Final Report Summary - TROFOCLIM (Tropical forests and climate change: understanding links to predict future responses)
Intact tropical forests have been reported to increase in biomass over time, thereby absorbing a significant fraction of human-caused CO2 emissions. It is commonly believed that the rise of atmospheric CO2 is fertilizing tropical trees, allowing trees to grow faster, but this hypothesis is controversial and poorly evaluated. In the TROFOCLIM project we investigated the extent to which CO2 rise modifies tree physiology and stimulates tree growth. We also evaluated the role of large-scale disturbances in driving forest dynamics. We used an innovative research approach that looks backwards in time to infer responses of tropical trees to long-term CO2 rise and past disturbances from growth rings and stable isotopes. We showed that the gradual anthropogenic CO2 rise since 1850 has strongly decreased water stress of tropical trees, but we did not find evidence for the hypothesized concurrent acceleration of tree growth. We also showed signs of large-scale disturbances and shifts in tree regeneration in tropical forests over the last 150 years. Overall, our results suggests that the widespread assumption of CO2 fertilization may not hold for tropical trees. As a result, current estimates of the risk of tropical forest dieback are likely too optimistic.