XVI International Botanical Congess

During severe drought, such as that provoked by el Nino events, even the dark, moist understory of primary Amazon forest can dry out sufficiently to catch fire. We measured the flammability of three types of Amazon forest across a 2,000-km transect by igniting experimental fires on the forest floor during the 1998 dry season. Two thirds of the 400 experimental fires ignited spread and had to be extinguished. Large portions of the forest floor become flammable as dry-induced leaf-shedding causes leaf area index to drop below 5 (from a wet season maximum of 6 to 6.5). Once forest burn, they are even more susceptible to further burning, thus creating the potential for a vicious positive feedback cycle, in which drought provokes forest burning, which leads to reduced evapotranspiration, further drought and repeated burning. During 1998, at least 30,000 square kilometers of Amazon forest experienced surface fires.