Plant responses to rising vapor pressure deficit

Leaf physiological and morphological constraints of water-use efficiency in C3 plants

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Improved weed mapping in corn fields by combining UAV-based spectral, textural, structural, and thermal measurements

BACKGROUND

Spatial-explicit weed information is critical for controlling weed infestation and reducing corn yield losses. The development of unmanned aerial vehicle (UAV)-based remote sensing presents an unprecedented opportunity for efficient, timely weed mapping. Spectral, textural, and structural measurements have been used for weed mapping, whereas thermal measurements-for example, canopy temperature (CT)-were seldom considered and used. In this study, we quantified the optimal combination of spectral, textural, structural, and CT measurements based on different machine-learning algorithms for weed mapping.

RESULTS

CT improved weed-mapping accuracies as complementary information for spectral, textural, and structural features (up to 5% and 0.051 improvements in overall accuracy [OA] and Marco-F1, respectively). The fusion of textural, structural, and thermal features achieved the best performance in weed mapping (OA = 96.4%, Marco-F1 = 0.964), followed by the fusion of structural and thermal features (OA = 93.6%, Marco-F1 = 0.936). The Support Vector Machine-based model achieved the best performance in weed mapping, with 3.5% and 7.1% improvements in OA and 0.036 and 0.071 in Marco-F1 respectively, compared with the best models of Random Forest and Naïve Bayes Classifier.

CONCLUSION

Thermal measurement can complement other types of remote-sensing measurements and improve the weed-mapping accuracy within the data-fusion framework. Importantly, integrating textural, structural, and thermal features achieved the best performance for weed mapping. Our study provides a novel method for weed mapping using UAV-based multisource remote sensing measurements, which is critical for ensuring crop production in precision agriculture. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Ensemble of CMIP6 derived reference and potential evapotranspiration with radiative and advective components

Assessing changes in future aridity requires an understanding of variations in the atmospheric demand for water. Such assessments are often driven by estimations of potential evapotranspiration (ET_P) and/or reference evapotranspiration (ET₀), yet no comprehensive and validated estimate of these climate metrics exists to date from the Coupled Model Intercomparison Project 6 (CMIP6). Here we describe the development and validation of a published dataset of global monthly estimates of the Penman-Monteith derived ET₀, its advective and radiation components, Priestley-Taylor derived ET_P, and vapor pressure deficit from 16 CMIP6 projections and four emissions scenarios. Historical validation of the ensemble of CMIP6 evaporative demand shows general agreement with observationally-derived baselines of ET₀ and ET_P from the Climate Research Unit (CRU) and ERA5-Land reanalysis products, with GCM biases driven primarily by regional differences in modeled humidity and advective contributions to ET₀. Overall, evaporative demand is projected to increase across all emissions scenarios, with the largest increases over polar regions, and with a larger contribution from advection particularly for regions with higher baseline ET₀.

A hybrid multifunctional physicochemical sensor suite for continuous monitoring of crop health

This work reports a first-of-its-kind hybrid wearable physicochemical sensor suite that we call PlantFit for simultaneous measurement of two key phytohormones, salicylic acid, and ethylene, along with vapor pressure deficit and radial growth of stem in live plants. The sensors are developed using a low-cost and roll-to-roll screen printing technology. A single integrated flexible patch that contains temperature, humidity, salicylic acid, and ethylene sensors, is installed on the leaves of live plants. The strain sensor with in-built pressure correction capability is wrapped around the plant stem to provide pressure-compensated stem diameter measurements. The sensors provide real-time information on plant health under different amounts of water stress conditions. The sensor suite is installed on bell pepper plants for 40 days and measurements of salicylic acid, ethylene, temperature, humidity, and stem diameter are recorded daily. In addition, sensors are installed on different parts of the same plant to investigate the spatiotemporal dynamics of water transport and phytohormone responses. Subsequent correlation and principal component analyses demonstrate the strong association between hormone levels, vapor pressure deficit, and water transport in the plant. Our findings suggest that the mass deployment of PlantFit in agricultural settings will aid growers in detecting water stress/deficiency early and in implementing early intervention measures to reduce stress-induced yield decline.

Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change

Trees continuously regulate leaf physiology to acquire CO₂ while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂ (iCO₂ ) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long-term records of tree-ring carbon isotope signatures with leaf physiological measurements of Quercus rubra (Quru) and Liriodendron tulipifera (Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net ), and stomatal conductance to water (g_s ) since 1940. We first show 16%-25% increases in tree iWUE since the mid-20th century, primarily driven by iCO₂ , but also document the individual and interactive effects of nitrogen (NO_x ) and sulfur (SO₂ ) air pollution overwhelming climate. We find evidence for Quru leaf gas exchange being less tightly regulated than Litu through an analysis of isotope-derived leaf internal CO₂ (C_i ), particularly in wetter, recent years. Modeled estimates of seasonally integrated A_net and g_s revealed a 43%-50% stimulation of A_net was responsible for increasing iWUE in both tree species throughout 79%-86% of the chronologies with reductions in g_s attributable to the remaining 14%-21%, building upon a growing body of literature documenting stimulated A_net overwhelming reductions in g_s as a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.

Satellite-based evidence highlights a considerable increase of urban tree cooling benefits from 2000 to 2015

Tree planting is a prevalent strategy to mitigate urban heat. Tree cooling efficiency (TCE), defined as the temperature reduction for a 1% tree cover increase, plays an important role in urban climate as it regulates the capacity of trees to alter the surface energy and water budget. However, the spatial variation and more importantly, temporal heterogeneity of TCE in global cities are not fully explored. Here, we used Landsat-based tree cover and land surface temperature (LST) to compare TCEs at a reference air temperature and tree cover level across 806 global cities and to explore their potential drivers with a boosted regression tree (BRT) machine learning model. From the results, we found that TCE is spatially regulated by not only leaf area index (LAI) but climate variables and anthropogenic factors especially city albedo, without a specific variable dominating the others. However, such spatial difference is attenuated by the decrease of TCE with tree cover, most pronounced in midlatitude cities. During the period 2000-2015, more than 90% of analyzed cities showed an increasing trend in TCE, which is likely explained by a combined result of the increase in LAI, intensified solar radiation due to decreased aerosol content, increase in urban vapor pressure deficit (VPD) and decrease of city albedo. Concurrently, significant urban afforestation occurred across many cities showing a global city-scale mean tree cover increase of 5.3 ± 3.8% from 2000 to 2015. Over the growing season, such increases combined with an increasing TCE were estimated to on average yield a midday surface cooling of 1.5 ± 1.3°C in tree-covered urban areas. These results are offering new insights into the use of urban afforestation as an adaptation to global warming and urban planners may leverage them to provide more cooling benefits if trees are primarily planted for this purpose.

Plant water use theory should incorporate hypotheses about extreme environments, population ecology, and community ecology

Plant water use theory has largely been developed within a plant-performance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models.

Land-atmosphere feedbacks contribute to crop failure in global rainfed breadbaskets

Global crop yields are highly dependent on climate variability, with the largest agricultural failures frequently occurring during extremely dry and hot years. Land-atmosphere feedbacks are thought to play a crucial role in agricultural productivity during such events: precipitation deficits cause soil desiccation, which reduces evaporation and enhances sensible heating from the land surface; the amplified local temperatures and moisture deficits can be detrimental to crop yield. While this impact of local land-atmosphere feedbacks on agricultural productivity has recently been reported, the dependency of crop yields on upwind regions remains understudied. Here, we determine the spatio-temporal origins of moisture and heat over the world's largest 75 rainfed breadbaskets, and illustrate the crop yield dependency on upwind regions. Further, we disentangle the role of local and upwind land-atmosphere interactions on anomalous moisture and heat transport during low-yield years. Our results indicate that crop failure increases on average by around 40% when both upwind and local land-atmosphere feedbacks cause anomalously low moisture and high heat transport into the breadbaskets. The impact of upwind land-atmosphere feedbacks on productivity deficits is the largest in water-limited regions, which show an increased dependency on moisture supply from upwind land areas. Better understanding these upwind-downwind dependencies in agricultural regions can help develop adaptation strategies to prevent food shortage in a changing climate.

