Predicting shifts in the functional composition of tropical forests under increased drought and CO 2 from trade‐offs among plant hydraulic traits

Optimising height-growth predicts trait responses to water availability and other environmental drivers

Future changes in climate, together with rising atmosphericCO 2 ${\text{CO}}_{2}$ , may reorganise the functional composition of ecosystems. Without long-term historical data, predicting how traits will respond to environmental conditions-in particular, water availability-remains a challenge. While eco-evolutionary optimality theory (EEO) can provide insight into how plants adapt to their environment, EEO approaches to date have been formulated on the assumption that plants maximise carbon gain, which omits the important role of tissue construction and size in determining growth rates and fitness. Here, we show how an expanded optimisation framework, focussed on individual growth rate, enables us to explain shifts in four key traits: leaf mass per area, sapwood area to leaf area ratio (Huber value), wood density and sapwood-specific conductivity in response to soil moisture, atmospheric aridity,CO 2 ${\text{CO}}_{2}$ and light availability. In particular, we predict that as conditions become increasingly dry, height-growth optimising traits shift from resource-acquisitive strategies to resource-conservative strategies, consistent with empirical responses across current environmental gradients of rainfall. These findings can explain both the shift in traits and turnover of species along existing environmental gradients and changing future conditions and highlight the importance of both carbon assimilation and tissue construction in shaping the functional composition of vegetation across climates.

Survival, growth, and functional traits of tropical wet forest tree seedlings across an experimental soil moisture gradient in Puerto Rico

Droughts are predicted to become more frequent and intense in many tropical regions, which may cause shifts in plant community composition. Especially in diverse tropical communities, understanding how traits mediate demographic responses to drought can help provide insight into the effects of climate change on these ecosystems. To understand tropical tree responses to reduced soil moisture, we grew seedlings of eight species across an experimental soil moisture gradient at the Luquillo Experimental Forest, Puerto Rico. We quantified survival and growth over an 8-month period and characterized demographic responses in terms of tolerance to low soil moisture-defined as survival and growth rates under low soil moisture conditions-and sensitivity to variation in soil moisture-defined as more pronounced changes in demographic rates across the observed range of soil moisture. We then compared demographic responses with interspecific variation in a suite of 11 (root, stem, and leaf) functional traits, measured on individuals that survived the experiment. Lower soil moisture was associated with reduced survival and growth but traits mediated species-specific responses. Species with relatively conservative traits (e.g., high leaf mass per area), had higher survival at low soil moisture whereas species with more extensive root systems were more sensitive to soil moisture, in that they exhibited more pronounced changes in growth across the experimental soil moisture gradient. Our results suggest that increasing drought will favor species with more conservative traits that confer greater survival in low soil moisture conditions.

Schizachyrium scoparium (C4) better tolerates drought than Andropogon gerardii (C4) via constant CO2 supply for photosynthesis during water stress

Climate change is dramatically altering global precipitation patterns across terrestrial ecosystems, making it critically important that we understand both how and why plant species vary in their drought sensitivities. Andropogon gerardii and Schizachyrium scoparium, both C4 grasses, provide a model system for understanding the physiological mechanisms that determine how species of a single functional type can differ in drought responses, an issue remains a critical gap in our ability to model and predict the impacts of drought on grassland ecosystems. Despite its greater lability of foliar water content, previous experiments have demonstrated that S. scoparium maintains higher photosynthetic capacity during droughts. It is therefore likely that the ability of S. scoparium to withstand drought instead derives from a greater metabolic resistance to drought. Here, we tested the following hypotheses: (H1) A. gerardii is more vulnerable to drought than S. scoparium at both the population and organismal levels, (H2) A. gerardii is less stomatally flexible than S. scoparium, and (H3) A. gerardii is more metabolically limited than S. scoparium. Our results indicate that it is actually stomatal limitations of CO2 supply that limit A. gerardii photosynthesis during drought. Schizachyrium scoparium was more drought-resistant than A. gerardii based on long-term field data, organismal biomass production and physiological gas exchange measurements. While both S. scoparium and A. gerardii avoided metabolic limitation of photosynthesis, CO2 supply of A. gerardii was greatly reduced during late-stage drought stress. That two common, co-occurring C4 species possess such different responses to drought highlights the physiological variability inherent within plant functional groups and underscores the need for more studies of C4 drought tolerance.

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.

