Precipitation mediates sap flux sensitivity to evaporative demand in the neotropics
Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.
KeywordsEvapotranspiration Plant functional traits Transpiration Vapor pressure deficit
This project was supported in part by the Next Generation Ecosystem Experiments Tropics, funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Sciences Program, under Award Number DE-SC-0011806. CG was supported by the Swiss National Science Foundation SNF (5231.00639.001.01). BC was supported in part by the Laboratory Directed Research and Development Program Project 8872 of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. This work has benefited from an “Investissements d’Avenir” grant managed by Agence Nationale de la Recherche (CEBA, ref. ANR-10-LABX-25-01). Data recorded in French Guiana (FRG) were collected at the Guyaflux sites which belong to the SOERE F-ORE-T and is supported annually by Ecofor, Allenvi and the French national research infrastructure, ANAEE-F. We thank Valentine Herrmann for building the probes for the Panamanian and Brazilian sites. We thank all technicians, students and post-docs who helped collect data at all sites.
Author contribution statement
CG, BC, JW and NGM planned the research. AA, HA, BB, BG, BW, CB, CB, CS, CF, DB, DC, MD, BF, CF, KJ, GRM, GWM, CV, JW, BW, NS, LA, TEW and LW contributed data. CG and BC analyzed the data and wrote a first draft of the manuscript, and all authors contributed to revisions.
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