Changes In Vapor Pressure Deficit Research Articles

The stomatal response to vapor pressure deficit drives the apparent temperature response of photosynthesis in tropical forests.

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.

Contrasting water-use strategies revealed by species-specific transpiration dynamics in the Caatinga dry forest.

In forest ecosystems, transpiration (T) patterns are important for quantifying water and carbon fluxes and are major factors in predicting ecosystem change. Seasonal changes in rainfall and soil water content can alter the sensitivity of sap flux density to daily variations in vapor pressure deficit (VPD). This sensitivity is species-specific and is thought to be related to hydraulic strategies. The aim of this work is to better understand how the sap flux density of species with low versus high wood density differ in their sensitivity to VPD and soil water content and how potentially opposing water-use strategies influence T dynamics, and ultimately, correlations to evapotranspiration (ET). We use hysteresis area analysis to quantify the sensitivity of species-specific sap flux density to changes in the VPD, breakpoint-based models to determine the soil water content threshold instigating a T response and multiscalar wavelet coherency to correlate T to ET. We found that low wood density Commiphora leptophloeos (Mart.) Gillett had a more dynamic T pattern, a greater sensitivity to VPD at high soil water content, required a higher soil water content threshold for this sensitivity to be apparent, and had a significant coherency correlation with ET at daily to monthly timescales. This behavior is consistent with a drought avoidance strategy. High wood density Cenostigma pyramidale (Tul.) E. Gagnon & G. P. Lewis, conversely, had a more stable T pattern, responded to VPD across a range of soil water content, tolerated a lower soil water content threshold to T, and had a significant coherency correlation with ET at weekly timescales. This behavior is consistent with a drought-tolerant strategy. We build on previous research to show that these species have contrasting water-use strategies that should be considered in large-scale modeling efforts.

Changes in groundwater irrigation withdrawals due to climate change in Kansas

Warming temperatures increase the evapotranspiration demand of crops, leading to an increase in irrigation and exacerbating water scarcity. Previous research relies on models of irrigation water requirements to understand the potential impacts of climate change, but these models have significant uncertainty and ignore the risk-averse behavior of irrigators. Here we develop regression models to estimate how changes in vapor pressure deficit and precipitation affect groundwater withdrawals for corn, soybeans, and wheat using well-level data from the Kansas portion of the High Plains Aquifer. Withdrawals are expected to increase for all three crops, with the largest increase for soybeans. Even after accounting for the CO2 improvements in transpiration efficiency, we find that total withdrawals are expected to increase by 5.9% (7.6%) by mid-century under RCP 4.5 (8.5). The increase in withdrawals is expected to accelerate the decline in aquifer water levels and is therefore important to consider when projecting future aquifer conditions.

Soil Moisture and Atmospheric Aridity Impact Spatio‐Temporal Changes in Evapotranspiration at a Global Scale

AbstractEvapotranspiration (ET) constitutes the water exchange from land to the atmosphere, which in turn modulates precipitation and soil moisture (SM). Multiple lines of evidence document complex feedbacks between changes in ET and temperature, atmospheric CO2 and vegetation greening. However, the existing analyses on global changes in ET do not account for the direct effects of SM supply and atmospheric water demand, expressed by vapor pressure deficit (VPD), while considering multiple environmental variables. Here we evaluated the performance of ET products using 140 flux towers included in the FLUXNET database. All ET products show reasonable performance, with an overall correlation higher than 0.7 and better performance at a higher latitude. From analysis of the ensemble mean of annual ET, we show insignificant (P = 0.06) trends in global ET during 1982–2020 and a significantly (P < 0.01) increasing trend during 2002–2020. Changes in GLEAM ET generally exert a positive response to changes in SM and a negative response to changes in VPD. Yet, these effects are not globally consistent and are largely determined by changes in vegetation transpiration. Using our finding as a benchmark, Earth System Models mostly reproduce the positive response of ET to SM with less coupling strength, while showing negative effects of VPD on ET with stronger coupling strength. Our study highlights that concurrent soil drying and atmospheric aridity could intensify water exchanges and the importance of realistically representing SM‐VPD‐ET interactions in models for accurate predictions of the hydrological cycle.

