Plant responses to rising vapor pressure deficit

Prolonged warming and drought reduce canopy-level net carbon uptake in beech and oak saplings despite photosynthetic and respiratory acclimation

Tree net carbon (C) uptake may decrease under global warming, as higher temperatures constrain photosynthesis while simultaneously increasing respiration. Thermal acclimation might mitigate this negative effect, but its capacity to do so under concurrent soil drought remains uncertain. Using a 5-yr open-top chamber experiment, we determined acclimation of leaf-level photosynthesis (thermal optimum Topt and rate Aopt) and respiration (rate at 25°C R25 and thermal sensitivity Q10) to chronic +5°C warming, soil drought, and their combination in beech (Fagus sylvatica L.) and oak (Quercus pubescens Willd.) saplings. Process-based modeling was used to evaluate acclimation impacts on canopy-level net C uptake (Atot). Prolonged warming increased Topt by 3.03-2.66°C, but only by 1.58-0.31°C when combined with soil drought, and slightly reduced R25 and Q10. By contrast, drought reduced Topt (-1.93°C in oak), Aopt (c. 50%), and slightly reduced R25 and Q10 (in beech). Mainly because of reduced leaf area, Atot decreased by 47-84% with warming (in beech) and drought, but without additive effects when combined. Our results suggest that, despite photosynthetic and respiratory acclimation to warming and soil drought, canopy-level net C uptake will decline in a persistently hotter and drier climate, primarily due to the prevalent impact of leaf area reduction.

Uncovering the role of solar radiation and water stress factors in constraining decadal intra-site spring phenology variability in diverse ecosystems across the Northern Hemisphere

The spring phenology has advanced significantly over recent decades with climate change, impacting large-scale biogeochemical cycles, climate feedback, and other essential ecosystem processes. Although numerous prognostic models have been developed for spring phenology, regional analyses of the optimality (OPT) strategy model that incorporate environmental variables beyond temperature and photoperiod remain lacking. We investigated the roles of solar radiation (SR) and three water stress factors (precipitation (P), soil moisture, and vapor pressure deficit (VPD)) on spring phenology from 1982 to 2015 using the OPT model with Global Inventory Modeling and Mapping Studies NDVI3g dataset and environmental data from TerraClimate, CRU_TS, and Global Land Data Assimilation System across the Northern Hemisphere (> 30°N). Our results show that SR and water stress factors significantly impacted intrasite decadal spring phenology variability, with water stress factors dominant in grassland ecosystems while SR dominated in the rest of the ecosystem types. Enhanced models incorporating SR (OPT-S) and VPD (OPT-VPD) outperformed the original OPT model, likely due to improved representation of the adaptive strategy of spring phenology to optimize photosynthetic carbon gain while minimizing frost risk. Our research enhances the understanding of the key environmental drivers influencing decadal spring phenology variation in the Northern Hemisphere and contributes to more accurate forecasts of ecological responses to global environmental change.

Stomata-Photosynthesis Synergy Mediates Combined Heat and Salt Stress Tolerance in Sugarcane Mutant M4209

Sugarcane (Saccharum officinarum L.) is an economically important long-duration crop which is currently facing concurrent heat waves and soil salinity. The present study evaluates an inducible salt-tolerant sugarcane mutant M4209, developed via radiation-induced mutagenesis of elite check variety Co 86032, under heat (42/30°C; day/night), NaCl (200 mM) or heat + NaCl (HS)-stress conditions. Though heat application significantly improved plant growth and biomass in both genotypes, this beneficial impact was partially diminished in Co 86032 under HS-stress conditions, coinciding with higher Na+ accumulation and lower triacylglycerol levels. Besides, heat broadly equalised the negative impact on NaCl stress in terms of various physiological and biochemical attributes in both the genotypes, indicating its spaciotemporal advantage. The simultaneous up- and downregulation of antagonistic regulators, epidermal patterning factor (EPF) 9 (SoEPF9) and SoEPF2, respectively attributed to the OSD (Open Small Dense) stomatal phenotype in M4209, which resulted into enhanced conductance, transpirational cooling and gaseous influx. This led to improved photoassimilation, which was supported by higher plastidic:nonplastidic lipid ratio, upregulation of SoRCA (Rubisco activase) and better source strength, resulting in overall plant growth enhancement across all the tested stress scenarios. Taken together, the present study emphasised the knowledge-driven harnessing of stomatal-photosynthetic synergy for ensuring global sugarcane productivity, especially under "salt-heat" coupled stress scenarios.

Seasonal Shifts in Tree Water Use and Non-Structural Carbohydrate Storage in a Tropical Dry Forest

Predictions of increased drought frequency and intensity have the potential to threaten to forest globally. The key to trees response to drought is an understanding of tree water use and carbohydrate storage. Our objective was to evaluate sap velocity and dynamics of non-structural carbohydrates (NSC) in native trees of a dry tropical forest, during rainy and drought periods. We evaluated six key species of the Caatinga: three deciduous species with low wood density (WD), two deciduous species with high WD and one evergreen species during the rainy and dry periods. We measured sap velocity, xylem water potential, stomatal conductance, phenology and NSC. We found that the evergreen specie had higher sap velocity and frequent NSC production. While the low deciduous WD species showed low sap velocity, store water and NSC mainly in the stem and roots, and have leaf sprouting and flowering at the end of the dry period. The deciduous high WD also showed low sap velocity, however, with low stored NSC. These results suggest that under longer dry seasons and an irregular rainy seasons, species with low WD that use part of the stored NSC to resprout still during dry season may be the most affected.

G, Anderegg WRL. Ring-specific vulnerability to embolism reveals accumulation of damage in the xylem

Human-caused climate change is predicted to bring more frequent droughts and higher temperatures in the western United States, which threaten ecologically important trembling aspen forests. We used ring-specific vulnerability curves of aspen branches along two climate gradients to determine whether damages to pit membranes accumulate as the xylem ages. We found that rings older than 3 yr have a significant decline in hydraulic conductivity, especially at average summer water potentials for the species. These differences were not due to differences in the diameter of the vessels, but a difference in how much xylem was active between rings older than 3 yr and 1 yr, suggesting the presence of accumulated damage to pit membranes impairing water transport. Vulnerability to embolism differs across ring age and between wetter and drier populations, underscoring that damages due to drought may accumulate to lethal levels if the xylem does not acclimate to climate change in newer growth.

Vegetation vulnerability is driven by either higher drought sensitivity or lower fog exposure in tropical cloud ecosystems

Both reduced precipitation and reduced fog uplift increase drought-driven plant mortality. However, it is still unclear how plant vulnerability to drought in cloud ecosystems depends on the role of fog in relieving water stress via foliar water uptake (FWU). To investigate how plants in contrasting montane vegetation rely on fog to alleviate drought impacts, we measured 11 morpho-physiological traits in 10 phylogenetic pairs of plants in a montane grassland (~2000 m a.s.l.) and in a submontane forest (~700 m a.s.l.), both in southeast Brazil. Forest species are more sensitive to drought (i.e., lower conservative trait values, lower resistance to embolism, and lower FWU) than grassland species. Nonetheless, decreased frequency of fog events in the montane grassland may expose these species to a higher risk of dehydration, despite higher FWU capacity. Both forest and grassland vegetation are vulnerable to drought, but the vulnerability is attributable to different causes: higher sensitivity to drought in forests and lower fog exposure in grasslands. Therefore, for a more accurate description of plant responses to drought, we recommend introduction of theoretical-experimental models to assess drought vulnerability to changes in both atmospheric and soil water availability.

