Temperature-driven vapor pressure deficit structures forest bryophyte communities across the landscape

Environmental conditions influence plant responses to ozone (O3), but few studies have evaluated individual factors directly. In this study, the effect of O3 at high and low atmospheric vapour pressure deficit (VPD) was evaluated in two genotypes of snap bean (Phaseolus vulgaris L.) (R123 and S156) used as O3 bioindicator plants. Plants were grown in outdoor controlled-environment chambers in charcoal-filtered air containing 0 or 60 nl l−1 O3 (12 h average) at two VPDs (1.26 and 1.96 kPa) and sampled for biomass, leaf area, daily water loss, and seed yield. VPD clearly influenced O3 effects. At low VPD, O3 reduced biomass, leaf area, and seed yield substantially in both genotypes, while at high VPD, O3 had no significant effect on these components. In clean air, high VPD reduced biomass and yield by similar fractions in both genotypes compared with low VPD. Data suggest that a stomatal response to VPD per se may be lacking in both genotypes and it is hypothesized that the high VPD resulted in unsustainable transpiration and water deficits that resulted in reduced growth and yield. High VPD- and water-stress-induced stomatal responses may have reduced the O3 flux into the leaves, which contributed to a higher yield compared to the low VPD treatment in both genotypes. At low VPD, transpiration increased in the O3 treatment relative to the clean air treatment, suggesting that whole-plant conductance was increased by O3 exposure. Ozone-related biomass reductions at low VPD were proportionally higher in S156 than in R123, indicating that differential O3 sensitivity of these bioindicator plants remained evident when environmental conditions were conducive for O3 effects. Assessments of potential O3 impacts on vegetation should incorporate interacting factors such as VPD.

Atmospheric vapor pressure deficit (VPD) measures the difference between saturation vapor pressure and actual vapor pressure, and its variability is closely related to fire activity in the western United States (US). Here, we assess the forecast skill of monthly VPD variability using a state-of-the-art dynamical forecast system and statistical predictions, such as the persistence forecast and model-analog forecasts. In the model-analog framework, we select analog states resembling the observed initial conditions from the model space, and the subsequent evolution of those initial model-analogs yields forecast ensembles. Dynamical forecasts demonstrate skillful predictions of VPD variability in the western US, exceeding the persistence forecast skill, which indicates additional sources of VPD predictability within the climate system. To quantify the contribution of different climate variables to VPD prediction, we develop a weighted model-analog forecast and evaluate its skill in comparison to VPD-only and unweighted forecasts. Our findings suggest that sea surface temperature is a critical source of VPD predictability over the western US. The optimally weighted model-analog exhibits forecast skill for VPD variability comparable to that of the dynamical forecast system.

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO(2) assimilation, stomatal conductance (g(s)), quantum yield of photosystem II (phi(II)), and nonphotochemical quenching of chlorophyll fluorescence (q(N)) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO(2) assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g(s) initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40 degrees C, CO(2) assimilation and g(s) were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g(s). The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO(2) and O(2)) versus nonphotorespiring (700 microbars CO(2) and 2% O(2)) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O(2) increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO(2) in the leaf (C(i)) where C(i) was estimated from gas exchange measurements. However, estimates of C(i) from measurements of phi(II) and q(N) suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C(i). The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C(i) from gas exchange measurements. Under low VPD, 30 degrees C, high light, and saturating CO(2), castor bean (C(3) tropical shrub) has a rate of photosynthesis (61 micromoles CO(2) per square meter per second) that is about 50% higher than that of tobacco (C(3)) or maize (C(4)) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.

