VPD ≠ VPD: Heat-driven increases in evaporative demand amplify water use sensitivity to soil drying in European beech (Fagus sylvatica L.)

Earth is currently undergoing global warming and “atmospheric drying” as a result of the increase in atmospheric water vapor pressure deficit (VPD). Rising heat and VPD are exposing plants to two problems: water and temperature stress. Through closing, stomata prevent excessive water loss when the VPD is high, thereby protecting their hydraulic integrity. Conversely, through opening stomata, plants avoid overheating. As high temperatures increase VPD, an emergent trade-off arises between water saving and latent cooling. In nature, VPD inherently covaries with both temperature and relative humidity. The resulting net effect on the degree of stomatal openness and its variability in relation to pedoclimatic conditions remains elusive.  

To close this knowledge gap, we grew European beech (Fagus sylvatica L.) in controlled climate chambers leveraging lysimeters filled with loam or sand to simulate contrasting soil hydraulic environments.. Trees were subjected to irrigated and drought-stressed conditions. Three VPD treatments were imposed: (1) low VPD (1.3 kPa) via decreasing relative humidity (RH) and increasing temperature, (2) elevated VPD (2.3 kPa) via decreasing RH at stable temperature, and (3) elevated VPD (2.3 kPa) via increasing temperature at stable RH. We measured the following parameters: soil water content and potential, transpiration via custom-made sensors (TransP), gas exchange using LI-6800, and leaf water potential via optical dendrometers calibrated against Scholander Bomb point measurements.

When elevated VPD was driven by increasing temperature, plants transpired linearly with rising VPD until higher thresholds in wet soil compared to humidity-driven elevated VPD, and consequently exhibited a more pronounced sensitivity to soil drying across both textures, i.e., reductions in transpiration rate and leaf water potential in wetter soil conditions. In the temperature increase treatment, trees also demonstrated enhanced thermal tolerance in both soil textures, as indicated by a higher temperature at which the plant's photosynthetic efficiency drops by 50% (T₅₀). No VPD treatment-induced differences emerged in above- and belowground morphology (e.g., root and leaf area), whole-plant hydraulic conductance, or pre-dawn stomatal conductance, suggesting primarily physiological rather than structural-hydraulic acclimation. Soil texture modulated response strength but not direction.

These results demonstrate that different drivers of increasing VPD profoundly alter plant water-use regulation: warming-induced rises in evaporative demand allow for sustained transpiration until higher VPD in wet soil but increases water use sensitivity to soil drying. Our results indicate the need for disentangling temperature- from humidity-mediated VPD, as VPD ≠ VPD.