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According to classic stomatal optimization theory, plant stomata are regulated to maximize carbon assimilation for a given water loss. A key component of stomatal optimization models is marginal water-use efficiency (mWUE), the ratio of the change of transpiration to the change in carbon assimilation. Although the mWUE is often assumed to be constant, variability of mWUE under changing hydrologic conditions has been reported. However, there has yet to be a consensus on the patterns of mWUE variabilities and their relations with atmospheric aridity. We investigate the dynamics of mWUE in response to vapor pressure deficit (VPD) and aridity index using carbon and water fluxes from 115 eddy covariance towers available from the global database FLUXNET. We demonstrate a non-linear mWUE-VPD relationship at a sub-daily scale in general; mWUE varies substantially at both low and high VPD levels. However, mWUE remains relatively constant within the mid-range of VPD. Despite the highly non-linear relationship between mWUE and VPD, the relationship can be informed by the strong linear relationship between ecosystem-level inherent water-use efficiency (IWUE) and mWUE using the slope, m*. We further identify site-specific m* and its variability with changing site-level aridity across six vegetation types. We suggest accurately representing the relationship between IWUE and VPD using Michaelis–Menten or quadratic functions to ensure precise estimation of mWUE variability for individual sites.

Solar-induced chlorophyll fluorescence (SIF) is widely accepted as a proxy for gross primary productivity (GPP). Among the various SIF measurements, tower-based SIF measurements allow for continuous monitoring of SIF variation at a canopy scale with high temporal resolution, making it suitable for monitoring highly variable plant physiological responses to environmental changes. However, because of the strong and close relationship between SIF and absorbed photosynthetically active radiation (aPAR), it may be difficult to detect the influence of environmental drivers other than light conditions. Among the drivers, atmospheric dryness (vapor pressure deficit, VPD) is projected to increase as drought becomes more frequent and severe in the future, negatively impacting plants. In this study, we evaluated the tower-based high-frequency SIF measurement as a tool for detecting plant response to highly variable VPD. The study was performed in a mixed temperate forest in Virginia, USA, where a 40-m-tall flux tower has been measuring gas and energy exchanges and ancillary environmental drivers, and the Fluospec 2 system has been measuring SIF. We show that a proper definition of light availability to vegetation can reproduce SIF response to changing VPD that is comparable to GPP response as estimated from eddy covariance measurement: GPP decreased with rising VPD regardless of how aPAR was defined, whereas SIF decreased only when aPAR was defined as the PAR absorbed by chlorophyll (aPARchl) or simulated by a model (Soil Canopy Observation, Photochemistry and Energy fluxes, SCOPE). We simulated the effect of VPD on SIF with two different simulation modes of fluorescence emission representing contrasting moisture conditions, ‘Moderate’ and ‘Soil Moisture (SM) Stress’ modes. The decreasing SIF to rising VPD was only found in the SM Stress mode, implying that the SIF-VPD relationship depends on soil moisture conditions. Furthermore, we observed a similar response of SIF to VPD at hourly and daily scales, indicating that satellite measurements can be used to study the effects of environmental drivers other than light conditions. Finally, the definition of aPAR emphasizes the importance of canopy structure research to interpret remote sensing observations properly.

Land-atmosphere coupling is a key factor for understanding the mean hydroclimatological conditions in South America. In this study, we analyzed how land-atmosphere coupling modulates the aridity conditions in the South American continent through experiments performed with a regional climate model. We assessed changes in precipitation (P), potential evaporation (PET), aridity index (ɸ = PET / P) and vapor pressure deficit (VPD) in experiments with and without soil moisture - atmosphere coupling. Additionally, we analyzed how air dry enthalpy (Hd), moist enthalpy (Hw) and total enthalpy (Ht) change between the coupled and uncoupled experiments, which help to identify patterns in aridity conditions. When analyzing ɸ, we found opposite responses in central-eastern Brazil and southeastern South America (SESA), the two main hotspots of land-atmosphere coupling. On the other hand, a widespread amplification of VPD is found. Both VPD and enthalpy are locally driven by changes in temperature and water vapor content and reflect wetter conditions in central-eastern Brazil and drier conditions in SESA. Changes in regional circulation, described in previous studies, increase moisture convergence in Brazil and help to explain the response of each individual metric when coupling is enabled. [ABSTRACT FROM AUTHOR]

Compound hot–dry events cause more severe impacts on terrestrial ecosystems than dry events, while the differences in recovery time (ΔRT) between hot–dry and dry events and their contributing factors remain unclear. Both remote sensing observations and eddy covariance measurements reveal that hot–dry events prolong the recovery time compared with dry events, with greater prolongation of recovery time in drylands than in humid regions. Random forest regression modeling demonstrates that the difference in vapor pressure deficit between hot–dry and dry events, with an importance score of 35%, is the major factor contributing to ΔRT. The severity of stomatal restriction exceeds that of non-stomatal limitation, which restricts the vegetation productivity that is necessary for the recovery process. These results emphasize the negative effect of vapor pressure deficit on vegetation recovery during hot–dry events and project an extension of drought recovery time considering elevated vapor pressure deficit in a warming world. [ABSTRACT FROM AUTHOR]

In recent decades, drought has intensified along with continuous global warming, significantly impacting terrestrial vegetation. High atmospheric water demand, indicated by vapor pressure deficit (VPD), and insufficient soil moisture (SM) are considered the primary factors causing drought stress in vegetation. However, the influences of VPD and SM on the autumn phenology are still unknown. Using satellite observations and meteorological data, we examined the impacts of VPD and SM on the end of the growing season (EOS) across the Northern Hemisphere (>30°N) from 1982 to 2022. We found that VPD and SM were as important as temperature, precipitation, and radiation in controlling the variations in the EOS. Moreover, the EOS was predominantly influenced by VPD or SM in more than one-third (33.8%) of the study area. In particular, a ridge regression analysis indicated that the EOS was more sensitive to VPD than to SM and the other climatic factors, with 25% of the pixels showing the highest sensitivity to VPD. In addition, the effects of VPD and SM on the EOS varied among biome types and climate zones. VPD significantly advanced the EOS in 25.8% of temperate grasslands, while SM had the greatest impact on advancing the EOS in 17.7% of temperate coniferous forests. Additionally, 27.7% of the midlatitude steppe (BSk) exhibited a significant negative correlation between VPD and the EOS, while 19.4% of the marine west coast climate (Cfb) showed a positive correlation between SM and the EOS. We also demonstrated that the correlation between VPD and the EOS was linearly affected by VPD and the leaf area index, while the correlation between SM and the EOS was affected by SM, precipitation, and the leaf area index. Our study highlights the importance of VPD and SM in regulating autumn phenology and enhances our understanding of terrestrial ecosystem responses to climate change. [ABSTRACT FROM AUTHOR]

• VPD and R g predominantly drive variations in tree-level transpiration. • Soil moisture availability did not limit tree-level transpiration. • Xylem sap flow exhibited a strong correlation with VPD, R g , and RH. • Significant correlation (P < 0.001) was observed between actual and potential transpiration. • Apple tree water usage was higher in the early season compared to mid- and late seasons. Understanding the environmental drivers and hydraulic dynamics of plants is pivotal for elucidating future water use strategies and refining precise irrigation techniques. In our investigation, we focused on the daily and seasonal fluctuations in sap flow (SF) using compensation heat-pulse techniques on 7-year-old 'Gamhong' apple trees. Predictions of stand transpiration were integrated using the Penman-Monteith grass reference evapotranspiration. Additionally, we measured leaf-level diurnal and seasonal stomatal conductance (g s), leaf water potential (Ψ Leaf), and transpiration rate (E) to explore the relationship between plant-level transpiration and orchard microclimate. Diurnal SF exhibited a pattern similar to global radiation (R g) and vapor pressure deficit (VPD), increasing in the morning with a peak in the afternoon and decreasing towards evening. Seasonal water use varied, with rates of 10.62 ± 0.282 L day-1 in early (May–June), 8.40 ± 0.222 L day-1 in mid- (July–August), and 5.96 ± 0.154 L day-1 in late season (September–October). Plant water consumption was influenced by weather conditions and evaporative demand driven by atmospheric VPD and R g. Clear and sunny days resulted in higher water use (10.23 ± 0.291 L day-1) compared to partially sunny but windy days (9.85 ± 0.32 L day-1) and cloudy/rainy days (5.37 ± 0.176 L day-1). Plant transpiration strongly correlated with VPD (R² = 0.565), R g (R² = 0.616), relative humidity (R ² = 0.623), and wind velocity (u, R ² = 0.684), while no significant correlation was observed with air temperature (T a , R ² = 0.19) or soil moisture status (Ψ soil , R² = 0.19). VPD and R g emerged as the primary drivers, with Ψ soil playing a minor role and not limiting tree-level transpiration. Leaf-level g s , Ψ Leaf , and E displayed similar diurnal trends. In our study, the integrated daily sap flow provided valuable insights into plant-water relations differently across growing seasons and its relationship with reference evapotranspiration (E o). The Penman–Monteith equation, using grass reference evapotranspiration indicated a positive correlation (R ² = 0.764, P < 0.001) between actual and estimated transpiration. However, the slope of the relationship differed significantly from 1.0, suggesting that E o per unit leaf area of grass corresponds to E 288 = E o /2.88 (L m-2 of leaf area d-1), which underestimated evapotranspiration. [ABSTRACT FROM AUTHOR]