Anthropogenic climate change contributes to wildfire particulate matter and related mortality in the United States

Climate change has increased forest fire extent in temperate and boreal North America. Here, we quantified the contribution of anthropogenic climate change to human mortality and economic burden from exposure to wildfire particulate matter at the county and state level across the contiguous US (2006 to 2020) by integrating climate projections, climate-wildfire models, wildfire smoke models, and emission and health impact modeling. Climate change contributed to approximately 15,000 wildfire particulate matter deaths over 15 years with interannual variability ranging from 130 (95% confidence interval: 64, 190) to 5100 (95% confidence interval: 2500, 7500) deaths and a cumulative economic burden of $160 billion. Approximately 34% of the additional deaths attributable to climate change occurred in 2020, costing $58 billion. The economic burden was highest in California, Oregon, and Washington. We suggest that absent abrupt changes in climate trajectories, land management, and population, the indirect impacts of climate change on human-health through wildfire smoke will escalate.

Advanced technologies in plant factories: exploring current and future economic and environmental benefits in urban horticulture

Plant factories (PFs), also known as vertical farms, are advanced agricultural production systems that operate independently of geographical and environmental conditions. They utilize artificial light and controlled environments to produce horticultural plants year-round. This approach offers a promising solution for the stable and efficient supply of high-quality horticultural produce in urban areas, enhancing resilient urban food systems. This review explores the economic and environmental impacts and potential of PFs. Breakthroughs in PF research and development are highlighted, including increased product yields and quality, reduced energy input and CO2 emissions through optimized growing conditions and automation systems, transitioning to clean energy, improved resource use efficiency, and reduced food transport distances. Moreover, innovations and applications of PFs have been proposed to address challenges from both economic and environmental perspectives. The proposed development of PF technologies for economic and environmental benefits represents a comprehensive and promising approach to urban horticulture, significantly enhancing the impact and benefits of fundamental research and industrial applications.

Physiological and biochemical responses of 'Divadona' peach on Rootpac 20 and Rootpac 40 under drought and heat stress adaptation and its recovery mechanisms

Climate change-induced drought and heat stress pose significant challenges to global peach production, threatening agricultural sustainability and food security. This study, therefore, investigated the morphological, physiological and biochemical responses of the 'Divadona'peach cultivar grafted onto two different rootstocks (Rootpac 20 and Rootpac 40) under drought stress, heat shock, and their combination. We aimed to identify superior rootstock performances and understand stress tolerance mechanisms for improved cultivation strategies. Our findings revealed that combined stress induced the most severe impacts, with Rootpac 40 demonstrating superior stress tolerance. Under combined stresses, relative shoot diameter decreased less in Rootpac 40/'Divadona' (19.75%) compared to Rootpac 20/'Divadona', while relative shoot length showed similar patterns. Antioxidant enzyme activities increased significantly, with POD showing the highest elevation in Rootpac 40/'Divadona' compared to Rootpac 20/'Divadona. Stress markers exhibited substantial accumulation, with MDA content rising more in Rootpac 20/'Divadona' than in Rootpac 40/'Divadona'. Nutrient analysis showed that Rootpac 40/'Divadona' maintained higher levels of essential nutrients under stress, with nitrogen content declining less compared to Rootpac 20/'Divadona'. The study demonstrated that Rootpac 40/'Divadona' possesses superior stress tolerance mechanisms through better maintenance of growth parameters, enhanced antioxidant defense systems, and improved nutrient retention capacity. These findings provide valuable insights for fruit growing, enabling informed rootstock selection for peach cultivation in drought-prone regions, ultimately contributing to more resilient and sustainable fruit production systems under changing climatic conditions.

Stomatal Plasticity Maintains Water Potential Homeostasis in Pinus radiata Needles

Vapour pressure deficit (VPD) is a primary determinant of stomatal behaviour and water balance in plants. With increasing global temperature, the accompanying rise in VPD is likely to have a significant impact on the performance of plant species in the future. However, the plasticity of stomatal response to VPD remains largely unexplored. This study examines the plasticity of whole plant stomatal conductance (gc) response to VPD in Pinus radiata plants grown under two temperatures and a water-deficient treatment over a period of 3 months. The soil-stem water potential gradient (ΔΨ), gc and soil-stem hydraulic conductance (Ks-s) were evaluated. The different treatment groups showed significant differences in maximum gc relating to differences in Ks-s, however, gc dynamic response to VPD was very similar in all treatments such that ΔΨ was conserved once VPD increased above an average threshold of 0.64 kPa. The ability to robustly quantify water potential regulation in Pinus presents opportunities to explore variation in this globally important tree genus as well as providing a new approach to characterize the regulation of gas exchange in response to VPD.

Discovery of anisiflupurin, an inhibitor of cytokinin dehydrogenase that mitigates heat-induced yield reduction in rice

BACKGROUND

In a screening of anilinopurine, anisiflupurin was identified as potent inhibitor of cytokinin dehydrogenase/oxidase (CKX). Inhibitors of CKX have been supposed to be potent plant growth regulators to alleviate the detrimental effects of abiotic stress on crop production. The aim of the study was to profile anisiflupurin in a set of physiological assays and to evaluate its potential for heat stress mitigation in rice field trials.

RESULTS

Anisiflupurin delayed dark-induced senescence and increased transpiration in detached maize leaves in a dose-dependent manner. Similarly, the transpiration of young rice plants under heat stress was increased for several days after application with anisiflupurin. Application of anisiflupurin during early phases of generative growth not only restored heat-induced pollen alterations it increased grain yield in field grown rice under heat conditions as demonstrated in a large field program conducted in southeast Asia. Thereby, efficacy of anisiflupurin was rate-dependent and most effective when applied during early generative growth phases prior heat stress.

CONCLUSIONS

Application of anisiflupurin secures seed setting by protecting pollen development and enhances grain weight under heat stress conditions in rice. The results of this research opens up a promising avenue for mitigating the adverse effects of heat stress in rice cultivation. © 2024 Society of Chemical Industry.

Stomatal Parameters in a Changing Environment

We recommend that stomatal slope parameters (g1) be inferred by inversion so that variations in g1 may be attributed to variations physiological and environmental conditions. Understanding g1 will advance predictions of plant gas exchange and performance under global climate.

Future Climate Shifts for Vegetation on Australia's Coastal Islands

Small coastal islands serve as replicated units of space that are useful for studying community assembly. Using a unique database holding information on comprehensive vegetation surveys on > 840 small coastal islands fringing the whole continent of Australia, we investigated the extent to which conditions will change for plants on Australia's islands over the next 80 years in terms of their temperature envelopes and inferred changes in vapour pressure deficit (VPD). We found ~40% of island plant populations will experience mean annual temperatures beyond their current envelope. However, envelopes defined by VPD and extreme monthly temperatures are unlikely to be exceeded, highlighting islands' potential to act as climate refugia. Large species with slow life histories and poor dispersal traits were most likely to experience warmer temperatures, although this proved to be driven by correlations of these traits with latitude (closer to the equator) and with smaller range sizes. We found no evidence of warm edge extinction or poleward migration across species in response to 0.5° of warming since the year 2000. These results have applications for monitoring and conservation efforts under climate change for fragmented habitats everywhere.

Using long-term field data to quantify water potential regulation in response to VPD and soil moisture in a conifer tree

The regulation of vascular water potential (Ψstem) by stomata is one of the most dynamic and important behaviours in vascular plants, playing a central role in determining gas exchange and vulnerability to drought. Yet, the species-specific characterization of Ψstem regulatory behaviour in response to soil or atmospheric dryness remains elusive. We hypothesize that Ψstem regulatory behaviour can only be defined when the combination of both vapour pressure deficit (VPD) and soil water potential (Ψsoil) effects is considered. To test this hypothesis, we collected a high-resolution time series of Ψstem using optical dendrometers from trees of a hardy conifer, Callitris rhomboidea, monitored across multiple highly variable growing seasons. The regulatory behaviour of Ψstem collected over a total of 571 d could be predicted on the basis of diurnal Ψsoil and VPD (R2 = 0.74) using five mechanism-aligned parameters that describe specific stomatal regulation. Our novel approach to predict species-specific water potential variation in response to seasonal change using data from a continuous Ψstem monitoring technique creates a new opportunity to quantitatively compare water use and climatic sensitivity between diverse species or genotypes in the field or laboratory.

