1. Department of Biology,
2. Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

Correspondence to: Brent R. Helliker¹ Correspondence and requests for materials should be addressed to B.R.H (Email: helliker@sas.upenn.edu).

Abstract

The oxygen isotope ratio (¹⁸O) of cellulose is thought to provide a record of ambient temperature and relative humidity during periods of carbon assimilation^{1, 2}. Here we introduce a method to resolve tree-canopy leaf temperature with the use of ¹⁸O 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 ¹⁸O to reconstruct climate history is that the temperature and relative humidity of an actively photosynthesizing leaf are the same as those of the surrounding air^{3, 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.

1. Department of Biology,
2. Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

Correspondence to: Brent R. Helliker¹ Correspondence and requests for materials should be addressed to B.R.H (Email: helliker@sas.upenn.edu).

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.