Harvard researchers are challenging the widely-held theory that climate change could cause Amazon forests to rapidly change from forests to savannah. A new model, based on the effect of water stress on individual trees, suggests the change would be a gradual transition from high-biomass forests to low-biomass forests and woodland ecosystems. The study is described in a recently published paper in the Proceedings of the National Academy of Sciences.

"In earlier approaches, they use an aggregated representation of the ecosystem," said Paul Moorcroft, Professor of Organismic and Evolutionary Biology, and a senior author of the study. One way to think about it is they're modeling an average tropical tree in an average tropical environment. But because of that, when the system responds, it all responds at once, because it's essentially all the same.

"In reality, ecosystems have a variety of individual plants, with different plants in different locations," he continued. "Our approach is to capture that heterogeneity, and what we were able to show is that...this predicts a more graded response to climate change."

The new predictions are the product of an advanced ecosystem model, dubbed Ecosystem Demography, or ED2, developed by Moorcroft and colleagues, including Naomi Levine, the Gabilan Assistant Professor of Biological Sciences and Earth Sciences at USC. While earlier studies had predicted longer and more intense dry seasons in the Amazon as a result of climate change, the effect on forests had been unclear.

Though earlier approaches had suggested a rapid shift from forest to grassland, Moorcroft and colleagues set out to improve on the all-or-nothing nature of those predictions.

"The way they described it, there was a tipping point," he said. "Below that point, essentially nothing was happening, but beyond it, things were catastrophic. In contrast our analysis predicts that as the climate changes, the ecosystem will respond almost immediately, but those changes will be less drastic, so in some sense it says the ecosystem is both more vulnerable and more resilient."

Unfortunately, Moorcroft said, it also means the Amazon forest is already responding to climate change, and those changes will be felt far sooner than many expected.

Changes in the forest could have impacts on everything from rainfall in the region - effects that would be felt both in agriculture and hydroelectric power - and on biodiversity.

"These systems have this resilience because not every tree in the forest is the same, or lives out their life in the same way," Moorcroft said. "What we're saying is that really matters when we predict how the ecosystem responds to perturbations."