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.