Key message: Red cedar chlorophyll fluorescence remains high during the nongrowing season, while transpiration is reduced. Evergreen trees retain their leaves during the winter and thus can photosynthesize when conditions permit. Eastern red cedar (Juniperus virginiana) is widely distributed in the eastern USA and is known to transpire and photosynthesize outside of the growing season. However, most recent work has been done in the Great Plains, which red cedar has recently invaded, while little work has been done in its original range. I hypothesized that red cedar would behave like other conifers from summer drought-free areas and show reduced chlorophyll fluorescence and transpiration during the winter. Four red cedar trees at a site near the Ohio River in Kentucky were equipped with Granier sap flow probes, while solar irradiance, temperature, relative humidity, and soil volumetric water content (VWC) were measured at the site. Dark chlorophyll fluorescence (Fv/Fm) was measured on northern and southern aspects on an approximately weekly basis from early 2019 through mid 2021. High values of Fv/Fm were measured in both growing (April-September) and nongrowing (October-March) seasons; median values in the nongrowing season were 94% of those in the growing season, which did not support my hypothesis. Fv/Fm data were fit to a Michaelis–Menten curve that used minimum temperature of the previous two nights, aspect, and maximum irradiance on the day of measurement taken before fluorescence was measured. Sap flow was explained by maximum daily temperature, vapor pressure deficit (VPD), mean daily irradiance, and VWC. Sap flow in the nongrowing season was 74% of that seen in the growing season, due to lower values of temperature, VPD, and irradiance, supporting my hypothesis. Thus, red cedar remains physiologically active during the nongrowing season. However, it does not appear to behave like other conifers from summer drought-free areas. [ABSTRACT FROM AUTHOR]

Water-use strategies play a crucial role in the adaptive capabilities of mangroves to the saline intertidal conditions, yet the intricacies of daily water-use patterns in mangrove species, which are pivotal for maintaining water balance, remain poorly understood. In this comprehensive study, we aimed to clarify the water use strategies of three co-occurring mangrove species, Avicennia marina , Aegiceras corniculatum and Kandelia obovata , through stem sap flow monitoring, leaf gas exchange and stem diameter change measurements. Our findings revealed that the daily sap flow density of Avicennia and Aegiceras reached the peak about 1 h earlier than that of Kandelia. When transpiration was strong, Kandelia and Aegiceras used stem storage to meet water demand, while Avicennia synchronized stem water storage. These three mangrove species adopted cross-peak water used and unique stem water storage to regulate their water balance. In Kandelia , the daily sap flow in per sapwood area was significantly lower, while water-use efficiency was significantly higher than those of Avicennia and Aegiceras , indicating that Kandelia adopted a more conservative and efficient water-use strategy. Sap flow in Avicennia was the most sensitive to environmental changes, while Kandelia limited water dissipation by tightly controlling stomata. Meteorological factors (photosynthetically active radiation, vapor pressure deficit and air temperature) were the main driving factors of sap flow. The increase of soil temperature can promote the water use of mangrove species, while the increase of salinity resulted in more conservative water use. Our results highlight the diversity of daily water-use strategies among the three co-occurring mangrove species, pinpointing Kandelia as the most adaptive at navigating the changing conditions of intertidal habitats in the future climate. In conclusion, our findings provide a mesoscale perspective on water-use characteristics of mangroves and also provides theoretical basis for mangroves afforestation and ecological restoration. [ABSTRACT FROM AUTHOR]

Monitoring the changes of ecosystem functioning is pivotal for understanding the global carbon cycle. Despite its size and contribution to the global carbon cycle, Africa is largely understudied in regard to ongoing changes of its ecosystem functioning and their responses to climate change. One of the reasons is the lack of long‐term in situ data. Here, we use eddy covariance to quantify the net ecosystem exchange (NEE) and its components—gross primary production (GPP) and ecosystem respiration (Reco) for years 2010–2022 for a Sahelian semiarid savanna to study trends in the fluxes. Significant negative trends were found for NEE (12.7 ± 2.8 g C m2 year−1), GPP (39.6 ± 7.9 g C m2 year−1), and Reco (32.2 ± 8.9 g C m2 year−1). We found that NEE decreased by 60% over the study period, and this decrease was mainly caused by stronger negative trends in rainy season GPP than in Reco. Additionally, we observed strong increasing trends in vapor pressure deficit, but no trends in rainfall or soil water content. Thus, a proposed explanation for the decrease in carbon sink strength is increasing atmospheric dryness. The warming climate in the Sahel, coupled with increasing evaporative demand, may thus lead to decreased GPP levels across this biome, and lowering its CO2 sequestration. [ABSTRACT FROM AUTHOR]

Tropical forest photosynthesis can decline at high temperatures due to (1) biochemical responses to increasing temperature and (2) stomatal responses to increasing vapor pressure deficit (VPD), which is associated with increasing temperature. It is challenging to disentangle the influence of these two mechanisms on photosynthesis in observations, because temperature and VPD are tightly correlated in tropical forests. Nonetheless, quantifying the relative strength of these two mechanisms is essential for understanding how tropical gross primary production (GPP) will respond to climate change, because increasing atmospheric CO2 concentration may partially offset VPD‐driven stomatal responses, but is not expected to mitigate the effects of temperature‐driven biochemical responses. We used two terrestrial biosphere models to quantify how physiological process assumptions (photosynthetic temperature acclimation and plant hydraulic stress) and functional traits (e.g., maximum xylem conductivity) influence the relative strength of modeled temperature versus VPD effects on light‐saturated GPP at an Amazonian forest site, a seasonally dry tropical forest site, and an experimental tropical forest mesocosm. By simulating idealized climate change scenarios, we quantified the divergence in GPP predictions under model configurations with stronger VPD effects compared with stronger direct temperature effects. Assumptions consistent with stronger direct temperature effects resulted in larger GPP declines under warming, while assumptions consistent with stronger VPD effects resulted in more resilient GPP under warming. Our findings underscore the importance of quantifying the role of direct temperature and indirect VPD effects for projecting the resilience of tropical forests in the future, and demonstrate that the relative strength of temperature versus VPD effects in models is highly sensitive to plant functional parameters and structural assumptions about photosynthetic temperature acclimation and plant hydraulics. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 concentration is elevated globally, which has "CO2 fertilization effects" and potentially improves plant photosynthesis, yield, and productivity. Despite the beneficial effect of CO2 fertilization being modulated by vapor pressure deficit (VPD), the underlying mechanism is highly uncertain. In the present study, the potential roles of hormones in determining CO2 fertilization effects under contrasting high and low VPD conditions were investigated by integrated physiological and transcriptomic analyses. Beneficial CO2 fertilization effects were offset under high VPD conditions and were constrained by plant water stress and photosynthetic CO2 utilization. High VPD induced a large passive water driving force, which disrupted the water balance and consequently caused plant water deficit. Leaf water potential, turgor pressure, and hydraulic conductance declined under high VPD stress. The physiological evidence combined with transcriptomic analyses demonstrated that abscisic acid (ABA) and jasmonic acid (JA) potentially acted as drought-signaling molecules in response to high VPD stress. Increased foliar ABA and JA content triggered stomatal closure to prevent excessive water loss under high VPD stress, which simultaneously increased the diffusion resistance for CO2 uptake from atmosphere to leaf intercellular space. High VPD also significantly increased mesophyll resistance for CO2 transport from stomatal cavity to fixation site inside chloroplast. The chloroplast "sink" CO2 availability was constrained by stomatal and mesophyll resistance under high VPD stress, despite the atmospheric "source" CO2 concentration being elevated. Thus, ABA- and JA-mediated drought-resistant mechanisms potentially modified the beneficial effect of CO2 fertilization on photosynthesis, plant growth, and yield productivity. This study provides valuable information for improving the utilization efficiency of CO2 fertilization and a better understanding of the physiological processes. [ABSTRACT FROM AUTHOR]

Rising temperatures can affect stomatal and nonstomatal control over photosynthesis, through stomatal closure in response to increasing vapor pressure deficit (VPD), and biochemical limitations, respectively. To explore the independent effects of temperature and VPD, we conducted leaf-level temperature-response measurements while controlling VPD on three tropical tree species. Photosynthesis and stomatal conductance consistently decreased with increasing VPD, whereas photosynthesis typically responded weakly to changes in temperature when a stable VPD was maintained during measurements, resulting in wide parabolic temperature-response curves. We have shown that the negative effect of temperature on photosynthesis in tropical forests across ecologically important temperature ranges does not stem from direct warming effects on biochemical processes but from the indirect effect of warming, through changes in VPD. Understanding the acclimation potential of tropical trees to elevated VPD will be critical to anticipate the consequences of global warming for tropical forests.