Stem growth of Amazonian species is driven by intra-annual variability in rainfall, vapor pressure and evapotranspiration

Intra-annual distribution of precipitation in central Amazonia often leads to a short mild dry season and an increase in irradiance, temperature, and vapor pressure deficit; however, the accurate effect of intra-annual microclimatic variability on stem growth is still under investigation. The objective of this study was to determine how stem growth responds to monthly variations in microclimatic factors in the central Amazon. During five years (2008-2012) we measured diameter stem growth of 109 trees (26 species) and used principal component regression to evaluate the effect of microclimatic variability on stem growth in diameter. We found that the mean stem growth in diameter across species increased in response to an increase in rainfall and reference evapotranspiration, but it decreased with a rise in mean and minimum vapor pressure deficit. A contribution of this study is to show that even when irradiance and temperature had no significant effect on stem growth, small changes in vapor pressure deficit significantly affect stem growth. If the dry season becomes longer, as predicted by models, trees currently more sensitive to microclimatic variability associated with droughts would be the most affected by climate changes.

Stomatal conductance in rice leaves and panicles responds differently to abscisic acid and soil drought.

Improvement of photosynthesis in non-foliar green tissues is beneficial for enhancing crop yield. Recently, we have demonstrated that panicle stomatal conductance is a major limiting factor for photosynthesis. However, mechanisms underlying the responses of panicle stomatal conductance (gs,panicle) and photosynthesis (Apanicle) to environmental stimuli remain unknown. In the present study, the responses of gs,panicle and leaf stomatal conductance (gs,leaf) to exogenous application of abscisic acid and step-changes in vapor pressure deficit were investigated at the anthesis stage in pot-grown rice plants. Furthermore, the effects of drought on Apanicle and leaf photosynthesis (Aleaf) were examined. Smearing and xylem feeding of abscisic acid significantly decreased gs,leaf. In contrast, while smearing of abscisic acid substantially increased gs,panicle, its xylem feeding dramatically decreased gs,panicle. In addition, both gs,leaf and gs,panicle effectively responded to step changes in vapor pressure deficit. Furthermore, both Aleaf and Apanicle were sensitive to plant dehydration; however, given the lower sensitivity of panicle water potential than leaf water potential to drought, Apanicle was less sensitive to soil drought than Aleaf. These findings indicate that gs,panicle is hydropassively regulated, while panicle photosynthesis is less sensitive to drought.

Temporal heterogeneity in photosystem II photochemistry in Artemisia ordosica under a fluctuating desert environment.

Acclimation strategies in xerophytic plants to stressed environmental conditions vary with temporal scales. Our understanding of environmentally-induced variation in photosystem II (PSII) processes as a function of temporal scales is limited, as most studies have thus far been based on short-term, laboratory-controlled experiments. In a study of PSII processes, we acquired near-continuous, field-based measurements of PSII-energy partitioning in a dominant desert-shrub species, namely Artemisia ordosica, over a six-year period from 2012-2017. Continuous-wavelet transformation (CWT) and wavelet coherence analyses (WTC) were employed to examine the role of environmental variables in controlling the variation in the three main PSII-energy allocation pathways, i.e., photochemical efficiency and regulated and non-regulated thermal dissipation, i.e., Φ PSII, Φ NPQ, and Φ NO, respectively, across a time-frequency domain from hours to years. Convergent cross mapping (CCM) was subsequently used to isolate cause-and-effect interactions in PSII-energy partitioning response. The CWT method revealed that the three PSII-energy allocation pathways all had distinct daily periodicities, oscillating abruptly at intermediate timescales from days to weeks. On a diurnal scale, WTC revealed that all three pathways were influenced by photosynthetically active radiation (PAR), air temperature (T a), and vapor pressure deficit (VPD). By comparing associated time lags for the three forms of energy partitioning at diurnal scales, revealed that the sensitivity of response was more acutely influenced by PAR, declining thereafter with the other environmental variables, such that the order of influence was greatest for T a, followed by VPD, and then soil water content (SWC). PSII-energy partitioning on a seasonal scale, in contrast, displayed greater variability among the different environmental variables, e.g., Φ PSII and Φ NO being more predisposed to changes in T a, and Φ NPQ to changes in VPD. CCM confirmed the causal relationship between pairings of PSII-energy allocation pathways, according to shrub phenology. A. ordosica is shown to have an innate ability to (i) repair damaged PSII-photochemical apparatus (maximum quantum yield of PSII photochemistry, with F v/F m > 0.78), and (ii) acclimatize to excessive PAR, dry-air conditions, and prolonged drought. A. ordosica is relatively sensitive to extreme temperature and exhibits photoinhibition.

