A rash of road construction is causing widespread change in the world's largest tropical forest — with potentially global consequences.
Next to a newly paved highway in the Peruvian Amazon, a discreet white-on-green sign urges travellers to protect the surrounding ecosystem. “Let's care for the environment, let's conserve the forest,” it reads. But the appeal comes too late for this spot in the region known as Madre de Dios. Before the route was paved a few years ago, tall trees lined the roadside, but the forest edge here now lies about half a kilometre away, beyond a jumble of underbrush and freshly cut trees where a cattle pasture was recently carved out of the woods.
As drivers head east and enter Brazil, the view is much the same for hundreds of kilometres. Such is the impact of the Interoceanic Highway, a route some 5,500 kilometres long that cuts clear across South America.
The highway is just one strand in a web of roads that now criss-cross the Amazon. So far, most have encroached on forest around the edges of the basin, but they are increasingly slicing through the middle. In Brazil alone, the Amazon road system grew by an average of almost 17,000 kilometres a year between 2004 and 2007 (ref. 1). Across the basin, estimates for the total length of roads vary widely from about 100,000 to 190,000 kilometres of paved and dirt roads cutting through the Amazon.
Once construction begins, road crews are quickly followed by land speculators, loggers, farmers, ranchers, gold miners and others who carve away the forest along the route. That activity leaves obvious scars on the landscape in the form of treeless expanses, but research is now showing that the building of roads also triggers a cascade of environmental changes in the remaining forest that can dry out trees, set the stage for wildfires and weaken the ecosystem.
“Put a road into a frontier area and it opens a Pandora's box,” says biologist William Laurance of the Centre for Tropical Environmental & Sustainability Science at James Cook University in Cairns, Australia.
The drying brought about by roads influences local atmospheric circulation patterns and can have farther-reaching effects that not only compromise the health of the Amazon but can also contribute to global warming by releasing carbon stored in the forest. Understanding those details is crucial, researchers say, for determining whether these effects — combined with severe droughts such as those that struck parts of the Amazon basin in 2005, 2007 and 2010 — could tip the world's largest expanse of tropical forest from being a net absorber of carbon dioxide to a net emitter2.
The first cut
It was a road that kicked off the pattern of destruction in the Amazon forest. In the 1970s, Brazil began building the Trans-Amazonian Highway from near the country's easternmost point on the Atlantic coast to its western border, where the state of Amazonas meets Peru. The route opened up the heart of the Amazon to logging, ranching and settlement, causing deforestation rates to soar. Extreme spells in the 1990s and early 2000s claimed more than 25,000 square kilometres a year — an area bigger than New Jersey. Since 2005, government measures, including crackdowns on illegal logging, have slowed forest loss. Throughout, roads have provided the means to penetrate the forest and erase large chunks of it. In an unpublished study of the Brazilian Amazon, Christopher Barber, a researcher at South Dakota State University in Brookings, found that 95% of deforestation in the region occurs within 7 kilometres of a road. And that is not the only problem: just as serious as outright deforestation is fragmentation, which happens when loggers, ranchers and farmers move in. In Brazil, up to 38,000 kilometres of new forest edge are created each year3.
Standing in a field in the western Brazilian state of Mato Grosso, Michael Coe can feel the difference that deforestation makes in the Amazon. An atmospheric scientist who heads the Amazon programme of the Woods Hole Research Center in Falmouth, Massachusetts, Coe is visiting an 80,000-hectare patch of former forest that was originally cleared some years ago to build a cattle ranch, which later morphed into a soya-bean farm. The air is noticeably hotter and drier in the field than in one of the few patches of forest left on the farm.
Coe and his colleagues are here to study how forest degradation and fires alter the flow of water and energy in the Amazon ecosystem. Evapotranspiration from trees provides moisture to the air and feeds much of the precipitation in the Amazon: when the trees disappear, so does a major source of moisture. A study using satellite data and models of atmospheric circulation suggests that air passing over tropical regions rich in vegetation produces at least twice as much rain as air moving over areas with little vegetation4.
Stripping away trees not only eliminates a source of moisture; it also changes the regional air flow. Heat rising from a bare field creates a low-pressure system that pulls in air from the surrounding area, sucking moisture out of the nearby forest, says Coe.
As the forest dries, it transfers less moisture to the atmosphere, changing rainfall patterns hundreds or thousands of kilometres downwind. That could affect not only forests and agriculture across the basin, but also the amount of water available to power hydroelectric dams. In a simulation using climate, hydrological and land-use models, Coe and his colleagues projected that reductions in rainfall caused by deforestation could drastically cut the power-generating capacity of Amazonian dams5. That would upset the plans of Brazil, Peru and Ecuador, which intend to increase hydropower to meet rapidly growing electricity demands.
Written By: Barbara Fraser
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