Summary: A long‐standing debate looks at whether air or soil dryness is more limiting to vegetation water use and productivity. The answer has large implications for future ecosystem functioning, as atmospheric dryness is predicted to increase globally while changes in soil moisture are predicted to be far more variable. Here, I review the complexities that contribute to this debate, including the strong coupling between atmospheric and soil dryness, and the widespread heterogeneity in vegetation hydraulic traits, acclimations, and adaptations to water stress. I discuss solutions to improve understanding and modeling of vegetation sensitivity to dryness, including how different types of observational data can be used together to gain insight into vegetation response to water stress across spatial and temporal scales. [ABSTRACT FROM AUTHOR]

As global temperatures rise, droughts are becoming more frequent and severe. To predict how drought might affect plant communities, ecologists have traditionally designed drought experiments with controlled watering regimes and rainout shelters. Both treatments have proven effective for simulating soil drought. However, neither are designed to directly modify atmospheric drought. Here, we detail the efficacy of a silica gel atmospheric drought treatment in outdoor mesocosms with and without a co‐occurring soil drought treatment. At California State University, Los Angeles, we monitored relative humidity, temperature, and vapor pressure deficit every 10 min for 5 months in bare‐ground, open‐top mesocosms treated with soil drought (reduced watering) and/or atmospheric drought (silica dehumidification packets suspended 12 cm above soil). We found that silica packets dehumidified these mesocosm microclimates most effectively (−5% RH) when combined with reduced soil water, regardless of the ambient humidity levels of the surrounding air. Further, packets increased microclimate vapor pressure deficit most effectively (+0.4 kPa) when combined with reduced soil water and ambient air temperatures above 20°C. Finally, packets simulated atmospheric drought most consistently when replaced within 3 days of deployment. Our results demonstrate the use of silica packets as effective dehumidification agents in outdoor drought experiments. We emphasize that incorporating atmospheric drought in existing soil drought experiments can improve our understandings of the ecological impacts of drought. [ABSTRACT FROM AUTHOR]

Increases in hydrological extremes, including drought, are expected for Amazon forests. A fundamental challenge for predicting forest responses lies in identifying ecological strategies which underlie such responses. Characterization of species-specific hydraulic strategies for regulating water-use, thought to be arrayed along an 'isohydric–anisohydric' spectrum, is a widely used approach. However, recent studies have questioned the usefulness of this classification scheme, because its metrics are strongly influenced by environments, and hence can lead to divergent classifications even within the same species. Here, we propose an alternative approach positing that individual hydraulic regulation strategies emerge from the interaction of environments with traits. Specifically, we hypothesize that the vertical forest profile represents a key gradient in drought-related environments (atmospheric vapor pressure deficit, soil water availability) that drives divergent tree water-use strategies for coordinated regulation of stomatal conductance (gs) and leaf water potentials (ΨL) with tree rooting depth, a proxy for water availability. Testing this hypothesis in a seasonal eastern Amazon forest in Brazil, we found that hydraulic strategies indeed depend on height-associated environments. Upper canopy trees, experiencing high vapor pressure deficit (VPD), but stable soil water access through deep rooting, exhibited isohydric strategies, defined by little seasonal change in the diurnal pattern of gs and steady seasonal minimum ΨL. In contrast, understory trees, exposed to less variable VPD but highly variable soil water availability, exhibited anisohydric strategies, with fluctuations in diurnal gs that increased in the dry season along with increasing variation in ΨL. Our finding that canopy height structures the coordination between drought-related environmental stressors and hydraulic traits provides a basis for preserving the applicability of the isohydric-to-anisohydric spectrum, which we show here may consistently emerge from environmental context. Our work highlights the importance of understanding how environmental heterogeneity structures forest responses to climate change, providing a mechanistic basis for improving models of tropical ecosystems. [ABSTRACT FROM AUTHOR]

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Due to the effects of global warming, the vapor pressure deficit (VPD) will inevitably and continuously rise in future, which is a fact that must be faced when discussing the sustainable development of vegetation ecosystems. Understanding the response of vegetation to changes in the atmospheric moisture state is necessary for minimizing the adverse effects of climate change. Using the leaf area index (LAI) and meteorological data in the Yellow River Basin collected over nearly 40 years, we explore the spatiotemporal variations in VPD and vegetation within the growing season and the differences in the effects of VPD on vegetation in different climatic zones. The results show the following: (1) both VPD and LAI in the Yellow River Basin show an increasing trend, in which VPD increases radially outward centered on the arid regions, while the vegetation develops favorably over almost the whole basin; (2) at the whole-basin scale, the area of VPD positively correlated with vegetation growth is greater than the area of negative correlation, and this overall trend is increasing significantly; (3) the VPD exerts heterogeneous effects on different vegetation in different climatic zones, and these heterogeneity findings suggest that the effects of VPD on vegetation are more influenced by differences among the climatic zones of the vegetation than by differences in the vegetation types themselves. Our results provide insights into the effects of atmospheric moisture on vegetation growth and provide references for the sustainable development of vegetation ecosystems and future safeguarding strategies of vegetation over large spatial scales. [ABSTRACT FROM AUTHOR]

The grasslands in high-latitude areas are sensitive to climate warming and drought. However, the drought stress effect on the long-term variability of grassland productivity at the continental scale still hinders our understanding. Based on aboveground net primary production (ANPP) surveys, satellite remote sensing Normalized Difference Vegetation Index (NDVI), and meteorological data, we comprehensively analyzed three Aridity metrics and their effect on ANPP in Eurasian grassland from 1982 to 2020. Our results showed that the ANPP had an overall uptrend from 1982 to 2020, increasing most in the Tibetan Plateau alpine steppe subregion (TPSSR). Among three Aridity indicators, vapor pressure deficit (VPD) had an overall uptrend, while the trend of Aridity and soil moisture (SM) was insignificant from 1982 to 2020. Soil drought had negative effects on ANPP for all Eurasian grassland, while the atmospheric VPD had a positive effect on ANPP for TPSSR and the Mongolian Plateau steppe subregion (MPSSR), but a negative effect for the Black Sea–Kazakhstan steppe subregion (BKSSR) which was the driest subregion. SM had been the predominant driving factor for the interannual variability of ANPP in MPSSR since 1997. The increasing VPD had facilitated grassland productivity in alpine grasslands due to its cascading effect with an increasing temperature after 2000. The cascading effects networks of climate factors—drought factors (VPD, Aridity, and SM)—ANPP (CDA–CENet) indicated that SM was the predominant driving factor of the interannual variability of ANPP in MPSSR and BKSSR, and the dominance of SM had enhanced after the year 1997. The inhibitory effect of VPD on ANPP transformed into a facilitating effect after 1997, and the facilitating effect of SM is weakening in TPSSR. [ABSTRACT FROM AUTHOR]

Increasing photosynthetic efficiency in fluctuating light is an important strategy to improve plant growth. In nature, fluctuating light often occurs concomitantly with the change in vapor pressure deficit (VPD) in the daytime, but how VPD affects dynamic photosynthesis is little known. In this study, we measured photosynthetic performance in fluctuating light under different atmospheric VPD conditions in the daytime for mature leaves of greenhouse-grown Rosa hybrida "Orange Reeva". A moderate increase of VPD significantly reduced photosynthetic efficiency in fluctuating light but hardly affected steady-state photosynthesis. With the increase of atmospheric VPD, stomatal conductance (gs) and mesophyll conductance (gm) in fluctuating light decreased simultaneously, but the maximum velocity of Rubisco carboxylation did not change. The significant effect of atmospheric VPD on gs and gm in fluctuating light was attributed to the change of relative humidity. Furthermore, photosynthesis in fluctuating light did not significantly differ between morning and afternoon under the same atmospheric VPD condition. These results indicate that atmospheric VPD is an overlooked environmental factor affecting photosynthetic efficiency under fluctuating light. [ABSTRACT FROM AUTHOR]

