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Subtropical to boreal convergence of tree-leaf temperatures


The oxygen isotope ratio (δ18O) of cellulose is thought to provide a record of ambient temperature and relative humidity during periods of carbon assimilation1,2. Here we introduce a method to resolve tree-canopy leaf temperature with the use of δ18O of cellulose in 39 tree species. We show a remarkably constant leaf temperature of 21.4 ± 2.2 °C across 50° 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 δ18O 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.

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Figure 1: Plot of predicted and observed cellulose Δ 18 O c against mean annual temperature.
Figure 2: Resolved tree-canopy temperature versus mean annual temperature.
Figure 3: Comparison of ambient and canopy-based relative humidity.


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We thank A. H. Johnson for discussion of the results; D. Vann and M. Dranoff for help with analysis; and B. Casper, P. Petraitis and D. Brisson for comments on the manuscript. This work was supported by a start-up grant from the University of Pennsylvania and a grant from the A.W. Mellon Foundation.

Author Contributions S.L.R. developed the framework for the sampling scheme and analysed the tree-ring cores. B.R.H. developed the framework for the modelling analysis and wrote the majority of the paper. Both authors discussed the results and commented on the manuscript.

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Correspondence to Brent R. Helliker.

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Helliker, B., Richter, S. Subtropical to boreal convergence of tree-leaf temperatures. Nature 454, 511–514 (2008).

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