Large diurnal compensatory effects mitigate the response of Amazonian forests to atmospheric warming and drying

Photosynthesis and evapotranspiration in Amazonian forests are major contributors to the global carbon and water cycles. However, their diurnal patterns and responses to atmospheric warming and drying at regional scale remain unclear, hindering the understanding of global carbon and water cycles. Here, we used proxies of photosynthesis and evapotranspiration from the International Space Station to reveal a strong depression of dry season afternoon photosynthesis (by 6.7 ± 2.4%) and evapotranspiration (by 6.1 ± 3.1%). Photosynthesis positively responds to vapor pressure deficit (VPD) in the morning, but negatively in the afternoon. Furthermore, we projected that the regionally depressed afternoon photosynthesis will be compensated by their increases in the morning in future dry seasons. These results shed new light on the complex interplay of climate with carbon and water fluxes in Amazonian forests and provide evidence on the emerging environmental constraints of primary productivity that may improve the robustness of future projections.

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.

Rubisco deactivation and chloroplast electron transport rates co-limit photosynthesis above optimal leaf temperature in terrestrial plants

Net photosynthetic CO₂ assimilation rate (A_n) decreases at leaf temperatures above a relatively mild optimum (T_opt) in most higher plants. This decline is often attributed to reduced CO₂ conductance, increased CO₂ loss from photorespiration and respiration, reduced chloroplast electron transport rate (J), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines in A_n at high temperature. We show that independent of species, and on a global scale, the observed decline in A_n with rising temperatures can be effectively accounted for by Rubisco deactivation and declines in J. Our finding that A_n declines with Rubisco deactivation and J supports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO₂ supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature.

Heat Acclimation under Drought Stress Induces Antioxidant Enzyme Activity in the Alpine Plant Primula minima

Heat and drought stresses are increasingly relevant topics in the context of climate change, particularly in the Alps, which are warming faster than the global average. Previously, we have shown that alpine plants, including Primula minima, can be gradually heat hardened under field conditions in situ to achieve maximum tolerance within a week. Here, we investigated the antioxidant mechanisms of P. minima leaves that had been heat hardened (H) without or with (H+D) additional drought stress. Lower free-radical scavenging and ascorbate concentrations were found in H and H+D leaves, while concentrations of glutathione disulphide (GSSG) were higher under both treatments without any change in glutathione (GSH) and little change in glutathione reductase activity. In contrast, ascorbate peroxidase activity in H leaves was increased, and H+D leaves had >two-fold higher catalase, ascorbate peroxidase and glucose-6-phosphate dehydrogenase activities compared with the control. In addition, the glutathione reductase activity was higher in H+D compared with H leaves. Our results highlight that the stress load from heat acclimation to maximum tolerance is associated with a weakened low-molecular-weight antioxidant defence, which may be compensated for by an increased activity of antioxidant enzymes, particularly under drought conditions.

Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

Nocturnal sap flow as compensation for water deficits: an implicit water-saving strategy used by mangroves in stressful environments

As part of the plant water-use process, plant nocturnal sap flow (Q _n) has been demonstrated to have important ecophysiological significance to compensate for water loss. The purpose of this study was to explore nocturnal water-use strategies to fill the knowledge gap in mangroves, by measuring three species co-occurring in a subtropical estuary. Sap flow was monitored over an entire year using thermal diffusive probes. Stem diameter and leaf-level gas exchange were measured in summer. The data were used to explore the different nocturnal water balance maintaining mechanisms among species. The Q _n existed persistently and contributed markedly over 5.5%~24.0% of the daily sap flow (Q) across species, which was associated with two processes, nocturnal transpiration (E _n) and nocturnal stem water refilling (R _n). We found that the stem recharge of the Kandelia obovata and Aegiceras corniculatum occurred mainly after sunset and that the high salinity environment drove higher Q _n while stem recharge of the Avicennia marina mainly occurred in the daytime and the high salinity environment inhibited the Q _n. The diversity of stem recharge patterns and response to sap flow to high salinity conditions were the main reasons for the differences in Q _n/Q among species. For Kandelia obovata and Aegiceras corniculatum, R _n was the main contributor to Q _n, which was driven by the demands of stem water refilling after diurnal water depletion and high salt environment. Both of the species have a strict control over the stomata to reduce water loss at night. In contrast, Avicennia marina maintained a low Q _n, driven by vapor pressure deficit, and the Q _n mainly used for E _n, which adapts to high salinity conditions by limiting water dissipation at night. We conclude that the diverse ways Q _n properties act as water-compensating strategies among the co-occurring mangrove species might help the trees to overcoming water scarcity.

Timing and magnitude of drought impacts on carbon uptake across a grassland biome

Although drought is known to negatively impact grassland functioning, the timing and magnitude of these impacts within a growing season remain unresolved. Previous small-scale assessments indicate grasslands may only respond to drought during narrow periods within a year; however, large-scale assessments are now needed to uncover the general patterns and determinants of this timing. We combined remote sensing datasets of gross primary productivity and weather to assess the timing and magnitude of grassland responses to drought at 5 km² temporal resolution across two expansive ecoregions of the western US Great Plains biome: the C₄ -dominated shortgrass steppe and the C₃ -dominated northern mixed prairies. Across over 700,000 pixel-year combinations covering more than 600,000 km² , we studied how the driest years between 2003-2020 altered the daily and bi-weekly dynamics of grassland carbon (C) uptake. Reductions to C uptake intensified into the early summer during drought and peaked in mid- and late June in both ecoregions. Stimulation of spring C uptake during drought was small and insufficient to compensate for losses during summer. Thus, total grassland C uptake was consistently reduced by drought across both ecoregions; however, reductions were twice as large across the more southern and warmer shortgrass steppe. Across the biome, increased summer vapor pressure deficit (VPD) was strongly linked to peak reductions in vegetation greenness during drought. Rising VPD will likely exacerbate reductions in C uptake during drought across the western US Great Plains, with these reductions greatest during the warmest months and in the warmest locations. High spatiotemporal resolution analyses of grassland response to drought over large areas provide both generalizable insights and new opportunities for basic and applied ecosystem science in these water-limited ecoregions amid climate change.

