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NSF REPORTS DROUGHT, FIRE IN AMAZON LEADS TO MORE DEAD TREES

Photo:  Amazon Forest.  Credit:  Wikimedia.  

FROM:  NATIONAL SCIENCE FOUNDATION 

Drought and fire in the Amazon lead to sharp increases in forest tree mortality

Deforestation and fragmentation of forests help create tinderbox conditions

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

Their 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, a geographer at Penn State University.

"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."

The researchers conclude in today's issue of the journal Proceedings of the National Academy of Sciences (PNAS) 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."

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, 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 researchers burned 50-hectare forest plots in the Southeastern Amazon, a region prone to the effects of climate change.

"If drought and wildfires happen in the same time-frame, there are far-reaching consequences for the health of Amazon forests," said Henry Gholz, a program director in the National Science Foundation Division of Environmental Biology, which funded the research. "When climate change becomes part of the mix, questions for these forests loom yet larger."

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 PNAS paper author Paulo Brando of the 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 effects 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 effects of fires on Amazon forests.

"Basically, none of the models used to evaluate future Amazon forest health includes fire, so most of these predictions underestimate the amount of tree death and overestimate overall forest health," said Michael Coe, a scientist at the 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 drier 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 of the Earth Innovation Institute, who also co-led the project. "We now know that severe drought makes fires more intense, creating a second tree mortality threshold."

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 are close to human settlements, 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 scientist Douglas Morton of NASA.

"These smaller forest fragments have more edges than large blocks of forest, which expose them to the hotter, drier conditions in the surrounding landscape and make them more vulnerable to escaped fires," said researcher Marcia Macedo of the Woods Hole Research Center.

By 2011, about eight 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 very susceptible to future fires.

The research was also funded by the Packard Foundation, NASA and the Max Planck Institute for Biogeochemistry.

-NSF-

PRE-COLOMBIAN HUMANS AND THEIR EFFECTS UPON THE AMAZON BASIN

Photo:  Blue Parrot.  Credit:  Wikipedia.
FROM:  NATIONAL SCIENCE FOUNDATION
Scientists Reconstruct Pre-Columbian Human Effects on the Amazon Basin
June 14, 2012
Small, shifting human populations existed in the Amazon before the arrival of Europeans, with little long-term effect on the forest.

That's the result of research led by Crystal McMichael and Mark Bush of the Florida Institute of Technology (FIT). The finding overturns the idea the Amazon was a cultural parkland in pre-Columbian times with large human populations that transformed vast tracts of the landscape.

The Amazon Basin is one of the highest biodiversity areas on Earth. Understanding how it was modified by humans in the past is important for conservation and for understanding the ecological processes in tropical rainforests.

McMichael, Bush and a team of researchers looked at how widespread human effects were in Amazonia before Europeans arrived. They published their results in this week's issue of the journal Science.

"The findings have major implications for how we understand the effect of the land-use change now occurring in Amazonia," said Alan Tessier, program director in the National Science Foundation's Division of Environmental Biology, which funded the research.
"Making the assumption that this system is resilient to deforestation, it turns out, isn't a position supported by historical evidence," Tessier said.

If the pre-Columbian Amazon was a highly altered landscape, then most of the Amazon's current biodiversity could have come from human effects.

The team retrieved 247 soil cores from 55 locations throughout the central and western Amazon, sampling sites that were likely disturbed by humans, such as river banks and other areas known from archaeological evidence to have been occupied by people.
They used markers in the cores to track the histories of fire, vegetation and human alterations of the soil.

The scientists conclude that people lived in small groups, with larger populations in the eastern Amazon--and most people lived near rivers.

They did not live in large settlements throughout the basin as was previously thought. Even sites of supposedly large settlements did not show evidence of high population densities and large-scale agriculture.

All the signs point to smaller, mobile populations before Europeans arrived. These small populations did not alter the forests substantially.

"The amazing biodiversity of the Amazon is not a by-product of past human disturbance," said McMichael. "We can't assume that these forests will be resilient to disturbance, because most of them have, at most, been lightly disturbed in the past.

"There is no parallel in western Amazonia for the scale of modern disturbance that accompanies industrial agriculture, road construction and the synergies of those disturbances with climate change."

Other co-authors of the paper are D.R. Piperno of the Smithsonian National Museum of Natural History; M.R. Silman of Wake Forest University; A.R. Zimmerman of the University of Florida; M.F. Raczka of FIT; and L.C. Lobato of the Federal University of Rondônia in Brazil.
-NSF-