Subtropical to boreal convergence of tree-leaf temperatures

The oxygen isotope ratio (d18O) of cellulose is thought to provide a record of ambient temperature and relative humidity during per- iods of carbon assimilation1,2. Here we introduce a method to resolve tree-canopy leaf temperature with the use of d18O of cellulose in 39 tree species. We show a remarkably constant leaf temperature of 21.4 6 2.2 6C across 506 of latitude, from subtropical to boreal biomes. This means that when carbon assimilation is maximal, the physiological and morphological properties of tree branches serve to raise leaf temperature above air temperature to a much greater extent in more northern latitudes. A main assumption underlying the use of d18O to reconstruct climate history is that the temperature and relative humidity of an actively photosynthesizing leaf are the same as those of the surrounding air3,4. Our data are contrary to that assumption and show that plant physiological ecology must be considered when reconstructing climate through isotope analysis. Furthermore, our results may explain why climate has only a modest effect on leaf economic traits5 in general.