Prof. Dr. Charlotte Grossiord

Plant water uptake from the soil is a crucial element of the global hydrological cycle and essential for vegetation drought resilience. Yet, knowledge of how the distribution of water uptake depth (WUD) varies across species, climates, and seasons is scarce relative to our knowledge of aboveground plant functions. With a global literature review, we found that average WUD varied more among biomes than plant functional types (i.e. deciduous/evergreen broadleaves and conifers), illustrating the importance of the hydroclimate, especially precipitation seasonality, on WUD. By combining records of rooting depth with WUD, we observed a consistently deeper maximum rooting depth than WUD with the largest differences in arid regions – indicating that deep taproots act as lifelines while not contributing to the majority of water uptake. The most ubiquitous observation across the literature was that woody plants switch water sources to soil layers with the highest water availability within short timescales. Hence, seasonal shifts to deep soil layers occur across the globe when shallow soils are drying out, allowing continued transpiration and hydraulic safety. While there are still significant gaps in our understanding of WUD, the consistency across global ecosystems allows integration of existing knowledge into the next generation of vegetation process models.

1. Enhancing tree diversity may be important to fostering resilience to drought-related climate extremes. So far, little attention has been given to whether tree diversity can increase the survival of trees and reduce its variability in young forest plantations.
2. We conducted an analysis of seedling and sapling survival from 34 globally distributed tree diversity experiments (363,167 trees, 168 species, 3744 plots, 7 biomes) to answer two questions: (1) Do drought and tree diversity alter the mean and variability in plot-level tree survival, with higher and less variable survival as diversity increases? and (2) Do species that survive poorly in monocultures survive better in mixtures and do specific functional traits explain monoculture survival?
3. Tree species richness reduced variability in plot-level survival, while functional diversity (Rao's Q entropy) increased survival and also reduced its variability. Importantly, the reduction in survival variability became stronger as drought severity increased. We found that species with low survival in monocultures survived comparatively better in mixtures when under drought. Species survival in monoculture was positively associated with drought resistance (indicated by hydraulic traits such as turgor loss point), plant height and conservative resource-acquisition traits (e.g. low leaf nitrogen concentration and small leaf size).
4. Synthesis. The findings highlight: (1) The effectiveness of tree diversity for decreasing the variability in seedling and sapling survival under drought; and (2) the importance of drought resistance and associated traits to explain altered tree species survival in response to tree diversity and drought. From an ecological perspective, we recommend mixing be considered to stabilize tree survival, particularly when functionally diverse forests with drought-resistant species also promote high survival of drought-sensitive species.

Increasing tree diversity is considered a key management option to adapt forests to climate change. However, the effect of species diversity on a forest's ability to cope with extreme drought remains elusive. In this study, we assessed drought tolerance (xylem vulnerability to cavitation) and water stress (water potential), and combined them into a metric of drought–mortality risk (hydraulic safety margin) during extreme 2021 or 2022 summer droughts in five European tree diversity experiments encompassing different biomes. Overall, we found that drought–mortality risk was primarily driven by species identity (56.7% of the total variability), while tree diversity had a much lower effect (8% of the total variability). This result remained valid at the local scale (i.e within experiment) and across the studied European biomes. Tree diversity effect on drought–mortality risk was mediated by changes in water stress intensity, not by changes in xylem vulnerability to cavitation. Significant diversity effects were observed in all experiments, but those effects often varied from positive to negative across mixtures for a given species. Indeed, we found that the composition of the mixtures (i.e., the identities of the species mixed), but not the species richness of the mixture per se, is a driver of tree drought–mortality risk. This calls for a better understanding of the underlying mechanisms before tree diversity can be considered an operational adaption tool to extreme drought. Forest diversification should be considered jointly with management strategies focussed on favouring drought-tolerant species.