Our previous work reveals that daily maximum temperature (Tmax) over land has warmed at an accelerated rate in recent decades, contrasting with the faster warming of daily minimum temperature (Tmin) observed in earlier periods. This faster warming of Tmax relative to Tmin globally has led to a broadening of the diurnal temperature range (DTR), defined as the difference between Tmax and Tmin. However, the impacts of this accelerated daytime warming and increased DTR on the water and carbon cycles have remained largely unexplored.Here, we show that the asymmetric warming rates between Tmax and Tmin have amplified the atmospheric vapor pressure deficit (VPD)—the difference between saturated vapor pressure (SVP) and actual vapor pressure (AVP). This amplification arises because a faster rise in Tmax compared to Tmin drives a larger SVP increase, due to the near-exponential relationship between temperature and SVP. Simultaneously, AVP is more strongly influenced by Tmin, as air is typically closer to saturation during the cooler nighttime hours. We quantified that the increase in DTR accounted for approximately 20% of the additional increase in global VPD over land.We further investigated the response of terrestrial net primary production (NPP) to changes in DTR in the extratropical Northern Hemisphere over the past two decades. Our findings reveal divergent impacts of increased DTR on vegetation productivity in humid and arid zones, mirroring the contrasting effects of VPD on vegetation productivity in these regions. In humid zones, increases in DTR have promoted NPP, while in arid zones, the opposite effect is observed. This contrast is largely explained by the greater impact of accelerated daytime warming on increased VPD in arid zones, which inhibits NPP.Additionally, we employed flux tower measurements to analyze the effects of DTR on net ecosystem carbon exchange (NEE, with negative values indicating net carbon uptake by the land) across various ecosystems. Our results demonstrate differential responses of ecosystems to changes in DTR. For example, in deciduous broadleaf forests, increases in DTR have had a dual negative impact on NEE, enhancing plant daytime photosynthesis driven by higher daytime temperatures while a more gradual rise in Tmin slows nighttime respiration increases. In evergreen needleleaf forests, the faster increase in Tmax relative to Tmin generally resulted in increased NEE, leading to a weak positive correlation between DTR and NEE.Our findings provide compelling evidence that accelerated daytime warming over recent decades has significantly contributed to increased atmospheric dryness and has had divergent impacts on vegetation productivity in humid and arid zones. These results underscore the importance of understanding the responses of land surface hydrological processes, ecosystem productivity, and extreme events such as drought and wildfires to recent asymmetric warming dynamics.

Air humidity indicated as vapour pressure deficit (VPD) is directly related to transpiration and stomatal function of plants. We studied the effects of VPD and nitrogen (N) supply on leaf metabolites, plant growth, and mineral nutrition with young micropropagated silver birches (Betula pendula Roth.) in a growth chamber experiment. Plants that were grown under low VPD for 26 d had higher biomass, larger stem diameter, more leaves, fewer fallen leaves, and larger total leaf area than plants that were grown under high VPD. Initially, low VPD increased height growth rate and stomatal conductance; however, the effect was transient and the differences between low and high VPD plants became smaller with time. Metabolic adjustment to low VPD reflected N deficiency. The concentrations of N, iron, chlorophyll, amino acids, and soluble carbohydrates were lower and the levels of starch, quercetin glycosides, and raffinose were higher in the leaves that had developed under low VPD compared with high VPD. Additional N supply did not fully overcome the negative effect of low VPD on nutrient status but it diminished the effects of low VPD on leaf metabolism. Thus, with high N supply, the glutamine to glutamate ratio and starch production under low VPD became comparable with the levels under high VPD. The present study demonstrates that low VPD affects carbon and nutrient homeostasis and modifies N allocation of plants.

Rising atmospheric vapor pressure deficit (VPD)—a measure of atmospheric dryness, defined as the difference between saturated vapor pressure (SVP) and actual vapor pressure (AVP)—has been linked to increasing daily mean near-surface air temperatures since the 1980s. However, it remains unclear whether the faster increases in daily maximum temperature (Tmax) relative to daily minimum temperature (Tmin) have contributed to rising VPD. Here, we show that the faster rise in Tmax compared with Tmin over land has intensified VPD from 1980 to 2023. This sub-diurnal asymmetric warming has driven a larger SVP increase than would occur under uniform temperature rise, while AVP is more strongly influenced by Tmin. Using reanalysis data, we estimate that asymmetric warming has contributed an additional ~18% to the increase in global land VPD. Sub-daily station observations corroborate this pattern, with asymmetric warming accounting for ~30% of VPD intensification across all stations. Our findings indicate that sub-diurnal asymmetric warming has substantially amplified global warming’s effect on atmospheric dryness over the past four decades, with significant implications for terrestrial water availability and carbon cycling.

