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 Ni{\~n}o-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 â??deforestation thresholdâ??? 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.
Retroalimentaciones Positivas entre Fragmentaci{\'o}n de Bosques, Sequ{\'i}a y Cambio Clim{\'a}tico en el Amazonas Resumen: La cuenca del Amazonas esta sujeta a una r{\'a}pida p{\'e}rdida y fragmentaci{\'o}n de bosques. Los bosques fragmentados son m{\'a}s propensos que los bosques intactos a da{\~n}os peri{\'o}dicos por las sequ{\'i}as de El Ni{\~n}o, las que causan un incremento en la mortalidad de {\'a}rboles, especialmente cerca de los bordes de bosque. M{\'a}s a{\'u}n, las retroalimentaciones positivas entre p{\'e}rdida de bosque, fragmentaci{\'o}n, fuego y cambio clim{\'a}tico regional son muy probables. La deforestaci{\'o}n reduce la evapotranspiraci{\'o}n de plantas, que a su vez constri{\~n}e la precipitaci{\'o}n pluvial regional, incrementando la vulnerabilidad de los bosques al fuego. Los fragmentos de bosque son especialmente vulnerables porque tienen bordes secos, propensos al fuego, son talados frecuentemente y a menudo son contiguos a pastizales que se queman regularmente. El resultado neto es que puede haber un â??umbral de deforestaci{\'o}nâ??? cr{\'i}tico sobre el que las selvas de la Amazon{\'i}a ya no pueden sostenerse, particularmente en {\'a}reas relativamente estacionales de la cuenca. El calentamiento global podr{\'i}a exacerbar este problema si promueve climas m{\'a}s secos o sequ{\'i}as de El Ni{\~n}o m{\'a}s fuertes. Los sinergismos entre muchos cambios ambientales simult{\'a}neos son amenazas sin precedentes para los bosques de la Amazon{\'i}a.

Laurance, William F. Williamson, G. Bruce
Conservation Biology