Examining the large-scale convergence of photosynthesis-weighted tree leaf temperatures through stable oxygen isotope analysis of multiple data sets.

摘要

The idea that photosynthesis-weighted tree canopy leaf temperature (T_{can delta} ) can be resolved through analysis of oxygen isotope composition in tree wood cellulose ( delta 18O_wc) has led to the observation of boreal-to-subtropical convergence of T_{can delta} to c. 20 degrees C. To further assess the validity of the large-scale convergence of T_{can delta} , we used the isotope approach to perform calculation of T_{can delta} for independent delta 18O_wc data sets that have broad coverage of climates. For the boreal-to-subtropical data sets, we found that the deviation of T_{can delta} from the growing season temperature systemically increases with the decreasing mean annual temperature. Across the whole data sets we calculated a mean T_{can delta} of 19.48 degrees C and an SD of 2.05 degrees C, while for the tropical data set, the mean T_{can delta} was 26.40+or-1.03 degrees C, significantly higher than the boreal-to-subtropical mean. Our study thus offers independent isotopic support for the concept that boreal-to-subtropical trees display conserved T_{can delta} near 20 degrees C. The isotopic analysis cannot distinguish between the possibility that leaf temperatures are generally elevated above ambient air temperatures in cooler environments and the possibility that leaf temperature equals air temperature, whereas the leaf/air temperature at which photosynthesis occurs has a weighted average of near 20 degrees C in cooler environments. Future work will separate these potential explanations.