Substantial capacitance found in the roots of 2 contrasting conifer species

High rates of photosynthesis require abundant water delivered to the canopy to replace water lost to transpiration. In addition to water drawn immediately from the soil, stem capacitance has been identified as an additional water source, particularly during transient transpiration states. However, little information is available about the potential of roots to contribute to plant capacitance because methodological constraints have made it challenging to quantify root capacitance. In this study, we present a method to measure the water storage capacity of the root system and assess its contribution to daytime transpiration. We used an optical dendrometer to obtain in situ measurements of water potential and transpiration in 2 contrasting conifer species, Oyster Bay pine (Callitris rhomboidea) and Monterey pine (Pinus radiata), allowing us to quantify diurnal changes in plant water deficit. We employed a modified flow meter to gauge the rehydration kinetics of the below-ground and above-ground systems separately. We observed that root capacitance is a major supplier to the water demands during transient changes in transpiration for both species. Notably, the total below-ground capacitance exceeded the above-ground capacitance in C. rhomboidea, while the 2 capacitances were similar in P. radiata. Our findings highlight the importance of measuring and including below-ground capacitance in hydraulic models to accurately predict diurnal plant water status and stomatal behavior.

Local atmospheric vapor pressure deficit as microclimate index to assess tropical rainforest riparian restoration success

Characterizing microclimatic variables, such as vapor pressure deficit (VPD), is crucial for monitoring ecological processes and biodiversity dynamics of forests, among other terrestrial ecosystems. Approaches using technologies such as remotely piloted aircraft (RPA) have demonstrated potential for assessing the biophysical interface between forests and the atmosphere by obtaining high-resolution microclimatic metrics in space and time. In the present study, we developed a microclimatic approach based on VPD modeling to quantify the success of forest restoration in a tropical rainforest landscape. We used the photogrammetric technique Structure from Motion (SfM) with RPA to estimate three-dimensional forest structures and evaluated its influence in obtaining metrics for VPD modeling. A total of 30 plots of 314 m2 were analyzed at five stages of riparian forest development, including areas of early-stage passive restoration (E10, 10 years and E14, 14 years), mid-stage natural forest regeneration (M26, 26 years and M29, 29 years), and an old-growth forest (REF). These plots were used to calibrate and validate the VPD model (∼70 % training data and ∼ 30 % test data, with k = 10). Old-growth forests exhibited an average VPD of 0.19 kPa, lower than younger forests that exceeded the 1.0 kPa threshold. The 50th and 75th percentiles of the height distribution explained 86 % and 83 % of the variance in VPD (RMSE of 0.34 kPa), respectively, and demonstrated the potential use of this metric to predict the effects of forest structure on VPD. Results show that early-stage restoration sites are exposed to higher threshold limits of VPD, which can affect ecosystem functioning. Spatial characterization allows for identifying target areas for interventions, increasing our capacity to support better decisions in forest management.

Unsaturation of Leaf Air Spaces Sheds New Light on the Role of Aquaporins

Aquaporins have long been known to facilitate water transport across membranes inside leaves. However, their role in regulating the water potential (ψ) difference between the cytosol and cell wall has been questioned, as the ψ of the cytosol and cell wall would be in equilibrium under the assumption of water‐vapour‐saturated leaf intercellular air spaces. Recent advances suggest that intercellular air spaces are unsaturated at high vapour pressure deficit (VPD) and that aquaporins that up‐regulate water transport might simultaneously down‐regulate CO2 transport in a competitive manner. Therefore, the currently assumed mechanisms of CO2 and water transport in the mesophyll under varying VPD must be re‐examined. We incorporated the competitive aquaporin hypothesis into the leaf gas exchange pathways with unsaturated intercellular air spaces. We show that the putative competitive control of CO2‐ and water‐facilitating aquaporins is fully effective only when there is a large ψ gradient between the cytosol and cell wall at high VPD. In this context, the down‐regulation of water‐facilitating aquaporins and the up‐regulation of CO2‐facilitating aquaporins could protect the cytosol from drying and maintain the CO2 supply for photosynthesis, respectively. While it remains unclear whether unsaturation drives aquaporin activity or vice versa, we identify research challenges that need to be addressed.

A loss of stomata exposes a critical vulnerability to variable atmospheric humidity in ferns

Stomata confer both benefits and costs to plants, but assessing the magnitude of these effects is challenging. Some ferns have entirely lost stomata on their leaves, providing an opportunity to understand functional limitations associated with the inability to regulate transpiration. Here, we show that the loss of stomata and a massive reduction in xylem tissue investment in a filmy fern (Hymenophyllum flabellatum Labill.) leaves its vascular system exposed to catastrophic failure during relatively small reductions in atmospheric humidity. Hydraulic limitation, together with a sensitivity to fast desiccation, sets a clear lethal vapor pressure deficit threshold. This threshold enables a quantitative prediction of range contraction in H. flabellatum using a simple physical model. According to this threshold and climate projections, H. flabellatum may disappear from most of its native habitat in mainland Australia by 2050.

Mast hindcasts reveal pervasive effects of extreme drought on a foundational conifer species

Predicting seed production is challenging because many plants produce highly variable crops among years (i.e. masting), but doing so can inform forest management, conservation, and our understanding of ecosystem trajectories in a changing climate. We evaluated the ability of an existing model to forecast masting in an ecologically and culturally important tree species in the southwestern United States, Pinus edulis. Annual seed cone production was predicted using cross-validation techniques on two unique out-of-sample datasets, representing different collection methods and spatial scales (cone scars and cone counts). We then hindcasted this model into the historical past to evaluate whether seed production has declined with the onset of extreme drought conditions in western North America. The evaluated model had fair skill, with root-mean-squared error of 6%. The model had better skill predicting the interannual variability within a site than among sites (i.e. within years). Hindcast analyses indicated recent (2000-2024) mean annual cone production was 30.6% lower than in the past century (1900-1999). Mast forecasts are within reach, but much room remains for improvement. Forecasts may be a powerful tool to anticipate the effects of climate change on forests and woodlands.

Increased Growth Temperatures Alter Arctic Plant Responses to Heat Wave and Drought

Persistent warming and higher frequency of heat waves in the Arctic are causing alterations in Arctic vegetation and plant functionality, potentially redefining the role of the Arctic ecosystem. Vegetation influences atmospheric composition through exchanges of CO2 and volatile organic compounds (VOCs), both processes exhibiting a strong response to temperature variations. However, our quantitative understanding of how increased temperatures interact with extreme weather events, namely heat waves and drought, to affect Arctic plant processes remains limited. Here, we measure phenology, photosynthesis, leaf fluorescence and VOC emissions from three widely distributed Arctic shrubs, Betula nana, Empetrum hermaphroditum and Salix spp., in response to future climate. We use state-of-the-art climate chambers to test the effects of warmer growth temperatures on Arctic shrub responses to heat waves and drought. Our results show that increased growth temperatures advance leaf unfolding by 24 days in B. nana and 17 days in E. hermaphroditum, and increase VOC emissions across species. For B. nana, photosynthesis decreased by 42% during the heat wave and by 72% during drought. In contrast, Salix spp. and E. hermaphroditum experienced decreased photosynthesis only during drought, by 62% and 71%, respectively. The VOC emissions during the heat wave shifted toward a less diverse compound profile: acetaldehyde emissions increased for both control and warmed plants in all species, and isoprene emissions increased in Salix spp. Additionally, plants grown at higher temperatures exhibited a twofold increase in emissions compared to control plants during the heat wave, suggesting a higher temperature sensitivity of emissions. Our study indicates that warming and increasingly frequent extreme weather events will significantly impact Arctic plant phenology, photosynthesis and the diversity and rates of VOCs emitted into the atmosphere, contributing to modifying the regional climate.

