UNIVERSITY PARK, Pa. -- Ongoing deforestation and fragmentation of forests in the Amazon help create tinderbox conditions for wildfires in remnant forests, contributing to rapid and widespread forest loss during drought years, according to a team of researchers.

The findings show that forests in the Amazon could reach a "tipping point" when severe droughts coupled with forest fires lead to large-scale loss of trees, making recovery more difficult, said Jennifer Balch, assistant professor of geography, Penn State.

"We documented one of the highest tree mortality rates witnessed in Amazon forests," Balch said. "Over the course of our experiment, 60 percent of the trees died with combined drought and repeated fire. Our results suggest that a perfect firestorm, caused by drought conditions and previous fire disturbance, crossed a threshold in forest resistance."

Balch noted that climate change is expected to warm the air in the Amazon region by several degrees and substantially reduce regional precipitation, making understanding the interactions between droughts and fires even more important. "However, even before any prediction of Amazon climate warming occurs, our study demonstrates that drought and fire are already driving forest dieback," she said.

The eight-year study is the largest and longest-running fire experiment in tropical forests. The team of researchers burned 50-hectare forest plots in the southeastern Amazon, a region prone to the effects of climate change.

The plots were burned every year, every three years or not at all. The timeframe for the study included 2007, a year of severe drought. By comparing the tree deaths for the plots each year, the researchers could assess the effect of drought on fire intensity and tree deaths.

"Drought causes more intense and widespread fires," said lead author Paulo Brando, Instituto de Pesquisa Ambiental da Amazônia, Carnegie Institution for Science and Woods Hole Research Center. "Four times more adult trees were killed by fire during a drought year, which means that there was also more carbon dioxide released to the atmosphere, more tree species loss and a greater likelihood of grasses invading the forest."

The researchers found that fragmented forests are more susceptible to the negative impacts of drought and fire and that drought leads to an increase in fuel such as leaves and branches. The findings are key, in part, because most climate change models have not included the impacts of fires on Amazonian forests.

"Basically, none of the models used to evaluate future Amazon forest health include fire, so most of these predictions grossly underestimate the amount of tree death and overestimate overall forest health," said Michael Coe, Woods Hole Research Center.

Fire as a forest management tool can contribute to an increase in severe fires because the resulting thinner canopy leads to dryer forest conditions. This lack of humidity does not dampen fires but does encourage airflow between fields and forests. Fragmented forests also have more edge space, which is susceptible to both fire and invasive grasses -- another potential fuel.

"These forests are tough and can take a lot, but if drought reaches a certain level, big trees begin to die," said Daniel Nepstad, Earth Innovation Institute, who also co-led the study. "We now know that severe drought also makes fires more intense, creating a second tree mortality threshold."

The researchers conclude in today's (April 14) issue of Proceedings of the National Academy of Science that "efforts to end deforestation in the Amazon must be accompanied by programs and policies that reduce the accidental spread of land management fires into neighboring forests and effectively control forest fires when started."

The results are important because large portions of the Amazon forest already experience droughts and are susceptible to fire -- they are broken into smaller blocks by agriculture and they are close to humans, who are the predominant source of fire in the Amazon. The researchers analyzed NASA satellite data to provide regional context for results from the experimental burns.

"In 2007, fires in Southeast Amazonia burned 10 times more forest than in an average climate year -- an area equivalent to a million soccer fields," said Douglas Morton, NASA.

"These smaller forest fragments have more edges than large blocks of forest, which exposes them to the hotter, dryer conditions in the surrounding landscape and makes them more vulnerable to escaped fires," said co-author Marcia Macedo, Woods Hole Research Center.

By 2011, around 8 percent of Southeast Amazonia's forests were less than 328 feet from an agricultural or pasture clearing. This lattice-like network of degraded forest edges is now extremely susceptible to future fire.

The National Science Foundation, Packard Foundation, National Aeronautics and Space Administration, and Max Planck Institute for Biogeochemistry supported this research.