Increasing bamboo dominance in southwestern Amazon forests following intensification of drought-mediated fires

Abstract

Since the late 1980s the Amazon rainforest has been affected by major forest fires every 3–5 years, mainly in the southwestern portion of the region. Besides the reduction of forest biomass and changes in structure and floristic composition, these forest fires favor the expansion of bamboo in forests in the southwestern Amazon. However, we know little about the impact of fire on bamboo expansion and changes in forest structure. The goal of this study is to quantify forest degradation by fire in areas with bamboo in the eastern portion of the state of Acre, Brazil, based upon a combination of forest-inventory and satellite remote-sensing data. The forest fires were defined by remote sensing as those in which the crowns of the trees were directly or indirectly affected by fire to the point that they cause a detectable impact on the optical satellite images in the 1984–2016 period. We measured trees and bamboo in 6 ha distributed in twelve 0.5-ha plots (100 m × 50 m) in unburned forest, forest burned in 2005, burned forest in 2010 and forest burned in both 2005 and 2010. Our results show change in the structure of the forest with a reduction in the number of live trees as the number of bamboo culms increases after the forest fires. The amount of breakage and damage to the trees by the bamboo culms can double or triple with the expansion of the bamboo after fire impact. Bamboo expansion was identified based on an increase of the proportion of pixels with near-infrared channel reflectance values > 3500. The impact of forest fires resulted in incursion and dominance of bamboo culms over an area of 120,000 ha, changing the forest type of this area to “bamboo-dominated forest.” Our results clearly show that drought-induced forest fires with anthropogenic sources are capable of shifting the structure of forest in southwestern Amazonia towards bamboo-dominated forest. With future climate scenarios indicating more frequent and extensive droughts due to global warming, which, together with the use of fire for new deforestation and for managing pasture and agricultural fields, can be expected to cause more forests in southwestern Amazonia to be exposed to extensive fires and potential increase in bamboo density.