Brief windows with more favorable atmospheric conditions explain patterns of Polylepis reticulata tree water use in a high-altitude Andean forest

Polylepis trees occur throughout the Andean mountain region, and it is the tree genus that grows at the highest elevation worldwide. In the humid Andes where moisture is rarely limiting, Polylepis trees must adapt to extreme environmental conditions, especially rapid fluctuations in temperature, ultraviolet radiation and vapor pressure deficit (VPD). However, Polylepis' water-use patterns remain largely unknown despite the importance of understanding their response to microclimate variation to determine their capacity to maintain resilience under future environmental change. We conducted a study in a Polylepis reticulata Kunth forest in the Ecuadorian Andes to evaluate its tree water-use dynamics and to identify the main environmental drivers of transpiration. Tree sap flow was monitored simultaneously with soil volumetric water content (VWC) and microclimate during 2 years for trees growing in forest edge and interior locations. We found that sap flow was primarily controlled by VPD and that VWC exerted a secondary role in driving sap flow dynamics. The highest values for sap flow rates were found when VPD > 0.15 kPa and VCW < 0.73 cm3 cm-3, but these threshold conditions only occurred during brief periods of time and were only found in 11% of our measurements. Moreover, these brief windows of more favorable conditions occurred more frequently in forest edge compared with forest interior locations, resulting in edge trees maintaining 46% higher sap flow compared with interior trees. Our results also suggest that P. reticulata has a low stomatal control of transpiration, as the sap flow did not decline with increasing VPD. This research provides valuable information about the potential impacts of projected future increases in VPD due to climate change on P. reticulata water-use dynamics, which include higher sap flow rates leading to greater transpirational water loss due to this species' poor stomatal control.

Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night

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.

Drought response in Arabidopsis displays synergistic coordination between stems and leaves

The synergy between drought-responsive traits across different organs is crucial in the whole-plant mechanism influencing drought resilience. These organ interactions, however, are poorly understood, limiting our understanding of drought response strategies at the whole-plant level. Therefore, we need more integrative studies, especially on herbaceous species that represent many important food crops but remain underexplored in their drought response. We investigated inflorescence stems and rosette leaves of six Arabidopsis thaliana genotypes with contrasting drought tolerance, and combined anatomical observations with hydraulic measurements and gene expression studies to assess differences in drought response. The soc1ful double mutant was the most drought-tolerant genotype based on its synergistic combination of low stomatal conductance, largest stomatal safety margin, more stable leaf water potential during non-watering, reduced transcript levels of drought stress marker genes, and reduced loss of chlorophyll content in leaves, in combination with stems showing the highest embolism resistance, most pronounced lignification, and thickest intervessel pit membranes. In contrast, the most sensitive Cvi ecotype shows the opposite extreme of the same set of traits. The remaining four genotypes show variations in this drought syndrome. Our results reveal that anatomical, ecophysiological, and molecular adaptations across organs are intertwined, and multiple (differentially combined) strategies can be applied to acquire a certain level of drought tolerance.

Changes in leaf functional traits with leaf age: when do leaves decrease their photosynthetic capacity in Amazonian trees?

Most leaf functional trait studies in the Amazon basin do not consider ontogenetic variations (leaf age), which may influence ecosystem productivity throughout the year. When leaf age is taken into account, it is generally considered discontinuous, and leaves are classified into age categories based on qualitative observations. Here, we quantified age-dependent changes in leaf functional traits such as the maximum carboxylation rate of ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) (Vcmax), stomatal control (Cgs%), leaf dry mass per area and leaf macronutrient concentrations for nine naturally growing Amazon tropical trees with variable phenological strategies. Leaf ages were assessed by monthly censuses of branch-level leaf demography; we also performed leaf trait measurements accounting for leaf chronological age based on days elapsed since the first inclusion in the leaf demography, not predetermined age classes. At the tree community scale, a nonlinear relationship between Vcmax and leaf age existed: young, developing leaves showed the lowest mean photosynthetic capacity, increasing to a maximum at 45 days and then decreasing gradually with age in both continuous and categorical age group analyses. Maturation times among species and phenological habits differed substantially, from 8 ± 30 to 238 ± 30 days, and the rate of decline of Vcmax varied from -0.003 to -0.065 μmol CO2 m-2 s-1 day-1. Stomatal control increased significantly in young leaves but remained constant after peaking. Mass-based phosphorus and potassium concentrations displayed negative relationships with leaf age, whereas nitrogen did not vary temporally. Differences in life strategies, leaf nutrient concentrations and phenological types, not the leaf age effect alone, may thus be important factors for understanding observed photosynthesis seasonality in Amazonian forests. Furthermore, assigning leaf age categories in diverse tree communities may not be the recommended method for studying carbon uptake seasonality in the Amazon, since the relationship between Vcmax and leaf age could not be confirmed for all trees.