Interactive effects of elevated temperature and drought on plant carbon metabolism: A meta-analysis

Elevated temperature (T_e ) and drought often co-occur and interactively affect plant carbon (C) metabolism and thus the ecosystem C cycling; however, the magnitude of their interaction is unclear, making the projection of global change impacts challenging. Here, we compiled 107 journal articles in which temperature and water availability were jointly manipulated, and we performed a meta-analysis of interactive effects of T_e and drought on leaf photosynthesis (A_growth ) and respiration (R_growth ) at growth temperature, nonstructural carbohydrates and biomass of plants, and their dependencies on experimental and biological moderators (e.g., treatment intensity, plant functional type). Our results showed that, overall, there was no significant interaction of T_e and drought on A_growth . T_e accelerated R_growth under well-watered conditions rather than under drought conditions. The T_e  × drought interaction on leaf soluble sugar and starch concentrations were neutral and negative, respectively. The effect of T_e and drought on plant biomass displayed a negative interaction, with T_e deteriorating the drought impacts. Drought induced an increase in root to shoot ratio at ambient temperature but not at T_e . The magnitudes of T_e and drought negatively modulated the T_e  × drought interactions on A_growth . Root biomass of woody plants was more vulnerable to drought than that of herbaceous plants at ambient temperature, but this difference diminished at T_e . Perennial herbs exhibited a stronger amplifying effect of T_e on plant biomass in response to drought than did annual herbs. T_e exacerbated the responses of A_growth and stomatal conductance to drought for evergreen broadleaf trees rather than for deciduous broadleaf and evergreen coniferous trees. A negative T_e  × drought interaction on plant biomass was observed on species-level rather than on community-level. Collectively, our findings provide a mechanistic understanding of the interactive effects of T_e and drought on plant C metabolism, which would improve the prediction of climate change impacts.

Integrating plant physiology into simulation of fire behavior and effects

Wildfires are a global crisis, but current fire models fail to capture vegetation response to changing climate. With drought and elevated temperature increasing the importance of vegetation dynamics to fire behavior, and the advent of next generation models capable of capturing increasingly complex physical processes, we provide a renewed focus on representation of woody vegetation in fire models. Currently, the most advanced representations of fire behavior and biophysical fire effects are found in distinct classes of fine-scale models and do not capture variation in live fuel (i.e. living plant) properties. We demonstrate that plant water and carbon dynamics, which influence combustion and heat transfer into the plant and often dictate plant survival, provide the mechanistic linkage between fire behavior and effects. Our conceptual framework linking remotely sensed estimates of plant water and carbon to fine-scale models of fire behavior and effects could be a critical first step toward improving the fidelity of the coarse scale models that are now relied upon for global fire forecasting. This process-based approach will be essential to capturing the influence of physiological responses to drought and warming on live fuel conditions, strengthening the science needed to guide fire managers in an uncertain future.

Factors Limiting Radial Growth of Conifers on Their Semiarid Borders across Kazakhstan

The forests of Central Asia are biodiversity hotspots at risk from rapid climate change, but they are understudied in terms of the climate-growth relationships of trees. This classical dendroclimatic case study was performed for six conifer forest stands near their semiarid boundaries across Kazakhstan: (1-3) Pinus sylvestris L., temperate forest steppes; (4-5) Picea schrenkiana Fisch. & C.A. Mey, foothills, the Western Tien Shan, southeast; (6) Juniperus seravschanica Kom., montane zone, the Western Tien Shan, southern subtropics. Due to large distances, correlations between local tree-ring width (TRW) chronologies are significant only within species (pine, 0.19-0.50; spruce, 0.55). The most stable climatic response is negative correlations of TRW with maximum temperatures of the previous (from -0.37 to -0.50) and current (from -0.17 to -0.44) growing season. The strength of the positive response to annual precipitation (0.10-0.48) and Standardized Precipitation Evapotranspiration Index (0.15-0.49) depends on local aridity. The timeframe of climatic responses shifts to earlier months north-to-south. For years with maximum and minimum TRW, differences in seasonal maximal temperatures (by ~1-3 °C) and precipitation (by ~12-83%) were also found. Heat stress being the primary factor limiting conifer growth across Kazakhstan, we suggest experiments there on heat protection measures in plantations and for urban trees, alongside broadening the coverage of the dendroclimatic net with accents on the impact of habitat conditions and climate-induced long-term growth dynamics.

The ecosystem wilting point defines drought response and recovery of a Quercus-Carya forest

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ_EWP ), a property that integrates the drought response of an ecosystem's plant community across the soil-plant-atmosphere continuum. Specifically, Ψ_EWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ_EWP of a Quercus-Carya forest from an "ecosystem pressure-volume (PV) curve," which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψ_pd ) was above Ψ_EWP (=-2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ_pd fell below Ψ_EWP , the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ_EWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ_pd observations fell below Ψ_EWP , the forest is commonly only 2-4 weeks of intense drought away from reaching Ψ_EWP , and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ_EWP , and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.

Hyposensitive canopy conductance renders ecosystems vulnerable to meteorological droughts

Increased meteorological drought intensity with rising atmospheric demand for water (hereafter vapor pressure deficit [VPD]) increases the risk of tree mortality and ecosystem dysfunction worldwide. Ecosystem-scale water-use strategy is increasingly recognized as a key factor in regulating drought-related ecosystem responses. However, the link between water-use strategy and ecosystem vulnerability to meteorological droughts is poorly established. Using the global flux observations, historic hydroclimatic data, remote-sensing products, and plant functional-trait archive, we identified potentially vulnerable ecosystems, examining how ecosystem water-use strategy, quantified by the percentage bias (δ) of the empirical canopy conductance sensitivity to VPD relative to the theoretical value, mediated ecosystem responses to droughts. We found that prevailing soil water availability substantially impacted δ in dryland regions where ecosystems with insufficient soil moisture usually showed conservative water-use strategy, while ecosystems in humid regions exhibited more pronounced climatic adaptability. Hyposensitive and hypersensitive ecosystems, classified based on δ falling below or above the theoretical sensitivity, respectively, achieved similar net ecosystem productivity during droughts, employing different structural and functional strategies. However, hyposensitive ecosystems, risking their hydraulic system with a permissive water-use strategy, were unable to recover from droughts as quickly as hypersensitive ones. Our findings highlight that processed-based models predicting current functions and future performance of vegetation should account for the greater vulnerability of hyposensitive ecosystems to intensifying atmospheric and soil droughts.

The detection and attribution of extreme reductions in vegetation growth across the global land surface

Negative extreme anomalies in vegetation growth (NEGs) usually indicate severely impaired ecosystem services. These NEGs can result from diverse natural and anthropogenic causes, especially climate extremes (CEs). However, the relationship between NEGs and many types of CEs remains largely unknown at regional and global scales. Here, with satellite-derived vegetation index data and supporting tree-ring chronologies, we identify periods of NEGs from 1981 to 2015 across the global land surface. We find 70% of these NEGs are attributable to five types of CEs and their combinations, with compound CEs generally more detrimental than individual ones. More importantly, we find that dominant CEs for NEGs vary by biome and region. Specifically, cold and/or wet extremes dominate NEGs in temperate mountains and high latitudes, whereas soil drought and related compound extremes are primarily responsible for NEGs in wet tropical, arid and semi-arid regions. Key characteristics (e.g., the frequency, intensity and duration of CEs, and the vulnerability of vegetation) that determine the dominance of CEs are also region- and biome-dependent. For example, in the wet tropics, dominant individual CEs have both higher intensity and longer duration than non-dominant ones. However, in the dry tropics and some temperate regions, a longer CE duration is more important than higher intensity. Our work provides the first global accounting of the attribution of NEGs to diverse climatic extremes. Our analysis has important implications for developing climate-specific disaster prevention and mitigation plans among different regions of the globe in a changing climate.

Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass

Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 °C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

Contrasting warming responses of photosynthesis in early- and late-successional tropical trees

The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (An), maximum rates of Rubisco carboxylation at 25°C (Vcmax25), stomatal conductance (gs) and the slope parameter of the stomatal conductance-photosynthesis model (g1), in ten early- (ES) and eight late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8°C in daytime ambient air temperature. The effect of seasonal drought on An was also investigated. We found that warm climate decreased wet-season An in LS species, but not in ES species. Values of Vcmax25 were lower at the warmest site across both successional groups, and An and Vcmax25 were higher in ES compared to LS species. Stomatal conductance exhibited no significant site differences and g1 was similar across both sites and successional groups. Drought strongly reduced An at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of An between successional groups may lead to shifts in species' competitive balance in a warmer world, to the disadvantage of LS trees.

