Slowforestation

Decelerating growth in tropical forest trees, thanks to accelerating carbon dioxide

Posted by Joseph Romm (Guest Contributor) at 7:02 AM on 22 Jan 2008

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!

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Related paper

I happened to see this article a few days ago, although note that the subject paper is from 2005.

by Steve Bloom at 8:59 AM on 22 Jan 2008

More

I now see the paper discussed in the link I provided above was the major reference in the concluding paragraph of the paper Joe discussed.

Following the links, I also see that both of these papers were done in the context of a much larger project called LBA-ECO (the ecological component of the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA); see project description here).

As LBA-ECO has finished data-gathering and is in what they call the synthesis phase, it appears we'll be seeing a lot more results soon.

by Steve Bloom at 4:22 PM on 22 Jan 2008

Interesting AMS abstract

Simulated carbon losses from the land surface exceed emissions from fossil fuel burning

Paul A. T. Higgins, American Meteorological Society, Washington, DC

Projections of future carbon storage by the land surface depend on three overarching factors: 1) how plant biomass and soil carbon respond to climate changes, 2) how plants respond to CO2 enrichment, and 3) how effectively plants can move in response to shifting climate patterns. The implications of CO2 enrichment on global carbon storage in vegetation and soil remain ambiguous despite nearly two decades of plot-level manipulative field experiments. Nevertheless, biogeochemical models often assume substantial plant fertilization in response to higher CO2 concentrations. Here I demonstrate that simulated climate changes associated with the A1FI greenhouse gas emission scenario could trigger further losses of carbon from the land surface that exceed the amount of carbon currently contained in the atmosphere. Furthermore, model results suggest that plausible constraints on plant migration could cause an additional carbon loss from the land surface in excess of cumulative historical anthropogenic emissions. Taken together, this release of carbon from the land surface is sufficient to push atmospheric concentrations to levels found in the highest IPCC emission scenario (A1FI) even if anthropogenic emissions correspond to the lowest emission levels (B1).

by Steve Bloom at 4:59 PM on 22 Jan 2008

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