Progressively warmer and drier climatic conditions impact tree phenology and carbon cycling with large consequences for forest carbon balance. However, it remains unclear how individual impacts of warming and drier soils differ from their combined effects and how species interactions modulate tree responses. Using mesocosms, we assessed the multiyear impact of continuous air warming and lower soil moisture alone or in combination on phenology, leaf-level photosynthesis, nonstructural carbohydrate concentrations, and aboveground growth of young European beech (Fagus sylvatica L.) and Downy oak (Quercus pubescens Willd.) trees. We further tested how species interactions (in monocultures and in mixtures) modulated these effects. Warming prolonged the growing season of both species but reduced growth in oak. In contrast, lower moisture did not impact phenology but reduced carbon assimilation and growth in both species. Combined impacts of warming and drier soils did not differ from their single effects. Under warmer and drier conditions, performances of both species were enhanced in mixtures compared to monocultures. Our work revealed that higher temperature and lower soil moisture have contrasting impacts on phenology vs. leaf-level assimilation and growth, with the former being driven by temperature and the latter by moisture. Furthermore, we showed a compensation in the negative impacts of chronic heat and drought by tree species interactions.

• Chronic reductions in soil moisture combined with high air temperatures can modify tree carbon and water relations. However, little is known about how trees acclimate their foliar structure to the individual and combined effects of these two climate drivers.
• We used open-top chambers to determine the multi-year effects of chronic air warming (+5 °C) and soil moisture reduction (−50%) alone and in combination on the foliar anatomy of two European tree species. We further investigated how these climate drivers affected the relationship between foliar anatomy and physiology/chemistry in young downy oak and European beech trees.
• After 4 years, reduced soil moisture led to development of thinner leaves with a narrower epidermis and lower gas exchange for oak and beech, but to a lesser extent in the latter. In contrast, prolonged warming did not affect the anatomical and physiological/chemical traits in either species. Warming also did not exacerbate the impacts of dry soils, highlighting soil moisture as the key driver in leaf anatomical shifts.
• While soil moisture altered oak foliar anatomy, and the physiology and chemistry of both species, our work revealed a limited acclimation potential towards more drought- and heat-tolerant leaves as conditions become drier and warmer, suggesting potentially high vulnerability of both species to future climate predictions.

Coastal cities are facing a rise in groundwater levels induced by sea level rise, further triggering saturation excess flooding where groundwater levels reach the topographic surface or reduce the storage capacity of the soil, thus stressing the existing infrastructure. Lowering groundwater levels is a priority for sustaining the long-term livelihood of coastal cities. In the absence of studies assessing the possibility of using tree-planting as a measure of alleviating saturation excess flooding in the context of rising groundwater levels, the multi-benefit nature of tree-planting programs as sustainable Nature-based solutions (NBSs) in coastal cities in the Global South is discussed. In environments where groundwater is shallow, trees uptake groundwater or reduce groundwater recharge, thereby contributing to lower groundwater levels and increasing the unsaturated zone thickness, further reducing the risk of saturation excess flooding. Tree-planting programs represent long-term solutions sustained by environmental factors that are complementary to conventional engineering solutions. The multi-benefit nature of such NBSs and the expected positive environmental, economic, and social outcomes make them particularly promising. Wide social acceptance was identified as crucial for the long-term success of any tree-planting program, as the social factor plays a major role in addressing most weaknesses and threats of the solution. In the case of Nouakchott City (Mauritania), where a rise in groundwater levels has led to permanent saturation excess flooding, a tree-planting program has the potential to lower the groundwater levels, thereby reducing flooding during the rainy season.

Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide-scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis A_net and stomatal conductance g_s), canopy temperature (T_can), and heat tolerance (leaf critical temperature T_crit and thermal safety margins TSM, i.e., the difference between maximum T_can and T_crit) in three oak species in forests along a latitudinal gradient (Quercus petraea in Switzerland, Quercus ilex in France, and Quercus coccifera in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (T_air) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in Q. ilex and Q. coccifera when T_air surpassed 30°C and soil moisture dropped below 14%. Across all seasons, T_can was mainly above T_air but increased strongly (up to 10°C > T_air) when A_net was null or negative. Although trees endured extreme T_air (up to 42°C), positive TSM were maintained during the growing season due to high T_crit in all species (average T_crit of 54.7°C) and possibly stomatal decoupling (i.e., A_net ≤0 while g_s >0). Indeed, Q. ilex and Q. coccifera trees maintained low but positive g_s (despite null A_net), decreasing Ψ passed embolism thresholds. This may have prevented T_can from rising above T_crit during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves.

