Canopy temperature is regulated by ecosystem structural traits and captures the ecohydrologic dynamics of a semiarid mixed conifer forest site

Authors: JAVADIAN, M. - University Of Arizona; SMITH, W.K. - University Of Arizona; LEE, K. - University Of Arizona; Knowles, John; Scott, Russell - Russ; FISHER, J.B. - Chapman University; MOORE, D.J.P. - University Of Arizona; VAN LEEUWEN, W. - University Of Arizona; BARRON-GAFFORD, G. - University Of Arizona; BEHRANGI, A. - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2022
Publication Date: 2/1/2022
Citation: Javadian, M., Smith, W., Lee, K., Knowles, J.F., Scott, R.L., Fisher, J., Moore, D., van Leeuwen, W., Barron-Gafford, G., Behrangi, A. 2022. Canopy temperature is regulated by ecosystem structural traits and captures the ecohydrologic dynamics of a semiarid mixed conifer forest site. Journal of Geophysical Research-Biogeosciences. 127(2). Article e2021JG006617. https://doi.org/10.1029/2021JG006617.
DOI: https://doi.org/10.1029/2021JG006617

Interpretive Summary: Plant canopy temperature is partly regulated via canopy evaporation and transpiration, similar to how the process of sweating works to cool the human body. In semiarid forests such as those of the southwest United States, the sensitivity of canopy water stress to changing environmental conditions may differ based on tree density and height. However, these links are not well understood due to the coarseness of currently available satellite-based estimates of canopy temperature and the challenges associated with field-based measurements of it in relatively inaccessible tall tree stands. As a result, we took advantage of recent advancements in temperature measurements on an unmanned aerial vehicle and the recent NASA ECOSTRESS satellite mission to quantify canopy temperature and inferred canopy water stress from previously inaccessible tall trees throughout the day. We found that the canopy temperature differed significantly among different tree densities and height classes. In particular, temperatures were higher in low density and short tree stands. Measurements demonstrated a strong water use signal from the trees even in winter due to the mild climate of the site in southern Arizona, USA. The results demonstrate the capability of linked site measurements and satellite observations to monitor tree water use over semiarid mixed conifer forest vegetation.

Technical Abstract: Plant canopy temperature (Tc) is partly regulated by evaporation and transpiration from the canopy surface and can be used to infer changes in stomatal regulation and vegetation water stress. In this study, we used a thermal Unmanned Aircraft Systems (UAS) in conjunction with eddy covariance, sap flow, and spectral reflectance data to assess the diurnal characteristics of Tc and water stress status over a semiarid mixed conifer forest in Arizona, USA. Diurnal Tc dynamics were closely related to tree sap flow and changes in spectral reflectance associated with stomatal regulation. Consistent with previously reported deviations, we found that on average Tc was 1.8 °C lower than the above canopy air temperature (Ta). However, the relationship between Tc and Ta varied significantly according to tree density and tree height classes, with taller and denser trees exhibiting relatively low Tc-Ta (2.4 and 2.1 °C cooler canopies, respectively) compared to shorter and less-dense tree stands (1.7 and 1.5 °C cooler canopies, respectively). We used these data to evaluate space-borne diurnal measurements of Tc and water stress from the ECOSTRESS mission. We found that ECOSTRESS observations of Tc accurately tracked seasonal shifts in diurnal surface temperatures and vegetation water stress, and that site-level observations of heterogeneity in forest composition and structure could be applied to separate the processes of canopy transpiration and soil evaporation within the ECOSTRESS footprint. This study demonstrates how proximal and satellite remote sensing approaches can be combined to reveal the diurnal and seasonally dynamic nature of Tc and water stress.