Vapour pressure deficit (VPD) has a significant effect on the amount of water required by the crop to maintain optimal growth. Data required to calculate the mean VPD on a daily basis are rarely available, and most models use approximations to estimate it. In APSIM (Agricultural Production Systems Simulator), VPD is estimated from daily maximum and minimum temperatures with the assumption that the minimum temperature equals dew point, and there is little change in vapour pressure or dew point during any one day. The accuracy of such VPD estimations was assessed using data collected every 15 min near Wagga Wagga in New South Wales, Australia. Actual vapour pressure of the air ranged from 0.5 to 2.5 kPa. For more than 75% of the time its variation was less than 20%, and the maximum variation was up to 50%. Daytime mean VPD ranged from 0 to 5.3 kPa. Daily minimum temperature was found to be a poor estimate of dew point temperature, being higher than dew point in summer and lower in winter. Thus the prediction of vapour pressure was poor. Vapour pressure at 0900 hours was a better estimate of daily mean vapour pressure. Despite the poor estimation of vapour pressure, daytime mean VPD was predicted reasonably well using daily maximum and minimum temperatures. If the vapour pressure at 0900 hours from the SILO Patched Point Dataset was used as the actual daily mean vapour pressure, the accuracy of daytime VPD estimation was further improved. Simulations using historical weather data for 1957–2002 show that such improved accuracy in daytime VPD estimation slightly increased simulated crop yield and deep drainage, while slightly reducing crop water uptake. Comparison of the APSIM RUE/TE and CERES-Wheat approaches for modelling potential transpiration revealed differences in crop water demand estimated by the two approaches. Although the differences had a small effect on the probability distribution of simulated long-term wheat yield, water uptake, and deep drainage, this finding highlights the need for a scientific re-appraisal of the APSIM RUE/TE and energy balance approaches for the estimation of crop demand, which will have implications for modelling crop growth under water-limited conditions and calculation of water required to maintain maximum growth.

The climatic causes of the major forest/grassland ecotone in the south central United States (Kansas, Missouri, Oklahoma, Texas) are still poorly understood. Grassland and forest vegetation types differ markedly in their ability to withstand water stress induced by vapor pressure deficit (VPD), the difference between saturation vapor pressure and actual vapor pressure in the atmosphere. VPD is an airmass characteristic induced by ambient temperatures higher than dewpoint temperature. Mean summer airmass movement is from the Gulf of Mexico onto the continent in the central states area, but mean VPD displays a strong gradient approximately parallel to the ecotone. A subset of days having the strongest VPD gradients across the ecotone also had a 500 mb pressure height pattern identical to the long-term mean (modal) pattern. This 500 mb pattern, with a ridge over the Rocky Mountains and a trough over the Great Lakes, induces subsidence, stability, warming, and high VPD in Great Plains airmasses. Farther east,...

Spatio-temporal changes in atmospheric aridity over the arid region of Central Asia during 1979–2019

The responses of tomato seedlings to different vapour pressure deficit (VPD) under low soil moisture were studied. Plants were grown in greenhouses with low and high VPD, under both well-watered and water stress conditions. Low VPD was effectively maintained below 1.5 kPa with a micro-fog system. Under well-watered conditions, low VPD resulted in reduced transpiration, but this did not affect plant water status or growth. Water stress induced leaf dehydration and inhibition of growth, but the adverse effects were significantly alleviated by a decrease in VPD. Under water stress, no difference in transpiration was observed between plants with or without the VPD regulation, but the whole-plant hydraulic conductance was higher under low VPD. Low VPD increased stomatal conductance in drought-stressed plants because it promoted stomatal development and increased stomatal aperture. Thus, stomatal limitation to photosynthesis was reduced by low VPD under water stress. The reduction in plant growth induced by water stress was moderated by low VPD, partially due to higher photosynthetic rate. These results suggest that decreasing VPD improves plant water status, which ultimately enhances photosynthesis and growth under water stress.