Climate change is predicted to increase atmospheric vapor pressure deficit, exacerbating soil drought, and thus enhancing tree evaporative demand and mortality. Yet, few studies have addressed the longer-term drought acclimation strategy of trees, particularly the importance of morphological versus hydraulic plasticity. Using a long-term (20 years) irrigation experiment in a natural forest, we investigated the acclimation of Scots pine (Pinus sylvestris) morpho-anatomical traits (stomatal anatomy and crown density) and hydraulic traits (leaf water potential, vulnerability to cavitation (ψ₅₀), specific hydraulic conductivity (K_s), and tree water deficit) to prolonged changes in soil moisture. We found that low water availability reduced twig water potential and increased tree water deficit during the growing season. Still, the trees showed limited adjustments in most branch-level hydraulic traits (ψ₅₀ and K_s) and needle anatomy. In contrast, trees acclimated to prolonged irrigation by increasing their crown density and hence the canopy water demand. This study demonstrates that despite substantial canopy adjustments, P. sylvestris may be vulnerable to extreme droughts because of limited adjustment potential in its hydraulic system. While sparser canopies reduce water demand, such shifts take decades to occur under chronic water deficits and might not mitigate short-term extreme drought events.

As air temperature and vapor pressure deficit (VPD) increase continuously, forests are losing more water through evapotranspiration, with large consequences for local and global hydrological cycles. In regions with high vegetation cover, soil warming can be more pronounced than warming of the air because of reduced air movement in the forest understory and high heat conduction in the soil. Hence, soil warming could further amplify climate change impacts on forest water uptake and use. Yet, the independent effects of persistent soil warming on tree transpiration remain highly uncertain, especially in sub-tropical forests where soil temperature rise has been considerable in recent years. We carried out a long-term soil warming experiment (+2 °C) in a mixed subtropical forest dominated by the relatively anisohydric Acacia auriculiformis and isohydric Schima superba. Throughout the dry and wet seasons, we tracked whole-tree transpiration (E_L), night-time water recharge (E_{L_night}), hydraulic conductivity (K), and canopy stomatal conductance (G_s) on mature trees of both species. We found that E_L and G_s of A. auriculiformis increased (+ 41 % and + 5 %, respectively) while it decreased in S. superba (- 5 % and - 21 %, respectively) with soil warming. However, no changes in K were found for either species. Moreover, soil warming did not change the sensitivity of E_L and G_s to VPD in both species, but the reference canopy stomatal conductance (G_sref) was influenced. Our results suggest that soil warming significantly alters tree canopy transpiration, independently of air temperature. In sub-tropical climates, species with anisohydric stomatal strategy, such as A. auriculiformis, could take up more water and continue to transpire under both high and low VPD, potentially leading to increased nutrient and carbon uptake, and growth. In contrast, isohydric species like S. superba seem adversely affected by soil warming, which may compromise its dominance in subtropical forests in the future.

In sub-Mediterranean ecosystems, shade-tolerant broadleaf evergreens, especially the invasive Trachycarpus fortunei, are spreading uncontrollably in the forest understorey, impeding the regeneration of the native deciduous woody vegetation. Most invasive species benefit from high-light environments, as they often have a resource-acquisitive strategy with high photosynthetic rates. Yet, evergreen neophytes, such as T. fortunei, grow mainly in dark understoreys, which raises the question about the physiological mechanisms allowing it to thrive and, eventually, outcompete the native vegetation. For one year, we compared the photosynthetic light-use efficiency of T. fortunei and seven competitors (one evergreen and six deciduous species) to understand possible physiological drivers of this invasion process. The work was done in invaded sub-Mediterranean forests in Southern Switzerland and Northern Italy. Measurements included the seasonal photosynthetic capacity (A_max), net assimilation (A_net), apparent quantum yield (q_LCP), light compensation point of photosynthesis (LCP), chlorophyll content (CC), chlorophyll fluorescence (F_v /F_m), leaf soluble sugar (SS_L) and starch concentrations (ST_L), nitrogen (LNC), and dry matter (LDMC) contents. During the entire year, T. fortunei had similar assimilation rates to the evergreen and deciduous competitors. However, in October, A_max and A_net of T. fortunei were higher than the other evergreen species, while most deciduous plants started losing their foliage. While no photosynthetic disadvantage was found for T. fortunei, we observed less seasonal variability in A_net, q_LCP, SS_L, ST_L, and LDMC, and a lower or similar q_LCP, F_v/F_m, CC, SS_L, ST_L, and LDMC in this species than in its competitors. We highlight that October was the only month when T. fortunei had a photosynthetic advantage over the native sub-Mediterranean evergreen vegetation, while during the rest of the year, it showed similar assimilation rates as evergreen and deciduous species. Forests with sparse canopies in October might be particularly favourable for T. fortunei, due to the benefits of high light conditions for its assimilation rates compared to native evergreens. Moreover, the low seasonal variability in physiological traits indicates that as winters get milder and growing seasons longer, the invasive palm may take a further competitive advantage over the native vegetation.