Constant hydraulic supply and ABA dynamics facilitate the trade-offs in water and carbon

Carbon-water trade-offs in plants are adjusted through stomatal regulation. Stomatal opening enables carbon uptake and plant growth, whereas plants circumvent drought by closing stomata. The specific effects of leaf position and age on stomatal behavior remain largely unknown, especially under edaphic and atmospheric drought. Here, we compared stomatal conductance (g_s ) across the canopy of tomato during soil drying. We measured gas exchange, foliage ABA level and soil-plant hydraulics under increasing vapor pressure deficit (VPD). Our results indicate a strong effect of canopy position on stomatal behavior, especially under hydrated soil conditions and relatively low VPD. In wet soil (soil water potential > -50 kPa), upper canopy leaves had the highest g_s (0.727 ± 0.154 mol m^-2 s^-1) and assimilation rate (A; 23.4 ± 3.9 µmol m^-2 s^-1) compared to the leaves at a medium height of the canopy (g_s : 0.159 ± 0.060 mol m² s^-1; A: 15.9 ± 3.8 µmol m^-2 s^-1). Under increasing VPD (from 1.8 to 2.6 kPa), g_s , A and transpiration were initially impacted by leaf position rather than leaf age. However, under high VPD (2.6 kPa), age effect outweighed position effect. The soil-leaf hydraulic conductance was similar in all leaves. Foliage ABA levels increased with rising VPD in mature leaves at medium height (217.56 ± 85 ng g^-1 FW) compared to upper canopy leaves (85.36 ± 34 ng g^-1 FW). Under soil drought (< -50 kPa), stomata closed in all leaves resulting in no differences in g_s across the canopy. We conclude that constant hydraulic supply and ABA dynamics facilitate preferential stomatal behavior and carbon-water trade-offs across the canopy. These findings are fundamental in understanding variations within the canopy, which helps in engineering future crops, especially in the face of climate change.

Earlier leaf senescence dates are constrained by soil moisture

The unprecedented warming that has occurred in recent decades has led to later autumn leaf senescence dates (LSD) throughout the Northern Hemisphere. Yet, great uncertainties still exist regarding the strength of these delaying trends, especially in terms of how soil moisture affects them. Here we show that changes in soil moisture in 1982-2015 had a substantial impact on autumn LSD in one-fifth of the vegetated areas in the Northern Hemisphere (>30° N), and how it contributed more to LSD variability than either temperature, precipitation or radiation. We developed a new model based on soil-moisture-constrained cooling degree days (CDD_SM ) to characterize the effects of soil moisture on LSD and compared its performance with the CDD, Delpierre and spring-influenced autumn models. We show that the CDD_SM model with inputs of temperature and soil moisture outperformed the three other models for LSD modelling and had an overall higher correlation coefficient (R), a lower root mean square error and lower Akaike information criterion (AIC) between observations and model predictions. These improvements were particularly evident in arid and semi-arid regions. We studied future LSD using the CDD_SM model under two scenarios (SSP126 and SSP585) and found that predicted LSD was 4.1 ± 1.4 days and 5.8 ± 2.8 days earlier under SSP126 and SSP585, respectively, than other models for the end of this century. Our study therefore reveals the importance of soil moisture in regulating autumn LSD and, in particular, highlights how coupling this effect with LSD models can improve simulations of the response of vegetation phenology to future climate change.

The influences of stomatal size and density on rice abiotic stress resilience

A warming climate coupled with reductions in water availability and rising salinity are increasingly affecting rice (Oryza sativa) yields. Elevated temperatures combined with vapour pressure deficit (VPD) rises are causing stomatal closure, further reducing plant productivity and cooling. It is unclear what stomatal size (SS) and stomatal density (SD) will best suit all these environmental extremes. To understand how stomatal differences contribute to rice abiotic stress resilience, we screened the stomatal characteristics of 72 traditionally bred varieties. We found significant variation in SS, SD and calculated anatomical maximal stomatal conductance (gsmax ) but did not identify any varieties with SD and gsmax as low as transgenic OsEPF1oe plants. Traditionally bred varieties with high SD and small SS (resulting in higher gsmax ) typically had lower biomasses, and these plants were more resilient to drought than low SD and large SS plants, which were physically larger. None of the varieties assessed were as resilient to drought or salinity as low SD OsEPF1oe transgenic plants. High SD and small SS rice displayed faster stomatal closure during increasing temperature and VPD, but photosynthesis and plant cooling were reduced. Compromises will be required when choosing rice SS and SD to tackle multiple future environmental stresses.

Thermal optimum of photosynthesis is controlled by stomatal conductance and does not acclimate across an urban thermal gradient in six subtropical tree species

Modelling the response of plants to climate change is limited by our incomplete understanding of the component processes of photosynthesis and their temperature responses within and among species. For ≥20 individuals, each of six common subtropical tree species occurring across steep urban thermal gradients in Miami, Florida, USA, we determined rates of net photosynthesis (A_net ), maximum RuBP carboxylation, maximum RuBP regeneration and stomatal conductance, and modelled the optimum temperature (T_opt ) and process rate of each parameter to address two questions: (1) Do the T_opt of A_net (T_optA ) and the maximum A_net (A_opt ) of subtropical trees reflect acclimation to elevated growth temperatures? And (2) What limits A_net in subtropical trees? Against expectations, we did not find significant acclimation of T_optA , A_opt  or the T_opt of any of the underlying photosynthetic parameters to growth temperature in any of the focal species. Model selection for the single best predictor of A_net both across leaf temperatures and at T_optA revealed that the A_net of most trees was best predicted by stomatal conductance. Our findings are in accord with those of previous studies, especially in the tropics, that have identified stomatal conductance to be the most important factor limiting A_net , rather than biochemical thermal responses.

Revegetation affects the response of land surface phenology to climate in Loess Plateau, China

Land surface phenology (LSP), defined as the plant's growth rhythm retrieved from satellite sensing products, is proven to shift with climate change and affect the carbon cycles of terrestrial ecosystems. Global afforested area is largely increasing and consequently affecting local and global climate. However, how and to what extent revegetation affects LSP remains relatively unexplored. Here we investigated the difference in four LSPs (i.e., greenup, maturity, senescence, and dormancy) and the response of LSP to climate between restored and native vegetation on Loess Plateau, China, where a remarkable process of vegetation restoration happened during 1982-2015. Most study regions showed a longer growing season (LOS) over time, specifically, with a slight delay in greenup but a relatively large delay in senescence. We found that air temperature was the dominant factor affecting greenup and maturity, while precipitation mostly controlled the senescence and dormancy in the study area. Under similar climate conditions, the LSP of restored vegetation (i.e., restored forest and grassland) showed a significant difference (p < 0.05) from native ones during 1999-2015. Compared to the native forest, restored forest from cropland and grassland showed a delayed greenup date by 0.3 and 3.6 days (p < 0.05) and an advanced dormancy date of 6.6 and 9.0 days (p < 0.05), respectively. Furthermore, the restored vegetation became less sensitive to air temperature than native vegetation, while the restored forest was more sensitive to precipitation, and its growth was affected by the water limitation to a larger extent in the study area. Our study highlights the necessity of considering land use management and its effect on the LSP change to better understand the effect of afforestation on global climate and carbon cycles.