A Global Synthesis of How Plants Respond to Climate Warming From Traits to Fitness

Despite intensive research, our understanding of how plants respond to warming by coordinating their full arsenal of traits to adjust fitness is lacking. To fill this gap, we applied a trait-based framework with three clusters (two functional clusters: "carbon-fixation rate" and "carbon-fixation area"; a third cluster: "total carbon fixation") to a global dataset compiled from 572 studies of warming experiments with 677 species and a comprehensive list of traits and fitness components. The pairwise correlation analysis complemented with SEM and PCA showed that plants increased biomass (the core variable in the third cluster) under warming by coordinating satellite traits in two functional clusters to adjust their core traits, net photosynthesis rate and total leaf area, respectively. In particular, the trait coordination was characterised by the maintenance of net photosynthesis rate and the increase of total leaf area, which was robust across ecological contexts although warming responses of the variables per se displayed context-dependences. Moreover, the trade-offs between biomass and reproduction (itself bearing mass vs. number trade-offs) in their warming responses scaled the coordination to enhance fitness except in the contexts where reproduction was reduced. These findings could help explain and predict plant form and function in a warming world.

Seed Production and 22 Years of Climatic Changes in an Everwet Neotropical Forest

Examining the cues and drivers influencing seed production is crucial to better understand forest resilience to climate change. We explored the effects of five climatic variables on seed production over 22 years in an everwet Amazonian forest, by separating direct effects of these variables from indirect effects mediated through flower production. We observed a decline in seed production over the study period, which was primarily explained by direct effects of rising nighttime temperatures and declining average vapour pressure deficits. Higher daytime temperatures were positively related to seed output, mainly through a flower-mediated effect, while rainfall effects on seed production were more nuanced, showing either positive or negative relationships depending on the seasonal timing of rains. If these trends continue, they are likely to lead to significant changes in forest dynamics, potentially impacting both forest structure and species composition.

Tree carbon allocation to root exudates: implications for carbon budgets, soil sequestration and drought response

Root carbon (C) exudation plays a central role in nutrient acquisition, microbially mediated organic matter decomposition and many other critical ecosystem processes. While it is well known that roots respond strongly to belowground resources, we have a limited quantitative understanding about C allocation to exudates and its fate in soil under changing water availability. This review synthesizes the importance of exudate C fluxes, summarizes studies quantifying mass-specific exudation rate (SER), total exudation rate (TER) and root exudate fraction (REF; the proportion of TER in a plant's C allocation), examines drought effects and highlights key research priorities to advance the understanding of C allocation to exudates in forest ecosystems. On average, SER is often <1 mg C gdry root-1 day-1, TER is 3.8 Pg C year-1 and REF varies between 1 and 17% of net primary production. Spatiotemporal variations in exudation, including seasonal and daily patterns and subsoil exudation, remain critical knowledge gaps. We show that many studies report a 1.2- to 11-fold increase in SER and REF in response to drought. However, TER often remains unchanged, suggesting that absolute exudate C inputs to the soil may stay constant under drought conditions. Disentangling the individual impacts of soil and air drought as well as drought legacy impacts on ecosystem C dynamics are overlooked aspects. By estimating the differences in rhizosphere formation and exudation across various forest biomes, we find that exudate-affected soil volumes are highest in tropical forests and lowest in boreal forests. While current research emphasizes significant C allocation from the canopy to soil via exudates, understanding exudation dynamics and biome-specific responses to drought by using standardized protocols is essential. Expanding these insights is critical for comprehending the role of root exudates in soil organic matter formation, ecosystem resilience and adaptation to climate change.

Orchard Microclimate Control as a Way to Prevent Kiwifruit Decline Syndrome Onset

A syndrome called "Kiwifruit Decline Syndrome" (KiDS) affects kiwifruit in several Mediterranean areas, causing growth arrest and wilt that rapidly progress to desiccation, scarce root growth, absence of fibrous roots, brown soft-rotting areas, and cortical detachment from the central cylinder. The origin is considered multifactorial, and a correlation with hydraulic conductance impairment caused by a high vapor pressure deficit (VPD) and temperature was detected. In this work, over-tree micro-sprinkler irrigation and shading nets were tested to protect leaves from overheating and locally decrease VPD. Leaf gas exchanges, leaf temperature, stem water potential, stem growth, root starch content, root xylem vessel diameter, density, and vulnerability to cavitation were assessed. A positive effect of over-tree irrigation associated with shading was observed: lower leaf temperature, higher stem water potential, stomatal conductance, and photosynthesis were detected; moreover, root starch content was higher in the summer. Narrow xylem vessel diameters were observed, indicating a long-term adaptation to rising VPD for lower vulnerability to cavitation, in all plants, but higher diameter, lower density, and higher vulnerability index indicated lower plant water stress under over-tree irrigation associated with shading. These results indicate that microclimate control by proper agronomic management can protect kiwifruit from climate stress, decreasing the risk of KiDS onset.

Ash dieback and hydrology affect tree growth patterns under climate change in European floodplain forests

Floodplain forests are currently undergoing substantial reorganization processes due to the combined effects of management-induced altered hydrological conditions, climate change and novel invasive pathogens. Nowadays, the ash dieback is one of the most concerning diseases affecting European floodplain forests, causing substantial tree mortality and threatening the loss of the dominant key tree species of the hardwood floodplain forest, Fraxinus excelsior. Understanding how the increased light availability caused by pathogen-driven mortality in combination with altered hydrological conditions and climate change affects growth responses in a diverse forest community is of crucial importance for conservation efforts. Thus, we examined growth of the main tree species in response to ash dieback and how it depended on altered hydrological conditions under novel climatic conditions for the lower and upper canopy in the floodplain forest of Leipzig, Germany. Our study period encompassed the consecutive drought years from 2018 to 2020. We found that tree growth responded mostly positively to increased light availability, but only on moist sites, while tree growth largely declined on dry sites, suggesting that water availability is a critical factor for tree species to be able to benefit from increased light availability due to canopy disturbances caused by ash dieback. This hydrological effect was species-specific in the lower canopy but not in the upper canopy. While, in the lower canopy, some species such as the competitive shade-tolerant but flood-intolerant Acer pseudoplatanus and Acer platanoides benefited from ash dieback on moist sites, others were less affected or suffered disproportionally, indicating that floodplain forests might turn into a novel ecosystem dominated by competitive Acer species, which may have detrimental effects on ecosystem functioning. Our results give hints on floodplain forests of the future and have important implications for conservation measures, suggesting that a substantial revitalization of natural hydrological dynamics is important to maintain a tree composition that resembles the existing one and thus sustain their conservation status.

Effectiveness of drone-based thermal sensors in optimizing controlled environment agriculture performance under arid conditions

Controlled environmental agriculture (CEA), integrated with internet of things and wireless sensor network (WSN) technologies, offers advanced tools for real-time monitoring and assessment of microclimate and plant health/stress. Drone applications have emerged as transformative technology with significant potential for CEA. However, adoption and practical implementation of such technologies remain limited, particularly in arid regions. Despite their advantages in agriculture, drones have yet to gain widespread utilization in CEA systems. This study investigates the effectiveness of drone-based thermal imaging (DBTI) in optimizing CEA performance and monitoring plant health under arid conditions. Several WSN sensors were deployed to track microclimatic variations within the CEA environment. A novel method was developed for assessing canopy temperature (Tc) using thermocouples and DBTI. The crop water stress index (CWSI) was computed based on Tc extracted from DBTI. Findings revealed that DBTI effectively distinguished between all treatments, with Tc detection exhibiting a strong correlation (R2 = 0.959) with sensor-based measurements. Results confirmed a direct relationship between CWSI and Tc, as well as a significant association between soil moisture content and CWSI. This research demonstrates that DBTI can enhance irrigation scheduling accuracy and provide precise evapotranspiration (ETc) estimates at specific spatiotemporal scales, contributing to improved water and food security.