Do stomata optimize turgor-driven growth? A new framework for integrating stomata response with whole-plant hydraulics and carbon balance

Every existing optimal stomatal model uses photosynthetic carbon assimilation as a proxy for plant evolutionary fitness. However, assimilation and growth are often decoupled, making assimilation less ideal for representing fitness when optimizing stomatal conductance to water vapor and carbon dioxide. Instead, growth should be considered a closer proxy for fitness. We hypothesize stomata have evolved to maximize turgor-driven growth, instead of assimilation, over entire plants' lifetimes, improving their abilities to compete and reproduce. We develop a stomata model that dynamically maximizes whole-stem growth following principles from turgor-driven growth models. Stomata open to assimilate carbohydrates that supply growth and osmotically generate turgor, while stomata close to prevent losses of turgor and growth due to negative water potentials. In steady state, the growth optimization model captures realistic stomatal, growth, and carbohydrate responses to environmental cues, reconciles conflicting interpretations within existing stomatal optimization theories, and explains patterns of carbohydrate storage and xylem conductance observed during and after drought. Our growth optimization hypothesis introduces a new paradigm for stomatal optimization models, elevates the role of whole-plant carbon use and carbon storage in stomatal functioning, and has the potential to simultaneously predict gross productivity, net productivity, and plant mortality through a single, consistent modeling framework.

Changes in trait covariance along an orographic moisture gradient reveal the relative importance of light- and moisture-driven trade-offs in subtropical rainforest communities

A range of functional trait-based approaches have been developed to investigate community assembly processes, but most ignore how traits covary within communities. We combined existing approaches - community-weighted means (CWMs) and functional dispersion (FDis) - with a metric of trait covariance to examine assembly processes in five angiosperm assemblages along a moisture gradient in Australia's subtropics. In addition to testing hypotheses about habitat filtering along the gradient, we hypothesized that trait covariance would be strongest at both ends of the moisture gradient and weakest in the middle, reflecting trade-offs associated with light capture in productive sites and moisture stress in dry sites. CWMs revealed evidence of climatic filtering, but FDis patterns were less clear. As hypothesized, trait covariance was weakest in the middle of the gradient but unexpectedly peaked at the second driest site due to the emergence of a clear drought tolerance-drought avoidance spectrum. At the driest site, the same spectrum was truncated at the 'avoider' end, revealing important information about habitat filtering in this system. Our focus on trait covariance revealed the nature and strength of trade-offs imposed by light and moisture availability, complementing insights gained about community assembly from existing trait-based approaches.

Towards a unified theory of plant photosynthesis and hydraulics

The global carbon and water cycles are governed by the coupling of CO₂ and water vapour exchanges through the leaves of terrestrial plants, controlled by plant adaptations to balance carbon gains and hydraulic risks. We introduce a trait-based optimality theory that unifies the treatment of stomatal responses and biochemical acclimation of plants to environments changing on multiple timescales. Tested with experimental data from 18 species, our model successfully predicts the simultaneous decline in carbon assimilation rate, stomatal conductance and photosynthetic capacity during progressive soil drought. It also correctly predicts the dependencies of gas exchange on atmospheric vapour pressure deficit, temperature and CO₂. Model predictions are also consistent with widely observed empirical patterns, such as the distribution of hydraulic strategies. Our unified theory opens new avenues for reliably modelling the interactive effects of drying soil and rising atmospheric CO₂ on global photosynthesis and transpiration.

Plant hydraulics, stomatal control, and the response of a tropical forest to water stress over multiple temporal scales

Many tropical regions are experiencing an intensification of drought, with increasing severity and frequency. The ecosystem response to these changes is still highly uncertain. On short time scales (from diurnal to seasonal), tropical forests respond to water stress by physiological controls, such as stomatal regulation and phenological adjustment, to cope with increasing atmospheric water demand and reduced water supply. However, the interactions among biological processes and co-varying environmental factors that determine the ecosystem-level fluxes are still unclear. Furthermore, climate variability at longer time scales, such as that generated by ENSO, produces less predictable effects because it depends on a highly stochastic combination of factors that might vary among forests and even between events in the same forest. This study will present some emerging patterns of response to water stress from 5 years of water, carbon, and energy fluxes observed on a seasonal tropical forest in central Panama, including an increase in productivity during the 2015 El Niño. These responses depend on the combination of environmental factors experienced by the forest throughout the seasonal cycle, in particular, increase in solar radiation, stimulating productivity, and increasing vapor pressure deficit (VPD) and decreasing soil moisture, limiting stomata opening. These results suggest a critical role of plant hydraulics in mediating the response to water stress over a broad range of temporal scales (diurnal, intraseasonal, seasonal, and interannual), by acclimating canopy conductance to light and VPD during different soil moisture regimes. A multilayer photosynthesis model coupled with a plant hydraulics scheme can reproduce these complex responses. However, results depend critically on parameters regulating water transport efficiency and the cost of water stress. As these costs have not been properly identified and quantified yet, more empirical research is needed to elucidate physiological mechanisms of hydraulic failure and recover, for example embolism repair and xylem regrowth.

