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

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"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 modellers, 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 this article or even this blog, which begins, “The study is contained in Nature magazine,” that the original study is from Nature. But, nooooo! 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. 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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3 Responses to Decelerating growth in tropical forest trees — thanks to accelerating carbon dioxide

  1. David B. Benson says:

    Bad scene. Thanks for posting about this.

  2. Lou Grinzo says:

    This kind of study, plus the frightening news of faster-than-expected polar melting, is why I’m so skeptical of all the claims that fossil fuel limits will prevent us from triggering massive GW effects. We simply don’t know with anything approaching the level of certainty we’d like how to map X part per million of CO2 to Y warming effect to Z impact on the biosphere to W impact on human beings. And virtually all the evidence is pointing to these effects being much more serious than even our best experts thought.

  3. Dano says:

    Joe, I’ve skimmed the paper and the causation offered by the authors isn’t acceleration of CO2 per se, it’s:

    In an attempt to identify the cause of the observed changes in growth rates, we tested for relationships between community-wide growth rates and climatic changes at BCI. For all stem size classes the strongest relationships were a significant increase in growth rates with increased frequency of rainfree days. This may be due to decreased cloudcover and increased light availability (Graham et al. 2003; Nemani
    et al. 2003) which is supported by the positive correlation between the number of rainfree days and insolation. There were also significant decreases in growth rates with decreased total precipitation and increased daily minimum temperatures.
    Climate data are not available from the Pasoh plot, but a nearby agricultural station (c. 5 km distant) recorded a steady increase in temperatures over the study period (Fig. 1) concurrent with the deceleration in growth rates (Table 3). Decreased growth at elevated temperatures is believed to be due to the increase in plant respiration relative to photosynthesis resulting in decreased net carbon
    assimilation (Amthor 2000; Clark et al. 2003; Clark 2004).

    Deforestation could be an issue here as well, causing the change in rainfall patterns and insolation (fewer clouds). Nonetheless, it is very likely that some sort of human activity is the cause.