Don't count on the tropical forest gobbling up our excess carbon. Such is the warning from a recent study by Harvard's Kenneth Feeley and others in Ecology Letters, which suggests that we may not be able to count on surging tropical forest growth to slow global warming by consuming some of the excess carbon (via carbon dioxide intake). Why not? Because warming temperatures, contrary to previous thought and hope, were found to actually slow tropical forest growth in this 25-year study in Panama and Malaysia. As Feeley notes in the article's abstract, "these patterns strongly contradict the hypothesized pantropical increase in tree growth rates caused by carbon fertilization. Decelerating tree growth will have important economic and environmental implications." Not good.
Tropical Forest Slows Down
Tropical forests play a major part in the global carbon cycle. An understanding of the responses of tropical forests to climate change is an essential element in predicting the trajectory of global environmental change in the coming decades. Some studies have found increasing growth rates of trees, consistent with model predictions of CO2 fertilization. However, others have suggested that growth rates might decrease, consistent with models of the effects of increasing temperature on tree respiration. Feeley et al. analyzed two detailed long-term data sets from forest plots in Panama and Malaysia, to reveal growth rates of individual species and whole communities over the past 25 years. In both places, growth rates decreased in the majority of species, and this pattern was also reflected at the community level. These decreases correlated with increasing temperature over the same period, suggesting the potential for positive feedbacks between decreasing tree growth and increasing atmospheric CO2 concentration.
And here's the full abstract from Ecology Letters:
The impacts of global change on tropical forests remain poorly understood. We examined changes in tree growth rates over the past two decades for all species occurring in large (50-ha) forest dynamics plots in Panama and Malaysia. Stem growth rates declined significantly at both forests regardless of initial size or organizational level (species, community or stand). Decreasing growth rates were widespread, occurring in 24–71% of species at Barro Colorado Island, Panama (BCI) and in 58–95% of species at Pasoh, Malaysia (depending on the sizes of stems included). Changes in growth were not consistently associated with initial growth rate, adult stature, or wood density. Changes in growth were significantly associated with regional climate changes: at both sites growth was negatively correlated with annual mean daily minimum temperatures, and at BCI growth was positively correlated with annual precipitation and number of rainfree days (a measure of relative insolation). While the underlying cause(s) of decelerating growth is still unresolved, these patterns strongly contradict the hypothesized pantropical increase in tree growth rates caused by carbon fertilization. Decelerating tree growth will have important economic and environmental implications.