exposed trees must be facilitated by low resistance to heat transfer, generating a large sensible heat flux, H. ΔT_leaf-air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but was dependent on canopy structure and within-canopy turbulence that enhanced the H. Nonevaporative 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'.

The effect of water stress on net primary productivity in northwest China

Net primary productivity (NPP) has been widely used as the indicator of vegetation function and exhibits large spatial and temporal variations caused by numerous factors. Northwest China (NWC) is one of the driest regions in China, and water supply is the key determinant of NPP here. However, studies on the effects of water stress on NPP in NWC at the regional scale are still relatively lacking. Thus, in this study, based on a set of Moderate-Resolution Imaging Spectroradiometer (MODIS) NPP and evapotranspiration (ET) datasets, we quantified the response of NPP to water stress, which is indicated by crop water stress index (CWSI). Regional average of annual NPP in NWC showed an increasing trend during the study period, at a rate of 0.84 g C m-2 yr-1. At the province level, the NPP increase rates increased in the order of Ningxia (7.7%), Shaanxi (6.5%), Gansu (4.5%), Qinghai (3.8%), and Xinjiang (1.7%). NPP was negatively correlated with CWSI (p<0.05) in 73% of areas, indicating the key role of water stress in constraining NPP over this arid region. The effect of water stress on NPP changes with elevation. Water stress has the strongest negative impact on NPP in areas with elevations around 2000 m. In elevations above 5000 m, NPP is not limited by water stress, mostly positively correlated with CWSI. Our findings further clarify the importance of water stress in dryland ecosystems, while highlighting that elevation gradients can significantly affect the correlation between NPP and water stress.

Assessing model performance via the most limiting environmental driver in two differently stressed pine stands

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.

Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake

The impact of extreme climate episodes such as heatwaves on plants physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO₂ , water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO₂ uptake fluxes (gross primary production, GPP) were significantly reduced during HW days, but only slightly decreased during INT days. By contrast, COS uptake flux and SIF_A (at 760 nm) decreased during both HW and INT days, which was reflected in leaf internal CO₂ concentrations and in nonphotochemical quenching, respectively. Intense (HW) heatwave conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, which allows the trees to preserve carbon assimilation during INT days but not during HW days.

Seeking the "point of no return" in the sequence of events leading to mortality of mature trees

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify this sequence, we used a 2016 tree mortality event in a semi-arid pine forest where dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the surviving trees, 8 months "prior to the visual signs of mortality" (PVSM; e.g., near complete canopy browning). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, 6 months PVSM. Third, cessation of stem sap flow 3 months PVSM. Eventual mortality could therefore be detected long before visual signs were observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capacity and soil water uptake. The results indicated that breakdown of stem radial water flow and phloem function is a critical element in defining the "point of no return" in the sequence of events leading to mortality of mature trees.

Heatwave frequency and seedling death alter stress-specific emissions of volatile organic compounds in Aleppo pine

Biogenic volatile organic compounds (BVOC) play important roles in plant stress responses and can serve as stress indicators. While the impacts of gradual environmental changes on BVOCs have been studied extensively, insights in emission responses to repeated stress and recovery are widely absent. Therefore, we studied the dynamics of shoot gas exchange and BVOC emissions in Pinus halepensis seedlings during an induced moderate drought, two four-day-long heatwaves, and the combination of drought and heatwaves. We found clear stress-specific responses of BVOC emissions. Reductions in acetone emissions with declining soil water content and transpiration stood out as a clear drought indicator. All other measured BVOC emissions responded exponentially to rising temperatures during heat stress (maximum of 43 °C), but monoterpenes and methyl salicylate showed a reduced temperature sensitivity during the second heatwave. We found that these decreases in monoterpene emissions between heatwaves were not reflected by similar declines in their internal storage pools. Because stress intensity was extremely severe, most of the seedlings in the heat-drought treatment died at the end of the second heatwave (dark respiration ceased). Interestingly, BVOC emissions (methanol, monoterpenes, methyl salicylate, and acetaldehyde) differed between dying and surviving seedlings, already well before indications of a reduced vitality became visible in gas exchange dynamics. In summary, we could clearly show that the dynamics of BVOC emissions are sensitive to stress type, stress frequency, and stress severity. Moreover, we found indications that stress-induced seedling mortality was preceded by altered methanol, monoterpene, and acetaldehyde emission dynamics.

