linking forests, climate and landuse activities
Fire has traditionally been used as a tool that is accessible to anyone who wishes to clear land in the Amazon basin. Historically, it was not as problematic as it has become. Because the dense, closed canopies of intact forests kept the litter layer on the forest floor too moist to burn, even during the long dry season, fires seldom traveled any distance through intact forest. However, as the landscape has become progressively more transformed through intentional activities, and as regional and global climate change provokes longer and more severe droughts, fires are much more widespread, causing more damage than in the past.
Because clear-cutting for timber is illegal in the Amazon, selective logging is the norm. Selective logging operations typically remove 10 to 20 percent of the trees in a given area of forest. The trees removed are often among the largest, so their removal may leave a big hole in the forest canopy, and these trees can also do a lot of damage to neighboring trees as they fall. The formerly closed canopy of the forest becomes perforated, letting in sunlight and warmer air to dry the fuel layer. More, and larger, fuel left behind from the limbs and branches discarded from the extracted tree and the branches damaged on surrounding trees. Removing the logs from the forest causes further damage, as skid paths and roads must be opened. Some of these may be used by settlers to gain access to the forest once the loggers leave. Fire is also used to clear areas for logging camps and loading areas, and these fires sometimes escape into the forest. When a forest made vulnerable by logging is later burned, the more open canopy and the trees killed by the fire increase its vulnerability to future fires, and unless the settlers who repeatedly set fires either move away or change their practices, recovery is difficult. Until a forest can regrow a closed canopy to re-establish the humid microclimate beneath, keeping litter too moist to burn, it will remain susceptible. As the forest becomes progressively degraded by repeated fire, it loses much of its value to the local settlers, so they are less likely to take steps to protect it.
Farmers and ranchers use fire to clear the forest to plant pastures or crops, and fire is also used to clear overgrown pastures. When these intentional fires escape, many people suffer economic losses: the small-holders lose their investments in fences, crops, and permanent plantations, and large ranch owners lose investments in pasture reform. Such losses further discourage landowners from investing in fire-sensitive forms of production and perpetuate reliance upon cattle pasture, subsistence agriculture, and the use of fire as a management tool. Because forests near extensive agriculture systems are at a greater risk of burning, loggers are not motivated to take risks to harvest them sustainably over long periods, as a future return is not guaranteed. Without a long-term commitment to the land, there is little incentive to practice less damaging timber extraction methods. For these reasons, extensive agriculture systems that use fire tend to be self-perpetuating once established in a region. Only through education and locally-based enforcement of strict fire control policies can more intensive, sustainable uses of the land take hold, once landowners are convinced that investments in permaculture, agroforestry, and sustainably managed forests will have a chance of paying off in the future.
The climate of the Amazon basin is inextricably linked to the rainforest. Most of the basin has an extended dry season where less than 20% of the annual rainfall may fall during 5-7 months of the year. The predominantly broad-leaved evergreen forest of the basin survives by sending roots deep into the soil to extract water stored there from previous rainy seasons, pumping it up through leaves and releasing it into the atmosphere, to coalesce into clouds and fall back to earth somewhere downwind. As more and more areas are cleared for agriculture, this ability to pump water from deep in the soil into the atmosphere will be diminished, and the areas downwind are thus deprived of moisture they customarily received from the natural system under which they formed. Smoke from fires has now been shown to inhibit the formation of clouds, so where there is burning, the smoke may also suppress rainfall in the region, rendering ever larger areas more vulnerable to fire.
The situation is further complicated by inputs from global-scale climate alterations. The increasingly frequent and more intense El Ni�o events associated with higher ocean surface temperatures in the Pacific bring very severe drought to the Amazon. In the 1997-1998 El Ni�o, the rainy season almost did not occur � rainfall in some areas barely reached 25 percent of normal levels. This meant that the deep soil water stocks were not replenished, and trees in these areas were unable to find water in the soil to sustain their full green canopies. They responded by shedding some of their leaves, opening the canopy to drying sunlight and wind, and increasing the load of fuel on the ground. The following dry season saw historic levels of burning.
Despite the Amazon forest�s remarkable resilience, with continued modifications of the global and regional climate, rainfall might one day be diminished enough to threaten the continuing survival of the native closed-canopy forests across large portions of the basin. More frequent or intense disruption of rainfall patterns may lead to increased fires, which in turn would damage the forest and make it even more vulnerable to future burns. These factors would certainly lead to negative impacts on biodiversity, diminish the amount of biomass stored in the forests, and would reduce the quantity of water liberated from the soil and returned to the atmosphere by vegetation, which is necessary to maintain the hydrologic cycle and climate of the region upon which the forest depends. In the long run, herbs, shrubs and those trees capable of resprouting after a fire might replace the species-rich forest in eastern, southern, and perhaps central Amazonia. In the face of drought and human disturbance, a large-scale shift to savanna-like vegetation is plausible for much of what is now rainforest.
©Woods Hole Research Center, 2007