Individual Versus Combined Effects of Warming, Elevated CO2 and Drought on Grassland Water Uptake and Fine Root Traits

Increasing warming, atmospheric CO2 and drought are expected to change the water dynamics of terrestrial ecosystems. Yet, limited knowledge exists about how the interactive effects of these factors will affect grassland water uptake, and whether adaptations in fine root production and traits will alter water uptake capacity. In a managed C3 grassland, we tested the individual and combined effects of warming (+3°C), elevated CO2 (eCO2; +300 ppm) and drought on root water uptake (RWU) as well as on fine root production, trait adaptation, and fine root-to-shoot production ratios, and their relationships with RWU capacity. High temperatures, amplified by warming, exacerbated RWU reductions under drought, with negligible water-sparing effects from eCO2. Drought, both under current and future (warming, eCO2) climatic conditions, shifted RWU towards deeper soil layers. Overall, RWU capacity related positively to fine root production and specific root length (SRL), and negatively to mean root diameters. Warming effects on traits (reduced SRL, increased diameter) and the ratio of fine root-to-shoot production (increased) were offset by eCO2. We conclude that under warmer future conditions, irrespective of shifts in water sourcing, it is particularly hot droughts that will lead to increasingly severe restrictions of grassland water dynamics.

The Response of Carbon Uptake to Soil Moisture Stress: Adaptation to Climatic Aridity

The coupling between carbon uptake and water loss through stomata implies that gross primary production (GPP) can be limited by soil water availability through reduced leaf area and/or stomatal conductance. Ecosystem and land-surface models commonly assume that GPP is highest under well-watered conditions and apply a stress function to reduce GPP as soil moisture declines. Optimality considerations, however, suggest that the stress function should depend on climatic aridity: ecosystems adapted to more arid climates should use water more conservatively when soil moisture is high, but maintain unchanged GPP down to a lower critical soil-moisture threshold. We use eddy-covariance flux data to test this hypothesis. We investigate how the light-use efficiency (LUE) of GPP depends on soil moisture across ecosystems representing a wide range of climatic aridity. 'Well-watered' GPP is estimated using the sub-daily P model, a first-principles LUE model driven by atmospheric data and remotely sensed vegetation cover. Breakpoint regression is used to relate daily β(θ) (the ratio of flux data-derived GPP to modelled well-watered GPP) to soil moisture estimated via a generic water balance model. The resulting piecewise function describing β(θ) varies with aridity, as hypothesised. Unstressed LUE, even when soil moisture is high, declines with increasing aridity index (AI). So does the critical soil-moisture threshold. Moreover, for any AI value, there exists a soil moisture level at which β(θ) is maximised. This level declines as AI increases. This behaviour is captured by universal non-linear functions relating both unstressed LUE and the critical soil-moisture threshold to AI. Applying these aridity-based functions to predict the site-level response of LUE to soil moisture substantially improves GPP simulation under both water-stressed and unstressed conditions, suggesting a route towards a robust, universal model representation of the effects of low soil moisture on leaf-level photosynthesis.

Decoupling of Tree-Ring Cellulose δ18O and δ2H Highlighted by Their Contrasting Relationships to Climate and Tree Intrinsic Variables

Oxygen (δ18O) and hydrogen (δ2H) stable isotope ratios are tightly coupled in precipitation and, albeit damped, in leaf water, but are often decoupled in tree-ring cellulose. The environmental and physiological conditions in which this decoupling occurs are not yet well understood. We investigated the relationships between δ18O and δ2H and tree-ring width (TRW), tree crown volume, tree age and climate in silver fir and Douglas-fir and found substantial differences between δ18O and δ2H. Overall, δ18O-δ2H correlations were weak to absent but became significantly negative under high summer vapour pressure deficit (VPD). δ18O and δ2H had positive and negative nonlinear relationships with TRW, respectively, with clear relationships at the site and tree levels for silver fir and, to a lesser extent, for Douglas-fir. Age trends for silver fir were weakly negative in δ18O but positive in δ2H. Tree crown volume and δ18O or δ2H had no significant relationships. Most strikingly, δ18O strongly depended on spring climate (precipitation and VPD), whereas δ2H depended on summer climate (temperature and VPD) for both species. Our study shows that the δ18O-δ2H decoupling in tree-ring cellulose in two temperate conifer species could be highlighted by their contrasting relationships to climate and tree intrinsic variables (TRW, age).

Warming Mitigates Ozone Damage to Wheat Photosynthesis in a FACE Experiment

Individual effects of elevated ozone (O3) and warming on wheat (Triticum aestivum L.) are well documented, their combined effects remain poorly understood. In the present study, we investigated the combined impacts of elevated O3 (1.5× ambient O3) and rising canopy temperature (+2°C) on the photosynthesis of wheat leaves in an open-air field experiment. We found that O3-induced oxidative stress reduced the biochemical capacity and inhibited leaf photosynthesis at the end of the grain-filling stage. Night-time warming (NW) increased leaf photosynthesis during the vegetative stage, but whole-day warming (WW) did not. Both WW and NW accelerated wheat development and decreased photosynthesis at the end of the reproductive stage. Neither elevated O3 nor warming stimulated antioxidant enzymes. Significant interaction between O3 and WW indicated that WW mitigated the adverse effect of O3 on leaf photosynthesis. Compared to NW, WW significantly increased daytime canopy temperature and canopy-to-air vapour pressure deficit across O3 treatments. Decreases in leaf water content and increases in grain oxygen isotope discrimination under warming suggested a link of WW-induced protection against O3 stress in photosynthesis with declines in stomatal O3 uptake rather than increases in the antioxidant capacity. Our results indicate the need to consider the warming-induced mitigation of O3 stress on leaf photosynthesis when predicting the effects of elevated O3 on crop growth under warmer climate in the future.

MaxEnt Modeling and Effects of Climate Change on Shifts in Habitat Suitability for Sorbus alnifolia in China

Anthropogenic climate change stands out as one of the primary forces expected to reshape Earth's ecosystems and global biodiversity in the coming decades. Sorbus alnifolia, which occurs in deciduous forests, is valued for its ornamental appeal and practical uses but is reported to be declining in the wild. Nevertheless, the distribution of this species' suitable range, along with the key ecological and environmental drivers that shape its habitat suitability, remains largely unknown. By analyzing 198 occurrence records and 54 environmental factors, we employed MaxEnt to project S. alnifolia's current and future habitat suitability. Our results showed that annual precipitation (37.4%), normalized difference vegetation index (30.0%), August water vapor pressure (20.8%), and temperature annual range (3.4%) were the most significant variables explaining S. alnifolia's environmental requirements. The suitable habitats were primarily scattered across eastern and central China. Under projected future climatic conditions, the total expanse of potential habitat is expected to increase. However, most of this expansion involves low-suitability habitats, whereas moderately and highly suitable habitats are likely to shrink, especially in southern and lower-altitude regions of China. Based on these findings, we propose several conservation strategies to support the long-term sustainability of S. alnifolia.

A high resolution, gridded product for vapor pressure deficit using Daymet

Vapor pressure deficit (VPD) is a critical variable in assessing drought conditions and evaluating plant water stress. Gridded products of global and regional VPD are not freely available from satellite remote sensing, model reanalysis, or ground observation datasets. We present two versions of the first gridded VPD product for the Continental US and parts of Northern Mexico and Southern Canada (CONUS+) at a 1 km spatial resolution and daily time step. We derived VPD from Daymet maximum daily temperature and average daily vapor pressure and scale the estimates based on (1) climate determined by the Köppen-Geiger classifications and (2) land cover determined by the International Geosphere-Biosphere Programme. Ground-based VPD data from 253 AmeriFlux sites representing different climate and land cover classifications were used to improve the Daymet-derived VPD estimates for every pixel in the CONUS+ grid to produce the final datasets. We evaluated the Daymet-derived VPD against independent observations and reanalysis data. The CONUS+ VPD datasets will aid in investigating disturbances including drought and wildfire, and informing land management strategies.

