Increasing Vapor Pressure Deficit Research Articles

Stomatal response to VPD in C4 plants with different biochemical sub-pathways.

C4 NAD-malic enzyme (NAD-ME) species occurs in drier regions and exhibit different drought responses compared to C4 NADP-malic enzyme (NADP-ME) species. However, a physiological mechanism explaining the geographical discrepancies remains uncertain. This study examined gas exchange patterns that might explain different distributions observed between two subtypes of C4 photosynthesis. We measured the response of leaf gas exchange to vapour pressure deficit (VPD) and CO2 in plants from six distinct C4 clades having closely related NAD-ME and NADP-ME species using a Li-Cor 6400 gas exchange system. We found that NAD-ME species exhibited greater relative reductions in stomatal conductance with increases in VPD than NADP-ME species but observed no consistent subtype differences in C4 cycle activity as indicated by the initial slope of the A response to intercellular CO2 concentration. Based on these results, we hypothesise the greater response of gs to increasing VPD may enable NAD-ME plants to outperform NADP-ME plants in hot, dry environments where VPD is normally high.

Recent trends in agriculturally relevant climate in Central America

AbstractThis study examines the climatology and trends in climate in Guatemala, Honduras, and El Salvador over the past four decades, against the background of potential impacts on rainfed agriculture, livelihoods, and migration. The results show that there has been a significant warming of surface temperatures, an earlier start to the monsoon, a drier and longer mid‐summer drought, and a delay in the second peak of precipitation from September to October. These changes have led to an increase in vapour pressure deficit (VPD) in northern Guatemala and along the Pacific coast in winter. High VPD can stress plants and lead to reduced yields. The study also finds that the thresholds that cause a decline in coffee yield have already been reached in El Salvador, but the average VPD has also risen in Guatemala and Honduras over the past 42 years. Maize yields have also been negatively affected with an inverse relationship with daily maximum temperatures during the summer flowering season. Observed changes and trends in these climate factors are believed to have direct implications for crop yields and livelihoods, potentially driving shifts in migration patterns.

Spatiotemporal variation and response of gross primary productivity to climate factors in forests in Qiannan state from 2000 to 2020

Accurate estimation of terrestrial gross primary productivity (GPP) is essential for quantifying the carbon exchange between the atmosphere and biosphere. Light use efficiency (LUE) models are widely used to estimate GPP at different spatial scales. However, difficulties in properly determining the maximum LUE (LUEmax) and downregulation of LUEmax into actual LUE result in uncertainties in the LUE-estimated GPP. The recently developed P model, a new LUE model, captures the adaptability of vegetation to the environment and simplifies parameterization. Site-level studies have proven the superior performance of the P model over LUE models. As a representative karst region with significant changes in forest cover in Southwest China, Qiannan is useful for exploring the spatiotemporal variation in forest GPP and its response to climate change for formulating forest management policies to address climate changes, e.g., global warming. Based on remote sensing and meteorological data, this study estimated the forest ecosystem GPP in Qiannan from 2000–2020 via the P model. This study explored the spatiotemporal changes in GPP in the study region over the past 20 years, used the Hurst index to predict future development trends from a time series perspective, and used partial correlation analysis to analyse the spatiotemporal GPP changes over the past 20 years in response to three factors: temperature, precipitation, and vapor pressure deficit (VPD). Our results showed that (1) the total amount of GPP and average GPP in Qiannan over the past 21 years (2000–2020) were 1.9 × 104 ± 2.0 × 103 MgC ha−1 year−1 and 1238.9 ± 107.9 gC m−2 year−1, respectively. The forest GPP generally increased at a rate of 6.1 gC m−2 year−1 from 2000 to 2020 in Qiannan, and this increase mainly occurred in the nongrowing season. (2) From 2000 to 2020, the forest GPP in Qiannan was higher in the southeast and lower in the northwest, indicating significant spatial heterogeneity. In the future, more than 70% of regional forest GPP will experience a weak increase in nonsustainability. (3) In Qiannan, forest GPP was positively correlated with both temperature and precipitation, with partial correlation coefficients of 0.10 and 0.11, respectively. However, the positive response of GPP to precipitation was approximately 70.47%, while that to temperature was 64.05%. Precipitation had a stronger restrictive effect on GPP than did temperature in this region, and GPP exhibited a negative correlation with VPD. The results showed that an increase in VPD inhibits GPP to some extent. Under rapid global change, the P model GPP provides new GPP data for global ecology studies, and the comparison of various stress factors allows for improvement of the GPP model in the future. The results of this study will aid in understanding the dynamic processes of terrestrial carbon. These findings are helpful for estimating and predicting the carbon budget of forest ecosystems in karst regions, clarifying the regional carbon absorption capacity, clarifying the main factors limiting vegetation growth in these regions, promoting sustainable regional forestry development and serving the “dual carbon goal.” This work has important guiding significance for policy formulation to mitigate climate change.