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.

Root pressure-volume curve traits capture rootstock drought tolerance

BACKGROUND AND AIMS

Living root tissues significantly constrain plant water uptake under drought, but we lack functional traits to feasibly screen diverse plants for variation in the drought responses of these tissues. Water stress causes roots to lose volume and turgor, which are crucial to root structure, hydraulics and growth. Thus, we hypothesized that root pressure-volume (p-v) curve traits, which quantify the effects of water potential on bulk root turgor and volume, would capture differences in rootstock drought tolerance.

METHODS

We used a greenhouse experiment to evaluate relationships between root p-v curve traits and gas exchange, whole-plant hydraulic conductance and biomass under drought for eight grapevine rootstocks that varied widely in drought performance in field trials (101-14, 110R, 420A, 5C, 140-Ru, 1103P, Ramsey and Riparia Gloire), grafted to the same scion variety (Vitis vinifera 'Chardonnay').

KEY RESULTS

The traits varied significantly across rootstocks, and droughted vines significantly reduced root turgor loss point (πtlp), osmotic potential at full hydration (πo) and capacitance (C), indicating that roots became less susceptible to turgor loss and volumetric shrinkage. Rootstocks that retained a greater root volume (i.e. a lower C) also maintained more gas exchange under drought. The rootstocks that previous field trials have classified as drought tolerant exhibited significantly lower πtlp, πo and C values in well-watered conditions, but significantly higher πo and πtlp values under water stress, than the varieties classified as drought sensitive.

CONCLUSIONS

These findings suggest that acclimation in root p-v curve traits improves gas exchange in persistently dry conditions, potentially through impacts on root hydraulics or root to shoot chemical signalling. However, retaining turgor and volume in previously unstressed roots, as these roots deplete wet soil to moderately negative water potentials, could be more important to drought performance in the deep, highly heterogenous rooting zones which grapevines develop under field conditions.

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

Maintaining phloem transport under water stress is expected to be crucial to whole-plant drought tolerance, but the traits that benefit phloem function under drought are poorly understood. Nearly half of surveyed angiosperm species, including important crops, use sucrose transporter proteins to actively load sugar into the phloem. Plants can alter transporter abundance in response to stress, providing a potential mechanism for active-loading species to closely regulate phloem loading rates to avoid drought-induced reductions or failures in phloem transport. We developed an integrated xylem-phloem-stomatal model to test this hypothesis by quantifying the joint impacts of transporter kinetics, phloem anatomy, and plant water status on sucrose export to sinks. We parameterized the model with phloem hydraulic resistances and sucrose transporter kinetic parameters compiled from the literature, and simulated loading regulation by allowing loading rates to decline exponentially with phloem pressure to prevent excessive sucrose concentrations from inducing viscosity limitations. In the absence of loading regulation, where loading rates were independent of phloem pressure, most resistance values produced unrealistic phloem pressures owing to viscosity effects, even under well-watered conditions. Conversely, pressure-regulated loading helped to control viscosity buildup and improved export to sinks for both lower and higher resistant phloem pathways, while maintaining realistic phloem pressures. Regulation also allowed for rapid loading and export in wet conditions while maintaining export and viable phloem pressures during drought. Therefore, we expect feedbacks between phloem pressure and loading to be critical to carbon transport in active-loading species, especially under drought, and for transporter kinetics to be strongly coordinated with phloem architecture and plant water status. This work provides an important and underexplored physiological framework to understand the ecophysiology of phloem transport under drought and to enhance the genetic engineering of crop plants.

