Fire-mediated dieback and compositional cascade in an Amazonian forest

Jos Barlow,Carlos A Peres

doi:10.1098/rstb.2007.0013

Abstract

The only fully coupled land–atmosphere global climate model predicts a widespread dieback of Amazonian forest cover through reduced precipitation. Although these predictions are controversial, the structural and compositional resilience of Amazonian forests may also have been overestimated, as current vegetation models fail to consider the potential role of fire in the degradation of forest ecosystems. We examine forest structure and composition in the Arapiuns River basin in the central Brazilian Amazon, evaluating post-fire forest recovery and the consequences of recurrent fires for the patterns of dominance of tree species. We surveyed tree plots in unburned and once-burned forests examined 1, 3 and 9 years after an unprecedented fire event, in twice-burned forests examined 3 and 9 years after fire and in thrice-burned forests examined 5 years after the most recent fire event. The number of trees recorded in unburned primary forest control plots was stable over time. However, in both once- and twice-burned forest plots, there was a marked recruitment into the 10–20 cm diameter at breast height tree size classes between 3 and 9 years post-fire. Considering tree assemblage composition 9 years after the first fire contact, we observed (i) a clear pattern of community turnover among small trees and the most abundant shrubs and saplings, and (ii) that species that were common in any of the four burn treatments (unburned, once-, twice- and thrice-burned) were often rare or entirely absent in other burn treatments. We conclude that episodic wildfires can lead to drastic changes in forest structure and composition, with cascading shifts in forest composition following each additional fire event. Finally, we use these results to evaluate the validity of the savannization paradigm.

Highlights

The only fully coupled land–atmosphere global climate model that is currently available predicts a large-scale and substantial reduction in precipitation over the Amazon Basin during the twenty-first century, leading to a widespread dieback of forest vegetation (Betts et al 2004; Cox et al 2004)
Climate change is likely to be accompanied by increased air temperature and dry season length over large regions of the Amazon forest (Cox et al 2004), and a probable increased frequency of severe seasonal droughts initiated by El Nino – Southern Oscillation (ENSO) events and Atlantic sea surface temperature (SST) anomalies (Li et al 2006)
We identified all common species of shrubs and saplings above 1 m in height and below 10 cm in diameter at breast height (DBH) in twelve 1!50 m subplots, with four subplots placed in each unburned and burned forest treatment

Summary

Introduction

The only fully coupled land–atmosphere global climate model that is currently available predicts a large-scale and substantial reduction in precipitation over the Amazon Basin during the twenty-first century, leading to a widespread dieback of forest vegetation (Betts et al 2004; Cox et al 2004). Climate change is likely to be accompanied by increased air temperature and dry season length over large regions of the Amazon forest (Cox et al 2004), and a probable increased frequency of severe seasonal droughts initiated by El Nino – Southern Oscillation (ENSO) events and Atlantic sea surface temperature (SST) anomalies (Li et al 2006). These severe supra-annual droughts are known to both augment and further desiccate the

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