Chromium stress influences several parameters of leaf dynamics and morphology in Oryza sativa L. genotypes

Chromium (Cr) stress is a prevalent abiotic stress that alters various physiological, growth and metabolic parameters in plants. The toxicity of chromium at concentrations up to 100 µM were studied in two common Indian Oryza sativa genotypes, Lalat upland and Swarnaprabha. More chromium accumulated in roots than in leaves in both genotypes, with Lalat upland taking up more chromium than Swarnaprabha. Higher chromium concentrations reduced various physiological traits including stomatal conductance, transpiration rate, photosynthetic water use efficiency, photosynthetic pigments content in both the genotypes, and this decline was more pronounced in Lalat upland. Increase in vapour pressure deficit, sub stomatal carbon dioxide concentration and malondialdehyde content was detected at 100 µM chromium treated genotypes, however the effects were higher in Lalat upland. Confocal microscopy showed stomatal guard cell deformation, greater stomatal area and closed stomatal aperture in the leaves of 100 µM chromium treated Lalat upland plants. Abscisic acid and Salicylic acid content and expression of NCED1 and NCED2 genes involved in stomata aperture regulation were enhanced in Lalat upland and down regulated in Swarnaprabha under chromium stress. Variation in glandular and non-glandular trichome density and length was found in the leaves of chromium stressed plants in both the genotypes. The results suggest that Lalat upland is more sensitive to chromium toxicity than Swarnaprabha at higher chromium concentrations.

The Contributions of Climate and Human Activities to Water Use Efficiency in China’s Drylands

In the context of global warming, terrestrial ecosystems have undergone significant variations. China has implemented a variety of ecological engineering methods to enhance carbon stocks. However, understanding the spatial and temporal dynamics of carbon and water in drylands under climate change remains limited. Here, our research elucidates carbon and water dynamics in China’s drylands over the last two decades, with a focus on understanding spatial–temporal changes and the effects of ecological engineering on the carbon–water cycle. Furthermore, this study investigates the relationships among climate change, water use efficiency (WUE), and its components—Gross Primary Productivity (GPP) and Evapotranspiration (ET)—identifying key climatic drivers and assessing possible directions for enhancing WUE under changing climate conditions. Our research indicates that both GPP and ET have significantly increased over the past 20 years, with growth rates of 4.96 gC·m−2·yr−1 and 4.26 mm·yr−1, respectively. Meanwhile, WUE exhibited a slight declining trend, at a rate of −0.004 gC·mmH2O·yr−1. This confirms the positive impact of vegetation restoration efforts. We found that fluctuations in interannual WUE were influenced by human activities and climate change. Precipitation (Prec) was the key climatic factor driving the GPP increase. Both solar radiation (Solra) and Prec were crucial for the interannual variation of WUE. Interestingly, WUE was the main factor affecting GPP development. The decline in WUE in drylands is linked to interannual variability in WUE and increased Vapor Pressure Deficit (VPD) due to warming. Seasonal variations in how WUE responds to climatic factors were also observed. For instance, fall rainfall increased WUE, while spring rainfall decreased it. Fall WUE was highly sensitive to VPD. Spatially, we found higher WUE in China’s eastern and Xinjiang regions and lower in inland areas and the Tibetan Plateau. Geomorphologic factors and soil conditions were the main drivers of this spatial variability in WUE. Temperature (Tem), Solra, VPD, and relative humidity (Relah) also played significant roles. Our results show a generalized inverse persistence in WUE variability. This suggests a potential for increased WUE in the eastern regions and a risk of decreased WUE on the Tibetan Plateau. Addressing the threat of vegetation decline in arid regions, particularly within the Tibetan Plateau, is crucial. It is essential to adapt forestry practices to complement the carbon and water cycles in these landscapes.

