Temperature is a primary control on plant processes at a range of spatial and temporal scales, affecting enzymatic reactions, ecosystem biogeochemistry, and species distributions. Although most focus is on air temperature, the radiative or skin temperature of plants is more relevant. Canopy skin temperature dynamics reflect biophysical, physiological, and anatomical characteristics and interactions with the environment, and can be used to examine forest responses to stresses like droughts and heat waves. Until recently, measurements of plant canopy temperatures have been challenging due to sensor limitations. Thermal infrared (TIR) imaging allows for extensive temporal and spatial sampling of canopy temperatures, particularly compared to spot measurements using thermocouples. We present results of TIR imaging of forest canopies at eddy covariance flux tower sites in the Pacific Northwest (USA) and in central Panama. These forests range from an old-growth temperate rainforest to a second growth semi-arid pine forest to a semi-deciduous tropical forest.
Canopy temperature regimes at these sites are highly variable. For example, during the 2014 growing season at the semi-arid pine forest site, the temporal variation of half-hourly mean canopy leaf temperature was large, ranging from –2.1 to 33.2 C. Canopy temperatures at all forest sites display frequent departures from air temperature, particularly during clear sky conditions. All forests exhibit elevated canopy temperatures during the day and depressed canopy temperatures at night compared to air temperature. Comparison of canopy temperatures to fluxes of carbon dioxide, water vapor, and energy reveals stronger relationships than those found with air temperature. Daytime growing season net ecosystem exchange at the pine forest site is better explained by canopy temperature (r2 = 0.61) than air temperature (r2 = 0.52). At the semi-deciduous tropical forest, gross primary productivity is highly correlated with canopy temperature (r2 = 0.51), and this site has a distinct optimum canopy temperature for photosynthesis (~31 °C) that agrees with leaf-level measurements. Finally, canopy temperatures at old-growth temperate rainforests during heat waves in 2015 exceeded 45 °C, well past the optimum temperature of photosynthesis and approaching temperatures that damage leaf tissue.