Seasonal spatial-temporal trends of vegetation recovery in burned areas across Africa

Africa is entering a new fire paradigm, with climate change and increasing anthropogenic pressure shifting the patterns of frequency and severity. Thus, it is crucial to use available information and technologies to understand vegetation dynamics during the post-fire recovery processes. The main objective of this study was to evaluate the seasonal spatio-temporal trends of vegetation recovery in response to fires across Africa, from 2001 to 2020. Non-parametric tests were used to analyze MODIS Normalized Difference Vegetation Index (NDVI) products comparing the following three-month seasonal periods: December-February (DJF), March-May (MAM), June-August (JJA), and September-November (SON). We evaluated the seasonal spatial trends of NDVI in burned areas by hemisphere, territory, or country, and by land cover types, and fire recurrences, with a focus on forested areas. The relationships between the seasonal spatial trend and three climatic variables (i.e. maximum air temperature, precipitation, and vapor pressure deficit) were then analyzed. For the 8.7 million km2 burned in Africa over the past 22 years, we observed several seasonal spatial trends of NDVI. The highest proportions of areas with increasing trend (p < 0.05) was recorded in MAM for both hemispheres, with 22.0% in the Northern Hemisphere and 17.4% in the Southern Hemisphere. In contrast, areas with decreasing trends (p < 0.05), showed 4.8-5.5% of burned area in the Northern Hemisphere, peaking in JJA, while the Southern Hemisphere showed a range of 7.1 to 10.9% with the highest proportion also in JJA. Regarding land cover types, 48.0% of fires occurred in forests, 24.1% in shrublands, 16.6% in agricultural fields, and 8.9% in grasslands/savannas. Consistent with the overall trend, the area exhibiting an increasing trend in NDVI values (p < 0.05) within forested regions had the highest proportion in MAM, with 19.9% in the Northern Hemisphere and 20.6% in the Southern Hemisphere. Conversely, the largest decreasing trend (p < 0.05) was observed in DJF in the Northern Hemisphere (2.7-2.9%) and in JJA in the Southern Hemisphere (7.2-10.4%). Seasonally, we found a high variability of regeneration trends of forested areas based on fire recurrences. In addition, we found that of the three climatic variables, increasing vapor pressure deficit values were more related to decreasing NDVI levels. These results indicate a strong component of seasonality with respect to fires, trends of vegetation increase or decrease in the different vegetation covers of the African continent, and they contribute to the understanding of climatic conditions that contribute to vegetation recovery. This information is helpful for researchers and decision makers to act on specific sites during restoration processes.

Relationship between wind speed and plant hydraulics at the global scale

Wind is an important ecological factor for plants as it can increase evapotranspiration and cause dehydration. However, the impact of wind on plant hydraulics at a global scale remains unclear. Here we compiled plant key hydraulic traits, including water potential at 50% loss of hydraulic conductivity (P50), xylem-specific hydraulic conductivity (KS), leaf area to sapwood area ratio (AL/AS) and conduit diameter (D) with 2,786 species-at-site combinations across 1,922 woody species at 469 sites worldwide and analysed their correlations with wind speed. Even with other climatic factors controlled (for example, moisture index, temperature and vapour pressure deficit), wind speed clearly affected plant hydraulics; for example, on average, species from windier sites constructed sapwood with smaller D and lower KS that was more resilient to drought (more negative P50), deploying less leaf total area for a given sapwood cross-section. Species with these traits may be at an advantage under future climates with higher wind speeds.

Acclimation of functional traits leads to biomass increases in leafy green species grown in aquaponics

As human population size continues to increase and climate change effects worsen, future food security has become a primary concern for agricultural industries worldwide. Yields of traditional agricultural methods are commonly limited by water and nutrient availability and many crop yields are predicted to decline. Alternative farming practices like aquaponics, which can alleviate these negative yield pressures, may become critical to reaching food production targets. Aquaponics approaches involve the cyclic joint production of fish and hydroponic plants where the fish efflux provides nutrients to plants that then purify the water to be recycled to the fish tanks. In this study, we investigated the acclimation of physiology and functional traits of plants grown in aquaponics versus soil for three leafy green species. We compared gas exchange, stomatal anatomy, water-use efficiency, and foliar chemistry on newly formed leaves across weekly measurements. Increased photosynthetic rate, driven by higher stomatal conductance and increases in tissue nitrogen, led to higher biomass production in aquaponics for all species. Aquaponics plants adjusted stomatal behavior and to a lesser degree stomatal anatomy to become less water-use efficient than plants grown in soil. Collectively, our findings demonstrate the ability of plants to acclimate quickly to aquaponics growing systems that largely remove water and nutrient limitations to plant growth. The increased biomass production of broccoli, pak choi, and salanova by 185%, 116%, and 362% in aquaponics compared to soil-grown plants demonstrates the potential of small-scale aquaponics systems as an efficient and sustainable alternative farming practice.

Leaf warming in the canopy of mature tropical trees reduced photosynthesis due to downregulation of photosynthetic capacity and reduced stomatal conductance

Tropical forests play a large role in the global carbon cycle by annually absorbing 30% of our annual carbon emissions. However, these forests have evolved under relatively stable temperature conditions and may be sensitive to current climate warming. Few experiments have investigated the effects of warming on large, mature trees to better understand how higher temperatures affect these forests in situ. We targeted four tree species (Endiandra microneura, Castanospermum australe, Cleistanthus myrianthus and Myristica globosa) of the Australian tropical rainforest and warmed leaves in the canopy by 4°C for 8 months. We measured temperature response curves of photosynthesis and respiration, and determined the critical temperatures for chloroplast function based on Chl fluorescence. Both stomatal conductance and photosynthesis were strongly reduced by 48 and 35%, respectively, with warming. While reduced stomatal conductance was likely in response to higher vapour pressure deficit, the biochemistry of photosynthesis responded to higher temperatures via reduced Vcmax25 (-28%) and Jmax25 (-29%). There was no shift of the Topt of photosynthesis. Concurrently, respiration rates at a common temperature did not change in response to warming, suggesting limited respiratory thermal acclimation. This combination of physiological responses to leaf warming in mature tropical trees may suggest a reduced carbon sink with future warming in tropical forests.

New insights into the mechanisms of plant isotope fractionation from combined analysis of intramolecular 13C and deuterium abundances in Pinus nigra tree-ring glucose

Understanding isotope fractionation mechanisms is fundamental for analyses of plant ecophysiology and paleoclimate based on tree-ring isotope data. To gain new insights into isotope fractionation, we analysed intramolecular 13C discrimination in tree-ring glucose (Δi', i = C-1 to C-6) and metabolic deuterium fractionation at H1 and H2 (εmet) combinedly. This dual-isotope approach was used for isotope-signal deconvolution. We found evidence for metabolic processes affecting Δ1' and Δ3', which respond to air vapour pressure deficit (VPD), and processes affecting Δ1', Δ2', and εmet, which respond to precipitation but not VPD. These relationships exhibit change points dividing a period of homeostasis (1961-1980) from a period of metabolic adjustment (1983-1995). Homeostasis may result from sufficient groundwater availability. Additionally, we found Δ5' and Δ6' relationships with radiation and temperature, which are temporally stable and consistent with previously proposed isotope fractionation mechanisms. Based on the multitude of climate covariables, intramolecular carbon isotope analysis has a remarkable potential for climate reconstruction. While isotope fractionation beyond leaves is currently considered to be constant, we propose significant parts of the carbon and hydrogen isotope variation in tree-ring glucose originate in stems (precipitation-dependent signals). As basis for follow-up studies, we propose mechanisms introducing Δ1', Δ2', Δ3', and εmet variability.