Peatland evaporation across hemispheres: contrasting controls and sensitivity to climate warming driven by plant functional types

Abstract. Peatlands store disproportionally large amounts of carbon per unit area, a function that is dependent on maintaining high and stable water tables. Climate change is likely to negatively impact carbon storage in peatlands, in part due to increases in vapour pressure deficit (VPD) driving higher evaporation (E) rates. However, the response of E to increasing VPD depends on the dominant vegetation type within peatlands. In this study, we used multiple years of eddy covariance (EC) measurements to compare E regimes at two peatlands with contrasting vegetation types – Kopuatai bog in Aotearoa / New Zealand, dominated by the vascular wire rush Empodisma robustum, and Mer Bleue bog in Canada, a “typical” shrub- and moss-dominated Northern Hemisphere peatland. We examined seasonal variability in E and equilibrium E (Eeq), energy balance partitioning, and the response of E, evaporative fraction (EF), and canopy conductance (gc) to VPD. Mean annual E was 45 % lower than mean annual Eeq at Kopuatai but only 16 % lower at Mer Bleue, demonstrating much greater limitations on E at Kopuatai. In addition, the mean midday (10:00–14:30 local standard time) dry-canopy Bowen ratio (β) at Kopuatai was 2.0 compared to 0.8 at Mer Bleue; therefore, the sensible heat flux (H) dominated over the latent heat flux (LE) at Kopuatai and vice versa at Mer Bleue. The responses of E, EF, and gc to increasing VPD at Kopuatai demonstrated stronger limitations on evaporative water loss for VPD > 0.7 kPa compared to Mer Bleue. The observed limitations at Kopuatai were attributed to strong stomatal control by E. robustum due to the rapid decrease in gc with increasing VPD; however, surface E could also be limited by its dense standing litter. At Mer Bleue, however, E was only weakly limited at VPD > 2 kPa, likely due to weak stomatal control over transpiration by the sparse shrub canopy and relatively large surface E from Sphagnum carpets. As such, the results of this study suggest that E. robustum drives a greater “hydrological resistance” to increasing VPD than the vegetation at Mer Bleue, leading to greater water retention at Kopuatai. This may enable greater resilience of the carbon sink function at Kopuatai to climatic warming and drying than at Mer Bleue.

An Unexpected Decline in Spring Atmospheric Humidity in the Interior Southwestern United States and Implications for Forest Fires

Abstract On seasonal time scales, vapor pressure deficit (VPD) is a known predictor of burned area in the southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970 to 2019 in the southwestern United States with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019. Significance Statement Burned forest area has increased significantly in the southwestern United States in recent decades, driven in part by an increase in atmospheric aridity [vapor pressure deficit (VPD)]. Increases in VPD can be caused by a combination of increasing temperature and decreasing specific humidity. As the atmosphere warms with climate change, its capacity to hold moisture increases. Despite this, there is a decrease in near-surface air humidity in the interior southwestern United States over 1970–2019, which during the summer is likely caused by a decline in early spring precipitation leading to limited soil moisture and evaporation in spring and summer. We estimate that this declining humidity alone, without an increase in temperature, would cause about one-quarter of the VPD-induced increase in burned forest area in this region over 1984–2019.

Assessment of CH4 flux and its influencing drivers in the rice-wheat agroecosystem of the Huai River Basin, China

To understand the CH4 flux variations and their climatic drivers in the rice-wheat agroecosystem in the Huai River Basin of China, the CH4 flux was observed by using open-path eddy covariance at a typical rice-wheat rotation system in Anhui Province from November 2019 to October 2021. The variations and their drivers were then analyzed with the Akaike information criterion method. CH4 flux showed distinct diurnal variations with single peaks during 9:00~13:00 local time. The highest peak was 2.15 µg m-2 s-1 which occurred at 11:00 in the vegetative growth stage in the rice growing season (RGS). CH4 flux also showed significant seasonal variations. The average CH4 flux in the vegetative growth stage in the RGS (193.8±74.2 mg m-2 d-1) was the highest among all growth stages. The annual total CH4 flux in the non-rice growing season (3.2 g m-2, 11.8%) was relatively small compared to that in the RGS (23.9 g m-2, 88.2%). CH4 flux increased significantly with increase in air temperature, soil temperature, and soil water content in both the RGS and the non-RGS, while it decreased significantly with increase in vapor pressure deficit in the RGS. This study provided a comprehensive understanding of the CH4 flux and its drivers in the rice-wheat rotation agroecosystem in the Huai River Basin of China. In addition, our findings will be helpful for the validation and adjustment of the CH4 models in this region.