Limited plasticity of anatomical and hydraulic traits in aspen trees under elevated CO2 and seasonal drought

The timing of abiotic stress elicitors on wood formation largely affects xylem traits that determine xylem efficiency and vulnerability. Nonetheless, seasonal variability of elevated CO2 (eCO2) effects on tree functioning under drought remains largely unknown. To address this knowledge gap, 1-year-old aspen (Populus tremula L.) trees were grown under ambient (±445 ppm) and elevated (±700 ppm) CO2 and exposed to an early (spring/summer 2019) or late (summer/autumn 2018) season drought event. Stomatal conductance and stem shrinkage were monitored in vivo as xylem water potential decreased. Additional trees were harvested for characterization of wood anatomical traits and to determine vulnerability and desorption curves via bench dehydration. The abundance of narrow vessels decreased under eCO2 only during the early season. At this time, xylem vulnerability to embolism formation and hydraulic capacitance during severe drought increased under eCO2. Contrastingly, stomatal closure was delayed during the late season, while hydraulic vulnerability and capacitance remained unaffected under eCO2. Independently of the CO2 treatment, elastic, and inelastic water pools depleted simultaneously after 50% of complete stomatal closure. Our results suggest that the effect of eCO2 on drought physiology and wood traits are small and variable during the growing season and question a sequential capacitive water release from elastic and inelastic pools as drought proceeds.

Drought Stress Mitigating Morphological, Physiological, Biochemical, and Molecular Responses of Guava (Psidium guajava L.) Cultivars

Guava (Psidium guajava L.), a major fruit crop of the sub-tropical region, is facing a production decline due to drought stress. Morphophysiological responses to drought stress and underlying transcriptional regulations in guava are, largely, unknown. This study evaluated the drought stress tolerance of two guava cultivars, viz. "Gola" and "Surahi," at morphological and physiological levels regulated differentially by ESTs (Expressed Sequence Tags). The treatments comprises three moisture regimes, viz. T_o = 100% (control), T₁ = 75%, and T₂ = 50% of field capacity. There was an overall decrease in both morphological and physiological attributes of studied guava cultivars in response to drought stress. Nonetheless, the water use efficiency of the "Surahi" cultivar increased (41.86%) speculating its higher drought tolerance based on enhanced peroxidase (402%) and catalase (170.21%) activities under 50% field capacity (T₂). Moreover, higher proline and flavonoid contents reinforced drought stress retaliation of the "Surahi" cultivar. The differential expression of a significant number of ESTs in "Surahi" (234) as compared to "Gola" (117) cultivar, somehow, regulated its cellular, biological, and molecular functions to strengthen morphophysiological attributes against drought stress as indicated by the upregulation of ESTs related to peroxidase, sucrose synthase (SUS), alcohol dehydrogenase (ADH), and ubiquitin at morphological, biochemical, and physiological levels. In conclusion, the drought stress acclimation of pear-shaped guava cultivar "Surahi" is due to the increased activities of peroxidase (POD) and catalase (CAT) complimented by the upregulation of related ESTs.

Shifting access to pools of shoot water sustains gas exchange and increases stem hydraulic safety during seasonal atmospheric drought

Understanding how plants acclimate to drought is crucial for predicting future vulnerability, yet seasonal acclimation of traits that improve drought tolerance in trees remains poorly resolved. We hypothesized that dry season acclimation of leaf and stem traits influencing shoot water storage and hydraulic capacitance would mitigate the drought-associated risks of reduced gas exchange and hydraulic failure in the mangrove Sonneratia alba. By late dry season, availability of stored water had shifted within leaves and between leaves and stems. While whole shoot capacitance remained stable, the symplastic fraction of leaf water increased 86%, leaf capacitance increased 104% and stem capacitance declined 80%. Despite declining plant water potentials, leaf and whole plant hydraulic conductance remained unchanged, and midday assimilation rates increased. Further, the available leaf water between the minimum water potential observed and that corresponding to 50% loss of stem conductance increased 111%. Shifting availability of pools of water, within and between organs, maintained leaf water available to buffer periods of increased photosynthesis and losses in stem hydraulic conductivity, mitigating risks of carbon depletion and hydraulic failure during atmospheric drought. Seasonal changes in access to tissue and organ water may have an important role in drought acclimation and avoidance.

