July 29, 2008 | David F. Coppedge

Leaf Assumption Challenged: Affects Climate Modeling

A reasonable-sounding assumption has been overturned, leaving climate models in upheaval.  The assumption was that leaf temperature stays in equilibrium with air temperature.  It doesn’t.  Leaves are hotter than assumed during active periods of growth, such as at midday in the growing season.  They maintain a relatively constant temperature through their own biological air conditioning, regardless of what the weather is doing.  This affects the interpretation of oxygen isotopes in wood extracted from tree rings, which in turn affects inferences about past climate.  The story came unraveled in Nature last week.1
    Scientists have used the ratio of oxygen-16 to its heavier sibling oxygen-18 as a proxy for climate variations.  The interpretation, however, assumed that leaves are in equilibrium with ambient temperature.  Helliker and Richter (U of Pennsylvania) found that leaves can be 10° C hotter than air temperature, and were almost uniformly warm across a wide range of habitats.  This also affects calculations of relative humidity – a function of temperature.  “Our results explain this observation over a broad climatic range and further suggest that the overarching trend is to maintain leaves at an optimal temperature irrespective of mean climate,” they said.  F. I. Woodward (U of Sheffield, UK), commenting on this paper in the same issue of Nature, 2 explained, “During the growing season, with photosynthesis at its peak, leaf temperatures remain constant over a wide latitudinal range.  This is a finding that overturns a common assumption and has various ramifications.”
    One key ramification relates to climate models.  Scientists have built models of past climate on the assumption that the oxygen ratios they measured in wood reflected the air temperature during the growing season.  Now, it appears that assumption is misguided.  Pine needles, for example, cannot be modeled in isolation, because they usually are in tight clusters.  It seemed reasonable that a pine needle, exposed on all sides to the air, would be the same temperature as the air.  But in the dense forest canopy, clusters of densely-packed needles create their own microclimate as the needles actively expend energy manufacturing sucrose in response to photosynthesis.  Leaf temperatures, therefore, can be much higher than air temperature – and relative humidity correspondingly lower.  This affects the rates at which oxygen-16 and oxygen-18 diffuse and become incorporated into the cellulose in tree rings.
    Woodward summarized the potential impact of this finding on climate models: “The fact that vegetation canopy rather than leaf morphology dominates temperature control in the forests sampled by Helliker and Richter suggests the need for greater emphasis on understanding how the canopy responds to climate change, and to global warming in particular.”  The authors also suggested that the finding will force researchers to modify estimates of water loss in the forest canopy.  In addition, theories about how climate affects leaf evolution have been called into question.

1.  Brent R. Helliker and Suzanna L. Richter, “Subtropical to boreal convergence of tree-leaf temperatures,” Nature 454, 511-514 (24 July 2008) | doi:10.1038/nature07031.
2.  F. I. Woodward, “Ecology: Forest air conditioning,” Nature 454, 422-423 (24 July 2008) | doi:10.1038/454422a.

Do you see how unquestioned assumptions become weak links in chains of reasoning on which politicians and societies put their trust?  Maybe the modifications to theory required will not be dramatic in this case, but they could be.  Scientists had treated oxygen ratios in tree rings as bona fide scientific evidence of past climate conditions.  Scenes of scientists measuring isotope ratios to high precision in labs make for impressive visuals in documentaries.  The lay public becomes persuaded that scientists have a virtual crystal ball into the past.  All the while they were not measuring climate – they were measuring local microclimates right at the leaf surfaces, which can be significantly warmer than the air just a meter away.  Plants have a thermostat of their own that maintains near-constant temperatures during the growing season.  These temperatures are the ones recorded in the wood – not the climate conditions.  This little “whoops” discovery should teach us a healthy caution and skepticism about proxy measurements employed matter-of-factly by scientists.  We can all learn, furthermore, to question our own assumptions, as reasonable as they may seem to us.

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