Popular evapotranspiration (ET) partitioning methods make assumptions that might not be well‐suited to dryland ecosystems, such as high sensitivity of plant water‐use efficiency (WUE) to vapor pressure deficit (VPD). Our objectives were to (a) create an ET partitioning model that can produce fine‐scale estimates of transpiration (T) in drylands, and (b) use this approach to evaluate how climate controls T and WUE across ecosystem types and timescales along a dryland aridity gradient. We developed a novel, semi‐mechanistic ET partitioning method using a Bayesian approach that constrains abiotic evaporation using process‐based models, and loosely constrains time‐varying WUE within an autoregressive framework. We used this method to estimate daily T and weekly WUE across seven dryland ecosystem types and found that T dominates ET across the aridity gradient. Then, we applied cross‐wavelet coherence analysis to evaluate the temporal coherence between focal response variables (WUE and T/ET) and environmental variables. At yearly scales, we found that WUE at less arid, higher elevation sites was primarily limited by atmospheric moisture demand, and WUE at more arid, lower elevation sites was primarily limited by moisture supply. At sub‐yearly timescales, WUE and VPD were sporadically correlated. Hence, ecosystem‐scale dryland WUE is not always sensitive to changes in VPD at short timescales, despite this being a common assumption in many ET partitioning models. This new ET partitioning method can be used in dryland ecosystems to better understand how climate influences physically and biologically driven water fluxes. Plain Language Summary: We developed a new model to better understand how plants use and lose water in drylands and applied it to seven dryland sites. Our model partitions evapotranspiration—the total water lost to the atmosphere from the Earth's surface— into its components. Evapotranspiration consists of both evaporation from wet surfaces, such as wet soil, and the water lost from plants when they photosynthesize. Currently, models assume a strong relationship between the efficiency with which plants use water ("water‐use efficiency") and the dryness of the atmosphere, but this violates what we know about how plants function in drylands. For example, in drylands many plants are adapted to very dry conditions and their water use can be less sensitive to increasing atmospheric dryness compared to plants from wet environments. Using this new model, we found that plant water‐use efficiency is only correlated with atmospheric dryness some of the time and that evapotranspiration is primary controlled by water lost from plants. This model allows us to better understand the importance of timescale and ecosystem type in governing plant water‐use dynamics and more accurately assess the potential impact of changing climate conditions on dryland water fluxes and ecosystem processes. Key Points: A new evapotranspiration partitioning model (DEPART) was developed using eddy covariance flux tower measurements in a Bayesian frameworkThis method produces daily estimates of transpiration and weekly estimates of plant water‐use efficiency at the ecosystem scaleThis method reveals water‐use efficiency is limited by moisture supply in more arid climates and moisture demand in less arid climates [ABSTRACT FROM AUTHOR]

Land management can moderate or intensify the impacts of a warming atmosphere. Since the early 1980s, nearly 116,000 km2 of cropland that was once held in fallow during the summer is now planted in the northern North American Great Plains. To simulate the impacts of this substantial land cover change on regional climate processes, convection‐permitting model experiments using the Weather Research and Forecasting model were performed to simulate modern and historical amounts of summer fallow. The control simulation was extensively validated using multiple observational data products as well as eddy covariance tower observations. Results of these simulations show that the transition from summer fallow to modern land cover led to ∼1.5°C cooler temperatures and decreased vapor pressure deficit by ∼0.15 kPa during the growing season across the study region, which is consistent with observed cooling trends. The cooler and wetter land surface with vegetation led to a shallower planetary boundary layer and lower lifted condensation level, creating conditions more conducive to convective cloud formation and precipitation. Our model simulations however show little widespread evidence of land surface changes effects on precipitation. The observed precipitation increase in this region is more likely related to increased moisture transport by way of the Great Plains Low Level Jet as revealed by the ERA5 reanalysis. Our results demonstrate that land cover change is consistent with observed regional cooling in the northern North American Great Plains but changes in precipitation cannot be explained by land management alone. Plain Language Summary: The northern part of the North American Great Plains have seen a globally‐unique early season climate cooling over the past five decades, with a corresponding increase in precipitation. This cooling and moistening trend has coincided with a large‐scale decrease in the practice of leaving fields bare ("fallow") during the growing season that was recommended in the early twentieth century to help conserve water for subsequent crops. Fallow fields can become quite hot in the absence of cooling from plant transpiration, and add less moisture to the atmosphere that can help contribute to rainfall. Cooler and moister air that arises from planted fields can make precipitation more likely, but is this the cause of observed climate trends in the northern Plains? We ran the Weather Research and Forecasting Model with historical and modern expanses of fallow on the land surface. Model results are consistent with observed early growing season climate cooling but do not predict substantial changes in precipitation, which instead are more consistent with additional moisture transport from the southern border of our study region. Key Points: Early growing season cooling in the northern North American Great Plains is consistent with summer fallow reductionWeather Research and Forecasting model predictions indicate that fallow reduction does not explain increases in precipitationIncreased moisture transport from the Great Plains Low Level Jet is consistent with observed increases in precipitation [ABSTRACT FROM AUTHOR]

Ecosystem water use efficiency (WUE) is a crucial parameter for understanding the interaction between carbon and water cycles. However, the spatio–temporal evolution and drivers of WUE remain unclear. This study utilized global annual scale global land surface satellite gross primary productivity and evapotranspiration data from 1982 to 2018 to estimate WUE and analyze its spatio–temporal characteristics. Additionally, the study investigated the response of WUE changes to five environmental factors (precipitation [PRE], soil moisture, temperature [TEM], palmer drought severity index, and vapor pressure deficit [VPD]) on WUE changes using partial correlation and structural equation modeling. The results suggested that the global annual WUE increased markedly over the study period, at an average rate of 0.0016 gC m−2 mm−1 H2O year−1. In contrast to the existing knowledge on the drivers of WUE change, climate change was found to have a larger contribution to WUE changes at the global and regional scales, especially in terms of TEM and VPD. A positive correlation between TEM and WUE was observed, but extreme TEM could lead to a decrease in WUE. VPD had the most significant direct effect on WUE, and its negative effect offset the positive influence of TEM especially in hyper‐arid, semi‐arid, and arid regions. These findings offer new insights into the impact of VPD and global warming on WUE. Plain Language Summary: Water use efficiency (WUE), defined as the ratio of gross primary productivity to evapotranspiration, is one of the key indicators for measuring the carbon‐water coupling in ecosystems. Analyzing the temporal and spatial evolution of global WUE and its influencing factors can offer valuable insights into addressing the challenges posed by global climate change. In this study, we examined the spatio‐temporal dynamics of global WUE over an extensive time span and utilized structural equation modeling to identify the primary drivers of WUE variation. Our findings reveal a consistent increasing trend in annual global WUE from 1982 to 2018, with an average annual increase of 0.0016 gC m−2 mm−1 H2O. Temperature and vapor pressure deficit (VPD) emerged as the key drivers of WUE changes, with the positive effect of temperature on WUE being counteracted by its negative influence through VPD (approximately 75%). This research contributes to our understanding of how climate change impacts carbon and water cycles within ecosystems and lays a foundation for enhancing carbon budget assessments. Key Points: Global water use efficiency (WUE) increased at an average rate of 0.0016 gC m−2 mm−1 H2O year−1 between 1982 and 2018Extreme temperatures that either excessively high (>26°C) or low (<2°C) can cause WUE to decreaseThe positive effect of temperature on WUE was offset by its negative effect on WUE through vapor pressure deficit [ABSTRACT FROM AUTHOR]

