Characterizing the fire regime evolution and land-use change in the Dry and Wet Chaco between 2001 and 2019

Wildfires play an essential role in the biogeophysical cycles of different world ecosystems, from dry savannas to humid wetlands. During the last decades, fire regimes of several global regions began to present significant alterations due to climate change and human land-use pressure. The South American Gran Chaco ecoregion contains one of the most important reservoirs of native forests and biodiversity in the world, including the largest continuous dry tropical forest and some of the most extensive wetlands. The area presents a marked precipitation gradient from the East (wet) to the West (dry), which is manifested in vegetation (from wetlands to dry forests and shrublands). In this work, we mapped natural vegetation with the European Space Agency (ESA) Climate Change Initiative (CCI) medium-resolution land cover maps (MRLC v2.0.7; annual - 300m) and fires with the ESA CCI Fire product (FireCCI51; monthly - 250m) in the Gran Chaco between 2001 and 2019 to establish the past and current effects and dynamics of fires in the area (which are primarily human ignited). To assess the region’s climatology, we used the ERA5 bias-corrected reanalysis dataset (WFDE5; daily - 0.5º). Our results highlight the distinct dynamics of fires in the wet and dry areas of the Gran Chaco, showing two fire seasons - summer and winter - in the wet areas (where grasses predominate) and one fire season - winter - in the dry areas (where shrubs and trees are more abundant). Examining the correlations between annual rain anomalies and burnt area, we find that precipitation anomalies have different effects in dry and wet areas throughout the region’s precipitation gradient. Correlations change from positive in the drier areas to negative in the wetter areas. These results may reflect that summer and winter fires do not have the same drivers and the key role of the available biomass limiting the fire expansion. Since biomass is more dependent on precipitation in dry areas compared to wetter ones, the correlation of winter fires with precipitation is positive in the drier regions. The negative correlations obtained in the summer season could be explained by the fact that summer fires essentially occurred in the wetter part of the Chaco and are intended (through human ignition) to increase the grasslands’ productivity; this practice could be more frequent during negative precipitation anomalies compared to positive ones. Further analysis will try to confirm these findings with biomass satellite data.