Changes in biomass allocation buffer low CO(2) effects on tree growth during the last glaciation

Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO(2)] (c(a)) was ~180 ppm, show the leaf-internal [CO(2)] (c(i)) was approaching the modern CO(2) compensation point for C(3) 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 c(i)/c(a) ratio because both vapour pressure deficit and temperature were decreased under glacial conditions at La Brea, and these have compensating effects on the c(i)/c(a) ratio. Reduced photorespiration at lower temperatures would partly mitigate the effect of low c(i) 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 [CO(2)] is consistent with increased below-ground allocation, and the apparent homoeostasis of radial growth, as c(a) increases today.