Evapotranspiration regulates leaf temperature and respiration in dryland vegetation

Authors: KIBLER, C.L. - University Of California Santa Barbara; TRUGMAN, A.Z. - University Of California Santa Barbara; ROBERTS, D - University Of California Santa Barbara; STILL, C.J. - Oregon State University; Scott, Russell - Russ; CAYLOR, K.K. - University Of California Santa Barbara; STELLA, J.C. - State University Of New York (SUNY); SINGER, M.B., - Cardiff University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2023
Publication Date: 6/21/2023
Citation: Kibler, C., Trugman, A., Roberts, D., Still, C., Scott, R.L., Caylor, K., Stella, J., Singer, M. 2023. Evapotranspiration regulates leaf temperature and respiration in dryland vegetation. Agricultural and Forest Meteorology. 339. Article 109560. https://doi.org/10.1016/j.agrformet.2023.109560.
DOI: https://doi.org/10.1016/j.agrformet.2023.109560

Interpretive Summary: One mechanism that plants use to cool their leaf temperature is through the regulation of evapotranspiration via tiny pores on the leaves. However, the relationship between evapotranspiration and leaf temperature is understudied. Here, we propose a physical model or relationship between these two quantities, and we test its validity using measurements from two shrubland sites in Arizona, USA that have the same tree species but with contrasting access to water. Both the observations and model predictions reveal that leaf temperature equilibrates with air temperature under high evapotranspiration rates. Leaf temperature exceeds air temperature when there is a net input of energy into the leaf tissue. The observations revealed that evaporative cooling reduced leaf temperature by around 1-5 °C, depending on the water status of the trees. Evaporative cooling also enhanced net carbon uptake by reducing leaf respiration by around 20% in the middle of the growing season. The regulation of leaf temperature by evapotranspiration and the resulting impacts on net carbon uptake represent an important link between plant water and carbon cycles that has received little attention in literature. The model presented here provides a mechanistic framework to quantify leaf evaporative cooling and incorporate its impacts into models of terrestrial ecosystem function.

Technical Abstract: Evapotranspiration regulates energy flux partitioning at the leaf surface, which in turn regulates leaf temperature. However, the mechanistic relationship between evapotranspiration and leaf temperature remains poorly constrained. In this study, we present a novel mechanistic model to predict leaf temperature as a linearized function of the evaporative fraction. The model is validated using measurements from infrared radiometers mounted on two flux towers in Arizona, USA, which measure stands of Prosopis velutina with contrasting water availability. Both the observations and model predictions reveal that leaf temperature equilibrates with air temperature when latent heat flux consumes all of the energy incident on the leaf surface. Leaf temperature exceeds air temperature when there is a net input of energy into the leaf tissue. The flux tower observations revealed that evaporative cooling reduced leaf temperature by ca. 1-5 °C, depending on water availability. Evaporative cooling also enhanced net carbon uptake by reducing leaf respiration by ca. 20% in the middle of the growing season. The regulation of leaf temperature by evapotranspiration and the resulting impacts on net carbon uptake represent an important link between plant water and carbon cycles that has received little attention in literature. The model presented here provides a mechanistic framework to quantify leaf evaporative cooling and incorporate its impacts into process-based models of terrestrial ecosystem function.