We tested the hypothesis that the canopy temperature when weighted by growing season gross primary production (GPP) is approximately 21 °C, for a range of forest types from subtropical to boreal. We used stemwood tree cores from a selection of AmeriFlux sites to calculate a resolved canopy temperature using δ18O of the stemwood cellulose (Tcanδ). We also used data from the same AmeriFlux sites to calculate an air temperature value weighted by GPP (Tgpp).
Our results confirm that values of Tcanδ are approximately 21 °C for a range of forested ecosystems, similar to previously published data. Our data also suggest that Tgpp could explain Tcanδ in temperate ecosystems. However, for ecosystems with extremely high or low mean annual temperature the difference between Tgpp and Tcanδ was as much as 8 °C. Prior eco-physiological studies have shown that plant canopy temperature can deviate substantially from air temperature either through convection for heating or transpiration for cooling. Therefore we employed infrared sensors to measure canopy temperature directly and will report these results as they compare to Tgpp, and Tcanδ.