Atmospheric pollution threatens human health worldwide, with tropospheric ozone (O₃) and particulate matter (PM) among the most harmful pollutants. Urban trees can reduce the concentration of air pollutants through dry deposition on their canopies and stomatal uptake. At the same time, urban trees can deteriorate air quality by emitting aerosol- and O₃ precursors in the form of biogenic volatile organic compounds (BVOCs). O₃ and PM removal, and BVOC emissions vary depending on the tree species. Therefore, the diversity and spatial distribution of urban trees significantly influence their impact on local air quality. This study employs a dual approach to assess and map the impact of urban trees on air quality in Geneva, Switzerland. Firstly, we use the i-Tree Eco model combined with a tree inventory (237,191 trees) to quantify BVOC emissions and PM₁₀ (PM < 10 µm) and O₃ removal at the genus level. Secondly, we develop a species-level parameterization using 51 common urban tree species to estimate the same variables and ozone-forming potential (OFP). Results show that the tree density is heterogeneous in the study area, leading to neighborhoods with greater biomass and, therefore, stronger influence by trees on local air quality. According to i-Tree Eco, urban trees in Geneva emitted 50 t of BVOCs, while removing 14 t of PM₁₀ and 52 t of O₃ in 2014. With the species-level parametrization, we estimate that urban trees removed about 66 ± 55 t of PM₁₀ and 150 ± 96 t of O₃ in 2014. However, they could also emit about 130 ± 52 t of BVOCs annually, which, under favorable conditions, can form 1153 ± 519 t of O₃. Depending on the method, urban trees removed between 4 and 19 % of the anthropogenic PM₁₀ emissions. The annual removal rates are comparable to findings in other European cities. The disparities between the two approaches are due to different parameterizations. This study could help urban planners to select adequate species for future planting programs in Geneva and more generally. It showed that the impact of urban trees on air quality is spatially heterogeneous but significant, and tightly linked to the species composition.

1. Tree species diversity in forest ecosystems could reduce their vulnerability to extreme droughts through improved microclimate and below-ground water source partitioning driven by contrasting species-specific water use patterns. However, little is known about the seasonal dynamics of belowground water uptake that determine whether diversity positively or negatively impacts tree carbon assimilation and water exchange.
2. Using a network of 30 permanent plots in Mediterranean forests with increasing tree species diversity (from monospecific to four-species mixtures), we examined the seasonal patterns of in-situ aboveground carbon and water relations and belowground water sources on 265 trees from four pine and oak species over 2 years using hydraulic and stable isotope approaches.
3. We found that increasing species diversity in broadleaf and conifer mixtures induced strong soil water source partitioning between oak and pine species. As conditions became drier during the summer in mixed stands, oak species took up water from deeper soil sources, while pines were systematically limited to shallow ones. Despite significant belowground moisture partitioning, stronger drought-induced reductions in photosynthesis, stomatal conductance, and leaf water potential were still observed in diverse compared with monospecific stands for pines but with some benefits for oaks.
4. Synthesis: Our findings reveal that tree species diversity promoted belowground water source partitioning in mixed oak and pine stands, potentially reducing competition for water in more diverse ecosystems. Yet, our results show that it is insufficient to buffer the adverse impacts of severe droughts on aboveground tree carbon and water use, leading to higher water stress, especially for pines.

Mixing species with contrasting resource use strategies could reduce forest vulnerability to extreme events. Yet, how species diversity affects seedling hydraulic responses to heat and drought, including mortality risk, is largely unknown. Using open-top chambers, we assessed how, over several years, species interactions (monocultures vs mixtures) modulate heat and drought impacts on the hydraulic traits of juvenile European beech and pubescent oak. Using modeling, we estimated species interaction effects on timing to drought-induced mortality and the underlying mechanisms driving these impacts. We show that mixtures mitigate adverse heat and drought impacts for oak (less negative leaf water potential, higher stomatal conductance, and delayed stomatal closure) but enhance them for beech (lower water potential and stomatal conductance, narrower leaf safety margins, faster tree mortality). Potential underlying mechanisms include oak's larger canopy and higher transpiration, allowing for quicker exhaustion of soil water in mixtures. Our findings highlight that diversity has the potential to alter the effects of extreme events, which would ensure that some species persist even if others remain sensitive. Among the many processes driving diversity effects, differences in canopy size and transpiration associated with the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations.

