Rising CO2 is making tropical trees photosynthesize more efficiently
A new study of tree rings spanning a century reveals that higher atmospheric carbon dioxide has boosted photosynthetic efficiency in tropical canopy trees—potentially increasing their carbon-absorbing capacity. The finding suggests tropical forests may naturally strengthen their role as global carbon sinks, but only if deforestation and other pressures don't undermine that benefit.
Originaltitel: Centennial-scale atmospheric CO<sub>2</sub> rise increased photosynthetic efficiency in a tropical tree species
<p>Tropical forests substantially influence the terrestrial carbon sink. Their contributions to the forest carbon sink may increase due to the stimulation of photosynthesis by rising atmospheric CO<sub>2</sub> (C<sub>a</sub>); however, the magnitude of this effect is poorly quantified for tropical canopy trees.</p><p>We measured the ratio of two deuterium isotopomers of glucose derived from tree rings to estimate how photosynthetic efficiency (photorespiration-to-photosynthesis ratio) has responded to C<sub>a</sub> rise at a centennial scale. Wood samples were obtained from <em>Toona ciliata</em> trees from three climatically distinct forests in Asia and Australia. We applied Bayesian mixed effect models to test how the isotopomer ratio changes with C<sub>a</sub>, tree diameter (as a proxy for crown exposure), temperature, and precipitation.</p><p>Across all sites, long-term C<sub>a</sub> rise increased photosynthetic efficiency, likely due to increased photosynthesis and the concurrent suppression of photorespiration. Increasing tree size reduced photosynthetic efficiency, likely due to reduced leaf internal CO<sub>2</sub> at higher irradiance and stronger hydraulic limitation. Associations of photosynthetic efficiency with temperature and precipitation were inconclusive.</p><p>Our study reveals a centennial-scale association between photosynthetic efficiency and increasing C<sub>a</sub> in canopy trees and provides a new and independent line of evidence for C<sub>a</sub>-induced stimulation of photosynthetic efficiency in tropical forests.</p>