The weakening of the interannual relationship between the rate of vegetation spring green-up and temperature related to warmer and drier preseason over the northern extratropics

Changes in the rate of spring green-up (RSP) caused by climate change can alter biosphere-atmosphere exchange of carbon, energy, and water cycles. Although the response of RSP to environmental conditions has been frequently discussed, little is known about the dynamic relationship between RSP and environmental variables over time. We investigated the interannual relationships between RSP and various environmental variables using the Ensemble Empirical Mode Decomposition (EEMD) detrending approach and partial correlation analyses. We then used 15-year moving windows to analyze the effects of accumulated green-up period temperature (AGT) on RSP and its temporal variation from 1982 to 2016.We found that: (1) AGT had a positive influence on RSP in 81.08% of the entire study regions and AGT dominated the interannual variation of RSP in 31.31% all pixels; (2) The percentage of moisture-controlled (vapor pressure deficit (VPD) and soil moisture) pixels was 28.71%, indicating that preseason moisture, especially vapor pressure deficit (negative correlations in 65.50% pixels), played critical roles in limiting RSP; (3) The linkage between RSP and AGT (RRSP–AGT) was significantly weakening over the Northern Extratropics with intensity decrease of −0.0017 year−1 (p < 0.05) and extent reduction of −0.4 × 105 km2 year−1 (p < 0.05), suggesting a decreasing influence of AGT on foliar development. (4) The decline in RRSP–AGT coincided with decreased chilling exposure (CE) and enhanced VPD during the preseason over the Northern Extrotrapics and Eurasia, but it only may be related to changes in VPD in the North America. Our findings highlighted the importance of the dynamic response of vegetation spring green-up to ongoing warming, for a better understanding of the vegetation seasonal cycle under climate change.

Vapor pressure deficit constrains transpiration and photosynthesis in holm oak: A comparison of three methods during summer drought.

High rates of vapor pressure deficit (VPD) can severely decrease plant productivity by reducing stomatal conductance, which might be exacerbated during Mediterranean summers due to soil water deficit. In this study, we monitored the response of holm oak, the archetype of Mediterranean trees, to changes in VPD during a summer drought period to evaluate the effects and consequences on gas exchange of the two water stresses (atmospheric and soil). Measurements were performed on trees growing in an experimental plantation over two summers with moderate drought stress by using three different methods: at the leaf level with an infrared gas analyzer, using a whole-plant chamber for short-term monitoring at the tree level, and measuring the canopy temperature for long-term monitoring. The three methods provided negative relationships between leaf conductance and VPD but with discrepancies probably associated with the measurement scale. Overall, the results showed that atmospheric and soil water stress had an additive effect. Under well-watered conditions, an increase in VPD was partially compensated by a reduction in stomatal conductance, resulting in a slight increase in the transpiration rates. With soil water deficit, the response to VPD resulted in a further decrease in stomatal conductance, reducing transpiration as a water saving strategy. The decrease in conductance in response to VPD was transitory, recovering to initial values as soon as the VPD decreased, both under well-watered and drought conditions. Due to this high sensitivity to atmospheric drought, the maximum carbon gain rates of holm oak were restricted to a short environmental window, which might modulate its physiological performance and natural distribution.