Water Use by Chinese Pine Is Less Conservative but More Closely Regulated Than in Mongolian Scots Pine in a Plantation Forest, on Sandy Soil, in a Semi-Arid Climate

The diversity of plant water use patterns among species and ecosystems is a matter of widespread debate. In this study, Chinese pine (Pinus tabuliformis, CP) and Mongolian Scots pine (Pinus sylvestris var. mongolica, MP), which is co-exist in the shelterbelt plantations in the Horqin Sandyland in northern China, were chosen for comparison of water use traits by monitoring xylem sap flow alongside recordings of the associated environmental factors over four growing seasons. Continuous sap flux density measurements were converted into crown projected area transpiration intensity (T_r) and canopy stomatal conductance (G_s). The results indicated that MP showed a higher canopy transpiration intensity than in CP, with T_r daily means (±standard deviation) of 0.84 ± 0.36 and 0.79 ± 0.43 mm⋅d^-1, respectively (p = 0.07). However, the inter-annual variability of daily T_r in MP was not significant, varying only approximately a 1.1-fold (p = 0.29), while inter-annual variation was significant for CP, with 1.24-fold variation (p < 0.01). In particular, the daily mean T_r value for CP was approximately 1.7-times higher than that of MP under favorable soil moisture conditions, with values for relative extractable soil water within the 0-1.0 m soil layer (REW) being above 0.4. However, as the soil dried out, the value of T_r for CP decreased more sharply, falling to only approximately 0.5-times the value for MP when REW fell to < 0.2. The stronger sensitivity of T_r and/or G_s to REW, together with the more sensitive response of G_s to VPD in CP, confirms that CP exhibits less conservation of soil water utilization but features a stronger ability to regulate water use. Compared with MP, CP can better adapt to the dry conditions associated with climate change.

Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2

Drought-induced tree mortality is expected to occur more frequently under predicted climate change. However, the extent of a possibly mitigating effect of simultaneously rising atmospheric [CO₂ ] on stress thresholds leading to tree death is not fully understood, yet. Here, we studied the drought response, the time until critical stress thresholds were reached and mortality occurrence of Pinus halepensis (Miller). In order to observe a large potential benefit from eCO₂ , the seedlings were grown with ample of water and nutrient supply under either highly elevated [CO₂ ] (eCO₂ , c. 936 ppm) or ambient (aCO₂ , c. 407 ppm) during 2 years. The subsequent exposure to a fast or a slow lethal drought was monitored using whole-tree gas exchange chambers, measured leaf water potential and non-structural carbohydrates. Using logistic regressions to derive probabilities for physiological parameters to reach critical drought stress thresholds, indicated a longer period for halving needle starch storage under eCO₂ than aCO₂ . Stomatal closure, turgor loss, the duration until the daily tree C balance turned negative, leaf water potential at thresholds and time-of-death were unaffected by eCO₂ . Overall, our study provides for the first-time insights into the chronological interplay of physiological drought thresholds under long-term acclimation to elevated [CO₂ ].

Sap Flow in Aleppo Pine in Greece in Relation to Sapwood Radial Gradient, Temporal and Climatic Variability

Research Highlights: The radial gradient of sap flux density (Js) and the effects of climatic factors on sap flow of Aleppo pine were assessed at different time scales in an eastern Mediterranean ecosystem to improve our understanding of the species water balance. Background and Objectives: Aleppo pine’s sap flow radial profile and responses to environmental parameters in the eastern Mediterranean were, to our best knowledge, originating to date from more arid planted forests. Information from natural forests in this region was lacking. Our objectives were to (a) determine the species’ radial variability in Js on a diurnal and seasonal basis and under different climatic conditions, (b) scale up to tree sap flow taking into account the radial profile of Js and (c) determine the responses of Aleppo pine’s sap flow over the year to climatic variability. Materials and Methods: Js was monitored in Aleppo pine in a natural forest in northern Greece with Granier’s method using sensors at three sapwood depths (21, 51, and 81 mm) during two periods differing in climatic conditions, particularly in soil water availability. Results: Js was the highest at 21 mm sapwood depth, and it declined with increasing depth. A steeper gradient of Js in deep sapwood was observed under drier conditions. The same patterns of radial variability in Js were maintained throughout the year, but the contribution of inner sapwood to sap flow was the highest in autumn when the lower seasonal Js was recorded in both study periods. Not taking into account the radial gradient of Js in the studied Aleppo pine would result in a c. 20.2–27.7 % overestimation of total sap flow on a sapwood basis (Qs), irrespective of climatic conditions. On a diurnal and seasonal basis, VPD was the strongest determinant of sap flux density, while at a larger temporal scale, the effect of soil water content was evident. At SWC > 20% sap flow responded positively to increasing solar radiation and VPD, indicating the decisive role of water availability in the studied region. Moreover, in drier days with VPD > 0.7 KPa, SWC controlled the variation of sap flow. Conclusions: There is a considerable radial variability in Js of the studied Aleppo pine and a considerable fluctuation of sap flow with environmental dynamics that should be taken into account when addressing the species water balance.

