Tropical forests absorb as much as 50% of global carbon dioxide emitted each year and are vitally important in fighting climate change. Climate scientists have commonly assumed that as these gases build up, forests will grow at an accelerated rate and absorb more carbon.
New research, however, has shown that this assumption may be incorrect.
The research, which was published in the scientific journal Nature Geoscience, was conducted by an international team of scientists that analyzed tropical tree ring samples from Bolivia, Cameroon and Thailand. The team found no correlation between increased carbon dioxide in the atmosphere and forest growth in the past 150 years.
While lead scientist Peter van der Sleen, of Wageningen University’s Forest Ecology and Management Group, maintains that forests still remain an important carbon sink, he believes his team’s new research calls into question tropical forests ability to mitigate climate change.
New research methodology is responsible for the conclusion, which differs from previous studies. Older studies were based on analyzing biomass material contained in small forest plots, while van der Sleen’s study was based on taking random samples from trees located throughout different forests.
His analysis also looked a tree growth over a longer time scale. Van der Sleen believes that if trees were indeed growing more due to higher carbon dioxide levels, their rings would thicken more with time. The trees studied showed no evidence of thicker rings.
However, the team did find that increased carbon dioxide exposure did affect trees in two other ways, by contributing to more efficient water absorption and photosynthesis. The team put forth three possibilities for why the trees didn’t show increased ring growth. First, rising temperatures may be inhibiting tree growth. Second, tree growth might be occurring, just in other places like the roots or fruit.
A third theory, which the team favors, is that tree growth is limited by other factors not connected to carbon such as nutrients in the soil. Even if individuals trees are not growing more, the number of trees could be increasing as a result of more carbon dioxide in the atmosphere. This would also have the effect of sequestering some atmospheric carbon. While the team’s research has called into doubt whether forests are compensating for increasing emissions, van der Sleen cautioned that his results are “not conclusive” and urged further study on the subject.