• Increasing tree species diversity in Mediterranean forests could reduce drought-induced hydraulic impairments through improved microclimate and reduced competition for water. However, it remains unclear if and how species diversity modulates tree hydraulic functions and how impacts may shift during the growing season.
• Using unmanaged Mediterranean forest stands composed of one (i.e., monospecific) or four (i.e., multispecific) tree species, we examined the seasonal dynamics of in-situ hydraulic traits (predawn and midday leaf water potential – Ψ_pd and Ψ_md, xylem- and leaf-specific hydraulic conductivity – K_S and K_L, percentage loss of conductivity – PLC, specific leaf area – SLA, and Huber value – HV) in four co-existing Pinus and Quercus species over two years.
• We mainly observed adverse impacts of species diversity with lower Ψ_pd, Ψ_md, K_S, K_L, and higher PLC in multispecific compared to monospecific stands, especially for the two pines. These impacts were observed all along the growing season but were stronger during the driest periods of the summer. Beneficial impacts of diversity were rare and only occured for oaks (Q. faginea) after prolonged and intense water stress.
• Our findings reveal that mixing oaks and pines could mainly enhance hydraulic impairments for all species during the dry season, suggesting a potential decline in mixed Mediterranean forests under future climate.

Micro-computed tomography (µCT) is a non-destructive X-ray imaging method used in plant physiology to visualize in-situ plant tissues that enables assessments of embolized xylem vessels. Whereas evidence for X-ray-induced cellular damage has been reported, the impact on plant physiological processes such as carbon (C) uptake, transport, and use are unknown. Yet, these damages could be particularly relevant for studies that track embolism and C fluxes over time. We examined the physiological consequences of µCT-scanning for xylem embolism over three months by monitoring net photosynthesis (A_net), diameter growth, chlorophyll concentration (Chl), and foliar non-structural carbohydrate (NSC) content in four deciduous tree species: hedge maple (Acer campestre), ash (Fraxinus excelsior), European hornbeam (Carpinus betulus), and sessile oak (Quercus petraea). C transport from the canopy to the roots was also assessed through ¹³C labelling. Our results show that monthly X-ray application did not impact foliar A_net, Chl, NSC content, and C transport. Although X-ray effects did not vary between species, the most pronounced impact was observed in sessile oak, marked by stopped growth and stem deformations around the irradiated area. The absence of adverse impacts on plant physiology for all the tested treatments indicates that laboratory-based µCT systems can be used with different beam energy levels and doses without threatening the integrity of plant physiology within the range of tested parameters. However, the impacts of repetitive µCT on the stem radial growth at the irradiated zone leading to deformations in sessile oak might have lasting implications for studies tracking plant embolism in the longer-term.

An exponential rise in the atmospheric vapour pressure deficit (VPD) is among the most consequential impacts of climate change in terrestrial ecosystems. Rising VPD has negative and cascading effects on nearly all aspects of plant function including photosynthesis, water status, growth and survival. These responses are exacerbated by land–atmosphere interactions that couple VPD to soil water and govern the evolution of drought, affecting a range of ecosystem services including carbon uptake, biodiversity, the provisioning of water resources and crop yields. However, despite the global nature of this phenomenon, research on how to incorporate these impacts into resilient management regimes is largely in its infancy, due in part to the entanglement of VPD trends with those of other co-evolving climate drivers. Here, we review the mechanistic bases of VPD impacts at a range of spatial scales, paying particular attention to the independent and interactive influence of VPD in the context of other environmental changes. We then evaluate the consequences of these impacts within key management contexts, including water resources, croplands, wildfire risk mitigation and management of natural grasslands and forests. We conclude with recommendations describing how management regimes could be altered to mitigate the otherwise highly deleterious consequences of rising VPD.

