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Sci Rep. 2017 Feb 24;7:43087. doi: 10.1038/srep43087.

Changes in biomass allocation buffer low CO2 effects on tree growth during the last glaciation.

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Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia.
School of Archaeology, Geography and Environmental Sciences (SAGES), Reading University, Reading, UK.
Geography Department, Kansas State University, Manhattan, KS 66505, USA.
Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA.
The La Brea Tar Pits Museum (George C. Page Museum), 5801 Wilshire Boulevard, Los Angeles, CA 90036, USA.
AXA Chair of Biosphere and Climate Impacts, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK.


Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO2] (ca) was ~180 ppm, show the leaf-internal [CO2] (ci) was approaching the modern CO2 compensation point for C3 plants. Despite this, stem growth rates were similar to today. Using a coupled light-use efficiency and tree growth model, we show that it is possible to maintain a stable ci/ca ratio because both vapour pressure deficit and temperature were decreased under glacial conditions at La Brea, and these have compensating effects on the ci/ca ratio. Reduced photorespiration at lower temperatures would partly mitigate the effect of low ci on gross primary production, but maintenance of present-day radial growth also requires a ~27% reduction in the ratio of fine root mass to leaf area. Such a shift was possible due to reduced drought stress under glacial conditions at La Brea. The necessity for changes in allocation in response to changes in [CO2] is consistent with increased below-ground allocation, and the apparent homoeostasis of radial growth, as ca increases today.

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