Deforestation: Carving up the Amazon..............

A rash of road construction is causing widespread change in the world's largest tropical forest — with potentially global consequences.

The Interoceanic Highway has spurred deforestation in the Peruvian Amazon. [click image to enlarge].

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 year.

Roads cause drying within the forest, making them more susceptible to burning.

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.

“We're looking at a tidal wave of road expansion happening in the next few decades.”

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.

The drying effect reaches well past the forest's edge. And the more fragmented the forest, the wider the impact, according to one study that found canopy drying 2.7 kilometres from the edge of a highly fragmented forest6.

The influence of roads in the Amazon could even reach around the world. Recent lines of research suggest that changes in several factors prevent trees in disrupted forests from storing as much carbon as they did in the past, a shift that could accelerate global warming.

Greg Asner, a tropical ecologist at the Carnegie Institution for Science at Stanford University in California, studies the chemistry of the tree canopy in the Amazon using ground plots and airborne spectrometers. He is finding that the forest canopy along the edges of open patches does not seem to hold as much water or pigment, such as chlorophyll, as trees in unbroken parts of the forest. “Not enough chlorophyll and not enough water keep the canopy from soaking up carbon dioxide at the rate that we know it can, compared to the more interior forest,” he says.

Patches of cleared land border the Interoceanic Highway as it runs through the Amazon.

Line of fire

Changes in the Amazon's fire potential are also impeding the forest's ability to store carbon. Conventional wisdom has long held that the rainforest was too humid to burn. But in 2005, when drought struck the western Amazon, wildfires in the Brazilian state of Acre merged into a line 11 kilometres long, with flames leaping to canopy height, recalls Foster Brown, an environmental geochemist at the Woods Hole Research Center who witnessed the fires.

The flames damaged more than a quarter of a million hectares of forest in that state alone and caused US$100 million in damages. Smoke blanketed Rio Branco, the state capital, and public-health concerns finally led to ordinances to control burning during times of drought.

Scientists considered the 2005 drought to be a once-in-a-century event; some 70 million hectares of forest suffered water stress7, and there was significant drying within the tree canopy. But five years later, a similar dry spell struck, triggering another extreme bout of fires. Because they have not evolved in an environment frequently beset by fires, trees in the Amazon forest are susceptible to heat and damage from flames.

Farther east, in Brazil's Xingu region, researchers saw similar results from experimental fires during a drought in 2007. Tree mortality from heat and fire damage that year was more than four times that of a normal year8, especially along the forest edge, which the researchers burn every three years in a cycle emulating traditional Amazonian agricultural practices, says ecologist Paulo Monteiro Brando of Brazil's Amazon Environmental Research Institute in Brasília. In the Amazon, burning is the cheapest and most effective way for farmers to clear fields and give them a nutrient boost before planting crops, or rid them of ticks that plague livestock.

Understanding the future of the Amazon means learning how to model not just physical and atmospheric processes, but also how humans are changing the land, researchers say.

And as the wider impact of Amazonian roads becomes clearer, planners and conservationists face a dilemma. Although roads threaten the forest's health, they also significantly lower costs for farmers and businesses, and can make a difference between life and death for people in remote areas far from hospitals.

But unrestricted road building could lead to irreparable environmental harm, say researchers. “We're looking at a tidal wave of road expansion happening in the next few decades,” Laurance says. “It's ecological Armageddon, and it's happening again and again.”

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bluesbaby5050: Drought Sensitivity of Amazonian Carbon Balance Revealed By