Estimating Bulk Stomatal Conductance in Grapevine Canopies

In response to changes in their environments, grapevines regulate transpiration using various physiological mechanisms that alter conductance of water through the soil-plant-atmosphere continuum. Expressed as bulk stomatal conductance at the canopy scale, it varies diurnally in response to changes in vapor pressure deficit and net radiation, and over the season to changes in soil water deficits and hydraulic conductivity of both the soil and plant. To help with future characterization of this dynamic response, a simplified method is presented for determining bulk stomatal conductance based on the crop canopy energy flux model by Shuttleworth and Wallace using measurements of individual vine sap flow, temperature and humidity within the vine canopy, and estimates of net radiation absorbed by the vine canopy. The methodology presented respects the energy flux dynamics of vineyards with open canopies, while avoiding problematic measurements of soil heat flux and boundary layer conductance needed by other methods, which might otherwise interfere with ongoing vineyard management practices. Based on this method and measurements taken on several vines in a non-irrigated vineyard in Bordeaux France, bulk stomatal conductance was estimated on 15-minute intervals from July to mid-September 2020 producing values similar to those presented for vineyards in the literature. Time-series plots of this conductance show significant diurnal variation and seasonal decreases in conductance associated with increased vine water stress as measured by predawn leaf water potential. Global sensitivity analysis using non-parametric regression found transpiration flux and vapor pressure deficit to be the most important input variables to the calculation of bulk stomatal conductance, with absorbed net radiation and bulk boundary layer conductance being much less important. Conversely, bulk stomatal conductance was one of the most important inputs when calculating vine transpiration, emphasizing the usefulness of characterizing its dynamic response for the purpose of estimating vine canopy transpiration in water use models.

Carbon reservoirs in shade-tolerant morphotype of Paubrasilia echinata are more susceptible to humidity and temperature changes than sun-tolerant morphotype

Given the eminent climate changes and the importance of carbon dynamics in the growth and survival of plants, we investigated how the non-structural carbohydrates – NSC – and cell wall polymers – CWP (cellulose, hemicellulose, and lignin) – from sun-tolerant and shade-tolerant plants will respond to changes in vapor pressure deficit (VPD) and temperature (temp.) of atmosphere predicted for the southeast region of Brazil for the next decades. We used plants of two morphotypes of Paubrasilia echinata as a biological model. This species presents intraspecific variations from each other in leaf morphology and growth concerning luminosity in the initial phase of growth. The small morphotype is shade-tolerant and the medium morphotype is tolerant to full sun. The work was carried out in greenhouses adjusted with the combination of the higher and lower VPD and temp., constituting 4 enviroments: 1) 0.4 KPa – 19.8 °C, 2) 0.5 KPa – 23.8 °C, 3) 2.0 KPa – 25.1 °C, and 4) 1.1 KPa – 14.9 °C. On day 125 after the start of the experiment, we determined the concentrations of NSC and CWP in the root, stem, and leaves. The shade-tolerant morphotype showed an increase in sugar concentration in the environment where the plants grew the most (2.0 KPa – 25.1 °C). In this same environment, the NSC concentrations of the shoot organs increased in the sun-tolerant morphotype. The 0.4 KPa – 19.8 °C reduced the cellulose and lignin concentrations of the shade-tolerant morphotype and promoted the hemicelluloses of the sun-tolerant morphotype. The NSC and CWP allocation proved to be more influenced by VPD and temp. variations in the shade-tolerant than the sun-tolerant morphotype. We conclude that warming and lower atmospheric humidity could inhibit the axial stem growth of sun-tolerant juvenile plants. Warming, regardless of atmospheric humidity variations, will not affect the cellulose and lignin levels of stem, and consequently, the wood quality of shade- and sun-tolerant plants. However, hemicelluloses concentrations of sun-tolerant plants may decline in a warmer and less humid atmosphere.

