Relationships of canopy skin temperatures to hydrologic cycling in a semi-arid coniferous forest in central Oregon

Water is a critical limiting factor to biomass growth and physiological function in the growing seasons in semi-arid forests. Dewfall can serve as an important water source to trees during dry periods in such ecosystems. Continuous temperature measurements on the tree canopies can enhance our understanding of canopy thermal states and their relationship with the timing and frequency of dewfall. Dew can be formed on the canopy surface when canopy temperature (Tcanopy) is lower than the dew point temperature (Tdew). In this study, we used a thermal camera to measure Tcanopy in a mature ponderosa pine forest in the US Pacific Northwest from September 2013 to October 2014. The objective of this study was to explore temporal changes in Tcanopy and the relationship of Tcanopy changes to the occurrence of dewfall during the growing seasons. The temporal variations of half-hourly mean Tcanopy were large, fluctuating from –5.5 to 20.3^oC in 2013 and from –2.0 to 33.2^oC in 2014. Comparison between half-hourly mean Tcanopy and Tdew indicated that dew was frequently formed in autumn 2013 (150 out of 682 measurements) and infrequently in summer and autumn 2014 (23 out of 3970 measurements). Dewfall primarily happened during the early morning and nighttime (89% of dewfall events in 2013 and 100% in 2014). Our results showed that thermographic measurements were suited to detect dew formation and helped to understand the thermal characteristics of forest canopies and their link to water supply and hydrologic cycles in semi-arid forest ecosystems. Importantly, other temperature measurements, including air temperature profiles and bulk surface temperature inferred from a 4-channel net radiometer, failed to detect the cooling of the canopy captured by thermal imaging. Further studies comparing to sapflow and energy and water fluxes will aid in interpretation.