Forest inventory data point to a CO2-induced stimulation of tree growth in the tropics, but direct evidence has been lacking. A tree-ring-based analysis spanning three continents suggests that tropical tree growth has remained stagnant on a centennial timescale, calling into question claims that old-growth tropical forests can help to soak up future CO2 emissions.
Peter van der Sleen, of Wageningen University, and colleagues examined changes in the growth and water-use efficiency of canopy and understorey trees in Bolivia, Cameroon and Thailand over the last 150 years using measurements of the width and stable carbon isotope signature of tree rings. In conjunction with a marked increase in intercellular CO2 concentrations, the researchers report a 30 to 35% increase in the water-use efficiency of the studied trees, indicative of a CO2-induced increase in photosynthesis, reduction in transpiration, or both. However, they find no evidence for a concomitant increase in tree growth.
Why increased concentrations of CO2 have failed to stimulate tree growth in the tropics remains unclear, but increasing temperatures, resource limitations and shifts in substrate allocation could hold the answer.
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Armstrong, A. Tropical tree growth: No signs of stimulation. Nature Plants 1, 14017 (2015). https://doi.org/10.1038/nplants.2014.17