Environmental controls on the light use efficiency of terrestrial gross primary production

Gross primary production (GPP) by terrestrial ecosystems is a key quantity in the global carbon cycle. The instantaneous controls of leaf-level photosynthesis are well established, but there is still no consensus on the mechanisms by which canopy-level GPP depends on spatial and temporal variation in the environment. The standard model of photosynthesis provides a robust mechanistic representation for C₃ species; however, additional assumptions are required to "scale up" from leaf to canopy. As a consequence, competing models make inconsistent predictions about how GPP will respond to continuing environmental change. This problem is addressed here by means of an empirical analysis of the light use efficiency (LUE) of GPP inferred from eddy covariance carbon dioxide flux measurements, in situ measurements of photosynthetically active radiation (PAR), and remotely sensed estimates of the fraction of PAR (fAPAR) absorbed by the vegetation canopy. Focusing on LUE allows potential drivers of GPP to be separated from its overriding dependence on light. GPP data from over 100 sites, collated over 20 years and located in a range of biomes and climate zones, were extracted from the FLUXNET2015 database and combined with remotely sensed fAPAR data to estimate daily LUE. Daytime air temperature, vapor pressure deficit, diffuse fraction of solar radiation, and soil moisture were shown to be salient predictors of LUE in a generalized linear mixed-effects model. The same model design was fitted to site-based LUE estimates generated by 16 terrestrial ecosystem models. The published models showed wide variation in the shape, the strength, and even the sign of the environmental effects on modeled LUE. These findings highlight important model deficiencies and suggest a need to progress beyond simple "goodness of fit" comparisons of inferred and predicted carbon fluxes toward an approach focused on the functional responses of the underlying dependencies.

Lichen ecophysiology in a changing climate

Lichens are one of the most iconic and ubiquitous symbioses known, widely valued as indicators of environmental quality and, more recently, climate change. Our understanding of lichen responses to climate has greatly expanded in recent decades, but some biases and constraints have shaped our present knowledge. In this review we focus on lichen ecophysiology as a key to predicting responses to present and future climates, highlighting recent advances and remaining challenges. Lichen ecophysiology is best understood through complementary whole-thallus and within-thallus scales. Water content and form (vapor or liquid) are central to whole-thallus perspectives, making vapor pressure differential (VPD) a particularly informative environmental driver. Responses to water content are further modulated by photobiont physiology and whole-thallus phenotype, providing clear links to a functional trait framework. However, this thallus-level perspective is incomplete without also considering within-thallus dynamics, such as changing proportions or even identities of symbionts in response to climate, nutrients, and other stressors. These changes provide pathways for acclimation, but their understanding is currently limited by large gaps in our understanding of carbon allocation and symbiont turnover in lichens. Lastly, the study of lichen physiology has mainly prioritized larger lichens at high latitudes, producing valuable insights but underrepresenting the range of lichenized lineages and ecologies. Key areas for future work include improving geographic and phylogenetic coverage, greater emphasis on VPD as a climatic factor, advances in the study of carbon allocation and symbiont turnover, and the incorporation of physiological theory and functional traits in our predictive models.

Bridging the gap between microclimate and microrefugia: A bottom-up approach reveals strong climatic and biological offsets

In the context of global warming, a clear understanding of microrefugia-microsites enabling the survival of species populations outside their main range limits-is crucial. Several studies have identified forcing factors that are thought to favor the existence of microrefugia. However, there is a lack of evidence to conclude whether, and to what extent, the climate encountered within existing microrefugia differs from the surrounding climate. To investigate this, we adopt a "bottom-up" approach, linking marginal disconnected populations to microclimate. We used the southernmost disconnected and abyssal populations of the circumboreal herbaceous plant Oxalis acetosella in Southern France to study whether populations in sites matching the definition of "microrefugia" occur in particularly favorable climatic conditions compared to neighboring control plots located at distances of between 50 to 100 m. Temperatures were recorded in putative microrefugia and in neighboring plots for approximately 2 years to quantify their thermal offsets. Vascular plant inventories were carried out to test whether plant communities also reflect microclimatic offsets. We found that current microclimatic dynamics are genuinely at stake in microrefugia. Microrefugia climates are systematically colder compared to those found in neighboring control plots. This pattern was more noticeable during the summer months. Abyssal populations showed stronger offsets compared to neighboring plots than the putative microrefugia occurring at higher altitudes. Plant communities demonstrate this strong spatial climatic variability, even at such a microscale approach, as species compositions systematically differed between the two plots, with species more adapted to colder and moister conditions in microrefugia compared to the surrounding area.

Plant hydraulic modelling of leaf and canopy fuel moisture content reveals increasing vulnerability of a Mediterranean forest to wildfires under extreme drought

Fuel moisture content (FMC) is a crucial driver of forest fires in many regions world-wide. Yet, the dynamics of FMC in forest canopies as well as their physiological and environmental determinants remain poorly understood, especially under extreme drought. We embedded a FMC module in the trait-based, plant-hydraulic SurEau-Ecos model to provide innovative process-based predictions of leaf live fuel moisture content (LFMC) and canopy fuel moisture content (CFMC) based on leaf water potential ( ψ Leaf ). SurEau-Ecos-FMC relies on pressure-volume (p-v) curves to simulate LFMC and vulnerability curves to cavitation to simulate foliage mortality. SurEau-Ecos-FMC accurately reproduced ψ Leaf and LFMC dynamics as well as the occurrence of foliage mortality in a Mediterranean Quercus ilex forest. Several traits related to water use (leaf area index, available soil water, and transpiration regulation), vulnerability to cavitation, and p-v curves (full turgor osmotic potential) had the greatest influence on LFMC and CFMC dynamics. As the climate gets drier, our results showed that drought-induced foliage mortality is expected to increase, thereby significantly decreasing CFMC. Our results represent an important advance in our capacity to understand and predict the sensitivity of forests to wildfires.

Developing future heat-resilient vegetable crops

Climate change seriously impacts global agriculture, with rising temperatures directly affecting the yield. Vegetables are an essential part of daily human consumption and thus have importance among all agricultural crops. The human population is increasing daily, so there is a need for alternative ways which can be helpful in maximizing the harvestable yield of vegetables. The increase in temperature directly affects the plants' biochemical and molecular processes; having a significant impact on quality and yield. Breeding for climate-resilient crops with good yields takes a long time and lots of breeding efforts. However, with the advent of new omics technologies, such as genomics, transcriptomics, proteomics, and metabolomics, the efficiency and efficacy of unearthing information on pathways associated with high-temperature stress resilience has improved in many of the vegetable crops. Besides omics, the use of genomics-assisted breeding and new breeding approaches such as gene editing and speed breeding allow creation of modern vegetable cultivars that are more resilient to high temperatures. Collectively, these approaches will shorten the time to create and release novel vegetable varieties to meet growing demands for productivity and quality. This review discusses the effects of heat stress on vegetables and highlights recent research with a focus on how omics and genome editing can produce temperature-resilient vegetables more efficiently and faster.