Effect of vapour pressure deficit on gas exchange of field-grown cotton

BackgroundPlants respond to changes in vapour pressure deficit (VPD) between the leaf and the atmosphere through changes in stomatal response, which can consequently affect transpiration, photosynthesis, and leaf-level water use efficiencies. With projected warmer air temperatures, changes in rainfall distribution and altered VPD in future climates, it is important to understand the potential effect of VPD on leaf-level physiology of field-grown crops. The aim of this study was to assess the impact of altered VPD on leaf-level physiology of field-grown cotton to improve the current understanding of the plant-by-environment interaction, thereby contributing to validation and improvement of physiological and yield response models. Different VPD environments in the field were generated by planting cotton on three dates within the sowing window (early-season (S1) = 5th October 2011; mid-season (S2) = 9th November 2011; and late-season (S3) = 30th November 2011). VPD was also modified by altering crop irrigations.ResultsVPDL accounted for the largest proportion of the explained variation in both stomatal conductance (32%∼39%) and photosynthetic (16%∼29%) responses of cotton. Generally, smaller percentages of variation were attributed to other main factors such as the individual plant (Plant), and accumulated temperature stress hours (ASH; a measure of plant water status over time) and interactive factors, including leaf vapour pressure deficit (VPDL) × Plant and Plant × ASH; however, a proportion of variation was unexplained. In addition, the Asat/E (instantaneous transpiration efficiency, ITE) model developed based on cotton grown in the glasshouse was applied to cotton grown in the field. We found that the modelled Asat/E and field-measured Asat/E were very similar, suggesting that the mechanistic basis for ITE was similar in both environments.ConclusionsThis study highlights the importance of accounting for VPD in climate change research, given that stomata are highly responsive to changes in VPD. This experiment provides a basis for physiology and production models, particularly in terms of cotton response to projected climatic environments.

High-Resolution Analysis of Growth and Transpiration of Quinoa Under Saline Conditions.

The Plantarray 3.0 phenotyping platform® was used to monitor the growth and water use of the quinoa varieties Pasto and selRiobamba under salinity (0–300 mM NaCl). Salinity reduced the cumulative transpiration of both varieties by 60% at 200 mM NaCl and by 75 and 82% at 300 mM NaCl for selRiobamba and Pasto, respectively. Stomatal conductance was reduced by salinity, but at 200 mM NaCl Pasto showed a lower reduction (15%) than selRiobamba (35%), along with decreased specific leaf area. Diurnal changes in water use parameters indicate that under salt stress, daily transpiration in quinoa is less responsive to changes in light irradiance, and stomatal conductance is modulated to maximize CO2 uptake and minimize water loss following the changes in VPD (vapor pressure deficit). These changes might contribute to the enhanced water use efficiency of both varieties under salt stress. The mechanistic crop model LINTUL was used to integrate physiological responses into the radiation use efficiency of the plants (RUE), which was more reduced in Pasto than selRiobamba under salinity. By the end of the experiment (eleven weeks after sowing, six weeks after stress), the growth of Pasto was significantly lower than selRiobamba, fresh biomass was 50 and 35% reduced at 200 mM and 70 and 50% reduced at 300 mM NaCl for Pasto and selRiobamba, respectively. We argue that contrasting water management strategies can at least partly explain the differences in salt tolerance between Pasto and selRiobamba. Pasto adopted a “conservative-growth” strategy, saving water at the expense of growth, while selRiobamba used an “acquisitive-growth” strategy, maximizing growth in spite of the stress. The implementation of high-resolution phenotyping could help to dissect these complex growth traits that might be novel breeding targets for abiotic stress tolerance.

To what extent can rising [CO2 ] ameliorate plant drought stress?