Differential Effects of Increasing Vapor Pressure Deficit on Photosynthesis at Steady State and Fluctuating Light

Differential Effects of Increasing Vapor Pressure Deficit on Photosynthesis at Steady State and Fluctuating Light

Impact of meteorological variability on diurnal and seasonal net ecosystem productivity in a desert riparian forest ecosystem.

The desert riparian forests are susceptible to meteorological changes and contribute significantly to the net ecosystem productivity (NEP) variations of arid ecosystems. However, the responsive patterns of their NEP variations to the meteorological variabilities remain inadequately comprehended. To address this gap, we utilized seven years of eddy covariance flux measurements in a representative desert riparian forest to investigate the NEP variations and its response to changing meteorological factors across diverse temporal scales. The results revealed significant periodic variations in half-hourly NEP, with dominant cycles spanning from five hours to one year, with a principal oscillation period of one day. Key meteorological factors including global solar radiation (Rg), relative humidity (RH), air temperature (Ta), soil temperature (Ts), and vapor pressure deficit (VPD) exhibited synchronization with NEP on daily scales. This synchronization, coupled with the observed one-day periodic NEP variations, provides robust evidence supporting the existence of a circadian rhythm in the ecosystem carbon exchange of desert riparian forest regulated by meteorological conditions. Seasonal patterns were significant in the impact of Rg phase, Ta diurnal amplitude, and VPD diurnal amplitude on NEP diurnal amplitude and phase. The NEP diurnal amplitude significantly, directly, and positively affected daily NEP in both the dormant and growing seasons, whereas its phase yielded significant negative effects (P< 0.05). The averages, amplitudes, and phases of diurnal meteorological conditions controlled the daily NEP by regulating NEP diurnal amplitude and phase. These findings provide evidence that the variability in circadian rhythms, caused by the increase in diurnal Ta and VPD, significantly impact the daily NEP at an ecosystem scale. This study enriches our comprehension of the meteorological mechanisms governing diurnal and seasonal carbon uptake dynamics within desert riparian forests, providing fresh insights into the direct and indirect roles of climate change in shaping patterns of ecosystem carbon exchange.

Differentiated responses of plant water use regulation to drought in Robinia pseudoacacia plantations on the Chinese Loess Plateau

Robinia pseudoacacia plantations play an important role in improving the ecological environment of the Chinese Loess Plateau (CLP). However, drought stress is emerging as the major threat of sustainable growth in R. pseudoacacia plantations under the background of global warming and increasing water scarcity. Investigating the responses of plant water use to drought and the associated regulating mechanism in R. pseudoacacia is helpful for improving understanding of plant survival strategies and developing sustainable forest management practices under climate change. In this study, we monitored the canopy transpiration (Tr) dynamics with synchronous observations of soil water content and leaf water potentials in R. pseudoacacia plantations during two different hydrological years (2021 and 2022) at two sites featuring semihumid (Changwu) and semiarid (Mizhi) climate conditions. Results showed that normalized Tr exhibited stronger relationships with meteorological variables at the Changwu site than Mizhi site, as well as under non-drought condition compared to drought condition. The canopy stomatal conductance (Gc) decreased significantly with increasing vapor pressure deficit (VPD) and soil drought at both sites. The sensitivity of Gc to VPD revealed more strict stomatal regulation of transpiration in response to drought at the Changwu site, and less strict stomatal regulation at the Mizhi site. Relationship between midday and predawn water potentials indicated a partial isohydric strategy in response to drought, and reflected stomatal closure tends to occur more rapidly than hydraulic conductivity loss in R. pseudoacacia. These results suggest that the Tr and Gc values of R. pseudoacacia and their sensitivity to climate weakened as soil drought progresses and varied with different climatic conditions, and R. pseudoacacia exhibited flexible stomatal regulation of transpiration and water use strategies in response to drought.