The sagebrush steppe ecosystem plays a critical role in water cycling in arid and semiarid landscapes of the western United States; yet, there is limited information regarding individual sagebrush plant water uptake. We used the stem heat balance (SHB) method to measure transpiration in mountain big sagebrush (Artemisia tridentata subsp. vaseyana) plants in a semiarid rangeland ecosystem in central Oregon, Pacific Northwest Region, USA. We evaluated the relationship between sagebrush transpiration and environmental factors from July 2022 to May 2023 for two individual plants representative of the average sagebrush stand height and crown width at the study site; transpiration rates varied by plant and by season. This study encompassed one below-average (2022; 278 mm) and one above-average (2023; 414 mm) precipitation years. Study results showed that the average water use during the entire period of study was 2.1 L d−1 for Plant 1 and 5.0 L d−1 for Plant 2. During the dry year, maximum transpiration was observed during the summer (Plant 1 = 4.8 L d−1; Plant 2 = 11.1 L d−1). For the wet year, both plants showed maximum transpiration levels at the end of the recording period in mid-May (Plant 1 = 9.6 L d−1; Plant 2 = 8.6 L d−1). The highest seasonal transpiration of both plants occurred in summer (2.87 L d−1), whereas the lowest transpiration was obtained in winter (0.21 L d−1). For all seasons but winter, soil moisture (SM), soil temperature (ST), and vapor pressure deficit (VPD) variables generally showed positive correlations with transpiration. Transpiration rates decreased in the summer of 2022 as the surface soil gradually dried. The two plants' most significant water uptake differences were obtained during the dry year. It is possible that the larger stem diameter of plant 2 may have contributed to its higher transpiration rates during times of limited water availability. The study results add to the understanding of water use by sagebrush and its potential impact on the water balance of cool-climate rangeland ecosystems. The findings also highlight the sensitivity of sagebrush to variations in seasonal soil moisture availability, soil temperature, and vapor pressure deficit. Future research should involve studying the combined effects of water use by various dominant vegetation species and its effects on the water budget at the watershed scale. [ABSTRACT FROM AUTHOR]

Atmospheric dryness (i.e., high vapor pressure deficit, VPD), together with soil moisture stress, limits plant photosynthesis and threatens ecosystem functioning. Regions where rainfall and soil moisture are relatively sufficient, such as the rainfed part of the U.S. Corn Belt, are especially prone to high VPD stress. With globally projected rising VPD under climate change, it is crucial to understand, simulate, and manage its negative impacts on agricultural ecosystems. However, most existing models simulating crop response to VPD are highly empirical and insufficient in capturing plant response to high VPD, and improved modeling approaches are urgently required. In this study, by leveraging recent advances in plant hydraulic theory, we demonstrate that the VPD constraints in the widely used coupled photosynthesis‐stomatal conductance models alone are inadequate to fully capture VPD stress effects. Incorporating plant xylem hydraulic transport significantly improves the simulation of transpiration under high VPD, even when soil moisture is sufficient. Our results indicate that the limited water transport capability from the plant root to the leaf stoma could be a major mechanism of plant response to high VPD stress. We then introduce a Demand‐side Hydraulic Limitation Factor (DHLF) that simplifies the xylem and the leaf segments of the plant hydraulic model to only one parameter yet captures the effect of plant hydraulic transport on transpiration response to high VPD with similar accuracy. We expect the improved understanding and modeling of crop response to high VPD to help contribute to better management and adaptation of agricultural systems in a changing climate. Key Points: Coupled photosynthesis‐stomatal conductance models alone underestimate vapor pressure deficit (VPD) stress effects on crop stomatal conductance and transpirationLimited plant hydraulic transport capability could play a role in plant response to high VPDA simplified representation of plant hydraulic model for capturing VPD stress on plants is proposed [ABSTRACT FROM AUTHOR]

Abstract We investigated the ecophysiological responses of the bromeliads Aechmea nudicaulis and Vriesea procera, seeking to assess their suitability to survive in a climate change scenario (optimistic scenario RCP 2.6 of the IPCC, 2021) in a Restinga environment. To carry out this investigation, we used open-top chambers (OTC). During a period of nine months (June 2022 to February 2023), the bromeliads A. nudicaulis and V. procera were subjected to the following treatments: treatment T: plants transplanted to the environmental conditions of the bare sand of the restinga and subjected to the microclimatic conditions of the OTC's; control C: plants transplanted to the environmental conditions of the bare sand of the restinga. The ecophysiological variables height, rosette diameter, relative water content, specific leaf area and total weight of the plants were evaluated. In addition, dead plants were counted. The OTC’s showed an average increase in temperature and VPD (Vapor Pressure Deficit) of 1.6°C and 0.5 Kpa, respectively, and an average reduction in RH (relative humidity) of 5.3%. The results of this study indicated that the increase in local temperature that occurred between the sixth and seventh months evaluated (November and December) created limiting conditions that exceeded the tolerance capacity of the bromeliads studied. Furthermore, the climatic conditions of the OTCs intensified the damage that occurred in the plants, verified here by the reductions in the values of the ecophysiological attributes evaluated in the bromeliads studied. In addition, the high mortality rate (above 50%) reinforces the idea that the climatic conditions of the OTC’s induced the bromeliads studied to a senescence process. Therefore, these results are important, as they indicate that even the most optimistic climate change scenario (IPCC 2021 RCP 2.6) can harm the growth and development of these bromeliads, which are essential for the structure and functioning of Restinga communities.

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.

Summary As temperature rises, net carbon uptake in tropical forests decreases, but the underlying mechanisms are not well understood. High temperatures can limit photosynthesis directly, for example by reducing biochemical capacity, or indirectly through rising vapor pressure deficit (VPD) causing stomatal closure. To explore the independent effects of temperature and VPD on photosynthesis we analyzed photosynthesis data from the upper canopies of two tropical forests in Panama with Generalized Additive Models. Stomatal conductance and photosynthesis consistently decreased with increasing VPD, and statistically accounting for VPD increased the optimum temperature of photosynthesis (Topt) of trees from a VPD‐confounded apparent Topt of c. 30–31°C to a VPD‐independent Topt of c. 33–36°C, while for lianas no VPD‐independent Topt was reached within the measured temperature range. Trees and lianas exhibited similar temperature and VPD responses in both forests, despite 1500 mm difference in mean annual rainfall. Over ecologically relevant temperature ranges, photosynthesis in tropical forests is largely limited by indirect effects of warming, through changes in VPD, not by direct warming effects of photosynthetic biochemistry. Failing to account for VPD when determining Topt misattributes the underlying causal mechanism and thereby hinders the advancement of mechanistic understanding of global warming effects on tropical forest carbon dynamics. [ABSTRACT FROM AUTHOR]

Understanding and accurately predicting how the sensitivity of terrestrial vegetation productivity to rising atmospheric CO2 concentration (β) is crucial for assessing carbon sink dynamics. However, the temporal characteristics and driving mechanisms of β remain uncertain. Here, observational and CMIP6 modeling evidence suggest a decreasing trend in β at the Northern Middle and High Latitudes during the historical period of 1982–2015 (−0.082 ± 0.005% 100 ppm−1 year−1). This decreasing trend is projected to persist until the end of the 21st century (−0.082 ± 0.005% 100 ppm−1 year−1 under SSP370 and −0.166 ± 0.006% 100 ppm−1 year−1 under SSP585). The declining β indicates a weakening capacity of vegetation to mitigate warming climates, posing challenges for achieving the temperature goals of the Paris Agreement. The rise in vapor pressure deficit (VPD), that triggers stomata closure and weakens photosynthesis, is considered as the dominated factor contributing to the historical and future decline in β, accounting for 62.3%–75.2% of the effect. Nutrient availability and water availability contribute 15.7%–21.4% and 8.5%–16.3%, respectively. These findings underscore the significant role of VPD in shaping terrestrial carbon sink dynamics, an aspect that is currently insufficiently considered in many climate and ecological models. [ABSTRACT FROM AUTHOR]

As constituents of one of the vital agricultural ecosystems, paddy fields exert significant influence on the global carbon cycle. Therefore, conducting observations and simulations of CO2 flux in rice paddy is of significant importance for gaining deeper insights into the functionality of agricultural ecosystems. This study utilized an eddy covariance system to observe and analyze the CO2 flux in a rice paddy field in Eastern China and also introduced and parameterized the Jarvis multiplicative model to predict the CO2 flux. Results indicate that throughout the observation period, the range of CO2 flux in the paddy field was −0.1 to −38.4 μmol/(m2·s), with a mean of −12.9 μmol/(m2·s). The highest CO2 flux occurred during the rice flowering period with peak photosynthetic activity and maximum CO2 absorption. Diurnal variation in CO2 flux exhibited a "U"-shaped curve, with flux reaching its peak absorption at 11:30. The CO2 flux was notably higher in the morning than in the afternoon. The nocturnal CO2 flux remained relatively stable, primarily originating from respiratory CO2 emissions. The rice canopy CO2 flux model was revised using boundary line analysis, elucidating that photosynthetically active radiation, temperature, vapor pressure deficit, phenological stage, time, and concentration are pivotal factors influencing CO2 flux. The simulation of CO2 flux using the parameterized model, compared with measured values, reveals the efficacy of the established parameter model in simulating rice CO2 flux. This study holds significant importance in comprehending the carbon cycling process within paddy ecosystems, furnishing scientific grounds for future climate change and environmental management endeavors. [ABSTRACT FROM AUTHOR]