Hot drought reduces the effects of elevated CO2 on tree water-use efficiency and carbon metabolism

Trees are increasingly exposed to hot droughts due to CO₂ -induced climate change. However, the direct role of [CO₂ ] in altering tree physiological responses to drought and heat stress remains ambiguous. Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO₂ ]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gas-exchange measurements were supplemented by primary metabolite analysis. Elevated [CO₂ ] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31-32°C) than drought (33-34°C) treatments unaffected by growth [CO₂ ]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO₂ ] was subtle and mainly reflected in enhanced root protein stability. The impact of elevated [CO₂ ] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO₂ ] cannot counterbalance the impacts of hot drought extremes in Aleppo pine.

Evidence for large carbon sink and long residence time in semiarid forests based on 15 year flux and inventory records

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.

Tree-Ring Analysis Reveals Density-Dependent Vulnerability to Drought in Planted Mongolian Pines

Population density influences tree responses to environmental stresses, such as drought and high temperature. Prolonged drought negatively affects the health of Mongolian pines in forests planted by the Three-North Shelter Forest Program in North China. To understand the relationship between stand density and drought-induced forest decline, and to generate information regarding the development of future management strategies, we analyzed the vulnerability to drought of planted Mongolian pines at three stand densities. A tree-ring width index for trees from each density was established from tree-ring data covering the period 1988–2018 and was compared for differences in radial growth. Resistance (Rt), recovery (Rc), resilience (Rs), and relative resilience (RRs) in response to drought events were calculated from the smoothed basal area increment (BAI) curves. The high-density (HDT) group showed a consistently lower tree-ring width than the border trees (BT) and low-density (LDT) groups. The BAI curve of the HDT group started to decrease five years earlier than the LDT and BT groups. Pearson correlation analysis revealed that the radial growth of all of the groups was related to precipitation, relative humidity (RH), potential evapotranspiration (ET0), and standardized precipitation evapotranspiration index (SPEI) in the previous October and the most recent July, indicating that Mongolian pine trees of different densities had similar growth–climate relationships. Over the three decades, the trees experienced three severe drought events, each causing reduced tree-ring width and BAI. All of the groups showed similar Rc to each drought event, but the HDT group exhibited significantly lower Rt, Rs, and RRs than the BT group, suggesting that the HDT trees were more vulnerable to repeated drought stress. The RRs of the HDT group decreased progressively after each drought event and attained <0 after the third event. All of the groups showed similar trends regarding water consumption under varying weather conditions, but the HDT group showed significantly reduced whole-tree hydraulic capability compared with the other two groups. From these results, HDT trees exhibit ecophysiological memory effects from successive droughts, including sap flux dysfunction and higher competition index, which may prevent recovery of pre-drought growth rates. HDT trees may be at greater risk of mortality under future drought disturbance.

Heat Waves Change Plant Carbon Allocation Among Primary and Secondary Metabolism Altering CO2 Assimilation, Respiration, and VOC Emissions

Processes controlling plant carbon allocation among primary and secondary metabolism, i.e., carbon assimilation, respiration, and VOC synthesis are still poorly constrained, particularly regarding their response to stress. To investigate these processes, we simulated a 10-day 38°C heat wave, analysing real-time carbon allocation into primary and secondary metabolism in the Mediterranean shrub Halimium halimifolium L. We traced position-specific ¹³C-labeled pyruvate into daytime VOC and CO₂ emissions and during light-dark transition. Net CO₂ assimilation strongly declined under heat, due to three-fold higher respiration rates. Interestingly, day respiration also increased two-fold. Decarboxylation of the C1-atom of pyruvate was the main process driving daytime CO₂ release, whereas the C2-moiety was not decarboxylated in the TCA cycle. Heat induced high emissions of methanol, methyl acetate, acetaldehyde as well as mono- and sesquiterpenes, particularly during the first two days. After 10-days of heat a substantial proportion of ¹³C-labeled pyruvate was allocated into de novo synthesis of VOCs. Thus, during extreme heat waves high respiratory losses and reduced assimilation can shift plants into a negative carbon balance. Still, plants enhanced their investment into de novo VOC synthesis despite associated metabolic CO₂ losses. We conclude that heat stress re-directed the proportional flux of key metabolites into pathways of VOC biosynthesis most likely at the expense of reactions of plant primary metabolism, which might highlight their importance for stress protection.

