A new paper has been published in the journal Nature entitled: 'Subtropical to boreal convergence of tree-leaf temperatures' by Brent R. Helliker & Suzanna L. Richter, which has implications for climate reconstructions using tree ring data.

The Abstract states:

The oxygen isotope ratio (18O) of cellulose is thought to provide a record of ambient temperature and relative humidity during periods of carbon assimilation. 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. 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 traits in general.

There is a summary of the paper in Nature News here.

The authors state in the text of the article:

“Our analysis shows that reconstructing ambient humidity by using tree-ring d18O becomes increasingly dubious as MAT [mean annual temperature] decreases. Caution is therefore advised when interpreting treering d18O data from high latitudes for both contemporary samples and samples of relictual wood from high-latitude forests of the past.”

and:

“The discovery of relatively invariant leaf temperatures has two important ramifications that transcend stable-isotope studies. First, elevated canopy temperature and depressed leaf relative humidity should have a large effect on real and modelled water loss from boreal ecosystems. Second, if the architectural controls of branches on leaf temperature are as widespread as our data suggest, then direct climatic selection on the evolution of leaf traits would be relaxed, whereas the selective force of climate on other plant organs (for example stems and roots) would remain. Our results therefore offer a possible explanation for the unexpected finding that climate is a minor correlate with global leaf economic traits.”

Roger Pileke Sr's take on this is:

This study has major implications with respect to the use of tree ring data to reconstruct long term air temperature trends, as the authors indicate in their text.

This study also illustrates the dynamic response of vegetation to their environment so as to maximize the ability to grow and compete within their ecological environment. This biological effect must be incorporated within climate models that seek to accurately simulate the response of the climate to human and natural effects, including the increase of the atmospheric concentration of CO2.  Source

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