Effect of increased air temperature and vapour pressure deficit on water relations, gas exchange, and stem increment in saplings of Norway spruce (Picea abies)

Whilst temperature (T ) increase on tree function has been well studied, the associated effect of vapour pressure deficit (VPD) is less clear. We investigated the impact of increasing T and VPD on canopy transpiration rate (E ), shoot gas exchange, and stem growth in Norway spruce (Picea abies ) saplings grown in organic and mineral soils in climate chambers with three treatment conditions for 12weeks: (1) 'ambient' (VPD≈0.5kPa); (2) 'highT' treatment (+3°C relative to ambient; VPD≈0.6kPa); and (3) 'highT/lowRH' treatment (+3°C and -7% RH relative to ambient; VPD≈0.8kPa). The stem diameter increment, assimilation rate (A ), and E were highest, and the needle-to-fine root biomass ratio was smallest in 'highT/lowRH' treatment (P A of trees grown in organic soil was higher (P <0.05) in 'highT/lowRH' treatment compared to ambient conditions, but no significant difference was found in mineral soil. Our findings indicate that the effect of a 3-°C temperature increase on spruce was marginal under well-watered conditions, and moderate VPD increase instead improved the tree's functioning. Thus, aside from temperature, the impact of the RH as a primary driver of the VPD should be considered when predicting spruce response to global warming.

Greening but enhanced vegetation water stress in the Yellow River Basin: A holistic perspective

The Yellow River Basin (YRB) has emerged as a focal point of global vegetation greening due to climate change and human activities. Given its ecological vulnerability and intense human activities, environmental sustainability has become an urgent concern for scholars. Current research on the hydrological effects of vegetation greening, from a reductionist perspective, still struggle to answer the crucial question that whether vegetation water stress is increasing or decreasing. Towards that, we adopt a holistic perspective to explore the relationships between monthly vegetation dynamics and multiple water stress indicators in the YRB from 1982 to 2018. Using statistical methods and the random forest model, we revealed that both gross primary productivity and water use efficiency showed an increasing trend, with rates of 5.83 g Cm-2 and 0.01 g Cmm-1m-2 per year, respectively. We identified that with increasing climatic aridity, the water stress factors for vegetation transition from monthly scale water conditions (vapor pressure deficit, VPD) to 1-2 months scale (soil water content, SWC) and seasonal scale (standardized precipitation evapotranspiration index-3, SPEI-3) water balance status. And with an aridity index of 0.35 as the threshold, the response of vegetation to water stress factors exhibits marked spatial differentiation. Furthermore, since 2000, despite a persistent greening trend in the YRB, there has been a noticeable expansion in the spatial range of intermediate and long-term water stress factors (SWC, SPEI-3), indicating an enhancing vegetation water stress. This suggests that a serious attention should be paid to the future ecological security of the YRB under the intensified climate change.

Dual controls of vapour pressure deficit and soil moisture on photosynthesis in a restored temperate bog

Despite only covering ~3 % of the land mass, peatlands store more carbon (C) per unit area than any other ecosystem. This is due to the discrepancy between C fixed by the plants (Gross primary productivity (GPP)) and decomposition. However, this C is vulnerable to frequent, severe droughts and changes in the peatland microclimate. Plants play a vital role in ecosystem C dynamics under drought by mediating water loss to the atmosphere (surface water vapour conductance) and GPP by the presence/absence of stomatal regulation. This is dependent on soil moisture, air temperature, and vapour pressure deficit (VPD). Although there is ample evidence of the role of VPD on stomatal regulation and GPP, the impact of soil moisture is still debated. We addressed this knowledge gap by investigating the role of bulk surface conductance of water vapour in shifts between climatic (Air temperature (Tair), incoming shortwave radiation (SWR) and VPD) and water limitation of GPP in a peat bog in Canada. A causal analysis process was used to investigate how environmental factors influenced GPP. The results suggested that stomatal regulation in response to increased VPD caused the reduction in GPP in 2016 (~2.5 gC m-2 day-1 as opposed to ~3 gC m-2 day-1 in 2018). In contrast, GPP was limited again in 2019 due to the dry surface. This was driven by the relaxed stomatal regulation adopted by the ecosystem following the initial drought to maximise C assimilation. We found the threshold at which surface water decline limited GPP was at about -8 cm water table depth (82.5 % soil moisture). The causal inference corroborated our findings. The temporal variations of water and energy limitation seen in this study could increasingly restrict GPP due to the projected climate warming.

Combined Impacts of Climate and Tree Physiology on Mercury Accumulation in Tropical and Subtropical Foliage and Robust Model Parametrization

Atmospheric elemental mercury (Hg0) assimilation by foliage contributes prevalently to the global atmospheric Hg0 sink in forests. Today, little is known about the mechanisms of foliar Hg accumulation and how climate factors and tree physiology interact to impact it. Here, we examined meteorological factors, foliar physiological traits, and Hg accumulation rates from leaf emergence to senescence in a tropical rainforest, tropical savanna, and subtropical evergreen broadleaf forest. Also, robust models for foliar Hg accumulation were parametrized. Generally, foliar Hg accumulation rate in subtropical evergreen forest was highest (16.4 ± 12.3 ng m-2 day-1), followed by the tropical rainforest (14.2 ± 9.8 ng m-2 day-1), and lowest in the tropical savanna (4.7 ± 4.9 ng m-2 day-1). Atmospheric relative humidity, stomatal conductance, and leaf photosynthesis are key drivers of spatial-temporal variations in foliar Hg accumulation. The canopy-structure-induced specific leaf physiological traits drive temporal variations in foliar Hg accumulation, and climate-controlled leaf physiological traits account for spatial variations among three forests. Finally, our robust models enable precise simulation of foliar Hg accumulation rates at both tree species and ecosystem scales facilitating particularly regional and global Hg transport and chemical models to quantify the vegetation's role as a sink for atmospheric Hg0 uptake.

Drought in a warmer, CO2-rich climate restricts grassland water use and soil water mixing

Soil water sustains terrestrial life, yet its fate is uncertain under a changing climate. We conducted a deuterium labeling experiment to determine whether elevated atmospheric carbon dioxide (CO2), warming, and drought impact soil water storage and transport in a temperate grassland. Elevated CO2 created a wetter rootzone compared with ambient conditions, whereas warming decreased soil moisture. Soil water remained well mixed in all global change treatments except for summer drought combined with warming and elevated CO2. These combined treatments caused the grassland to conserve water and restricted soil water flow to large, rapidly draining pores without mixing with small, slowly draining pores. Our results suggest that drought in a warmer, more CO2-rich climate can severely alter grassland ecohydrology by constraining postdrought soil water flow and grassland water use.

Short-term impact of low air pressure on plants' functional traits

Lower atmospheric pressure affects biologically relevant physical parameters such as gas partial pressure and concentration, leading to increased water vapor diffusivity and greater soil water content loss through evapotranspiration. This might impact plant photosynthetic activity, resource allocation, water relations, and growth. However, the direct impact of low air pressure on plant physiology is largely unknown. This study examined the effects of low air pressure, alone and combined with two water inputs, on different functional traits of three plant species transplanted from montane grasslands at 1,500 m a.s.l. during the first four weeks of their early phenological stage: Trifolium pratense, Hieracium pilosella, and Brachypodium rupestre. Using the terraXcube Ecotron facility which can simulate different climatic conditions, we isolated the effect of air pressure from those of other, related environmental factors (temperature, humidity, and solar radiation) by simulating three different elevations with corresponding air pressures: 1,500 m a.s.l. (85 kPa, control scenario), 2,500 m a.s.l. (75 kPa), and 4,000 m a.s.l. (62 kPa) and we used two different water regimes to observe the combined effect of low air pressure and the impact of varying water inputs on plants. In T. pratense and H. pilosella, we observed an increase in stomatal conductance but a reduction in aboveground biomass at the lowest pressure compared to the control scenario after four weeks of incubation. Contrastingly, B. rupestre showed an interactive effect of air pressure and water treatment on chlorophyll and biomass nitrogen content, which were reduced under higher soil water conditions at 85kPa. This study serves as an initial step in isolating the specific impact of air pressure on plant physiology, demonstrating the potential of the facility for future research. The mixed response patterns across species highlight that atmospheric pressure could be a driving factor to consider when assessing plant responses along elevational gradient.