Grasslands provide important ecosystem services to society, including biodiversity, water security, erosion control, and forage production. Grasslands are also vulnerable to droughts, rendering their future vitality under climate change uncertain. Yet, the grassland response to drought is not well understood, especially for heterogeneous Central European grasslands. We here fill this gap by quantifying the spatiotemporal sensitivity of grasslands to drought using a novel remote sensing dataset from Landsat/Sentinel‐2 paired with climate re‐analysis data. Specifically, we quantified annual grassland vitality at fine spatial scale and national extent (Germany) from 1985 to 2021. We analyzed grassland sensitivity to drought by testing for statistically robust links between grassland vitality and common drought indices. We furthermore explored the spatiotemporal variability of drought sensitivity for 12 grassland habitat types given their different biotic and abiotic features. Grassland vitality maps revealed a large‐scale reduction of grassland vitality during past droughts. The unprecedented drought of 2018–2019 stood out as the largest multi‐year vitality decline since the mid‐1980s. Grassland vitality was consistently coupled to drought (R2 =.09–.22) with Vapor Pressure Deficit explaining vitality best. This suggests that high atmospheric water demand, as observed during recent compounding drought and heatwave events, has major impacts on grassland vitality in Central Europe. We found a significant increase in drought sensitivity over time with highest sensitivities detected in periods of extremely high atmospheric water demand, suggesting that drought impacts on grasslands are becoming more severe with ongoing climate change. The spatial variability of grassland drought sensitivity was linked to different habitat types, with declining sensitivity from dry and mesic to wet habitats. Our study provides the first large‐scale, long‐term, and spatially explicit evidence of increasing drought sensitivities of Central European grasslands. With rising compound droughts and heatwaves under climate change, large‐scale grassland vitality loss, as in 2018–2019, will thus become more likely in the future. [ABSTRACT FROM AUTHOR]

Ongoing climate change is projected to intensify drought stress globally. Understanding the response mechanisms of Phyllostachys edulis (Carrière) J. Houz. (moso bamboo) to long-term drought is crucial, given its significance as a carbon sequestration resource. In this study, precipitation exclusion was implemented to simulate drought stress and we investigated the effects of long-term drought on the photosynthetic parameters, stomatal conductance, and water use efficiency of moso bamboo. The results showed that throughout all growth seasons, the maximum net photosynthetic rates (Pmax) of bamboo at all ages under long-term drought conditions (after 8 years of precipitation exclusion treatment) were significantly lower than those of the control (p < 0.05). It can be concluded that long-term drought reduced the maximum photosynthetic capacity of the bamboo at all ages. Under long-term drought conditions, there were many seasons where the light saturation point (LSP) of first-degree (1–2 years old) bamboo and third-degree (5–6 years old) bamboo under drought was significantly lower than those of the control, while the LSP value of second-degree (3–4 years old) bamboo under drought was significantly higher than that of the control. This suggests that long-term drought reduced the ability of first-degree and third-degree bamboo to utilize strong light, while improving the ability of second-degree bamboo to utilize strong light in summer, autumn, and winter. Under long-term drought conditions, the light compensation point (LCP) and the apparent quantum efficiency (AQY) of the bamboo decreased. It can be concluded that long-term drought reduced the ability of first-degree bamboo to utilize weak light in all seasons, as well as the ability of second-degree bamboo to utilize weak light in spring and autumn; meanwhile, it improved the ability of second-degree bamboo to utilize weak light in summer and winter, and the ability of third-degree bamboo to utilize weak light in spring, summer, and autumn. In the high light range (PARi > 1000 µmol · m−2 · s−1), there were significant differences in stomatal conductance (gs) among different the different treatments of bamboo, which were influenced by both the growing season and the forest age. Compared to the control, under drought conditions, the stomatal conductance of third-degree bamboo increased in spring and that of the second-degree bamboo increased in autumn. The correlation analysis showed that the relationship between the stomatal conductance and vapor pressure deficit (VPDL) of bamboo under long-term drought conditions showed a significant polynomial relationship in both high and low light ranges. The correlation between the instantaneous water use efficiency (iWUE) and VPDL for the drought and control treatments of bamboo also showed a significant polynomial relationship in high light ranges. It was found that long-term drought changed the photosynthetic parameters of the bamboo, reflecting its ability to tolerate and adapt to drought in different seasons. Age-related differences in photosynthetic parameters should be fully considered in forest age structure adjustments and forest thinning procedures to strengthen the light intensity and maintain the opening of the stoma. These results provide a theoretical basis for the efficient and sustainable cultivation of bamboo under global climate change. [ABSTRACT FROM AUTHOR]

A forest is vulnerable to drought and plays important roles in the regulation of carbon and water cycling in a terrestrial ecosystem. Atmospheric vapor pressure deficit (VPD) has been identified as an increasingly major factor in plant functioning and has been established as a main contributor to recent drought-induced plant mortality, independent of other drivers associated with climate change. However, most previous studies have focused on the effects of climate warming and CO2 enrichment on vegetation growth, without considering the effects of an increased VPD on vegetation growth and evapotranspiration (ET) in forest ecosystems. This could lead to a large uncertainty in estimating the variability in forest carbon sinks. Based on the long-term satellite data, we investigated the response of the leaf area index (LAI) and ET to the VPD via a partial correlation analysis in this study. We also examined the temporal variability in the partial coefficients within a ten-year moving window. The results showed that over 50% of the region displayed a negative partial correlation between the LAI, ET, and VPD, and those pixels were mainly concentrated in North America and the plains of Eastern Europe. Regions with a negative trend of partial correlation in both the LAI and ET are mostly located in the plains of Eastern Europe and the Siberian Plain of western Russia, while the positive trend is mainly in South America. The plains of Eastern Europe are becoming drier, which was proved by the interannual trend of the Standardized Precipitation Evapotranspiration Index (SPEI) and soil water content (SWC). Additionally, the LAI and ET in those areas exhibited a significant positive correlation with the SWC based on the moving window average. This study suggests that the role of the VPD on vegetation will become increasingly prominent in the context of future climate change for the forest ecosystem. [ABSTRACT FROM AUTHOR]

Abstract As global temperatures rise, droughts are becoming more frequent and severe. To predict how drought might affect plant communities, ecologists have traditionally designed drought experiments with controlled watering regimes and rainout shelters. Both treatments have proven effective for simulating soil drought. However, neither are designed to directly modify atmospheric drought. Here, we detail the efficacy of a silica gel atmospheric drought treatment in outdoor mesocosms with and without a co‐occurring soil drought treatment. At California State University, Los Angeles, we monitored relative humidity, temperature, and vapor pressure deficit every 10 min for 5 months in bare‐ground, open‐top mesocosms treated with soil drought (reduced watering) and/or atmospheric drought (silica dehumidification packets suspended 12 cm above soil). We found that silica packets dehumidified these mesocosm microclimates most effectively (−5% RH) when combined with reduced soil water, regardless of the ambient humidity levels of the surrounding air. Further, packets increased microclimate vapor pressure deficit most effectively (+0.4 kPa) when combined with reduced soil water and ambient air temperatures above 20°C. Finally, packets simulated atmospheric drought most consistently when replaced within 3 days of deployment. Our results demonstrate the use of silica packets as effective dehumidification agents in outdoor drought experiments. We emphasize that incorporating atmospheric drought in existing soil drought experiments can improve our understandings of the ecological impacts of drought.