Impact of rainfall and vapor pressure deficit on latewood growth and water stress in Douglas-fir in a Mediterranean climate

Both elevated temperature and reduced precipitation have been related to growth declines in Douglas-fir (Pseudotsuga menziesii) in the northwest U.S. However, the impact of high vapor pressure deficit (VPD) on Douglas-fir growth and physiological stress is not fully understood. We investigated how inter- and intra-annual rainfall and VPD correlated to the radial growth and carbon isotope signature (δ13C) of latewood for ∼ 50-year-old Douglas-fir trees in the western Cascade Mountains in Oregon. Latewood δ13C reflects variation in stomatal restriction of photosynthetic gas exchange and, therefore, was used as a proxy for the relative degree of physiological water stress. We cored three trees at each of nine sampling sites (n = 27 trees) and used a moving window analysis to test the period of the year in which VPD and rainfall best predicted mean latewood radial growth and δ13C. Latewood growth, measured as the basal area increment, was more sensitive to daytime VPD than the timing and amounts of rainfall, especially in early summer. In contrast, δ13C was equally sensitive to the average daytime VPD and total rainfall during spring and summer. We used the results of the moving window analysis in a linear mixed effects model to test how the effect of VPD and rainfall on yearly latewood growth and δ13C differed among our nine sites. We found no evidence for statistical differences in the effects of VPD and rainfall on growth (p = 0.93 and p = 0.91) or δ13C (p = 0.31 and p = 0.81) among our nine sites. However, the marginal effects of VPD on latewood growth at each site were weakly related to soil moisture deficits at 100 cm suggesting that site-to-site differences in soil moisture availability may be important in buffering the negative effects of seasonal aridity on growth. In contrast, there was no evidence that soil moisture differences among sites influenced the marginal effects of VPD on latewood δ13C. We conclude that increases in VPD during summer are likely to reduce latewood growth and increase water stress in Douglas-fir in the Pacific Northwest region. However, more research is needed to better understand the magnitude of this effect across sites with variable subsurface water storage and microclimate.

On the Pattern and Attribution of Pan Evaporation over China (1951–2021)

Abstract Pan evaporation (Epan) serves as a monitorable method for estimating potential evaporation, evapotranspiration, and reference crop evapotranspiration, providing crucial data and information for fields such as water resource management and agricultural irrigation. Based on the PenPan model, the monthly Epan was calculated over China during 1951–2021, resulting in an average R2 of 0.93 ± 0.045 and an RMSE of 21.48 ± 6.06 mm month−1. The trend of Epan over time was characterized by an initial increase before 1961, followed by a decrease from 1961 to 1993, and a subsequent increase from 1994 to 2021. However, the sustained duration and magnitude of the decreasing trend led to an overall decreasing trend in the long-term dataset. To better understand the drivers of Epan trends, the Epan process was decomposed into radiative and aerodynamic components. While radiation was found to be the dominant component, its trend remained relatively stable over time. In contrast, the aerodynamic component, although smaller in proportion, exhibited larger fluctuations and played a crucial role in the trend of Epan. The primary influencing factors of the aerodynamic component were found to be wind speed and vapor pressure deficit (VPD). Wind speed and VPD jointly promoted Epan before 1961, and the significant decrease in wind speed from 1961 to 1993 led to a decrease in Epan. From 1994 to 2021, the increase in VPD was found to be the main driver of the observed increase in Epan. These results show the complex and dynamic nature of Epan and underscore the need for continued monitoring and in-depth analysis of its drivers. Significance Statement The primary objective of this study is to explore the spatiotemporal patterns and potential driving factors of pan evaporation in China based on constructing a comprehensive dataset of pan evaporation. This is important because pan evaporation is an important indicator of the water cycle, which is currently undergoing modifications and is expected to become more pronounced as the climate continues to warm. Our findings showed that the patterns of pan evaporation were characterized by its drivers. As the drivers are numerous and continuously changing under climate change, it is necessary to pay attention to the pattern and attribution of pan evaporation.

