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The ongoing enrichment of the atmosphere with CO₂ raises the question of whether growth of forest trees, which represent close to 90% of the global biomass carbon, is still carbon limited at current concentrations of close to 370 p.p.m. As photosynthesis of C3 plants is not CO₂‐saturated at such concentrations, enhanced ‘source activity’ of leaves could stimulate ‘sink activity’ (i.e. growth) of plants, provided other resources and developmental controls permit. I explore current levels of non‐structural carbon in trees in natural forests in order to estimate the potential for a carbon‐driven stimulation of growth.

The concentration of non‐structural carbohydrates (NSC) in tree tissues is considered a measure of carbon shortage or surplus for growth. A periodic reduction of NSC pools indicates either that carbon demand exceeds con‐current supply, or that both source and sink activity are low. A steady, very high NSC …