Response mechanism of water status and photosynthetic characteristics of Cotoneaster multiflorus under drought stress and rehydrated conditions

Introduction

Plant physiology response and adaptation to drought stress has become a hotspot in plant ecology and evolution. Cotoneaster multiflorus possesses high ecological, ornamental and economic benefits. It has large root system and tolerance to cold, drought and poor soil. Therefore, C. multiflorus is considered as one of the most important tree species for ecological restoration in arid and semi-arid areas. However, little is known about the physiological mechanisms, molecular mechanisms and drought strategies of how C. multiflorus responds to drought stress. Therefore, exploring the physiological response mechanisms, molecular mechanisms and adaptive strategies of C. multiflorus in response to drought is important for its growth in arid and semi-arid regions.

Methods

We investigated the response and coupling mechanisms of water status, photosynthetic properties and chloroplast fluorescence parameters in C. multiflorus in response to drought and rehydrated after drought, especially the importance of nocturnal sap flow and nocturnal water refilling to maintain its own water balance in response to drought stress. In addition, we studied the stress response of C. multiflorus transcriptome factors, and we also discussed drought adaptation strategies of C. multiflorus.

Results

C. multiflorus adapted to drought stress by a series of structural and physiological mechanisms, such as promoting closing stomata, increasing nocturnal sap flow. When rehydrated after undergoing severe drought stress, its physiological activities such as photosynthesis, water status, chlorophyll fluorescence parameters and other physiological activities have rapidly resumed. This showed C. multiflorus had strong tolerance to drought. In addition, water status, photosynthetic characteristics, and chloroplast fluorescence parameters of C. multiflorus were highly coupled. Nocturnal sap flow and nocturnal water refilling were very important for C. multiflorus to maintain its own water balance in response to drought stress. Finally, C. multiflorus will strengthen the drought defense mechanism by gene regulation of various metabolisms, such as promoting stomatal closure, reducing transpiration water loss, and vigorously regulating water balance. C. multiflorus responded to drought stress by avoiding or reducing water deficit in plant organs and tissues. Therefore, the shrub C. multiflorus is a drought-tolerant plant.

Discussion

We explored the response mechanisms of water status, photosynthetic characteristics, and chloroplast fluorescence parameters of C. multiflorus in drought and rehydrated after drought stress, especially the response mechanisms of nocturnal sap flow and nocturnal water refilling in response to drought stress, and identified the physiological coupling mechanisms, molecular mechanisms and drought types of C. multiflorus in response to drought.

Advancing diurnal analysis of vegetation responses to drought events in the Yangtze River Basin using next-generation satellite data

Extreme climate events, particularly droughts, pose significant threats to vegetation, severely impacting ecosystem functionality and resilience. However, the limited temporal resolution of current satellite data hinders accurate monitoring of vegetation's diurnal responses to these events. To address this challenge, we leveraged the advanced satellite ECOSTRESS, combining its high-resolution evapotranspiration (ET) data with a LightGBM model to generate the hourly continuous ECOSTRESS-based ET (HC-ETECO) for the middle and lower reaches of the Yangtze River Basin (YRB) from 2015 to 2022. This dataset showed strong agreement with both ground-based and satellite observations. Utilizing the SPEI, we identified the significant drought period: September to November 2019 and August to September 2022. By integrating hourly Solar-Induced Chlorophyll Fluorescence (SIF) data, we observed that during drought period, the typical afternoon peak in SIF was absent. In contrast to non-drought period, morning photosynthesis and SIF-based Water Use Efficiency (WUESIF) anomalies were primarily driven by high Vapor Pressure Deficit (VPD), while the afternoon reductions were influenced by both high VPD and low Soil Moisture (SM) as the drought progressed. Our simulated HC-ETECO data revealed that ET in the middle and lower reaches of the YRB was consistently lower than normal during drought period. Attribution analysis indicated that this reduction was primarily driven by midday temperature increases and high VPD, suggesting that vegetation in the region copes with drought stress predominantly by limiting water loss. These findings highlight the utility of the generated high-resolution ET dataset in advancing our understanding of vegetation dynamics under drought climate conditions. This work provides critical insights for enhancing climate adaptation strategies and enhancing ecosystem management practices in the face of increasing climate variability.

Species-specific responses of canopy greenness to the extreme droughts of 2018 and 2022 for four abundant tree species in Germany

Germany experienced extreme drought periods in 2018 and 2022, which significantly affected forests. These drought periods were natural experiments, providing valuable insights into how different tree species respond to drought. The quantification of species-specific drought responses may help to identify the most climate-change-resilient tree species, thereby informing effective forest regeneration strategies. In this study, we used remotely sensed peak-season canopy greenness as a proxy for tree vitality to estimate the drought response of four widely abundant tree species in Germany (oak, beech, spruce, and pine). We focused on two questions: (1) How were the four tree species affected by these two droughts? (2) Which environmental parameters primarily determined canopy greenness? To address these questions, we combined a recently published tree species classification map with remotely sensed canopy greenness and environmental variables related to plant available water capacity (PAWC) and atmospheric vapor pressure deficit (VPD). Our results indicate that the more isohydric species featured a greater decline in canopy greenness under these droughts compared to the more anisohydric species despite similar soil moisture conditions. Based on spatial lag models, we found that the influence of PAWC on canopy greenness increases with increasing isohydricity while the influence of VPD decreases. Our statistical analysis indicates that oak was the only species with significantly higher canopy greenness in 2022 compared to 2018. Yet, all species are likely to be susceptible to accumulated drought effects, such as insufficient recovery time and increased vulnerability to biotic pathogens, in the coming years. Our study provides critical insights into the diverse responses of different tree species to changing environments over a large environmental gradient in Central Europe and sheds light on the complex interactions between soil moisture, climate variables, and canopy greenness. These findings contribute to understanding forests' climate-change resilience and may guide forest management and conservation strategies.

Light intensity moderates photosynthesis by optimizing photosystem mechanisms under high VPD stress

In recent decades, the global increase in vapor pressure deficit (VPD) has significantly inhibited plant growth and photosynthesis. Light intensity, a crucial environmental regulator, plays a vital role in stress response and photosynthetic adjustment. This study investigated whether increasing light intensity under high VPD conditions could optimise the photosystem and thereby enhance photosynthesis. We designed experiments using factorial combinations of two VPD levels (HVPD; high VPD, AVPD; appropriate VPD) and two irradiance gradients (L300; 300 μmol photons m-2 s-1, L600; 600 μmol photons m-2 s-1). Under high VPD, plants protect their photosystems by reducing light energy absorption and limiting photosynthetic electron flow, which results in reduced photosynthesis. However, when exposed to HVPD + L600, plants exhibited increased light energy absorption, as evidenced by elevated chlorophyll b and carotenoid levels, enhanced response to irradiance, and decreased NPQ and Y(NO). This regimen also enhanced photosynthetic electron transport by increasing the total driving force and plastoquinone pool, consequently improving the photochemical efficiency of the photosystem and ultimately boosting the net photosynthetic rate by 46.9%. This study confirmed that modulating light intensity under high VPD stress can improve photosynthesis by optimizing the photosystem. This novel approach can be utilized to enhance tomato production in arid regions.