Raised emissions of biologically reactive nitrogen (N) have intensified N deposition, enhancing tree productivity globally. Nonetheless, the drivers of forest sensitivity to N deposition remain unknown. We used stem growth data from 62,000 trees across Europe combined with N deposition data to track the effects of air temperature and precipitation on tree growth's sensitivity to N deposition and how it varied depending on leaf form over the past 30 years. Overall, N deposition enhanced conifer growth (until 30 kg N ha⁻¹ yr⁻¹) while decreasing growth for broadleaved angiosperms. Lower temperatures led to higher growth sensitivity to N deposition in conifers potentially exacerbated by N limitation. In contrast, higher temperatures stimulated growth sensitivity to N deposition for broadleaves. Higher precipitation equally increased N deposition sensitivity in all leaf forms. We conclude that air temperature and leaf form are decisive in disentangling the effect of N deposition in European forests, which provides crucial information to better predict the contribution of N deposition to land carbon sink enhancement.

1. Temperature rise and more severe and frequent droughts will alter forest transpiration, thereby affecting the global water cycle. Yet, tree responses to increased atmospheric vapour pressure deficit (VPD) and reduced soil water content (SWC) are not fully understood due to long-term tree adjustments to local environmental conditions that modify transpiration responses to short-term VPD and SWC changes.
2. We analysed sap flux density (SFD) of Fagus sylvatica, Picea abies, Pinus sylvestris, and Quercus ilex from 25 sites across Europe to understand how daily variation in SWC affects the sensitivity of SFD to VPD (β_VPD) and the maximum SFD (S₉₅). Furthermore, we tested whether long-term adjustments to site climatic conditions and stand characteristics affect β_VPD and S₉₅.
3. The studied species showed contrasting β_VPD and S₉₅ with the largest values in F. sylvatica, followed by Q. ilex, which surpassed the two conifers that showed low β_VPD and low S₉₅. We observed that β_VPD and S₉₅ dropped during days of low SWC in F. sylvatica, P. sylvestris, and Q. ilex, but not in P. abies. Both β_VPD and S₉₅ were driven by tree height, and the temperature and precipitation at the sites. However, stand basal area was the most important driver of β_VPD and S₉₅, explaining 30% of their total variance.
4. Synthesis and applications: A future warmer and drier climate will restrict tree transpiration and thereby heavily affect the soil-plant-atmosphere coupling. However, the effect of basal area, being the largest driver of tree transpiration sensitivity to vapour pressure deficit across a broad range of conditions, provides the opportunity to pre-adapt European forests to future climate conditions. While stand thinning can increase the soil water availability for remaining trees, it also increases transpiration sensitivity to high air temperatures and may thereby amplify tree vulnerability to heat and drought.

Global warming and droughts push forests closer to their thermal limits, altering tree carbonuptake and growth. To prevent critical overheating, trees can adjust their thermotolerance(T_crit), temperature and photosynthetic optima (T_opt and A_opt), and canopy temperature (T_can) to stay below damaging thresholds. However, we lack an understanding of how soil droughts affect photosynthetic thermal plasticity and T_can regulation.
In this study, we measured the effect of soil moisture on the seasonal and diurnal dynamics of net photosynthesis (A), stomatal conductance (g_s), and T_can, as well as the thermal plasticityof photosynthesis (T_crit, T_opt, and A_opt), over the course of 1 yr using a long-term irrigationexperiment in a drought-prone Pinus sylvestris forest in Switzerland.
Irrigation resulted in higher needle-level A, g_s, T_opt, and A_opt compared with naturally drought-exposed trees. No daily or seasonal differences in T_can were observed between treatments. Trees operated below their thermal thresholds (T_crit), independently of soil moisture content.
Despite strong T_can and T_air coupling, we provide evidence that drought reduces trees’ temperature optimum due to a substantial reduction of g_s during warm and dry periods of the year. These findings provide important insights regarding the effects of soil drought on thethermal tolerance of P. sylvestris.

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (G_c) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of G_c are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between G_c and the ability of stem tissues to rehydrate at night remains unclear.
We investigated whether species-specific G_c responses aim to prevent branch embolisms, or enable night-time stem rehydration, which is critical for turgor-dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch-vulnerability curves of six common European tree species.
Species-specific G_c reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P₅₀). Instead, we found a stronger relationship with stem rehydration. Species with a stronger G_c control were less effective at refilling stem-water storage as the soil dries, which appeared related to their xylem architecture.
Our findings highlight the importance of stem rehydration for water-use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety–efficiency stomatal control paradigm.