Tree rings provide an annual record of plant responses to inter-annual climatic variability and the oxygen isotope ratio of cellulose is thought to provide a record of leaf-to-air vapor pressure deficit.  We recently developed a method to resolve tree-canopy leaf temperature using the oxygen isotope ratio of tree-ring cellulose and ambient relative humidity. We found a remarkably constant leaf temperature of 21.4 ± 2.2 °C in 39 tree species across 50° of latitude, from subtropical to boreal biomes.  There are two general mechanisms for this perceived leaf temperature homeostasis: (i) Most photosynthesis occurs when ambient temperatures are near 21 °C; in warmer biomes photosynthesis occurs before and after the hottest part of the day and in colder biomes most photosynthesis occurs around midday.  (ii) When carbon assimilation is maximal, the physiological and morphological properties of tree leaves and branches serve to maintain canopy leaf temperatures near 21 °C.

Results/Conclusions

To address these questions we reanalyzed field-based photosynthetic optima for trees published over the last 40 years and compared them to prevailing ambient temperature data.  We further calculated a temperature-weighted value for daytime net ecosystem exchange during the growing season for several biomes from published eddy covariance data.  The results, while preliminary, support the observation of a photosynthesis-weighted temperature near 21 °C determined through our isotope analysis.  The results further indicate that the controlling mechanism is likely a combination of points (i) and (ii) above.