Fire disturbance in Amazonian blackwater floodplain forests

Background: Large fire scars were detected in floodplain forests of the middle Rio Negro in dry years of the 1990s, using satellite data. Aim: To relate fire years and river level anomalies to the Tropical Pacific Southern Oscillation Index (SOI); to measure fire damage and post-fire succession rate. Methods: We analysed the relationship between the SOI and water levels for 1968-2010. In Landsat images of the 1990s we determined fire scar ages. Using QuickBird images, we measured forest cover loss in 36 of these scars, covering 873 ha. This was validated by field measurements of tree mortality in 15 scars. As a metric of post-fire succession, we compared change in the Enhanced Vegetation Index (EVI) in 10 large floodplain fire scars and 12 terra firme slash-and-burn fallows. Results: SOI explained 32% of the variation in annual low water level. Forest cover loss in the 36 burn scars was 88% ± 8% (mean ± SD), range 67-98%. Post-fire tree mortality was 91%, ±8%, range 75-100%. Correlation between cover loss and mortality was 73% (P < 0.002). Forest recovery was very slow. EVI values typical of bare soil were still present 13 years after the fires, indicating a successional stage similar to the first year in terra firme sites. Conclusions: Results suggest a very low resistance and resilience of blackwater floodplain forests to fire disturbance associated with drought during El Niño events. © 2014 Copyright 2012 Botanical Society of Scotland and Taylor & Francis