Drought-Induced Reductions and Limited Recovery in the Radial Growth, Transpiration, and Canopy Stomatal Conductance of Mongolian Scots Pine (Pinus sylvestris var. mongolica Litv): A Five-Year Observation

Determining plant–water relationships in response to drought events can provide important information about the adaptation of trees to climate change. The Mongolian Scots pine (Pinus sylvestris var. mongolica Litv), as one of the major tree species to control soil loss and desertification in northern China, has experienced severe degradation in recent decades. Here, we aimed to examine the impacts of a two-year consecutive drought and another year of drought on the radial growth, transpiration, and canopy stomatal conductance of Mongolian Scots pine over a five-year period, especially in terms of its recovery after drought. The study period during 2013–2017 consisted of a ‘normal’ year, a ‘dry year’, a ‘very dry’ year, a ‘wet’ year, and a ‘dry’ year, according to annual precipitation and soil moisture conditions. Based on measurements of the sap flow and diameters at breast height of 11 sample trees as well as the concurrent environmental factors, we quantified the reductions in tree radial growth, transpiration, and canopy stomatal conductance during the drought development as well as their recovery after the drought. The results showed that the tree radial growth, transpiration, and canopy stomatal conductance of Mongolian Scots pines decreased by 33.8%, 51.9%, and 51.5%, respectively, due to the two consecutive years of drought. Moreover, these reductions did not fully recover after the two-year drought was relieved. The minimum difference of these parameters between before and after the two-year consecutive drought period was 8.5% in tree radial growth, 45.1% in transpiration levels, and 42.4% in canopy stomatal conductance. We concluded that the two consecutive years of drought resulted in not only large reductions in tree radial growth and water use, but also their lagged and limited recoveries after drought. The study also highlighted the limited resilience of Mongolian Scots pine trees to prolonged drought in semi-arid sandy environmental conditions.

Quantification of leaf-scale light energy allocation and photoprotection processes in a Mediterranean pine forest under extensive seasonal drought

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 CO2 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 CO2 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.

Contrasting stomatal sensitivity to temperature and soil drought in mature alpine conifers

Conifers growing at high elevations need to optimize their stomatal conductance (g_s ) for maximizing photosynthetic yield while minimizing water loss under less favourable thermal conditions. Yet the ability of high-elevation conifers to adjust their g_s sensitivity to environmental drivers remains largely unexplored. We used 4 years of sap flow measurements to elucidate intraspecific and interspecific variability of g_s in Larix decidua Mill. and Picea abies (L.) Karst along an elevational gradient and contrasting soil moisture conditions. Site- and species-specific g_s response to main environmental drivers were examined, including vapour pressure deficit, air temperature, solar irradiance, and soil water potential. Our results indicate that maximum g_s of L. decidua is >2 times higher, shows a more plastic response to temperature, and down-regulates g_s stronger during atmospheric drought compared to P. abies. These differences allow L. decidua to exert more efficient water use, adjust to site-specific thermal conditions, and reduce water loss during drought episodes. The stronger plasticity of g_s sensitivity to temperature and higher conductance of L. decidua compared to P. abies provide new insights into species-specific water use strategies, which affect species' performance and should be considered when predicting terrestrial water dynamics under future climatic change.

Sun-induced fluorescence and gross primary productivity during a heat wave

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.

Water Balance of Mediterranean Quercus ilex L. and Pinus halepensis Mill. Forests in Semiarid Climates: A Review in A Climate Change Context

Forests provide many environmental services, especially those related to the water cycle. In semiarid areas where water is a limiting factor for ecosystem functioning, forested areas can have a strong impact on ground water recharge. In these areas, proper knowledge of forests’ water balance is necessary to promote management practices that may ensure ecosystem properties and environmental services like water or carbon fixation. In this article, we review several ecohydrology topics within the framework of Mediterranean water-limited environments in two representative ecosystems: Kermes oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill.) forests. Both are the commonest species in countries that surround the Western Mediterranean Basin. We analysed the Blue and Green water components, i.e., green water is the water demand of forests, represented by evapotranspiration and interception; while blue water is the part of the balance involving runoff and deep percolation, which can be regarded as water directly usable by society. In general, different studies conducted in Mediterranean areas have pointed out that the water balances of Q. ilex and P. halepensis forests have low values for the Blue to Green water (B/G) ratios. Adaptive forest management like forest thinning can compensate for these ratios. Thinning has demonstrated to reduce losses by interception, but at same time, it can also increase individual tree transpiration and evaporation rates. However, these practices lead to higher B/G ratios when considering the whole stand. In future global change scenarios, in which drought conditions are expected to intensify, management practices can improve the water balance in these ecosystems by minimizing the risk of plant mortality and species replacement due to intense competence by water resources.

