One of the oddest arguments against taking action to tackle climate change has been that the extra CO2 would increase plant productivity. While on paper a case can be made that an increased concentration of CO2 increases plant productivity, the reality of the situation is much more complex.
Now new research (PDF) claims that climate change is having a substantial negative impact on growth rates in tropical forests.
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…
Finally, we stress the potential for positive feedbacks to cause further declines in tropical forest growth rates. If decelerated stem growth results in slower rates of carbon uptake, the rise in atmospheric CO2 concentrations could accelerate. This may in turn lead to even higher temperatures and lower net productivity. Another potential feedback might occur if reduced timber yields force loggers to compensate by enlarging the amount of area harvested, resulting in higher CO2 emissions through deforestation and associated fires, as well as increased rates of habitat fragmentation/degradation and species extinctions
This is significant and could be another area where many models (including those used by the IPCC) are too conservative.
(h/t Climate Progress)