Hydraulically-vulnerable trees survive on deep-water access during droughts in a tropical forest

Deep-water access is arguably the most effective, but under-studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep-water access may delay plant dehydration. Here, we tested the role of deep-water access in enabling survival within a diverse tropical forest community in Panama using a novel data-model approach. We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990-2015) to vapor pressure deficit, water potentials in the whole-soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water-access depths. Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981-2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress-related mortality risk through deep-water access. The role of deep-water access in mitigating mortality of hydraulically-vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

Mechanisms driving plant functional trait variation in a tropical forest

Plant functional trait variation in tropical forests results from taxonomic differences in phylogeny and associated genetic differences, as well as, phenotypic plastic responses to the environment. Accounting for the underlying mechanisms driving plant functional trait variation is important for understanding the potential rate of change of ecosystems since trait acclimation via phenotypic plasticity is very fast compared to shifts in community composition and genetic adaptation. We here applied a statistical technique to decompose the relative roles of phenotypic plasticity, genetic adaptation, and phylogenetic constraints. We examined typically obtained plant functional traits, such as wood density, plant height, specific leaf area, leaf area, leaf thickness, leaf dry mass content, leaf nitrogen content, and leaf phosphorus content. We assumed that genetic differences in plant functional traits between species and genotypes increase with environmental heterogeneity and geographic distance, whereas trait variation due to plastic acclimation to the local environment is independent of spatial distance between sampling sites. Results suggest that most of the observed trait variation could not be explained by the measured environmental variables, thus indicating a limited potential to predict individual plant traits from commonly assessed parameters. However, we found a difference in the response of plant functional traits, such that leaf traits varied in response to canopy-light regime and nutrient availability, whereas wood traits were related to topoedaphic factors and water availability. Our analysis furthermore revealed differences in the functional response of coexisting neotropical tree species, which suggests that endemic species with conservative ecological strategies might be especially prone to competitive exclusion under projected climate change.

Trade-offs between xylem water and carbohydrate storage among 24 coexisting subtropical understory shrub species spanning a spectrum of isohydry

Hydraulic capacitance and carbohydrate storage are two drought adaptation strategies of woody angiosperms. However, we currently lack information on their associations and how they are associated with species' degree of isohydry. We measured total stem xylem nonstructural carbohydrate (NSC) concentration in the dry and wet seasons, xylem hydraulic capacitance, native leaf water potentials, pressure-volume curve parameters and photosynthetic performance in 24 woody understory species differing in their degree of isohydry. We found a trade-off between xylem water and carbohydrate storage both in storage capacitance and along a spectrum of isohydry. Species with higher hydraulic capacitance had lower native NSC storage. The less isohydric species tended to show greater NSC depletion in the dry season and have more drought-tolerant leaves. In contrast, the more isohydric species had higher hydraulic capacitance, which may enhance their drought avoidance capacity. In these species, leaf flushing in the wet season and higher photosynthetic rates in the dry season resulted in accumulation rather than depletion of NSC in the dry season. Our results provide new insights into the mechanisms through which xylem storage functions determine co-occurring species' drought adaptation strategies and improve our capacity to predict community assembly processes under drought.

Optimal leaf life strategies determine Vc,max dynamic during ontogeny

Leaf photosynthetic properties, for example the maximum carboxylation velocity or V_c,max , change with leaf age due to ontogenetic processes. This study introduces an optimal dynamic allocation scheme to model changes in leaf-level photosynthetic capacity as a function of leaf biochemical constraints (costs of synthesis and defence), nitrogen availability and other environmental factors (e.g. light). The model consists of a system of equations describing RuBisCO synthesis and degradation within chloroplasts, defence and ageing at leaf levels, nitrogen transfer and carbon budget at plant levels. Model results show that optimal allocation principles explained RuBisCO dynamics with leaf age. An approximated analytical solution can reproduce the basic pattern of RuBisCO and V_c,max in rice and in two tropical tree species. The model also reveals leaf life complementarities that remained unexplained in previous approaches, as the interplay between V_c,max at maturation, life span and the decline in photosynthetic capacity with age. Furthermore, it explores the role of defence, which is not implemented in current models. This framework covers some of the existing gaps in integrating multiple processes across plant organs (chloroplast, leaf and whole plant) and is a first-step towards representing mechanistically leaf ontogenetic processes into physiological and ecosystem models.

Optimization can provide the fundamental link between leaf photosynthesis, gas exchange and water relations

Tight coordination in the photosynthetic, gas exchange and water supply capacities of leaves is a globally conserved trend across land plants. Strong selective constraints on leaf carbon gain create the opportunity to use quantitative optimization theory to understand the connected evolution of leaf photosynthesis and water relations. We developed an analytical optimization model that maximizes the long-term rate of leaf carbon gain, given the carbon costs in building and maintaining stomata, leaf hydraulics and osmotic pressure. Our model demonstrates that selection for optimal gain should drive coordination between key photosynthetic, gas exchange and water relations traits. It also provides predictions of adaptation to drought and the relative costs of key leaf functional traits. Our results show that optimization in terms of carbon gain, given the carbon costs of physiological traits, successfully unites leaf photosynthesis and water relations and provides a quantitative framework to consider leaf functional evolution and adaptation.

