Defining climatic drivers for the prediction of summer wildfires in northern Italy

Wildfires are natural phenomena affecting ecosystems and causing negative impacts on human health and biodiversity. In the Mediterranean region, wildfire regimes are strongly influenced by local climatic conditions, leading to pronounced inter and intra-annual variability in wildfire occurrence.

Owing this link, the study explores for the first time the climatic drivers influencing the monthly burned area (BA) during the summer fire season (May - September) in northern Italy at the three scales of spatial resolution: 0.11, 0.25 and 0.50 degrees. We then build multi-regression data-driven models to define the main BA predictors for the investigated area. The summer monthly percentages of burned area at the three resolution for the 2008-2022 period were derived from the GPS-based BA perimeters. A total of 150 daily precipitation and maximum and minimum ground station series were collected, converted at monthly scale, reconstructed, homogenised and spatialised at 0.11°, 0.25° and 0.50° resolution using the Universal Kriging with auxiliary variables. Several climatic indices were subsequently computed for precipitation, temperature and drought. To identify the best BA predictors, we first performed the Pearson’s correlation test, for each pixel, between the monthly BA series and the climatic indices calculated for three different aggregation periods: concurrent summer (2008-2022), 6 months before the fires (winter 2007-2021) and 12 months before the fires (summer 2007-2021). Multilinear regressions models were computed using every possible combination of the best predictors. The best regression models were selected through an out-of-sample procedure, and the model performance was tested by comparing the predicted BA with the observed data, estimating explained variance and correlation. Finally based on the CORINE Land Cover map, the vegetation classes that were most susceptible to wildfires, and their typical elevation ranges, were identified.

This study shows that summer fires in northern Italy are concentrated in July and August and are predominantly located in the southern part of the study area, at elevations between 100 and 600 m a.s.l. In particular, the lower rates of the Ligurian and Tuscan Apennines exhibit a fire return period of 1 to 2 years, in contrast to the Alps, where it exceeds 6 years. Sclerophyllous, Sparse, and Open Forests appear to be the vegetation classes most susceptible to fire in these fire-prone regions. Modelling results for the 2008–2022 period indicate that the most accurate predictions were performed at 0.11° of resolution and fires are driven by drought conditions caused by water stress than by high temperatures. Indeed, the most significant predictors of burned area were the two drought indices and water balance, recorded both for the current period (June to July) and for the preceding 6 months period (December to March).