Bioclimatic atlas of the terrestrial Arctic

The Arctic is the region on Earth that is warming at the fastest rate. In addition to rising means of temperature-related variables, Arctic ecosystems are affected by increasingly frequent extreme weather events causing disturbance to Arctic ecosystems. Here, we introduce a new dataset of bioclimatic indices relevant for investigating the changes of Arctic terrestrial ecosystems. The dataset, called ARCLIM, consists of several climate and event-type indices for the northern high-latitude land areas > 45°N. The indices are calculated from the hourly ERA5-Land reanalysis data for 1950-2021 in a spatial grid of 0.1 degree (~9 km) resolution. The indices are provided in three subsets: (1) the annual values during 1950-2021; (2) the average conditions for the 1991-2020 climatology; and (3) temporal trends over 1951-2021. The 72-year time series of various climate and event-type indices draws a comprehensive picture of the occurrence and recurrence of extreme weather events and climate variability of the changing Arctic bioclimate.

Water yield response to plant community conversion caused by vegetation degradation and improvement in an alpine meadow on the northeastern Tibetan Plateau

Water provision is an important ecological function of alpine meadows on the Tibetan Plateau. Quantitative assessment of the effects of vegetation change induced by vegetation degradation and improvement on water yield (WY) in alpine meadows is urgent for rational water and grassland resources conservation and management. Previous studies mainly focused on the effects of vegetation coverage. What is less clear is how the WY of alpine meadow changes under plant community conversion caused by vegetation degradation and improvement. To test the hypotheses that lysimeter drainage (LD) decreases in the vegetation-degraded meadow and recovers in the vegetation-improved meadow, and the LD decreases as the stress tolerance of dominant strategy decreases, in situ lysimeters with intact monoliths of well-vegetated alpine meadows subjected to vegetation intact (sedge-dominated), degraded (forb-dominated) and improved (fast-growing grass-dominated) were employed, and then plant communities among treatments were compared based on the quantitative competitor, stress tolerator, and ruderal (CSR) theory. Compared to the vegetation-intact monoliths, the LD of vegetation-degraded monoliths was 59 % lower owing to the deeper roots and greater aboveground growth. The LD of vegetation-improved monoliths was 83 % higher than that of vegetation-degraded monoliths due to the shallower roots but was 25 % lower than that of vegetation-intact monoliths due to the greater aboveground growth. The LD decreased along a plant community conversion gradient in which the S-selection of the dominance strategy decreased (R² = 0.34, P = 0.022) and the C-selection increased (R² = 0.71, P < 0.001), likely due to the significant covariation between community-weighted CSR strategy with eco-hydrological plant and soil properties. These results indicated that the community conversion caused by vegetation degradation reduces the WY of alpine meadows, and sowing fast-growing grasses can only partly restore this function. Application of stress-tolerant plants for vegetation improvement may be more efficient in recovering the WY of degraded meadows, especially in flat meadows under humid climate.

Changes in the relationship between vapour pressure deficit and water use efficiency with the drought recovery time: A case study of the Yellow River Basin

Drought is a major driver of interannual variability in the gross primary productivity (GPP) of global terrestrial ecosystems, and drought recovery time has been widely used to assess ecosystem responses to drought. However, the response of the carbon-water coupled cycle to drought, especially changes in the correlation between drought intensity and carbon-water coupling throughout the recovery time, remains unclear. In this study, the Yellow River Basin (YRB) located mostly in drylands was the study area. We assessed the correlation between the standardized water vapour pressure deficit (VPD) and the water use efficiency of ecosystems (WUEe) and water use efficiency of canopies (WUEc) every month with the drought recovery time of GPP. We found that the drought intensity in the middle reach of the YRB (MYRB) was greater and the drought recovery time was longer than those in the upper reach (UYRB) and lower reach (LYRB) during the period from 2003 to 2017. In terms of the correlation between drought intensity and carbon-water coupling, the greater the VPD was, the lower the WUEc. In addition, the correlation of WUEc with VPD was higher than that of WUEe in most areas of the YRB, especially in the LYRB. On the watershed level, the correlation between the two types of WUE and VPD increased gradually with the recovery time, while the correlation between WUEc and VPD (mostly negative) changed more than the correlation between WUEe and VPD (mostly positive). Therefore, the response of WUEc to meteorological drought should be given more attention, especially during the middle and late stages of drought, since it exhibited an opposite signal compared to that of WUEe during drought recovery.

Contrasting impacts of dry versus humid heat on US corn and soybean yields

The impact of extreme heat on crop yields is an increasingly pressing issue given anthropogenic climate warming. However, some of the physical mechanisms involved in these impacts remain unclear, impeding adaptation-relevant insight and reliable projections of future climate impacts on crops. Here, using a multiple regression model based on observational data, we show that while extreme dry heat steeply reduced U.S. corn and soy yields, humid heat extremes had insignificant impacts and even boosted yields in some areas, despite having comparably high dry-bulb temperatures as their dry heat counterparts. This result suggests that conflating dry and humid heat extremes may lead to underestimated crop yield sensitivities to extreme dry heat. Rainfall tends to precede humid but not dry heat extremes, suggesting that multivariate weather sequences play a role in these crop responses. Our results provide evidence that extreme heat in recent years primarily affected yields by inducing moisture stress, and that the conflation of humid and dry heat extremes may lead to inaccuracy in projecting crop yield responses to warming and changing humidity.

The ratio of electron transport to assimilation (ETR/AN): underutilized but essential for assessing both equipment's proper performance and plant status

ETR/A_N ratios should be in the range 7.5-10.5 for non-stressed C₃ plants. Ratios extremely out of this range can be reflecting both uncontrolled plant status and technical mistakes during measurements. We urge users to explicitly refer to this ratio in future studies as a proof for internal data quality control. For the last few decades, the use of infra-red gas-exchange analysers (IRGAs) coupled with chlorophyll fluorometers that allow for measurements of net CO₂ assimilation rate and estimates of electron transport rate over the same leaf area has been popularized. The evaluation of data from both instruments in an integrative manner can result in additional valuable information, such as the estimation of the light respiration, mesophyll conductance and the partitioning of the flux of electrons into carboxylation, oxygenation and alternative processes, among others. In this review, an additional and more 'straight' use of the combination of chlorophyll fluorescence and gas exchange-derived parameters is presented, namely using the direct ratio between two fully independently estimated parameters, electron transport rate (ETR)-determined by the fluorometer-and net CO₂ assimilation rate (A_N)-determined by the IRGA, i.e., the ETR/A_N ratio, as a tool for fast detection of incongruencies in the data and potential technical problems associated with them, while checking for the study plant's status. To illustrate this application, a compilation of 75 studies that reported both parameters for a total of 178 species under varying physiological status is presented. Values of ETR/A_N between 7.5 and 10.5 were most frequently found for non-stressed C₃ plants. C₄ species showed an average ETR/A_N ratio of 4.7. The observed ratios were larger for species with high leaf mass per area and for plants subjected to stressful factors like drought or nutritional deficit. Knowing the expected ETR/A_N ratio projects this ratio as a routinary and rapid check point for guaranteeing both the correct performance of equipment and the optimal/stress status of studied plants. All known errors associated with the under- or overestimation of ETR or A_N are summarized in a checklist that aims to be routinely used by any IRGA/fluorometer user to strength the validity of their data.

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (T_leaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy T_leaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging T_leaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high T_leaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback.

