Positive feedbacks among forest fragmentation, drought, and climate change in the Amazon

Abstract

The Amazon basin is experiencing rapid forest loss and fragmentation. Fragmented forests are more prone than intact forests to periodic damage from El Niño-Southern Oscillation (ENSO) droughts, which cause elevated tree mortality, increased litterfall, shifts in plant phenology, and other ecological changes, especially near forest edges. Moreover, positive feedbacks among forest loss, fragmentation, fire, and regional climate change appear increasingly likely. Deforestation reduces plant evapotranspiration, which in turn constrains regional rainfall, increasing the vulnerability of forests to fire. Forest fragments are especially vulnerable because they have dry, fire-prone edges, are logged frequently, and often are adjoined by cattle pastures, which are burned regularly. The net result is that there may be a critical "deforestation threshold" above which Amazonian rainforests can no longer be sustained, particularly in relatively seasonal areas of the basin. Global warming could exacerbate this problem if it promotes drier climates or stronger ENSO droughts. Synergisms among many simultaneous environmental changes are posing unprecedented threats to Amazonian forests.