Cumulative growth and stress responses to the 2018-2019 drought in a European floodplain forest

Droughts increasingly threaten the world's forests and their potential to mitigate climate change. In 2018-2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine tree growth and physiological stress responses (increase in carbon isotope composition; Δδ¹³ C) to this consecutive drought based on tree rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress responses in 2018 were comparable to former single drought years but the hotter drought in 2018 induced drought legacies in tree growth while former droughts did not. We observed strong decreases in tree growth and increases in Δδ¹³ C across all tree species in 2019, which are likely driven by the cumulative stress both consecutive hotter droughts exerted. Our results show that consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with comparably high levels of water supply.

Foliar respiration is related to photosynthetic, growth and carbohydrate response to experimental drought and elevated temperature

Short-term plant respiration (R) increases exponentially with rising temperature, but drought could reduce respiration by reducing growth and metabolism. Acclimation may alter these responses. We examined if species with different drought responses would differ in foliar R response to +4.8°C temperature and -45% precipitation in a field experiment with mature piñon and juniper trees, and if any differences between species were related to differences in photosynthesis rates, shoot growth and nonstructural carbohydrates (NSCs). Short-term foliar R had a Q₁₀ of 1.6 for piñon and 2.6 for juniper. Piñon foliar R did not respond to the +4.8°C temperatures, but R increased 1.4× for juniper. Across treatments, piñon foliage had higher growth, lower NSC content, 29% lower photosynthesis rates, and 44% lower R than juniper. Removing 45% precipitation had little impact on R for either species. Species differences in the response of R under elevated temperature were related to substrate availability and stomatal response to leaf water potential. Despite not acclimating to the higher temperature and having higher R than piñon, greater substrate availability in juniper suggests it could supply respiratory demand for much longer than piñon. Species responses will be critical in ecosystem response to a warmer climate.

Dose-dependent methyl jasmonate effects on photosynthetic traits and volatile emissions: biphasic kinetics and stomatal regulation

Exogenous application of methyl jasmonate (MeJA) has been extensively used to study jasmonate-dependent signaling events triggered by biotic stresses. MeJA application leads to complex jasmonate-dependent physiological responses, including changes in stomatal openness and induction of emissions of a multitude of volatile compounds. Whether the alterations in stomatal conductance and emissions of MeJA-induced volatiles are quantitatively associated with MeJA dose, and whether the induced volatile emissions are regulated by modifications in stomatal conductance had been poorly known until recently. Our latest studies highlighted a biphasic kinetics of jasmonate-dependent volatile emissions induced by MeJA treatment in the model species cucumber (Cucumis sativus), indicating induction of an immediate stress response and subsequent gene-expression level response. Both the immediate and delayed responses were MeJA dose-dependent. The studies further demonstrated that stomata modulated the kinetics of emissions of water-soluble volatiles in a MeJA dose-dependent manner. These studies contribute to understanding of plant short- and long-term responses to different biotic stress severities as simulated by treatments with a range of MeJA doses corresponding to mild to acute stress.