Bark water vapour conductance is associated with drought performance in tropical trees

Bark water vapour conductance (g_bark) is a rarely considered functional trait. However, for the few tree species measured to date, it appears high enough to create stem water deficits associated with mortality during droughts, when access to water is limited. I tested whether g_bark correlates with stem water deficit during drought conditions in two datasets of tropical trees: one of saplings in forest understories during an annual dry season and one of potted saplings in a shadehouse during extreme drought conditions. Among all 14 populations of eight species measured, g_bark varied more than 10-fold (0.86-12.98 mmol m^-2 s^-1). In the forest understories, g_bark was highly correlated with stem water deficit among four deciduous species, but not among evergreen species that likely maintained access to soil water. In the shadehouse, g_bark was positively correlated with stem water deficit and mortality among all six species. Overall, tree species with higher g_bark suffer higher stem water deficit when soil water is unavailable. Incorporating g_bark into soil-plant-atmosphere hydrodynamic models may improve projections of plant mortality under drought conditions.

Hydraulic acclimation in a Mediterranean oak subjected to permanent throughfall exclusion results in increased stem hydraulic capacitance

Stem water storage capacity and hydraulic capacitance (C_S ) play a crucial role in tree survival under drought-stress. To investigate whether C_S adjusts to increasing water deficit, variation in stem water content (StWC) was monitored in vivo for 2 years and related to periodical measurements of tree water potential in Mediterranean Quercus ilex trees subjected either to permanent throughfall exclusion (TE) or to control conditions. Seasonal reductions in StWC were larger in TE trees relative to control ones, resulting in greater seasonal C_S (154 and 80 kg m^-3 MPa^-1 , respectively), but only during the first phase of the desorption curve, when predawn water potential was above -1.1 MPa. Below this point, C_S decreased substantially and did not differ between treatments (<20 kg m^-3 MPa^-1 ). The allometric relationship between tree diameter and sapwood area, measured via electrical resistivity tomography, was not affected by TE. Our results suggest that (a) C_S response to water deficit in the drought-tolerant Q. ilex might be more important to optimize carbon gain during well-hydrated periods than to prevent drought-induced embolism formation during severe drought stress, and (b) enhanced C_S during early summer does not result from proportional increases in sapwood volume, but mostly from increased elastic water.

The response of stomatal conductance to seasonal drought in tropical forests

Stomata regulate CO₂ uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (g_s ) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water-use-efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψ_leaf ), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf-mass-per-area (LMA), leaf carboxylation capacity (V_c,max ), leaf water content, the degree of isohydry, and predawn Ψ_leaf . We first investigated how the choice of four commonly used leaf-level g_s models with and without the inclusion of Ψ_leaf as an additional predictor variable influence the ability to predict g_s , and then explored the abiotic (i.e. month, site-month interaction) and biotic (i.e. tree-species-specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψ_leaf did not improve model performance and that the models that represent the response of g_s to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each g_s model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R²  = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of g_s dynamics over seasonal drought, and identifies a practical, trait-based approach to improve modeling of carbon and water exchange in tropical forests.

Potential Mechanisms of Abiotic Stress Tolerance in Crop Plants Induced by Thiourea

Abiotic stresses, such as temperature extremes, drought, salinity, and heavy metals are major factors limiting crop productivity and sustainability worldwide. Abiotic stresses disturb plant growth and yield formation. Several chemical compounds, known as plant growth regulators (PGRs), modulate plant responses to biotic and abiotic stresses at the cellular, tissue, and organ levels. Thiourea (TU) is an important synthetic PGR containing nitrogen (36%) and sulfur (42%) that has gained wide attention for its role in plant stress tolerance. Tolerance against abiotic stresses is a complex phenomenon involving an array of mechanisms, and TU may modulate several of these. An understanding of TU-induced tolerance mechanisms may help improve crop yield under stress conditions. However, the potential mechanisms involved in TU-induced plant stress tolerance are still elusive. In this review, we discuss the essential role of TU-induced tolerance in improving performance of plants growing under abiotic stresses and potential mechanisms underlying TU-induced stress tolerance. We also highlight exploitation of new avenues critical in TU-induced stress tolerance.