Modeling of historical and current distributions of lone star tick, Amblyomma americanum (Acari: Ixodidae), is consistent with ancestral range recovery

The lone star tick, Amblyomma americanum L., is a three-host hard tick notorious for aggressive feeding behavior. In the early to mid-20th century, this species' range was mostly limited to the southern USA. Since the 1950s, A. americanum has been detected in many new localities in the western, northcentral, and northeastern regions of the country. To examine the influence of climate on this apparent expansion, we used historical (1748-1950) lone star locations from the literature and museum records to model areas suitable for this species based on past environmental conditions in the late 1800s - early 1900s. We then projected this model forward using present (2011-2020) climatic conditions and compared the two for evidence of climate-associated distributional shifts. A maximum entropy distribution or Maxent model was generated by using a priori selected climatic variables including temperature, precipitation, and vapor pressure deficit. Temperature and vapor pressure deficit were selected as the most important factors in creating a sensitive and specific model (success rate = 82.6 ± 6.1%) that had a good fit to the existing data and was significantly better than a random model [partial ROC (receiver operating characteristic) to AUC (area under the ROC curve) ratio = 1.97 ± 0.07, P < 0.001]. The present projected model was tested with an independent dataset of curated museum records (1952-2020) and found to be 95.6% accurate. Comparison of past and present models revealed > 98% A. americanum niche overlap. The model suggests that some areas along the western fringe are becoming less suitable for A. americanum, whereas areas in some Great Lakes and coastal northeastern regions are becoming more suitable, results that are compatible with possible effects of climate change. However, these changes are minor, and overall climate in North America does not appear to have changed in ways significant to A. americanum's distribution. These findings are consistent with an alternative hypothesis that recent changes in A. americanum's distribution are a result of this species re-occupying its historical range, driven predominantly by factors other than climate, such as shifts in land use and population densities of major hosts.

Determination of drought tolerance of different strawberry genotypes

Strawberry production future depends on productive, high quality and drought tolerant varieties. The goal of this study was to determine the most suitable variety by determining the yield and photosynthetic responses (net photosynthesis (Pn), stomatal conductance (g_s), and transpiration rate (E)) of four strawberry genotypes with different characteristics (Rubygem, Festival; 33, and 59) at two different irrigation levels (IR50: water stress (WS), IR100: well-watered (WW)). It was also aimed to prepare the irrigation program by making use of the crop water stress index (CWSI). The trial was conducted at the Agronomic Research Area, University of Çukurova, Turkey during 2019-2020 experimental year. The trial was implemented as a 4 × 2 factorial scheme of genotypes and irrigation levels, in a split-plot design. Genotype Rubygem had the highest canopy temperature (Tc)-air temperature (Ta), whereas genotype 59 had the lowest, indicating that genotype 59 has better ability to thermoregulate leaf temperatures. Moreover, yield, Pn, and E were found to have a substantial negative relationship with Tc-Ta. WS reduced yield, Pn, g_s, and E by 36%, 37%, 39%, and 43%, respectively, whereas it increased CWSI (22%) and irrigation water use efficiency (IWUE) (6%). Besides, the optimal time to measure leaf surface temperature of strawberries is around 1:00 pm and strawberry irrigation management might be maintained under the high tunnel in Mediterranean utilizing CWSI values between 0.49 and 0.63. Although genotypes had varying drought tolerance, the genotype 59 had the strongest yield and photosynthetic performances under both WW and WS conditions. Furthermore, 59 had highest IWUE and lowest CWSI in the WS conditions, proving to be the most drought tolerant genotype in this research.

Environment-oriented selection criteria to overcome controversies in breeding for drought resistance in wheat

Wheat is one of the most important cereal crops, representing a fundamental source of calories and protein for the global human population. Drought stress (DS) is a widespread phenomenon, already affecting large wheat-growing areas worldwide, and a major threat for cereal productivity, resulting in consistent losses in average grain yield (GY). Climate change is projected to exacerbate DS incidence and severity by increasing temperatures and changing rainfall patterns. Estimating that wheat production has to substantially increase to guarantee food security to a demographically expanding human population, the need for breeding programs focused on improving wheat drought resistance is manifest. Drought occurrence, in terms of time of appearance, duration, frequency, and severity, along the plant's life cycle varies significantly among different environments and different agricultural years, making it difficult to identify reliable phenological, morphological, and functional traits to be used as effective breeding tools. The situation is further complicated by the presence of confounding factors, e.g., other concomitant abiotic stresses, in an open-field context. Consequently, the relationship between morpho-functional traits and GY under water deficit is often contradictory; moreover, controversies have emerged not only on which traits are to be preferred, but also on how one specific trait should be desired. In this review, we attempt to identify the possible causes of these disputes and propose the most suitable selection criteria in different target environments and, thus, the best trait combinations for breeders in different drought contexts. In fact, an environment-oriented approach could be a valuable solution to overcome controversies in identifying the proper selection criteria for improving wheat drought resistance.

Does leaf gas exchange correlate with petiole xylem structural traits in Ulmus laevis seedlings under well-watered and drought stress conditions?

Several studies have shown that petiole xylem structure could be an important predictor of leaf gas exchange capacity, but the question of how petiole xylem structure relates to leaf gas exchange under different environment conditions remains unresolved. Moreover, knowledge of the amount of leaf gas exchange and structural variation that exists within a single species is also limited. In this study, we investigated the intraspecies coordination of leaf gas exchange and petiole xylem traits in 2-year-old seedlings of Ulmus laevis Pall. under well-watered and drought conditions. It was found that all studied petiole xylem traits of the elm seedlings were positively correlated with each other. This shows that the development of petiole xylem structure is internally well-coordinated. Nevertheless, the lower correlation coefficients between some petiole xylem traits indicate that the coordination is also individually driven. Drought stress reduced all studied leaf gas exchange traits and significantly increased intraspecies variation. In addition, drought stress also shifted the relationships between physiological traits and exhibited more structure-function relationships. This indicates the importance of petiole xylem structure in dictating water loss during drought stress and could partly explain the inconsistencies between leaf structure-function relationships studied under optimal conditions. Although several structure-function traits were related, the wide ranges of correlation coefficients indicate that the internal coordination of these traits substantially differs between individual elm seedlings. These findings are very important in the context of expected climatic change, as some degree of intraspecies variation in structure-function relationships could ensure the survival of some individuals under different environmental conditions.

New tree-ring data from Canadian boreal and hemi-boreal forests provide insight for improving the climate sensitivity of terrestrial biosphere models

Understanding boreal/hemi-boreal forest growth sensitivity to seasonal variations in temperature and water availability provides important basis for projecting the potential impacts of climate change on the productivity of these ecosystems. Our best available information currently comes from a limited number of field experiments and terrestrial biosphere model (TBM) simulations of varying predictive accuracy. Here, we assessed the sensitivity of annual boreal/hemi-boreal forest growth in Canada to yearly fluctuations in seasonal climate variables using a large tree-ring dataset and compared this to the climate sensitivity of annual net primary productivity (NPP) estimates obtained from fourteen TBMs. We found that boreal/hemi-boreal forest growth sensitivity to fluctuations in seasonal temperature and precipitation variables changed along a southwestern to northeastern gradient, with growth limited almost entirely by temperature in the northeast and west and by water availability in the southwest. We also found a lag in growth climate sensitivity, with growth largely determined by the climate during the summer prior to ring formation. Analyses of NPP sensitivity to the same climate variables produced a similar southwest to northeast gradient in growth climate sensitivity for NPP estimates from all but three TBMs. However, analyses of growth from tree-ring data and analyses of NPP from TBMs produced contrasting evidence concerning the key climate variables limiting growth. While analyses of NPP primarily indicated a positive relationship between growth and seasonal temperature, tree-ring analyses indicated negative growth relationships to temperature. Also, the positive effect of precipitation on NPP derived from most TBMs was weaker than the positive effect of precipitation on tree-ring based growth: temperature had a more important limiting effect on NPP than tree-ring data indicated. These mismatches regarding the key climate variables limiting growth suggested that characterization of tree growth in TBMs might need revision, particularly regarding the effects of stomatal conductance and carbohydrate reserve dynamics.

