Monitoring canopy temperatures of old and second growth coniferous forests in the HJ Andrews Experimental Forest

Printer-friendly version

Recent global warming has dramatically changed the severity and length of heat waves and drought periods during the growing seasons. Forest canopy surface temperatures reflect the thermal status of trees’ leaves and stems. This thermal status is related to heat, water, and carbon exchanges of the canopies, and is affected by climatic and environmental conditions. Canopy temperature quickly responds to heat and drought events in the warm seasons, and its response varies between old and young forest stands due to differences in canopy structure, tree height and volume, and rates of photosynthesis and transpiration. In this study, we monitored canopy temperatures in old (~600-year-old) and second growth (~50-year-old) Douglas-fir forests of the HJ Andrews Forests, OR from May to July 2015 using a thermal infrared (TIR) camera. We installed a TIR camera and its platform on an old-growth tree at 55-m height above the ground. The camera collected one image every 10 minutes; each image captures temperatures of both canopies (old and second growth forests). The temperatures were used to examine the characteristics of temporal and spatial changes and correlations to climatic variables in these forests. Over the three months, mean canopy temperature in the old and second growth forests was 16.2ºC, while the second growth forest contained the higher daytime peak temperatures (up to 40.2ºC). Minimum temperatures in both forests were found in the morning of early May, and the maximum temperatures occurred in the daytime of early June during a heat wave. The temporal variation in temperature was similar between the old and second growth forest, whereas the spatial variation, indicated by standard deviation of canopy temperatures, was larger in the second growth forest. The higher daytime peak temperatures and larger spatial variation in the second growth forest suggest that heating response of younger canopies is less buffered. This is likely due to more closed and denser canopy structure in the young forest compared to the old one. Overall, canopy temperatures in both forests were significantly correlated to air temperatures, radiation, and relative humidity (p < 0.01) but non-significantly correlated with precipitation and wind (p > 0.05). Physiology of the coniferous forests, influenced by canopy thermal status, could be thus driven by climatic conditions during springs and summers.