Decelerating growth in tropical forest trees, thanks to accelerating carbon dioxide
I meant to blog on this earlier, but lost track of it after failing to find the original study (for reasons that will become clear). The bottom line is:
Global warming could cut the rate at which trees in tropical rainforests grow by as much as half, a new study based on more two decades of data from forests in Panama and Malaysia shows.
The effects, so far largely overlooked by climate modelers, Nature magazine said, could severely erode or even remove the ability of tropical rainforests to remove carbon dioxide from the air as they grow.
More evidence that the carbon sinks in the ocean and on the land may saturate sooner than scientists expected, which will inevitably lead to an acceleration of atmospheric concentrations of carbon dioxide (see below).
You might think from the article — or this post, which begins, "The study is contained in Nature magazine" — that the original study is from Nature. But, nooo! Someone — we won’t name names — could waste a lot of time looking for it there before they found out that it was only written about in Nature.
The actual study is from Ecology Letters, and here is a preprint (PDF). The abstract is sobering:
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.
And let’s throw in the article’s final paragraph, since it identifies two new amplifying feedbacks to worry about:
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.
More evidence that the time to act is now!