Canopy Temperature Is Regulated by Ecosystem Structural Traits and Captures the Ecohydrologic Dynamics of a Semiarid Mixed Conifer Forest Site. Issue 2 (7th February 2022)

• Record Type: Journal Article
• Authors: Javadian, Mostafa; Smith, William K.; Lee, Kangsan; Knowles, John F.; Scott, Russell L.; Fisher, Joshua B.; Moore, David J. P.; van Leeuwen, Willem J. D.; Barron‐Gafford, Greg; Behrangi, Ali
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-07
• Subjects: canopy temperature -- thermal UAS -- ECOSTRESS -- diurnal vegetation dynamics -- ecohydrologic dynamics -- semiarid mixed conifer forest; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2021JG006617
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Plant canopy temperature ( T c ) 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 in conjunction with eddy covariance, sap flow, and spectral reflectance data to assess the diurnal characteristics of T c and water stress status over a semiarid mixed conifer forest in Arizona, USA. Diurnal T c 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 T c was 1.8°C lower than the above canopy air temperature ( T a ). However, the relationship between T c and T a 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 T c and water stress from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) mission. We found that ECOSTRESS observations of T c 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 T c and water stress. Plain Language Summary: Plant canopy temperature ( T c ) 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 T c and the challenges associated with field‐based measurements of T c in relatively inaccessible tall tree stands. As a result, we took advantage of recent advancements in thermal Unmanned Aircraft Systems and the recent NASA ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station satellite mission to quantify T c and inferred canopy water stress from previously inaccessible tall trees throughout the day. We found that T c differed significantly among different tree density and height classes. In particular, T c was higher in low density and short tree stands. Our measurements even demonstrated a strong water use signal from the trees during the winter due to the mild climate of the site in southern Arizona, USA. These results demonstrate the capability of linked site measurements and satellite observations to monitor tree water use over semiarid mixed conifer forest vegetation. Key Points: Canopy temperature differed significantly among different tree density and height classes at a semiarid mixed conifer forest site Diurnal canopy temperature dynamics were closely related to canopy photochemical reflectance and sap flow Satellite observations from ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station captured key aspects of diurnal canopy temperature dynamics and were consistent with site‐level data