Atmospheric measurements. Feedbacks between land carbon pools and climate provide one of the largest sources of uncertainty in our predictions of global climate1, 2. Estimates of the sensitivity of the terrestrial carbon budget to climate anomalies in the tropics and the identification of the mechanisms responsible for feedback effects remain uncertain3, 4. The Amazon basin stores a vast amount of carbon5, and has experienced increasingly higher temperatures and more frequent floods and droughts over the past two decades6. Here we report seasonal and annual carbon balances across the Amazon basin, based on carbon dioxide and carbon monoxide measurements for the anomalously dry and wet years 2010 and 2011, respectively. We find that the Amazon basin lost 0.48 ± 0.18 petagrams of carbon per year (Pg C yr−1) during the dry year but was carbon neutral (0.06 ± 0.1 Pg C yr−1) during the wet year. Taking into account carbon losses from fire by using carbon monoxide measurements, we derived the basin net biome exchange (that is, the carbon flux between the non-burned forest and the atmosphere) revealing that during the dry year, vegetation was carbon neutral. During the wet year, vegetation was a net carbon sink of 0.25 ± 0.14 Pg C yr−1, which is roughly consistent with the mean long-term intact-forest biomass sink of 0.39 ± 0.10 Pg C yr−1 previously estimated from forest censuses7. Observations from Amazonian forest plots suggest the suppression of photosynthesis during drought as the primary cause for the 2010 sink neutralization. Overall, our results suggest that moisture has an important role in determining the Amazonian carbon balance. If the recent trend of increasing precipitation extremes persists6, the Amazon may become an increasing carbon source as a result of both emissions from fires and the suppression of net biome exchange by drought. Chou, W. W. et al. Net fluxes of CO2 in Amazonia derived from aircraft observations. J. Geophys. Res. 107, 4614, http://dx.doi.org/10.1029/2001JD001295 (2002) ] 16.Aragão, L. E. O. C. et al. Spatial patterns and fire response of recent Amazonian droughts. Geophys. Res. Lett. 34, L07701-1. http://dx.doi.org/10.1029/2006gl028946 (2007).

2.Espinoza, J. C. et al. Climate variability and extreme drought in the upper Solimões River (western Amazon Basin): understanding the exceptional 2010 drought. Geophys. Res. Lett. 38, L13406, http://dx.doi.org/10.1029/2011gl047862 (2011).

3.Miller, J. B. et al. Airborne measurements indicate large methane emissions from the eastern Amazon basin. Geophys. Res. Lett. 34, L10809, http://dx.doi.org/10.1029/2006GL029213 (2007).
4.European Commission. Emission Database for Global Atmospheric Research (EDGAR) version 4.0, http://edgar.jrc.ec.europa.eu/overview.php?v = 40 (Joint Research Centre/Netherlands Environmental Assessment Agency, 2009).

bluesbaby5050: The Amazon Basin In Transition.........

Agricultural expansion and climate variability have become important agents of disturbance in the Amazon basin. Recent studies have demonstrated considerable resilience of Amazonian forests to moderate annual drought, but they also show that interactions between deforestation, fire and drought potentially lead to losses of carbon storage and changes in regional precipitation patterns and river discharge. Although the basin-wide impacts of land use and drought may not yet surpass the magnitude of natural variability of hydrologic and biogeochemical cycles, there are some signs of a transition to a disturbance-dominated regime. These signs include changing energy and water cycles in the southern and eastern portions of the Amazon basin. Amazon drought raises research doubts. A once-in-a-century drought struck much of the Amazon rainforest in 2005, reducing rainfall by 60–75% in some areas — and giving scientists a window on to a future coloured by climate change.

The drought foreshadowed the Amazon drying that many climate modellers expect to see in a warmer world. But five years on, a spate of research, including 13 papers published on 20 July in a special issue of the journal New Phytologist , shows that researchers are still grappling with the impact of drought and what it could reveal about the fate of the world's largest tropical forest, a major carbon storehouse.

The debate began with a 2007 study1 that used data gathered by NASA's Terra satellite to argue that the canopy of the Amazon rainforest grew and "greened up" during the drought — suggesting that the rainforest could be resilient to dryness, at least for short periods. The phenomenon can be attributed to fewer clouds and more sunlight. But in March, a study using the same satellite data added confusion to the issue when it failed to find excessive greening2.

These uncertainties are exposing the limits of space-based sensors that were designed 20 years ago, says Gregory Asner, an expert in remote sensing at the Carnegie Institution's global ecology programme at Stanford University in California. "It's nobody's fault — these guys are trying to squeeze blood from a stone," says Asner, co-author of a review of Amazon drought research in the New Phytologist issue3. "These issues are not going to be resolved by technology that is in orbit today."