A thorough understanding of how crops respond to water stress is essential for effective irrigation management under changing climate conditions and declining water resources. Amongst the various methods of irrigation, crop water stress index (CWSI)-based irrigation is popular due to its non-invasive techniques that require less data and can precisely indicate the severity of crop water deficiency. The present study focuses on establishing the upper- and lower-baselines of canopy-air temperature difference (Tc-Ta) for the estimation of the CWSI and to determine the CWSI threshold for maize and wheat crops. The upper- and lower-baseline of (Tc-Ta) was established using the relationship between (Tc-Ta) and vapor pressure deficit (VPD) for different irrigation treatments such as 60 %ETc, 70 %ETc, 80 %ETc, 90 %ETc, and 100 %ETc with irrigation interval of 1-day, 2-days, and 3-days. The study showed that the baselines varied with the growth stages of the crop and with irrigation treatments and reached the maximum in Anthesis and Silking growth stage for wheat and maize, respectively. The threshold of CWSI was estimated by developing a relationship between the normalized values of grain yield (GY) and irrigation water productivity (WPI). For the Rabi wheat, a quadratic correlation was identified between CWSI and WPI (r2 = 0.89), and GY (r2 = 0.93). Similarly, for Kharif maize, a quadratic relationship was noted between CWSI and both WPI (r2 = 0.75) and GY (r2 = 0.85). From the study, it was found that the CWSI thresholds of Rabi wheat and Kharif maize are 0.29 and 0.31, respectively. Based on the study, it can be concluded that by utilizing the thresholds for a winter wheat-summer maize cropping system, high GY and WPI can be achieved.

Abstract Ecosystem water use efficiency (WUE) is a crucial parameter for understanding the interaction between carbon and water cycles. However, the spatio–temporal evolution and drivers of WUE remain unclear. This study utilized global annual scale global land surface satellite gross primary productivity and evapotranspiration data from 1982 to 2018 to estimate WUE and analyze its spatio–temporal characteristics. Additionally, the study investigated the response of WUE changes to five environmental factors (precipitation [PRE], soil moisture, temperature [TEM], palmer drought severity index, and vapor pressure deficit [VPD]) on WUE changes using partial correlation and structural equation modeling. The results suggested that the global annual WUE increased markedly over the study period, at an average rate of 0.0016 gC m−2 mm−1 H2O year−1. In contrast to the existing knowledge on the drivers of WUE change, climate change was found to have a larger contribution to WUE changes at the global and regional scales, especially in terms of TEM and VPD. A positive correlation between TEM and WUE was observed, but extreme TEM could lead to a decrease in WUE. VPD had the most significant direct effect on WUE, and its negative effect offset the positive influence of TEM especially in hyper‐arid, semi‐arid, and arid regions. These findings offer new insights into the impact of VPD and global warming on WUE.

Increases in air temperature lead to increased dryness of the air and potentially develops increased dryness in the soil. Extreme dryness (in the soil and/or in the atmosphere) affects the capacity of ecosystems for functioning and for modulating the climate. Here, we used long-term high temporal resolution (daily) soil moisture (SM) and vapor pressure deficit (VPD) data of high spatial resolution (∼0.1° × 0.1°) to show that compared to the reference period (1950–1990), the overall frequency of extreme soil dryness, extreme air dryness, and extreme compound dryness (i.e., co-occurrence of extreme soil dryness and air dryness) has increased by 1.2-fold [0.8,1.6] (median [10th,90th percentile], 1.6-fold [1,2.3], and 1.7-fold [0.9,2.5], respectively, over the last 31 years (1991–2021) across Europe. Our results also indicate that this increase in frequency of extreme compound dryness (between reference and 1991–2021 period) is largely due to increased SM-VPD coupling across Northern Europe, and due to decreasing SM and/or increasing VPD trend across Central and Mediterranean Europe. Furthermore, under the RCP8.5 (Representative Concentration Pathways 8.5) emission scenario, this increase in frequency of extreme compound dryness would be 3.3-fold [2.0,5.8], and 4.6-fold [2.3,11.9] by mid-21st century (2031–2065) and late-21st century (2066–2100), respectively. Additionally, we segregated the changes in frequency of extreme dryness across the most recent (year 2021) land cover types in Europe to show that croplands, broadleaved forest, and urban areas have experienced more than twice as much extreme dryness during 1990–2021 compared to the reference period of 1990–2021, which based on the future projection data will increase to more than three-fold by mid 21st century. Such future climate-change induced increase in extreme dryness could have negative implications for functioning of ecosystems and compromise their capacity to adapt to rapidly rising dryness levels.

Flash droughts threaten ecosystems substantially because of the fast onset and low predictability. Soil and atmospheric water stress are two main factors reducing ecosystem productivity during flash droughts. However, the long-term trends in the soil and atmospheric water stress on vegetation during flash droughts are unclear. By conducting long-term land surface model simulations, this study investigated the impact of atmospheric and soil water stress on gross primary productivity (GPP) during flash droughts and hot periods of flash droughts, as well as the long-term changes in water stress from 1961 to 2022 over China. The areas dominated by soil and atmospheric stress were 65.2% and 19.9% during flash droughts, respectively. During the hot periods of flash droughts, the areas dominated by atmospheric water stress were raised to 39.4%, and the areas dominated by soil water stress were reduced to 48.7%. During 1961–2022, the frequency, intensity, and duration of flash droughts all showed significant upward trends (p

The article titled "Tree Physiology" discusses the impact of climate change on gas exchange in tropical forests. It emphasizes the importance of vapor pressure deficit (VPD) in determining the photosynthetic temperature optimum in tropical trees. The study conducted on 42 species of trees in Panama concludes that VPD is the main driver of photosynthesis reduction in tropical trees, surpassing the influence of temperature alone. The authors highlight the need to consider VPD in accurately predicting carbon stock and forest growth in tropical forests under future climates. This research provides valuable insights into the potential effects of climate change on tropical forests and calls for further investigation in this area. [Extracted from the article]

Accurate assessment and monitoring of plant water stress are essential for optimizing irrigation strategies, improving water use efficiency. This article explores the multifaceted issue of water stress, encompassing both agricultural and environmental contexts. It emphasizes the pivotal role of precise water stress detection in effectively managing water resources and fostering sustainable agricultural practices. The primary focus is on the progression of sensors designed specifically to detect water stress, with particular attention given to two approaches: Vapor Pressure Deficit (VPD) and Crop Water Stress Index (CWSI). The article thoroughly investigates the underlying principles, operational mechanisms, advantages, and limitations of these sensor technologies. It vividly showcases their wide-ranging applications across agriculture, horticulture, and environmental monitoring, elucidating their significance in each domain. Moreover, it delves into the integration of VPD and CWSI sensors and introduces emerging technologies like thermal imaging and chlorophyll fluorescence sensors, expanding the horizon of water stress detection methodologies. Addressing the challenges linked to calibration and data interpretation, the article proposes potential pathways for future research endeavors. In essence, the overarching goal of this article is to propel the development of advanced sensor technologies, ultimately facilitating precise water stress detection. It aims to bolster sustainable water resource management practices while fortifying resilient agricultural methods in the face of evolving environmental challenges. VPD and CWSI-based approaches offer precise water stress insights in agriculture, aiding irrigation management.

Recent research has demonstrated that global warming significantly enhances peak vegetation growth on the Tibetan Plateau (TP), underscoring the influence of climatic factors on vegetation dynamics. Nevertheless, the effects of different drought types on peak vegetation growth remain underexplored. This study utilized satellite-derived gross primary productivity (GPP) and the normalized difference vegetation index (NDVI) to assess the impacts of soil moisture (SM) and vapor pressure deficit (VPD) on peak vegetation growth (GPPmax and NDVImax) across the TP from 2001 to 2022. Our findings indicate that NDVImax and GPPmax exhibited increasing trends in most regions, displaying similar spatial patterns, with 65.28% of pixels showing an increase in NDVImax and 72.98% in GPPmax. In contrast, the trend for SM primarily showed a decrease (80.86%), while VPD showed an increasing trend (74.75%). Through partial correlation analysis and ridge regression, we found that peak vegetation growth was significantly affected by SM or VPD in nearly 20% of the study areas, although the magnitude of these effects varied considerably. Furthermore, we revealed that hydrothermal conditions modulated the responses of peak vegetation growth to SM and VPD. In regions with annual precipitation less than 650 mm and an annual mean temperature below 10 °C, decreased SM and increased VPD generally inhibited peak vegetation growth. Conversely, in warm and humid areas, lower SM and higher VPD promoted peak vegetation growth. These findings are crucial for deepening our understanding of vegetation phenology and its future responses to climate change.