Coupled whole-tree optimality and xylem hydraulics explain dynamic biomass partitioning

Trees partition biomass in response to resource limitation and physiological activity. It is presumed that these strategies evolved to optimize some measure of fitness. If the optimization criterion can be specified, then allometry can be modeled from first principles without prescribed parameterization. We present the Tree Hydraulics and Optimal Resource Partitioning (THORP) model, which optimizes allometry by estimating allocation fractions to organs as proportional to their ratio of marginal gain to marginal cost, where gain is net canopy photosynthesis rate, and costs are senescence rates. Root total biomass and profile shape are predicted simultaneously by a unified optimization. Optimal partitioning is solved by a numerically efficient analytical solution. THORP's predictions agree with reported tree biomass partitioning in response to size, water limitations, elevated CO₂ and pruning. Roots were sensitive to soil moisture profiles and grew down to the groundwater table when present. Groundwater buffered against water stress regardless of meteorology, stabilizing allometry and root profiles as deep as c. 30 m. Much of plant allometry can be explained by hydraulic considerations. However, nutrient limitations cannot be fully ignored. Rooting mass and profiles were synchronized with hydrological conditions and groundwater even at considerable depths, illustrating that the below ground shapes whole-tree allometry.

A theoretical and empirical assessment of stomatal optimization modeling

Optimal stomatal control models have shown great potential in predicting stomatal behavior and improving carbon cycle modeling. Basic stomatal optimality theory posits that stomatal regulation maximizes the carbon gain relative to a penalty of stomatal opening. All models take a similar approach to calculate instantaneous carbon gain from stomatal opening (the gain function). Where the models diverge is in how they calculate the corresponding penalty (the penalty function). In this review, we compare and evaluate 10 different optimization models in how they quantify the penalty and how well they predict stomatal responses to the environment. We evaluate models in two ways. First, we compare their penalty functions against seven criteria that ensure a unique and qualitatively realistic solution. Second, we quantitatively test model against multiple leaf gas-exchange datasets. The optimization models with better predictive skills have penalty functions that meet our seven criteria and use fitting parameters that are both few in number and physiology based. The most skilled models are those with a penalty function based on stress-induced hydraulic failure. We conclude by proposing a new model that has a hydraulics-based penalty function that meets all seven criteria and demonstrates a highly predictive skill against our test datasets.

The influence of soil temperature and water content on belowground hydraulic conductance and leaf gas exchange in mature trees of three boreal species

Understanding stomatal regulation is fundamental to predicting the impact of changing environmental conditions on vegetation. However, the influence of soil temperature (ST) and soil water content (SWC) on canopy conductance (g_s ) through changes in belowground hydraulic conductance (k_bg ) remains poorly understood, because k_bg has seldom been measured in field conditions. Our aim was to (a) examine the dependence of k_bg on ST and SWC, (b) examine the dependence of g_s on k_bg and (c) test a recent stomatal optimization model according to which g_s and soil-to-leaf hydraulic conductance are strongly coupled. We estimated k_bg from continuous sap flow and xylem diameter measurements in three boreal species. k_bg increased strongly with increasing ST when ST was below +8°C, and typically increased with increasing SWC when ST was not limiting. g_s was correlated with k_bg in all three species, and modelled and measured g_s were well correlated in Pinus sylvestris (a model comparison was only possible for this species). These results imply an important role for k_bg in mediating linkages between the soil environment and leaf gas exchange. In particular, our finding that ST strongly influences k_bg in mature trees may help us to better understand tree behaviour in cold environments.

Day length regulates seasonal patterns of stomatal conductance in Quercus species

Vapour pressure deficit is a major driver of seasonal changes in transpiration, but photoperiod also modulates leaf responses. Climate warming might enhance transpiration by increasing atmospheric water demand and the length of the growing season, but photoperiod-sensitive species could show dampened responses. Here, we document that day length is a significant driver of the seasonal variation in stomatal conductance. We performed weekly gas exchange measurements across a common garden experiment with 12 oak species from contrasting geographical origins, and we observed that the influence of day length was of similar strength to that of vapour pressure deficit in driving the seasonal pattern. We then examined the generality of our findings by incorporating day-length regulation into well-known stomatal models. For both angiosperm and gymnosperm species, the models improved significantly when adding day-length dependences. Photoperiod control over stomatal conductance could play a large yet underexplored role on the plant and ecosystem water balances.