Heavy mowing enhances the effects of heat waves on grassland carbon and water fluxes

Heat waves (HWs) are a type of extreme weather event that is of growing concern in the scientific community. Yet field data based on sound experiment on the variation of ecosystem CO₂ levels under HWs remain rare. Additionally, ecosystems react to HWs and the coupled human activities (such as grazing in grasslands) are unknown. Thus, a 3-year field experiment was conducted to simulate HWs in conjunction with different mowing intensities that mimicking grazing in a Stipa krylovii steppe on the Mongolian Plateau. HWs significantly decreased ecosystem exchange (NEE) of CO₂, ecosystem respiration (Re) and gross ecosystem productivity (GEP) by 31%, 5% and 16%, respectively, over the three years. Continuous HWs over multiple years produced cumulative effects by reducing NEE at 20%, 34% and 40% in the first, second and third HW years, respectively. During three pre-defined three periods of HWs (during HW period, after HW period in the same year, and after HW period in the next year), variations in water use efficiency indicated that the grassland ecosystem exhibited a strategy for adapting to the continuous HWs to a certain extent, by adjusting community structure or increasing litter biomass. Finally, mowing increased the effects of HWs by extending the legacy effect, such that restoration of the grassland required a greater amount of time under the combination of HWs and mowing.

Forests growing under dry conditions have higher hydrological resilience to drought than do more humid forests

More frequent and intense droughts are projected during the next century, potentially changing the hydrological balances in many forested catchments. Although the impacts of droughts on forest functionality have been vastly studied, little attention has been given to studying the effect of droughts on forest hydrology. Here, we use the Budyko framework and two recently introduced Budyko metrics (deviation and elasticity) to study the changes in the water yields (rainfall minus evapotranspiration) of forested catchments following a climatic drought (2006-2010) in pine forests distributed along a rainfall gradient (P = 280-820 mm yr^-1 ) in the Eastern Mediterranean (aridity factor = 0.17-0.56). We use a satellite-based model and meteorological information to calculate the Budyko metrics. The relative water yield ranged from 48% to 8% (from the rainfall) in humid to dry forests and was mainly associated with rainfall amount (increasing with increased rainfall amount) and bedrock type (higher on hard bedrocks). Forest elasticity was larger in forests growing under drier conditions, implying that drier forests have more predictable responses to drought, according to the Budyko framework, compared to forests growing under more humid conditions. In this context, younger forests were shown more elastic than older forests. Dynamic deviation, which is defined as the water yield departure from the Budyko curve, was positive in all forests (i.e., less-than-expected water yields according to Budyko's curve), increasing with drought severity, suggesting lower hydrological resistance to drought in forests suffering from larger rainfall reductions. However, the dynamic deviation significantly decreased in forests that experienced relatively cooler conditions during the drought period. Our results suggest that forests growing under permanent dry conditions might develop a range of hydrological and eco-physiological adjustments to drought leading to higher hydrological resilience. In the context of predicted climate change, such adjustments are key factors in sustaining forested catchments in water-limited regions.

Environmental conditions regulate the impact of plants on cloud formation

The terrestrial vegetation emits large amounts of volatile organic compounds (VOC) into the atmosphere, which on oxidation produce secondary organic aerosol (SOA). By acting as cloud condensation nuclei (CCN), SOA influences cloud formation and climate. In a warming climate, changes in environmental factors can cause stresses to plants, inducing changes of the emitted VOC. These can modify particle size and composition. Here we report how induced emissions eventually affect CCN activity of SOA, a key parameter in cloud formation. For boreal forest tree species, insect infestation by aphids causes additional VOC emissions which modifies SOA composition thus hygroscopicity and CCN activity. Moderate heat increases the total amount of constitutive VOC, which has a minor effect on hygroscopicity, but affects CCN activity by increasing the particles' size. The coupling of plant stresses, VOC composition and CCN activity points to an important impact of induced plant emissions on cloud formation and climate.

Volatile organic compounds (VOC) emitted by vegetation influence cloud formation, yet the impact of environmental stresses remains little known. Here, manipulation experiments reveal insect infestation and heat stress are linked to induced VOC and constitutive VOC emissions shifts, respectively.