Unlike the commonly used relative humidity, vapor pressure deficit (VPD) is an absolute measure of the difference between the water vapor content of the air and its saturation value and an accurate metric of the ability of the atmosphere to extract moisture from the land surface. VPD has been shown to be closely related to variability in burned forest areas in the western United States. Here, the climatology, variability, and trends in VPD across the United States are presented. VPD reaches its climatological maximum in summer in the interior southwest United States because of both high temperatures and low vapor pressure under the influence of the northerly, subsiding eastern flank of the Pacific subtropical anticyclone. Maxima of variance of VPD are identified in the Southwest and southern plains in spring and summer and are to a large extent driven by temperature variance, but vapor pressure variance is also important in the Southwest. La Niña–induced circulation anomalies cause subsiding, northerly flow that drives down actual vapor pressure and increases saturation vapor pressure from fall through spring. High spring and summer VPDs can also be caused by reduced precipitation in preceding months, as measured by Bowen ratio anomalies. Case studies of 2002 (the Rodeo–Chediski and Hayman fires, which occurred in Arizona and Colorado, respectively) and 2007 (the Murphy Complex fire, which occurred in Idaho and Nevada) show very high VPDs caused by antecedent surface drying and subsidence warming and drying of the atmosphere. VPD has increased in the southwest United States since 1961, driven by warming and a drop in actual vapor pressure, but has decreased in the northern plains and Midwest, driven by an increase in actual vapor pressure.

Effects of soil moisture on water transport, photosynthetic carbon gain and water use efficiency in tomato are influenced by evaporative demand

Photosynthetic efficiency of three C4 species is maintained under low light and high vapour pressure deficit

Evapotranspiration (ET) or water loss to the atmosphere is one of the largest components of the hydrologic cycle, and its estimation is subject to uncertainties. Most ET estimation methods depend on vapor pressure deficit estimation. Improvements in saturation vapor pressure, actual vapor pressure, and vapor pressure deficit computations contribute to reducing errors in estimating ET. Using high-resolution meteorological data, various vapor pressure computations methods were compared. High-resolution saturation vapor pressure can be computed from high-resolution meteorological data reflecting diurnal fluctuations. In the absence of high-resolution meteorological data, daily average saturation vapor pressure is best estimated from the daily 24-h average relative humidity and the 24-h average air temperature followed by the average of daily maximum and minimum air temperature. Actual vapor pressure is best estimated from the 24-h mean air temperature and relative humidity. With some error, the average of the maximum and minimum air temperature and relative humidity can be applied to estimate actual vapor pressure. In this study, application of many equations is presented with correlation of the results with “true” estimates.

Vapour pressure deficit (VPD) and blue light (BL) are import signals for stomatal regulation and highly relevant during cultivation of economically important crops such as cucumber in controlled environments. However, the information about combined effects of VPD and BL and the regulation of abscisic acid (ABA) in this respect, is limited and seems to vary between species. This study investigated how moderate (1.12 kPa) or low (0.28 kPa) VPD combined with 5% or 30% BL affect the foliar ABA metabolism, transpiration rate, nutrient uptake, and growth in well-watered cucumber plants (Cucumis sativus ‘Quatro’). Additional BL, regardless of VPD, resulted in a decrease in the nighttime ABA content (52%), as well as increased adaxial stomatal density (8–19%), transpiration rate during the day (40–50%) and foliar nitrogen content (30%). ABA regulation was unaffected by VPD, but the BL levels during the day influenced the ABA inactivation at night; both the transpiration rate and the ABA-glucose ester (ABA-GE) content then increased in 30% BL compared to 5% BL. An increase in starch degradation due to additional BL led to higher content of the transport carbohydrate raffinose in source leaves and increased the fruit appearance rate (fruit day−1) in low VPD but reduced it in moderate VPD. This study provides novel insight into the regulation of ABA under different VPDs and BL proportions in cucumber, and reveals that under well-watered conditions, BL is a stronger determinant of stomatal aperture regulation than ABA. Also, a combination of high VPD and high BL can act as a weak stressor and affect cucumber growth and yield.