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

The idea that photosynthesis-weighted tree canopy leaf temperature (T(canδ)) can be resolved through analysis of oxygen isotope composition in tree wood cellulose (δ(18) O(wc)) has led to the observation of boreal-to-subtropical convergence of T(canδ) to c. 20°C. To further assess the validity of the large-scale convergence of T(canδ), we used the isotope approach to perform calculation of T(canδ) for independent δ(18) O(wc) data sets that have broad coverage of climates. For the boreal-to-subtropical data sets, we found that the deviation of T(canδ) from the growing season temperature systemically increases with the decreasing mean annual temperature. Across the whole data sets we calculated a mean T(canδ) of 19.48°C and an SD of 2.05°C, while for the tropical data set, the mean T(canδ) was 26.40 ± 1.03°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(canδ) near 20°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°C in cooler environments. Future work will separate these potential explanations.