The stomatal response to rising CO2 concentration and drought is predicted by a hydraulic trait-based optimization model

Modeling stomatal control is critical for predicting forest responses to the changing environment and hence the global water and carbon cycles. A trait-based stomatal control model that optimizes carbon gain while avoiding hydraulic risk has been shown to perform well in response to drought. However, the model's performance against changes in atmospheric CO2, which is rising rapidly due to human emissions, has yet to be evaluated. The present study tested the gain-risk model's ability to predict the stomatal response to CO2 concentration with potted water birch (Betula occidentalis Hook.) saplings in a growth chamber. The model's performance in predicting stomatal response to changes in atmospheric relative humidity and soil moisture was also assessed. The gain-risk model predicted the photosynthetic assimilation, transpiration rate and leaf xylem pressure under different CO2 concentrations, having a mean absolute percentage error (MAPE) of 25%. The model also predicted the responses to relative humidity and soil drought with a MAPE of 21.9% and 41.9%, respectively. Overall, the gain-risk model had an MAPE of 26.8% compared with the 37.5% MAPE obtained by a standard empirical model of stomatal conductance. Importantly, unlike empirical models, the optimization model relies on measurable physiological traits as inputs and performs well in predicting responses to novel environmental conditions without empirical corrections. Incorporating the optimization model in larger scale models has the potential for improving the simulation of water and carbon cycles.

A stomatal control model based on optimization of carbon gain versus hydraulic risk predicts aspen sapling responses to drought

Empirical models of plant drought responses rely on parameters that are difficult to specify a priori. We test a trait- and process-based model to predict environmental responses from an optimization of carbon gain vs hydraulic risk. We applied four drought treatments to aspen (Populus tremuloides) saplings in a research garden. First we tested the optimization algorithm by using predawn xylem pressure as an input. We then tested the full model which calculates root-zone water budget and xylem pressure hourly throughout the growing season. The optimization algorithm performed well when run from measured predawn pressures. The per cent mean absolute error (MAE) averaged 27.7% for midday xylem pressure, transpiration, net assimilation, leaf temperature, sapflow, diffusive conductance and soil-canopy hydraulic conductance. Average MAE was 31.2% for the same observations when the full model was run from irrigation and rain data. Saplings that died were projected to exceed 85% loss in soil-canopy hydraulic conductance, whereas surviving plants never reached this threshold. The model fit was equivalent to that of an empirical model, but with the advantage that all inputs are specific traits. Prediction is empowered because knowing these traits allows knowing the response to climatic stress.

A steady-state stomatal model of balanced leaf gas exchange, hydraulics and maximal source-sink flux

Trees must simultaneously balance their CO2 uptake rate via stomata, photosynthesis, the transport rate of sugars and rate of sugar utilization in sinks while maintaining a favourable water and carbon balance. We demonstrate using a numerical model that it is possible to understand stomatal functioning from the viewpoint of maximizing the simultaneous photosynthetic production, phloem transport and sink sugar utilization rate under the limitation that the transpiration-driven hydrostatic pressure gradient sets for those processes. A key feature in our model is that non-stomatal limitations to photosynthesis increase with decreasing leaf water potential and/or increasing leaf sugar concentration and are thus coupled to stomatal conductance. Maximizing the photosynthetic production rate using a numerical steady-state model leads to stomatal behaviour that is able to reproduce the well-known trends of stomatal behaviour in response to, e.g., light, vapour concentration difference, ambient CO2 concentration, soil water status, sink strength and xylem and phloem hydraulic conductance. We show that our results for stomatal behaviour are very similar to the solutions given by the earlier models of stomatal conductance derived solely from gas exchange considerations. Our modelling results also demonstrate how the 'marginal cost of water' in the unified stomatal conductance model and the optimal stomatal model could be related to plant structural and physiological traits, most importantly, the soil-to-leaf hydraulic conductance and soil moisture.

No evidence of general CO2 insensitivity in ferns: one stomatal control mechanism for all land plants?