High temperatures (HTs) and high vapor pressure deficits (VPDs) have significant impacts on maize yields, partly due to the high sensitivity of maize tassels. However, there are few studies quantifying the impacts of HTs and VPDs on maize tassel performance under changing environments. Therefore, we carried out a two-year field experiment that included 20 inbred lines and four sowing dates. Compared with the first sowing date, the seed set in the second sowing date decreased by ~80% in both years. The opened spikelet ratio (OSR) and pollen viability (PV) were the key determinants of seed set, and their respective correlation coefficients with seed set were 0.58 and 0.90. The OSR and PV decreased by ~20% and ~50%, respectively, under high-temperature stress. When Tmax exceeded 32.5 °C or the VPD exceeded 0.91 KPa, PV began to decline; when Tmax exceeded 33.8 °C or VPD exceeded 1.10 KPa, the OSR began to decline. The OSR was more dependent on genotypic background than PV (28.4% vs. 19.7%). The maize tassel water content was significantly correlated with the OSR and PV. Based on the OSR and PV values, the 20 genotypes were divided into three different groups, namely the high H, middle M, and low L groups. The H group, on average, had the highest kernel number per ear and seed set, followed by the M and L groups. The average seed sets of the H, M, and L genotypes under the second sowing date were 17.4%, 10.9%, and 0%, respectively, in 2019 and 13.8%, 7.9%, and 0.6%, respectively, in 2020. The present results indicate that selecting maize varieties with a high OSR is an effective approach for improving maize yield under heat stress.

In recent decades, drought has intensified along with continuous global warming, significantly impacting terrestrial vegetation. High atmospheric water demand, indicated by vapor pressure deficit (VPD), and insufficient soil moisture (SM) are considered the primary factors causing drought stress in vegetation. However, the influences of VPD and SM on the autumn phenology are still unknown. Using satellite observations and meteorological data, we examined the impacts of VPD and SM on the end of the growing season (EOS) across the Northern Hemisphere (>30°N) from 1982 to 2022. We found that VPD and SM were as important as temperature, precipitation, and radiation in controlling the variations in the EOS. Moreover, the EOS was predominantly influenced by VPD or SM in more than one-third (33.8%) of the study area. In particular, a ridge regression analysis indicated that the EOS was more sensitive to VPD than to SM and the other climatic factors, with 25% of the pixels showing the highest sensitivity to VPD. In addition, the effects of VPD and SM on the EOS varied among biome types and climate zones. VPD significantly advanced the EOS in 25.8% of temperate grasslands, while SM had the greatest impact on advancing the EOS in 17.7% of temperate coniferous forests. Additionally, 27.7% of the midlatitude steppe (BSk) exhibited a significant negative correlation between VPD and the EOS, while 19.4% of the marine west coast climate (Cfb) showed a positive correlation between SM and the EOS. We also demonstrated that the correlation between VPD and the EOS was linearly affected by VPD and the leaf area index, while the correlation between SM and the EOS was affected by SM, precipitation, and the leaf area index. Our study highlights the importance of VPD and SM in regulating autumn phenology and enhances our understanding of terrestrial ecosystem responses to climate change.

Earth's ecosystems are increasingly threatened by "hot drought," which occurs when hot air temperatures coincide with precipitation deficits, intensifying the hydrological, physiological, and ecological effects of drought by enhancing evaporative losses of soil moisture (SM) and increasing plant stress due to higher vapor pressure deficit (VPD). Drought-induced reductions in gross primary production (GPP) exert a major influence on the terrestrial carbon sink, but the extent to which hotter and atmospherically drier conditions will amplify the effects of precipitation deficits on Earth's carbon cycle remains largely unknown. During summer and autumn 2020, the U.S. Southwest experienced one of the most intense hot droughts on record, with record-low precipitation and record-high air temperature and VPD across the region. Here, we use this natural experiment to evaluate the effects of hot drought on GPP and further decompose those negative GPP anomalies into their constituent meteorological and hydrological drivers. We found a 122 Tg C (>25%) reduction in GPP below the 2015-2019 mean, by far the lowest regional GPP over the Soil Moisture Active Passive satellite record. Roughly half of the estimated GPP loss was attributable to low SM (likely a combination of record-low precipitation and warming-enhanced evaporative depletion), but record-breaking VPD amplified the reduction of GPP, contributing roughly 40% of the GPP anomaly. Both air temperature and VPD are very likely to continue increasing over the next century, likely leading to more frequent and intense hot droughts and substantially enhancing drought-induced GPP reductions.

Accurate estimation of desert vegetation transpiration is key to regulating desert water resources of desert ecosystems. Sap flow density (SFD) can indirectly reflect a tree's transpiration consumption, and it has been affected by climate warming and groundwater depths in desert ecosystems. Sap flow responses to meteorological conditions and groundwater depths are further affected by tree of different sizes. However, how meteorological factors and groundwater depths affects tree sap flow among tree sizes remains poorly understand. In this study, a 50 × 50 m P. euphratica stand was selected as a sample plot in the hinterland of the Taklamakan Desert, and the SFD of P. euphratica of different sizes was measured continuously using the thermal diffusion technique from May to October of 2021 and 2022. The results showed that SFD of large P. euphratica was consistently higher than that of small P. euphratica in 2021 and 2022. and the SFD of P. euphratica was significantly and positively correlated with solar radiation (Rad) and vapor pressure deficit (VPD), and the correlation was higher than that of the air temperature (Ta) and relative humidity (RH), and also showed a strong non-linear relationship. Analysis of the hour-by-hour relationship between P. euphratica SFD and VPD and Rad showed a strong hysteresis. Throughout the growing season, there was no significant relationship between SFD of P. euphratica and groundwater depth, VPD and Rad were still the main controlling factors of SFD in different groundwater depths. However, during the period of relative groundwater deficit, the effect of groundwater depth on the SFD of P. euphratica increased, and the small P. euphratica was more sensitive, indicating that the small P. euphratica was more susceptible to groundwater changes. This study emphasized that Rad and VPD were the main drivers of SFD during the growing season, as well as differences in the response of different sizes of P. euphratica to groundwater changes. The results of the study provide a scientific basis for future modeling of transpiration consumption in P. euphratica forests in desert oases, as well as the regulation and allocation of water resources. [ABSTRACT FROM AUTHOR]

There has been substantial research on shade in cacao agroforestry systems, with most studies focusing on the impact of the shade cast by the associated shade trees on microclimate and yield. However, to our knowledge, no cacao agroforestry studies have explored how shade and its microclimate-modifying capacity influence the agrosystem’s pest and disease regulating service. Utilizing thermal hygrometers and hemispherical photographs, we measured temperature and relative humidity during the dry and wet seasons as well as the shade of associated trees (associated shade), combining the latter with cacao self-shade (total shade). This approach enabled us to uncover how each shade type influences microclimate, yields, and pests and diseases beneath cacao trees. Additionally, we developed a novel method to estimate attainable yield, actual yield, and yield loss due to pests and diseases. Using yield loss as a proxy of the pest and disease regulating service and structural equation modeling, we built a model depicting the interaction network between shade types and their role in cacao pest and disease regulation. Our results showed that each shade type uniquely influenced cacao agroecosystem outcomes, with the associated shade negatively impacting attainable yield and total shade having a positive effect. Associated shade also mitigated the dry season microclimate and limited pest and disease occurrence. Notably, shade alone was not the sole pest and disease-related yield loss driver; it is part of a complex interaction network. These innovative shade measurement and yield loss estimation methods have enhanced enhance our understanding of pest and disease regulation. Conclusively, different management approaches for associated shade and total shade are crucial for optimizing yields and pest and disease regulation in cacao agroforestry systems. [ABSTRACT FROM AUTHOR]

Spatiotemporal variations in Central Asian vegetation phenology provide insights into arid ecosystem behavior and its response to environmental cues. Nevertheless, comprehensive research on the integrated impact of meteorological factors (temperature, precipitation, soil moisture, saturation vapor pressure deficit), topography (slope, aspect, elevation), and greenhouse gases (carbon dioxide, methane, nitrous oxide) on the phenology of Central Asian vegetation remains insufficient. Utilizing methods such as partial correlation and structural equation modeling, this study delves into the direct and indirect influences of climate, topography, and greenhouse gases on the phenology of vegetation. The results reveal that the start of the season decreased by 0.239 days annually, the length of the season increased by 0.044 days annually, and the end of the season decreased by 0.125 days annually from 1982 to 2021 in the arid regions of Central Asia. Compared with topography and greenhouse gases, meteorological factors are the dominant environmental factors affecting interannual phenological changes. Temperature and vapor pressure deficits (VPD) have become the principal meteorological elements influencing interannual dynamic changes in vegetation phenology. Elevation and slope primarily regulate phenological variation by influencing the VPD and soil moisture, whereas aspect mainly affects the spatiotemporal patterns of vegetation phenology by influencing precipitation and temperature. The findings of this study contribute to a deeper understanding of how various environmental factors collectively influence the phenology of vegetation, thereby fostering a more profound exploration of the intricate response relationships of terrestrial ecosystems to environmental changes. [ABSTRACT FROM AUTHOR]