Assessing carbon and water fluxes in a mixed Mediterranean protected forest under climate change: An integrated bottom – up and top – down approach

This paper proposes a framework based on the integration of bottom-up and top-down approaches to assess and quantify the effect of local climate change on carbon and water fluxes in a protected Mediterranean mixed forest. The forest, with an extent of 50 ha, is in a coastal zone and is in decline because of severe summer drought stress. Since 2018, it has been involved in the LIFE PRIMED ecological restoration project.The bottom-up approach used gas exchange measurements and the MOCA model (MOdelling for Carbon Assessment) to quantify photosynthesis and transpiration of the main species in the forest (P. latifolia, P. lentiscus, Q. cerris, and F. angustifolia sub. oxycarpa). The top-down approach used the satellite models C-fix and NDVI-CWS, to quantify and spatialise gross primary production (GPP), net primary production (NPP), and evapotranspiration (ETA) at the forest stand scale. The up-scaling process from leaf to forest stand was made possible by calibrating two key parameters: light use efficiency (ε) and vegetation transpiration coefficient (Kcveg).The results indicate a decline in GPP, NPP, and ETA during the study period (2018–2022) due to the summer drought conditions caused by rising temperatures (16.60 °C in 2018 vs. 16.98 °C in 2022), increased vapor pressure deficit, reduced soil water content, and rainfall (1328 mm in 2018 vs. 468 mm in 2022). In 2018, the annual GPP was 1186.83 ± 351.40 gC/m2y, which decreased to 1128.67 ± 331.21 gC/m2y in 2022. During the dry summer period, GPP ranged from 401.76 ± 119.54 gC/m2y in 2018 to 347.39 ± 101.62 gC/m2y in 2022. Annual ETA rates varied between 1184.86 ± 351.86 mmH₂O/m2y (2018) and 1304.92 ± 384.07 mmH₂O/m2y (2022). The summer ETA rates were significantly influenced by drought conditions, decreasing from 638.73 ± 190.67 mmH₂O/m2y in 2018 to 390.35 ± 108.83 mmH₂O/m2y in 2022.Based on the observed trend in other Mediterranean areas, species-specific responsiveness to local climate change suggests that P. latifolia and P. lentiscus, which are better adapted to water-limited environments, could gradually replace Q. cerris and F. angustifolia subs. oxycarpa.This study provides useful quantitative indications for the management and restoration goals of the area and suggests a methodology that can be applicable in several management practices, particularly for small heterogeneous and fragmented forested areas such as the Mediterranean ones.

Vapour pressure deficit was not a primary limiting factor for gas exchange in an irrigated, mature dryland Aleppo pine forest.

Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (gbr ), transpiration rate (E) and net photosynthesis (Anet ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and gbr to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the gbr and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

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

Polylepis trees occur throughout the Andean mountain region, and it is the tree genus that grows at the highest elevation worldwide. In the humid Andes where moisture is rarely limiting, Polylepis trees must adapt to extreme environmental conditions, especially rapid fluctuations in temperature, ultraviolet radiation and vapor pressure deficit (VPD). However, Polylepis' water-use patterns remain largely unknown despite the importance of understanding their response to microclimate variation to determine their capacity to maintain resilience under future environmental change. We conducted a study in a Polylepis reticulata Kunth forest in the Ecuadorian Andes to evaluate its tree water-use dynamics and to identify the main environmental drivers of transpiration. Tree sap flow was monitored simultaneously with soil volumetric water content (VWC) and microclimate during 2 years for trees growing in forest edge and interior locations. We found that sap flow was primarily controlled by VPD and that VWC exerted a secondary role in driving sap flow dynamics. The highest values for sap flow rates were found when VPD>0.15kPa and VCW<0.73cm3cm-3, but these threshold conditions only occurred during brief periods of time and were only found in 11% of our measurements. Moreover, these brief windows of more favorable conditions occurred more frequently in forest edge compared with forest interior locations, resulting in edge trees maintaining 46% higher sap flow compared with interior trees. Our results also suggest that P. reticulata has a low stomatal control of transpiration, as the sap flow did not decline with increasing VPD. This research provides valuable information about the potential impacts of projected future increases in VPD due to climate change on P. reticulata water-use dynamics, which include higher sap flow rates leading to greater transpirational water loss due to this species' poor stomatal control.