The debate over the greening fuelled accusations that the Intergovernmental Panel on Climate Change had exaggerated when it suggested in its 2007 report that 40% of the Amazon region is highly vulnerable to drought. But longer-term studies on the ground, although limited in range, have consistently showed that the forest is sensitive to drying. A 2009 analysis of ground plots4 found decreased growth and increased tree death from the 2005 drought, and one of the New Phytologist papers documents similar results from the longest-running Amazon drought experiment5. After siphoning off half of the rain that fell across a 1-hectare plot, researchers observed a 30% reduction in tree growth and a doubling in tree death over the course of 7 years.

But there might have been more to 2005 than just drought, according to a separate report published online last month6. Using a combination of satellite images and ground-based tree counts, the report estimated that an unusual cluster of powerful thunderstorms in January of that year knocked down more than half a billion trees. Those results do not necessarily undercut the drought research, which took place on plots unaffected by the storms. Nonetheless, it is a reminder that "there are many factors at play", says Carlos Nobre, a scientist at Brazil's National Institute for Space Research in São José Dos Campos, who was not involved in the study. "Any time you try to explain basin-wide phenomena with only one factor, you are probably going to be wrong," he says.

The uncertainties in understanding drought in the Amazon won't be reduced without better sensors in space, says Asner. In particular, he is pushing for NASA to prioritize a mission called the Hyperspectral Infrared Imager (HyspIRI), which would measure the subtle colour and chemical changes in the rainforest canopy in much more detail than the Terra satellite. Despite being high on the priority list of the Earth-science community, HyspIRI is not scheduled for launch until 2020. Asner laments, "We are going to be cruising for another decade with myopic goggles on."

Although drought has often been the focus of researchers gauging the impact of global warming on the Amazon, recent models suggest that the largest uncertainties stem from a different factor: how the forest responds to rising atmospheric carbon dioxide. By easing the burden of CO2 uptake, higher concentrations of the gas reduce water loss during photosynthesis. A strong CO2 fertilization effect not only boosts growth and carbon uptake, but could also offset reductions in precipitation — thus increasing resilience to drought, says Peter Cox of the University of Exeter, UK, a co-author on four modelling studies in the New Phytologist issue.

But in the Northern Hemisphere, fertilization experiments — which involve pumping tonnes of CO2 into forest plots — indicate that the effect is limited by the availability of nutrients such as nitrogen. Tropical scientists want to know if the same is true in the Amazon. "CO2 fertilization is absolutely critical in the tropics, and we know absolutely nothing about it because we haven't done the experiments," says Cox. Climate change crisis for rainforests. The tropical forests of South America, Africa and Asia take up and release huge amounts of carbon each year. On the whole, they are a significant 'sink' for atmospheric carbon dioxide, but their future role in sequestering the greenhouse gas is uncertain. If rainforests are hit by serious drought, as they were in the Amazon basin during 2005, they could turn into a carbon 'source' sooner than we thought. So, are we in danger of losing our closest allies in the fight against climate change?

How is climate change affecting the growth of the forests?

Atmospheric CO2 levels are now 40% above what plants were experiencing just a century or so ago. Plants may have benefited from the availability of extra CO2, which they convert, through photosynthesis, into biomass. But plant growth benefits from elevated CO2 levels only up to a point, and more negative aspects of climate warming may still be ahead.

The biggest worry is drought. Scientists who have for the first time determined the drought sensitivity of a tropical forest report in Science1 today alarming results from the Amazon basin: the unusual 2005 drought there has apparently turned some of the affected areas of the Amazon from a carbon sink to a carbon source.

A comparison of plots that were monitored regularly before and after the drought revealed that forest patches subjected to a 100-milimetre decrease in rainfall released on average 5.3 tonnes of carbon per hectare as trees in the area died.

Basin-wide, between 1.2 billion and 1.6 billion tonnes of carbon were released as a result of the intense dry season and weakened wet season during 2005, the team estimates. The exceptional growth in atmospheric CO2 concentrations in 2005 may actually have been caused by these releases.

So does climate change mean that rainforests will not be carbon sinks in the future?

