Farming, logging, and strip mining has long altered much of the Amazon rainforest, with slash-and-burn land-clearing techniques turning large portions of the forest into patchworks of pastures, second-growth forest, and degraded land. Now, rural people are increasingly moving to booming Amazonian cities; paradoxically, the land they're leaving behind is being ravaged by wildfires.
A new paper published in PNAS shows that in the Peruvian Amazon, land use changes and depopulation have let large wildfires fly through converted land. It puts a damper on those optimistic that the urbanization of the Amazon may allow parts of the forest to recover, by centralizing populated areas and leaving old converted land to be slowly gobbled up by the encroaching forest.
Amazonian wildfires are nearly all manmade. Pristine forest is simply too dense and too wet for natural fires to occur regularly, unlike in U.S. forest systems. As such, slash-and-burn techniques has razed massive swaths of forest in recent decades.
A video from the Columbia Earth Institute discussing rural burns.
The practice has been heavily criticized for myriad reasons: Aside from clearing pristine forest and eliminating habitats for native species, the burning of such huge quantities of plant material pumps out massive amounts of carbon in to the atmosphere, and floods the soil with minerals and nutrients that are quickly washed away. It takes a long time for essential nutrients to be fixed in a rainforest, and releasing them all at once means pastures are viable for a few years before going totally bare.
At that point, ranchers tend to clear more pasture land, and leave barren pastures behind. From that point, recovery is a long, slow process into becoming a healthy second-growth forest again. Now that farmers and other rural folk are moving into cities, they've left behind pastureland that ecologists hoped would have a chance to recover. But, as the Columbia team found, fewer people managing fires in converted land means those fires get out of control more often.
"Farmers are often blamed for deforestation and environmental destruction, but they are fairly sophisticated in managing fire—they plan when, how and where to burn the land," said lead author María Uriarte, a forest ecologist in Columbia's Department of Ecology, Evolution and Environmental Biology, in a release. "However, when you have more fallow land and fewer people around to work at control, that combination generates these big fires."
Proportion of Amazonian residents living in urban areas, via the paper
The population of Amazonian cities have boomed, and in many Amazonian countries, more than half of Amazonian residents live in cities. According to Uriarte's team, the population of the Peruvian Amazon grew by 20 percent between 1993 and 2007, the date of the most recent census. That period saw a huge population shift towards cities, with some Peruvian provinces seeing up to 60 percent of rural people leaving for cities. That trend, the authors report, holds true for much of the Amazon.
The catch then is that, while urban populations are more concentrated in terms of area, the resulting infrastructure needed to sustain those populations is much more massive, with bigger, higher-impact roads cutting through more of the forest and allowing more access for logging and farming operations.
Tack on to that huge droughts in 2005 and 2010, which the authors note are only supposed to occur every 100 years, raising the familiar specter of climate change. What you get is an urban Amazon that's being increasingly divided by large infrastructure, with more new land available to be cleared, and old pastureland that's easily burned and that has fewer people managing it. The end result is more fire.
Previous studies have shown that Amazonian fires increase with drought (of course) and proximity to roads (due to human factors, as well as opening up forest edges for fires to take hold on), but by adding evidence from Peru that rural population declines lead to more fires, the Columbia team offers insight into what management strategies could be effective. New logging and farming enterprises are often carved out of old forest, rather than repurposing old converted land. By showing that abandoned farmland is more susceptible to destructive fires, the team hopes that local governments will put more emphasis on repurposing old land rather than clearing the new.
For Americans weary of a country that's been gripped by a number of massive wildfires in recent years, and that can expect only more drought and more fires in the future, Amazonian fires might seem like a bit of an abstraction. Indeed, they're different. American wildfires are often caused by a combination of huge fuel buildup caused by a lack of regular low-intensity burns, while in the Amazon its a situation rooted in turning land that won't burn into land that will.
But there's one connecting thread between the two: drought. And like the U.S., Amazonian regions are expected to experience more drought, which has been linked in previous research with warmer temperatures in the tropical Atlantic Ocean, an effect that's likely to increase as the world warms. So while a proper management strategy is relatively clear–either continue using converted land or rehabilitate it to be less fire-prone, and build a more efficient fire monitoring network–the difficulty of funding such a network while convincing people to give up slash-and-burn technique, along with a changing climate, means the changing Amazon can likely expect more fires in the future.
Follow Derek Mead on Twitter: @derektmead