It’s the chicken and the egg all over again. We know the importance of rainforests in sequestering carbon; an average tree in a tropical rainforest pulls in and stores about 50 pounds of carbon from the atmosphere each year. New growth and rapidly growing trees tend to absorb more carbon.
However, changing weather patterns are leading to increased drought, including in the rainforests themselves, and researchers are concerned about its effect on the health of the trees and on their growth rates. If drought means slower growth, does that mean the trees will be less efficient absorbers of carbon? And will that, in turn, eventually contribute to the conditions that led to the drought?
Researchers from Michigan State University recently used lidar imagery to help determine how drought is affecting growth patterns in the rainforests of Brazil. Specifically, they were interested in the structure of the leaf canopy. Lead researcher Marielle Smith notes, “The activity of a forest as a whole—its growth and exchanges of gas and energy with the atmosphere—is largely determined by how leaves are distributed in the mosaic of environments that the forest itself creates.”
Researchers surveyed the forest during drought and non-drought years; the results were not quite what they expected. They knew from earlier observations that when the upper canopy of the rainforest increases—as it tends to do during a drought, since the taller trees have deeper roots and better access to water—the plants in the understory tend to lose more leaves. The researchers believed this was probably because the shorter trees weren’t getting as much sunlight. What they actually found, though, was that shorter trees in sunnier areas—those whose light was not blocked by the upper canopy—lost the most leaves. Shorter trees that were shaded by the taller ones did just fine, maintaining the same leaf area.
The research team concluded that when exposed to hotter, drier drought conditions, small trees with shallow roots suffer the most damage. “The research is helping build a picture of how canopy micro-environments, tree heights, seasonality, and drought come together to determine which trees will win and lose under drier climates. This is crucial to understanding the future resilience of the Amazon to climate change,” the university says.
The research has been published in the journal New Phytologist.