Patterns of compound-leaf form and deciduous-leaf habit across forests in China: Their association and key climatic factors

Leaf form (compound vs. simple) and habit (evergreen vs. deciduous) are key functional traits of trees to adapt to various climates and are vital in determining plant response to climate change. However, their association and climatic determinants remain uncertain, especially in East Asian forests in the largest monsoon region on earth. To fill these knowledge gaps, we compiled a dataset comprising 42 intact forests and over 2200 angiosperm tree species across China (spanning 30 latitudes and 47 longitudes). The geographical and climatic patterns of leaf form and habit were analyzed. The association between compound leaf and deciduousness was tested for tropical, subtropical and temperate climatic zones. We found that both the percentage of compound leaf (CT%) and deciduous tree species (DT%) increased with latitude and decreased with mean annual precipitation (MAP). For all forests, DT% was negatively related to mean annual temperature (MAT), whereas CT% was not. Nevertheless, both DT% and CT% increased with increasing MAT in the tropics, possibly owing to the high vapor pressure deficits (VPD) and canopy water deficits associated with high temperatures. A positive linear relationship between CT% and DT% was found across all forests and within different climatic zones except for temperate, and the intercept of the regression line was significantly higher in the tropics than in the subtropics. Overall, as supported by principal component analysis, deciduousness was negatively associated with both temperature and precipitation, while CT negatively with precipitation only across zones and positively with temperature in the tropics. Different relationships in different climatic zones suggest potentially different selective forces. Our findings provide novel insights into the linkage between leaf form and habit, as well as how climate shapes the landscape of broadleaf forests, which has important implications regarding the response of forest composition to climate change.

Higher sowing density of pearl millet increases productivity and water use efficiency in high evaporative demand seasons

INTRODUCTION

Pearlmillet is themain subsistence crop for smallholder farmers systemswhere it is grown at low plant density. Intensifying pearl millet cultivation could boost productivity although it may have trade-offs. Increasing planting density would indeed increase the leaf area and the related water budget, whereas a denser canopy could create a more favorable canopymicroclimate to the benefit of the water use efficiency (WUE) of the crops. The first aim of this work was to test the yield response of popular pearlmillet varieties to an increased density and to assess possible genotypic variation in this response. The second aim was to measure the water use and the WUE of the crop in different densities.

METHOD

To this end we designed several field and lysimetric experiments To increase the robustness of the results, these trials were carried out in India and Senegal, using two independent sets of genotypes adapted to both sites.

RESULTS

In the field, the higher sowing density significantly increased yield in all genotypes when trials were carried out in high evaporative demand conditions. There was no genotype x density interaction in these trials, suggesting no genotypic variation in the response to density increase. The high-density treatment also decreased the vapor pressure deficit (VPD) in the canopies, both in the field and in the lysimeter experiments. In the lysimeter trials, although the higher density treatment increased water use, the resulting increase in biomass was proportionally higher, hence increasingWUE of the crops in all genotypes under high density. The increase in yield under high density was closely related to the increase in WUE, although this link was more tight in the high- than in the low evaporative demand seasons. This confirmed a strong environmental effect on the response to density of all genotypes tested.

DISCUSSION

Although they did not open a scope for breeding density tolerant cultivars, these results highlight the possibility to improve pearl millet yield by increasing the density, targeting specifically areas facing high evaporative demand.

Turgor loss point and vulnerability to xylem embolism predict species-specific risk of drought-induced decline of urban trees

Increasing frequency and severity of drought events is posing risks to trees' health, including those planted in urban settlements. Drought-induced decline of urban trees negatively affects ecosystem services of urban green spaces and implies cost for maintenance and removal of plants. We aimed at identifying physiological traits that can explain and predict the species-specific vulnerability to climate change in urban habitats. We assessed the relationships between long-term risk of decline of different tree species in a medium-sized town and their key indicators of drought stress tolerance, i.e. turgor loss point (TLP) and vulnerability to xylem embolism (P₅₀ ). Starting from 2012, the study area experienced several summer seasons with positive anomalies of temperature and negative anomalies of precipitation. This trend was coupled with increasing percentages of urban trees showing signs of crown die-back and mortality. The species-specific risk of decline was higher for species with less negative TLP and P₅₀ values. The relationship between species-specific risk of climate change-induced decline of urban trees and key physiological indicators of drought tolerance confirms findings obtained in natural forests and highlights that TLP and P₅₀ are useful indicators for species selection for tree plantation in towns, to mitigate negative impacts of climate change.

Drivers of Decadal Carbon Fluxes Across Temperate Ecosystems

Long-running eddy covariance flux towers provide insights into how the terrestrial carbon cycle operates over multiple timescales. Here, we evaluated variation in net ecosystem exchange (NEE) of carbon dioxide (CO₂) across the Chequamegon Ecosystem-Atmosphere Study AmeriFlux core site cluster in the upper Great Lakes region of the USA from 1997 to 2020. The tower network included two mature hardwood forests with differing management regimes (US-WCr and US-Syv), two fen wetlands with varying levels of canopy sheltering and vegetation (US-Los and US-ALQ), and a very tall (400 m) landscape-level tower (US-PFa). Together, they provided over 70 site-years of observations. The 19-tower Chequamegon Heterogenous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors 2019 campaign centered around US-PFa provided additional information on the spatial variation of NEE. Decadal variability was present in all long-term sites, but cross-site coherence in interannual NEE in the earlier part of the record became weaker with time as non-climatic factors such as local disturbances likely dominated flux time series. Average decadal NEE at the tall tower transitioned from carbon source to sink to near neutral over 24 years. Respiration had a greater effect than photosynthesis on driving variations in NEE at all sites. Declining snowfall offset potential increases in assimilation from warmer springs, as less-insulated soils delayed start of spring green-up. Higher CO₂ increased maximum net assimilation parameters but not total gross primary productivity. Stand-scale sites were larger net sinks than the landscape tower. Clustered, long-term carbon flux observations provide value for understanding the diverse links between carbon and climate and the challenges of upscaling these responses across space.

isocalcR: An R package to streamline and standardize stable isotope calculations in ecological research

The use of stable isotopes to characterize ecosystem dynamics and infer leaf gas exchange processes has become increasingly prevalent over the last few decades within the ecological community. While advancements in theory and our understanding of the physiological processes controlling isotopic signatures in plants has been well-documented, no standardized tool currently exists to facilitate the computation of common isotope-derived plant physiological indices. Here, we present isocalcR, an R package intended to facilitate the use of stable isotope data from plant tissues by providing an integrated collection of functions and recommended reference data. The isocalcR R package contains a suite of functions that compute leaf carbon isotope discrimination (∆¹³ C), leaf intercellular [CO₂ ], the ratio of leaf intercellular to atmospheric [CO₂ ], the difference between atmospheric and leaf intercellular [CO₂ ], and intrinsic water use efficiency from carbon isotope signatures in leaf or wood tissue with minimal inputs from the user. isocalcR also implements and provides recommended input atmospheric [CO₂ ] (ppm) and atmospheric δ¹³ CO₂ (‰) data for the period 0-2021 C.E. A major goal of isocalcR is to provide a standardized, open-source tool to streamline the calculation of reproducible physiological indices from stable isotope signatures in plant tissues, incorporating the most up-to-date theory, while simultaneously eliminating potential errors associated with complex calculations. isocalcR can be used for any location globally as long as the user provides information regarding temperature and elevation to the main workhorse functions.