Canopy skin temperature variations in relation to climate, soil temperature, and carbon flux at a ponderosa pine forest in central Oregon

Abstract

Thermal infrared (TIR) techniques to collect thermal imagery have been useful for recording quasi-continuous plant surface temperatures. In this study, we applied a thermal camera to measure canopy skin temperatures in a mature ponderosa pine forest in central Oregon over one growing season from May to September 2014. This study had the following objectives: (1) to examine spatial and temporal variations of canopy temperature; (2) to explore the effects of climate and soil conditions on canopy temperature; and (3) to quantify the relationships of canopy temperatures to forest-atmosphere heat and carbon fluxes. The temporal variation of 30-min mean canopy temperature was large, and leaf temperatures ranged from −2.1 to 33.6°C during the study period. The temperature difference was small between the whole canopy and leaf regions, while tree stems had warmer temperatures than leaves, especially during the afternoon (12:00–19:59h). The canopy thermal regime was largely controlled by climatic conditions and related to the soil thermal states. Air temperature, relative humidity, longwave radiation, and soil temperature at 2-cm depth were tightly correlated with 30-min and daily/sub-daily mean canopy leaf temperatures (r≥0.6 or ≤−0.6, p<0.01). The daily/sub-daily mean canopy temperatures contained stronger relationships with the climatic and soil variables than the 30-min mean temperatures. During the afternoon, the mean leaf temperature was more closely related to net ecosystem exchange (r2=0.69) than air temperature, driven by the strong relationship between tissue temperature and photosynthesis and respiration. Our results show that canopy thermal conditions can be monitored almost continuously for extended time periods to better characterize how canopies respond to environmental conditions. Finally, thermal measurements show great promise for quantifying linkages to carbon exchange in forest ecosystems.