Stomatal regulation of plant carbon uptake and water loss under changing environmental conditions was a crucial evolutionary step in the colonization of land by plants. There are currently two conflicting models describing the nature of stomatal regulation across terrestrial vascular plants: the first is characterized by a fundamental mechanistic similarity across all lineages, and the second is characterized by the evolution of major differences in angiosperms compared with more ancient lineages. Specifically, the second model posits that stomata of ferns lack a response to elevated atmospheric CO2 concentration (ca ) and therefore cannot regulate leaf intercellular CO2 concentration (ci ). We compared stomatal sensitivity to changes in ca in three distantly related fern species and a representative angiosperm species. Fern and angiosperm stomata responded strongly and similarly to changes in ca . As a result, ci /ca was maintained within narrow limits during ca changes. Our results challenge the model in which stomata of ferns generally lack a response to elevated ca and that angiosperms evolved new dynamic mechanisms for regulating leaf gas exchange that differ fundamentally from ferns. Instead, the results are consistent with a universal stomatal control mechanism that is fundamentally conserved across ferns and angiosperms, and therefore likely all vascular plant divisions.

Transpirational demand affects aquaporin expression in poplar roots

Isohydric plants tend to maintain a water potential homeostasis primarily by controlling water loss via stomatal conductance. However, there is accumulating evidence that plants can also modulate water uptake in a dynamic manner. The dynamics of water uptake are influenced by aquaporin-mediated changes in root hydraulics. Most studies in this area have been conducted on herbaceous plants, and less is known about responses of woody plants. Here a study was conducted to determine how roots of hybrid poplar plants (Populus trichocarpa×deltoides) respond to a step change in transpirational demand. The main objective was to measure the expression of selected aquaporin genes and to assess how transcriptional responses correspond to changes in root water flow (Q R) and other parameters of water relations. A subset of plants was grown in shade and was subsequently exposed to a 5-fold increase in light level. Another group of plants was grown at ~95% relative humidity (RH) and was then subjected to lower RH while the light level remained unchanged. Both plant groups experienced a transient drop in stem water potentials. At 28h after the increase in transpirational demand, water potentials recovered. This recovery was associated with changes in the expression of PIP1 and PIP2 subfamily genes and an increase in Q R. Stomata of plants growing at high RH were larger and showed incomplete closure after application of abscisic acid. Since stomatal conductance remained high and unchanged in these plants, it is suggested that the recovery in water potential in these plants was largely driven by the increase in Q R.

Smaller, faster stomata: scaling of stomatal size, rate of response, and stomatal conductance

Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (g (op)) correlates with minimum stomatal conductance prior to morning light [g (min(dawn))]; (ii) stomatal size (S) is negatively correlated with g (op) and the maximum rate of stomatal opening in response to light, (dg/dt)(max); and (iii) g (op) correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc (max)) and light-saturated rate of electron transport (J (max)). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g (op) can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk.

Cavitation induced by a surfactant leads to a transient release of water stress and subsequent 'run away' embolism in Scots pine (Pinus sylvestris) seedlings

Cavitation decreases the hydraulic conductance of the xylem and has, therefore, detrimental effects on plant water balance. However, cavitation is also hypothesized to relieve water stress temporarily by releasing water from embolizing conduits to the transpiration stream. Stomatal closure in response to decreasing water potentials in order to avoid excessive cavitation has been well documented in numerous previous studies. However, it has remained unclear whether the stomata sense cavitation events themselves or whether they act in response to a decrease in leaf water potential to a level at which cavitation is initiated. The effects of massive cavitation on leaf water potential, transpiration, and stomatal behaviour were studied by feeding a surfactant into the transpiration stream of Scots pine (Pinus sylvestris) seedlings. The stomatal response to cavitation in connection with the capacitive effect was also studied. A major transient increase in leaf water potential was found due to cavitation in the seedlings. As cavitation was induced by lowering the surface tension, the two mechanisms could be uncoupled, as the usual relation between xylem water potential and the onset of cavitation did not hold. Our results indicate that the seedlings responded more to leaf water potential and less to cavitation itself, as stomatal closure was insufficient to prevent the seedlings from being driven to 'run-away' cavitation in a manner of hours.