Fires in Amazonian Blackwater Floodplain Forests: Causes, Human Dimension, and Implications for Conservation

Abstract

The Amazon basin is being increasingly affected by anthropogenic fires, however, most studies focus on the impact of fires on terrestrial upland forests and do not consider the vast, annually inundated floodplains along the large rivers. Among these, the nutrient-poor, blackwater floodplain forests (igapós) have been shown to be particularly susceptible to fires. In this study we analyzed a 35-year time series (1982/1983–2016/2017) of Landsat Thematic Mapper from the Jaú National Park (Central Amazonia) and its surroundings. Our overall objective was to identify and delineate fire scars in the igapó floodplains and relate the resulting time series of annual burned area to the presence of human populations and interannual variability of regional hydroclimatic factors. We estimated hydroclimatic parameters for the study region using ground-based instrumental data (maximum monthly temperature–Tmax, precipitation–P, maximum cumulative water deficit–MCWD, baseflow index–BFI, minimum water level–WLmin90 of the major rivers) and large-scale climate anomalies (Oceanic Niño Index–ONI), considering the potential dry season of the non-flooded period of the igapó floodplains from September to February. Using a wetland mask, we identified 518,135 ha of igapó floodplains in the study region, out of which 17,524 ha (3.4%) burned within the study period, distributed across 254 fire scars. About 79% of the fires occurred close to human settlements (<10 km distance), suggesting that human activities are the main source of ignition. Over 92.4% of the burned area is associated with El Niño events. Non-linear regression models indicate highly significant relationships (p < 0.001) with hydroclimatic parameters, positive with Tmax (R2adj. = 0.83) and the ONI (R2adj. = 0.74) and negative with P (R2adj. = 0.88), MCWD (R2adj. = 0.90), WLmin90 (R2adj. = 0.61) and BFI (R2adj. = 0.80). Hydroclimatic conditions were of outstanding magnitude in particular during the El Niño event in 2015/2016, which was responsible for 42.8% of the total burned floodplain area. We discuss these results under a historical background of El Niño occurrences and a political, demographic, and socioeconomic panorama of the study region considering the past 400 years, suggesting that disturbance of igapós by fires is not a recent phenomenon. Concluding remarks focus on current demands to increase the conservation to prevent and mitigate the impacts of fire in this vulnerable ecosystem.

A Review of the Ecological and Biogeographic Differences of Amazonian Floodplain Forests

“…Although most fires in the Amazon basin have anthropogenic origins [153,154], the occurrence of wildfires through lightning cannot be discounted as a selective force for floodplain tree species [155]. Igapó forests, and here especially the highly flooded elevations, are more frequently subject to fires than upland or várzea forests [121,156] because of a combination of several factors.…”

“…Igapó forests, and here especially the highly flooded elevations, are more frequently subject to fires than upland or várzea forests [121,156] because of a combination of several factors. First, the low water retention potential of sandy igapó substrates is especially prominent at the end of the dry season, particularly in El Niño years [155]. Second, the slow decomposition rates of leaves at highly flooded topographies lead to the accumulation of a thick litter layer.…”

“…Combined with the fine root mats that establish at or near the soil surface, these factors provide a layer of fine fuel, which is about twice as large than in upland forests [116]. A comparatively low canopy and open understory characterizes particularly highly flooded topographies, where high radiation incidence results in a dry microclimate near the forest floor [151,155]. Once burned, the slow recovery times keep burnt areas open for several years, potentially exposing them to recurring fires, and the forests may be trapped relatively easily into a fire-dominated savanna state [156,157].…”

“…Carvalho et al [155] investigated fire scars in igapó forests of the Jaú National Park (lower Negro River) over 35 years by remote-sensing techniques and found that 79% of the 254 detected fire scars occurred close (<10 km distance) to human settlements, suggesting that human activities are the main source of ignition. Over 90% of the burned area was associated to extreme hydroclimatic conditions during El Niño years.…”

“…Over 90% of the burned area was associated to extreme hydroclimatic conditions during El Niño years. Because of the continuous increase of both more intense and more frequent drought events in large parts of the Amazon basin [158], fires might act as an increasing environmental filter for many fire-intolerant igapó tree species, particularly those of the highest flood levels [155].…”