What Lies Beneath: Vertical Heterogeneity in Vegetation Canopy Temperatures

Abstract:

Canopy temperature exerts fundamental controls on plant-level physiological processes such as photosynthesis, respiration, transpiration, and growth and development, and consequently on ecosystem fluxes of carbon, water, and energy. New thermal remote sensing instruments and algorithms are offering unprecedented measurements of surface canopy temperature across space and time. However, airborne and satellite sensors only measure canopies from above, and the vertical heterogeneity of canopy temperatures remains largely unexplored. We combined data from the ECOSTRESS mission, lidar, and a FLIR thermal camera deployed in a Californian woodland savanna to answer the following questions: (1) What is the vertical heterogeneity of tree canopy temperature in a Mediterranean woodland savanna, and how does it vary diurnally, seasonally, and in response to environmental conditions?, and (2) How do blue oak (Quercus douglasii) tree canopy temperatures as measured by ECOSTRESS correspond with FLIR-measured, vertically-resolved temperatures? At any particular time, locations within a single blue oak canopy can vary by more than 4°C, while the difference in temperature between the top quarter and bottom quarter of the canopy can range ±2°C. Canopy temperatures are considerably more homogeneous at night and at other times with low incoming solar radiation. Comparisons with the newly-available ECOSTRESS thermal measurements will also be presented. Based on results from the thermal imaging camera, we predict that ECOSTRESS measurements are a good proxy for bulk canopy temperature under conditions of low incoming radiation, but are comparatively non-representative of canopy temperature when the temperature at the bottom of the canopy is significantly warmer or cooler than the temperature at the top, such as when the understory grass is dry, hot, and strongly radiating.