Plant responses to elevated atmospheric carbon dioxide (eCO2 ) have been hypothesized as a key mechanism that may ameliorate the impact of future drought. Yet, despite decades of experiments, the question of whether eCO2 reduces plant water use, yielding 'water savings' that can be used to maintain plant function during periods of water stress, remains unresolved. In this Viewpoint, we identify the experimental challenges and limitations to our understanding of plant responses to drought under eCO2 . In particular, we argue that future studies need to move beyond exploring whether eCO2 played 'a role' or 'no role' in responses to drought, but instead more carefully consider the timescales and conditions that would induce an influence. We also argue that considering emergent differences in soil water content may be an insufficient means of assessing the impact of eCO2 . We identify eCO2 impact during severe drought (e.g. to the point of mortality), interactions with future changes in vapour pressure deficit and uncertainty about changes in leaf area as key gaps in our current understanding. New insights into CO2 × drought interactions are essential to better constrain model theory that governs future climate model projections of land-atmosphere interactions during periods of water stress.

Past Variance and Future Projections of the Environmental Conditions Driving Western U.S. Summertime Wildfire Burn Area.

Increases in vapor pressure deficit (VPD) have been hypothesized as the primary driver of future fire changes. The Coupled Model Intercomparison Project Phase 5 (CMIP5) models agree that western U.S. surface temperatures and associated dryness of air as defined by the VPD will increase in the 21st century for Representative Concentration Pathways (RCPs) 4.5 and 8.5. However, we find that averaged over seasonal and regional scales, other environmental variables demonstrated to be relevant to flammability, moisture abundances, and aridity—such as precipitation, evaporation, relative humidity, root zone soil moisture, and wind speed—can be used to explain observed variance in wildfire burn area as well or better than VPD. However, the magnitude and sign of the change of these variables in the 21st century are less certain than the predicted changes in VPD. Our work demonstrates that when objectively selecting environmental variables to maximize predictive skill of linear regressions (minimize square error on unseen data) VPD is not always selected and when it is not, the magnitude of future increases in burn area becomes less certain. Hence, this work shows that future burn area predictions are sensitive to what environmental predictors are chosen to drive burn area.

Observed changes in vapor pressure deficit suggest a systematic drying of the atmosphere in Xinjiang of China

The vapor pressure deficit (VPD) is the difference between the atmospheric water vapor holding capacity at saturation (es) and actual amount of water vapor (ea) in the air at a given temperature. In this study, we investigated the changes in VPD and related variables (air temperature, relative humidity, es and ea), and assessed the relationship between VPD and atmospheric conditions in Xinjiang over the period 1961–2017. The annual VPD records showed increasing trends during all seasons. In terms of spatial distribution, the increasing and decreasing trends of annual VPD were 79% (57% significant) and 21% (9% significant) of the observation stations, respectively. A trend reversal from downward to upward was observed in 1993, and the VPD records indicated rapid drying of the atmosphere in Xinjiang for 1994–2017, especially in the summer months. The differences in es and ea resulted in changes in the VPD, and the ea generally increased less than the es. We suggest that the increasing VPD in Xinjiang is due to increasing es from rising temperatures and decreasing ea from decreasing relative humidity since 1994. The decreased relative humidity has a strong influence on the increased VPD and accounts for ~74% of the variance in Xinjiang. In different environments, the VPD trends followed changes in air temperature and ea, with the greatest VPD increase in the desert, followed by the oasis and mountain environments. The increased VPD observed in Xinjiang between 1994 and 2017 is consistent with the global intensification of drought and adverse ecological effects of the 21st century. Thus, VPD change can be an accurate measure of regional drought and vegetation dynamics in Xinjiang.

