Modelling the influence of regional landscape drivers on spatio-temporal patterns of wildfire activity
Authors: Castel-Clavera Jorge , Pimont François , Opitz Thomas , Ruffault Julien , Dupuy Jean-Luc 
Published: October 2022

Identifying the drivers of fire activityâ€™s spatio-temporal variability is challenging in densely populated and fire prone landscapes. Human usage and climate affect the local fire regime in contrasting ways. The identification of these drivers is further complicated due to the stochastic nature of fire activity. Fire regimes in Mediterranean France show contrasted spatial patterns and temporal changes at decadal scales. While overall, the number of fires decreased over the last thirty years, certain zones suffered local increases in fire activity. To describe and understand the drivers of those changes and the spatial variability, we introduced several improvements in the Firelihood model - a probabilistic framework capable of prediction fire occurrence of >1ha fires, and exceedance probabilities of 10 and 100 ha thresholds - by incorporating Land-Use Land-Cover (LULC) explanatory variables, as well as by enhancing its spatio-temporal components to account for unexplained variability in models. The novel model - fitted on a 2km-pixel grid, but relying on variables aggregated at various spatial aggregations (2, 4, 8 and 16km) - is used to explain the observed spatial patterns of fire activity during the last 27 years, as well as the regional and local changes observed between two decades with contrasted fire activities by running counterfactual scenarios. LULC variables, including road density, wildland-urban interface, or expert-based fuel type rating explain a significant part (as much as fire-weather) of the variability in fire occurrence (>1ha), thereby reducing the effect of unexplained spatial variability. The selected occurrence model uses only 2km-resolution variables, as local factors have a high influence on fire ignition and initial spread. The occurrence of larger fire (>10 ha or >100 ha) is largely driven by fire-weather, followed by unexplained spatial variability; selected models for larger fires uses a few LULC variables aggregated at 4, 8 and/or 16 km. This indicates the influence of surrounding factors on fire size extension. The spatial effect for fire occurrence presents contrasted hot and cold-spots throughout the area, while it has a clear east to west decreasing trend for fire size. Regarding temporal changes in fire activity between the two decades, changes in fire weather induced a strong increase in fire probability in many hot spots throughout the region, but this effect was overcompensated by a negative trend associated with unexplained temporal factors (and of larger magnitude than fire weather). LULC variables had negligible effect on the fire regimeâ€™s temporal trends. Moreover, an east-to-west gradient appears for the spatial trends of the larger fires, and for the temporal trends in all sizes, highlighting the increase in fire activity in the western side of the region. Those results suggest that observed temporal changes in fire activity are the result of a changing socio-economic or policy frame, probably related to reinforced suppression policies following the year 2003, and the increasing agricultural abandonment.