Effects of hot versus dry vapor pressure deficit on ecosystem carbon and water fluxes

Authors: JOHNSTON, M. - University Of Iowa; BARNES, M.L. - University Of Indiana; PREISLER, Y. - Harvard University; SMITH, W.K. - University Of Arizona; Biederman, Joel; Scott, Russell; WILLIAMS, A.P. - University Of California (UCLA); DANNENBERG, M.P. - University Of Iowa

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2025
Publication Date: 1/25/2025
Citation: Johnston, M., Barnes, M., Preisler, Y., Smith, W., Biederman, J.A., Scott, R.L., Williams, A., Dannenberg, M. 2025. Effects of hot versus dry vapor pressure deficit on ecosystem carbon and water fluxes. Journal of Geophysical Research-Biogeosciences. 130(1). Article e2024JG008146. https://doi.org/10.1029/2024JG008146.
DOI: https://doi.org/10.1029/2024JG008146

Interpretive Summary: Plants need water for basic functioning, so the dryness of the air affects many plant processes. A lot of research has focused on how air dryness (aka “vapor pressure deficit” or VPD) affects plant functions such as photosynthesis. However, air can seem dry to plants for two non-exclusive reasons: (1) moisture is low, and/or (2) temperature is high (warm air can hold more moisture, so it will be drier than cool air that has the same amount of moisture). We first asked: at specific locations across the U.S.A., are particular values of VPD always caused by the same combination of temperature and relative humidity? We found that they were not: the same degree of air dryness arises from many combinations of moisture and temperature. We then asked: does this “dryness” vs. “hotness” character of VPD matter for plant functioning (net and gross photosynthesis and transpiration of water to the atmosphere)? We report that it is relevant in about half of the ecosystems we studied. Our results emphasize the need to experimentally manipulate VPD by changing moisture and temperature independently to better understand how increasing VPD is affecting plants and ecosystems.

Technical Abstract: Vapor pressure deficit (VPD) has increased and will likely continue increasing, with wide-ranging effects on ecosystems. Future VPD increases will largely be driven by warming, yet most experiments examining VPD effects on plants have done so by changing humidity. Here, we used meteorological data and carbon and water fluxes measured at 26 climatically-diverse eddy covariance sites to quantify the extent to which VPD has been driven by variation in air temperature versus humidity. We fit generalized additive models (GAMs) at each site to quantify effects of hotter (and wetter) and cooler (and drier) versus typical VPD on ecosystem-scale fluxes of carbon and water. We found that VPD has occurred under diverse combinations of temperature and humidity: >50% of a site's daytime growing season temperature range and >35% of its relative humidity range have often combined to define a particular VPD. We found moderate evidence that hotter versus drier VPD of the same magnitude differentially affect gross primary productivity (GPP), net ecosystem productivity (NEP), and latent heat flux (LE): Selected GPP and NEP GAMs at about half of sites and LE GAMs at about a third of sites included a VPD-temperature interaction. The magnitude of the interaction varied, but was generally 29%–57% of the effect attributable solely to VPD. The direction of the interaction also varied, but hot VPD was commonly associated with higher carbon fluxes. These effects were not strongly modified by soil moisture. Overall, results emphasize the relevance of VPD-temperature interactions at a critical time of rapid VPD increase.