Drought Effects on Tectona grandis Water Regulation Are Mediated by Thinning, but the Effects of Thinning Are Temporary

Results from tropical planted forests have demonstrated that tree plantations can have variable growth and water use patterns in response to drought. Yet research on how specific species will perform during a drought and whether this response can be mediated through forest management is still poorly understood. We took advantage of the 2015-2016 El Nino drought in central Panama to test the effects of thinning on sap flux density, transpiration, and growth of planted Tectona grandis (teak), a non-native species introduced to Panama for timber production. Despite a reduction in growth of teak during drought for control and thinned sites, tree sap flux density of thinned sites significantly temporarily increased after the thinning. Sap flux density (Js) for teak is strongly driven by changes in vapor pressure deficit (VPD), temperature, and radiation; however, Js declines as temperature rises above 28C and VPD is above 0.5, suggesting a temperature threshold that could be problematic as droughts and temperatures increase in unison in the future. At the stand-level, all sites reduced transpiration during the drought. Although diameter growth and transpiration declined during drought, the leaf area index after the drought ended returned to pre-drought levels.

Are stomata in ferns and allies sluggish? Stomatal responses to CO2 , humidity and light and their scaling with size and density.

Fast stomatal reactions enable plants to successfully cope with a constantly changing environment yet there is an ongoing debate on the stomatal regulation mechanisms in basal plant groups. We measured stomatal morphological parameters in 29 fern and allied species from temperate to tropical biomes and two outgroup angiosperm species. Stomatal dynamic responses to environmental drivers were measured in 16 ferns and the two angiosperms using a gas-exchange system. Principal components analyses were used to further reveal the structure-function relationships in stomata. We show a > 10-fold variation for stomatal opening delays and 20-fold variation for stomatal closing delays in ferns. Across species, stomatal responses to vapor pressure deficit (VPD) were the fastest, while light and [CO2 ] responses were slower. In most cases the outgroup species' reaction speeds to changes in environmental variables were similar to those of ferns. Correlations between stomatal response rate and size were apparent for stomatal opening in light and low [CO2 ] while not evident for closing reactions and changes in VPD. No correlations between stomatal density and response speed were observed. Together, this study demonstrates different mechanisms controlling stomatal reactions in ferns at different environmental stimuli, which should be considered in future studies relating stomatal morphology and function.

Contrasting Evaporative Responses of Ecosystems to Heatwaves Traced to the Opposing Roles of Vapor Pressure Deficit and Surface Resistance

AbstractUnderstanding the evaporative response of ecosystems to heatwaves is critical for managing ecosystem services and water resources, especially under a changing climate. In this study, we examine the land‐atmosphere exchange of water and heat fluxes under heatwave and nonheatwave conditions across five different land cover types, including grasslands, shrublands, croplands, deciduous broadleaf forests, and evergreen needleleaf forests, using data from eddy covariance towers. Results show that net radiation and sensible heat flux increase from nonheatwave to heatwave conditions across all five land cover types but latent heat flux shows contrasting responses to heatwaves. An attribution analysis further demonstrates that heatwave‐induced changes in evaporative fraction are mainly caused by changes in vapor pressure deficit (positive contribution) and changes in surface resistance (negative contribution). The imbalance between the positive and negative contributions varies across the five land cover types and is responsible for their contrasting evaporative responses to heatwaves.

Evapotranspiration in High‐Yielding Maize and under Increased Vapor Pressure Deficit in the US Midwest

Core Ideas Higher maize yields do not require more water. The transpiration efficiency approach results in biased estimates of ET in high‐yielding maize production. Evapotranspiration does not change in higher plant populations if water supply is adequate. Projected changes in vapor pressure deficit will not lead to greater water use in maize. This paper describes the results of an analysis demonstrating that high yields in maize can be obtained without additional water under current and projected vapor pressure deficits. The objective of the study was to quantify evapotranspiration (ET) in high‐yielding maize under current and projected vapor pressure deficits using the energy balance contrasted with the transpiration efficiency (TE) approach. This study indicates a lack of accuracy and bias in the TE approach when future crop water requirements were estimated. High maize yields are achievable using on average 700 mm of water as demonstrated by the current record maize grain yield of 34 Mg ha−1, which is ∼23 Mg ha−1 higher than the US average. These yields are achievable with approximately the same ET even under projected changes in vapor pressure deficit, through improved genetics and optimum agronomic management.