That's not clear, because current climate models are not very good at simulating rainfall. The formation and distribution of clouds and precipitation are controlled by atmospheric processes that occur on smaller scales than existing climate models can resolve. As a result, climate models reproduce observed temperatures reasonably well but diverge rather wildly when it comes to rainfall, and particularly so in the tropics. Projections of rainfall must therefore be taken with a pinch of salt.

Nonetheless, many scientists do strongly suspect that, in a warmer climate, dry conditions such as those of 2005 will become more frequent in the Amazon region and around the tropics. If they are right, tropical forests could gradually cease to act as a solid buffer against climate change.

How large a carbon sink are the world's tropical forests at the moment?

Scientists estimate that mature tropical forests, which cover about 10% of Earth's land, take up as much as 1.3 billion tonnes of carbon per year. This is a substantial amount, equivalent to almost 20% of carbon emissions from fossil-fuel burning. Tropical forest thus accounts for around 40% of the global terrestrial carbon sink. The good news is that undisturbed old forests keep getting better at sequestering carbon from the atmosphere. Over the past couple of decades, mature tropical forests in Africa and South America seem to have taken up an extra 0.6 tonnes of carbon per hectare each year on average2,3. Tropical forests in Asia are likely to have improved their carbon uptake as well, although probably at a lower rate.

How reliable are these figures?

Measuring tree growth is notoriously difficult, not least because tropical observation networks are pitifully few, particularly in Africa. Problems related to plot selection, comparability and converting tree-diameter measurements to carbon content have led to an intense debate about the size and fate of the tropical (and global) terrestrial carbon sink. Given the many uncertainties, forests have been excluded from national carbon budgets under the 1997 Kyoto Protocol on Climate Change.

However, data gathered over the past decade suggest that undisturbed old-growth forests — in and outside the tropics — do indeed continue to grow and accumulate carbon. There is little doubt that tropical forests have acted as a substantial carbon sink for at least the past couple of decades. Old-growth boreal forests, which were long suspected to be carbon-neutral, have recently been found to keep accumulating carbon as well4.

How long will old forests continue to get better at taking up CO2?

That is a key question. Deforestation and forest degradation, through logging, clearing and fire, are only the most obvious problems. Between 2000 and 2005, South America and Africa have each lost around 4,000 square kilometres of forest annually. But even undisturbed forests cannot continue to grow for ever. Their accelerated growth in recent decades is probably a temporary phenomenon, explained either by the fertilization effect of elevated CO2 levels or by the fact that they are still in the process of growing back from major disturbances in past centuries.

What does all this mean for forest management and the politics of climate change?

Climate change and deforestation pose a double threat to rainforests. Keeping alive large amounts of forest will require big areas to remain undisturbed from logging and clearing. Fragmented forest areas are more vulnerable and more likely to be overrun by climate change.

"These forests have given us a subsidy for a long time, but this cannot be taken for granted," says Oliver Phillips, an ecologist at the University of Leeds, UK, who coordinates the Amazon Forest Inventory Network, which was responsible for the latest study. "So when putting a carbon value on them we'd rather be conservative." Modelling conservation in the Amazon basin, and Expansion of the cattle and soy industries in the Amazon basin has increased deforestation rates and will soon push all-weather highways into the region's core1, 2, 3, 4. In the face of this growing pressure, a comprehensive conservation strategy for the Amazon basin should protect its watersheds, the full range of species and ecosystem diversity, and the stability of regional climates. Here we report that protected areas in the Amazon basin—the central feature of prevailing conservation approaches5, 6, 7, 8—are an important but insufficient component of this strategy, based on policy-sensitive simulations of future deforestation. By 2050, current trends in agricultural expansion will eliminate a total of 40% of Amazon forests, including at least two-thirds of the forest cover of six major watersheds and 12 ecoregions, releasing 32 ± 8 Pg of carbon to the atmosphere. One-quarter of the 382 mammalian species examined will lose more than 40% of the forest within their Amazon ranges. Although an expanded and enforced network of protected areas could avoid as much as one-third of this projected forest loss, conservation on private lands is also essential. Expanding market pressures for sound land management and prevention of forest clearing on lands unsuitable for agriculture are critical ingredients of a strategy for comprehensive conservation3.

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