Drivers of chaparral photosynthetic rate reduction under modern drought conditions

Terrestrial vegetation communities are experiencing rapid and novel changes to photosynthetic rates under the changing climate. Chaparral, a semi-arid shrubland ecosystem of the Southwestern United States and Northern Mexico, is projected to experience substantial increases in aridity and stochastic precipitation. This study identifies the primary meteorological drivers of photosynthesis for three widespread chaparral shrub species—Adenostoma sparsifolium, Adenostoma fasciculatum, and Ceanothus perplexans—from 2019 to 2021. Monthly leaf-level carbon exchange rates, water potentials (WPs), and meteorological conditions were collected for each species. Average monthly primary productivity (n = 25) demonstrated vapor pressure deficit (VPD) as a significant limit to photosynthetic rates for A. sparsifolium and A. fasciculatum. VPD was also the most influential predictor of WP for all three species. These results suggest increasing atmospheric dryness as a key predictor for reduction in chaparral primary productivity, particularly for deeply-rooted, resprouting species. There are additional indications that VPD could exacerbate drought-related mortality for C. perplexans and A. sparsifolium by pushing WP to novel extremes. This study concludes that atmospheric dryness, across 3 years of differing soil water stress levels, was consistently a substantial physiological limitation for three common, chaparral species. Although this experiment occurred over a limited window and cannot assess climatic response trends, acute increases in air temperature and VPD within the region would exacerbate photosynthetic limitation for these species and may contribute to declining primary productivity in broader chaparral ecosystems.

The Divergent Resistance and Resilience of Forest and Grassland Ecosystems to Extreme Summer Drought in Carbon Sequestration

It is projected that extreme drought events will become more frequent and more severe across many regions of the globe by the end of the 21st century. Despite the substantial efforts that have been made to explore the impacts of droughts on terrestrial ecosystems, our understanding of the response of diverse ecosystems, including resistance and resilience, remains unclear. A total of 16 site years of eddy covariance-based carbon flux data were used to reveal the different responses of forest and grassland ecosystems to two extreme summer droughts. We found that the carbon fluxes of the forest, namely gross primary productivity (GPP), ecosystem respiration (Re), and the net ecosystem carbon exchange (NEE), exhibited distinct seasonal patterns with a single peak. However, GPP and NEE of grassland showed multiple peaks owing to hay harvesting throughout one year. Meanwhile, all climate factors jointly affected the seasonal dynamics in the NEE of the forest, whereas solar radiation only dominated the variability in the NEE of grassland. Moreover, the optimal response relationship was quadratic between the vapor pressure deficit (VPD) and the NEE, with the thresholds being 5.46 and 5.84 for forest and grassland, respectively. Owing to the large increase in VPD during the droughts of 2003 and 2018, the carbon sequestration of forest and grassland reduced sharply and even altered from carbon sink to carbon source. Compared with grassland, forest GPP showed stronger resistance with weaker resilience to droughts. However, larger resilience appeared for both forest and grassland NEE relative to their resistance. All analyses reflect the different adaptive strategies among plant functional types, which is crucial to evaluate ecosystem carbon sequestration to overcome future climate change.

Sharp decline in future productivity of tropical reforestation above 29°C mean annual temperature.

Tropical reforestation is among the most powerful tools for carbon sequestration. Yet, climate change impacts on productivity are often not accounted for when estimating its mitigation potential. Using the process-based forest growth model 3-PGmix, we analyzed future productivity of tropical reforestation in Central America. Around 29°C mean annual temperature, productivity sharply and consistently declined (-11% per 1°C of warming) across all tropical lowland climate zones and five tree species spanning a wide range of ecological characteristics. Under a high-emission scenario (SSP3-7.0), productivity of dry tropical reforestation nearly halved and tropical moist and rain forest sites showed moderate losses around 10% by the end of the century. Under SSP2-4.5, tropical moist and rain forest sites were resilient and tropical dry forest sites showed moderate losses (-17%). Increased vapor pressure deficit, caused by increasing temperatures, was the main driver of growth decline. Thus, to continue following high-emission pathways could reduce the effectiveness of reforestation as climate action tool.

Global water use efficiency saturation due to increased vapor pressure deficit.

The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUEeco), is widely expected to increase because of the rising atmospheric carbon dioxide concentration (Ca). However, little is known about the interactive effects of rising Ca and climate change on WUEeco. On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUEeco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.