Forests are thought to act to mitigate global warming, caused by greenhouse gases, by removing carbon dioxide from the atmosphere. However, when trees die, they not only cease to absorb carbon dioxide, they also decompose, releasing carbon dioxide.
If the Amazon rainforest dies, that's a lot of carbon dioxide, which will greatly accelerate global warming. Whether the rainforest lives or dies in the long run depends in large part on their resistance to long-term drought brought upon by global climate change.
Is the rainforest resistant to long-term drought? Recent research suggests that it can withstand short-term drought.
Drought conditions result in fewer clouds. The rainforest may use the extra sunlight to grow even more.
However, large-scale tests of such theories have not been performed. Furthermore, whether or not short-term survival capacity translates to a long-term survival capacity is unknown.
Oliver Phillips (University of Leeds, UK) and coworkers, from 41 different institutions around the world, have conducted such a large-scale test of the Amazon rainforest's sensitivity to drought. As part of their 25-year large-scale study of the rainforest, using the unusually severe 2005 drought as a model, the scientists have shown that the forest is not resilient in the face of short-term drought, which suggests that it will not be resilient in the face of a long-term drought either.
Measuring sensitivity to drought.
By 2005, the scientists were monitoring 136 separate forest plots. They were all old-growth plots, located in 44 distinct landscapes.
They calculated total biomass through wood density, tree diameter, computer modelling, and measured death and growth. They focused on the 55 forest plots that were regularly monitored both before and after the 2005 drought.
Moisture stress within each plot was evaluated from available weather data. The scientists were thus able to calculate sensitivity to drought, by relating forest growth dynamics to moisture stress over the same period of time.
Before and after the drought.
Before the 2005 drought, the scientists found that 76% of the plots increased in biomass, giving a net increase. This is consistent with the work of other scientists.
However, during the 2005 drought, the scientists found that only 51% of the plots increased in biomass, giving no net increase. There were in fact net losses in biomass for especially stressed plots.
Furthermore, the forest plots that are normally unstressed (such as in the northwest, typically experiencing little interseasonal climate variability) exhibited the most biomass loss. In contrast, those plots that are normally stressed (such as in the far south, typically experiencing strong interseasonal climate variability) didn't fare as badly.
What this says is that regions of the forest already used to regular droughts survived better than those regions which are not used to regular droughts. However, the rainforest suffered overall.
Implications for forest ecology.
Were there any types of trees that fared worse than others? If some are more sensitive to drought than others, this may lead to widespread changes in the ecology of the rainforest.
The scientists found that the trees which died after the drought were on average 5% lighter than those that died before the drought. This suggest that fast-growing, light-wooded trees are less resistant to drought conditions, which in turn suggests that ecological changes are possible in the face of a long-term drought.
Long-term prospects for the Amazon rainforest.
It is clear that a short-term drought is detrimental to the survival of the Amazon rainforest. If short-term droughts turn into long-term droughts due to global climate change, the Amazon rainforest may not survive.
This may not lead to an immediate large-scale dumping of carbon dioxide into the atmosphere, because a drought can inhibit decomposition. However, decomposition will happen eventually, and the resultant accelerating effect on global warming will be huge.
This doesn't even address the cultural and habitat loss that will accompany the destruction of the Amazon rainforest (which is unlikely to simply "grow back" quickly). The long-term impact on civilization is just one reason why global warming needs to be taken seriously worldwide.
for more information:
Phillips, O. L.; et. al.
Drought sensitivity of the Amazon rainforest.
Science 2009, 323, 1344-1347.