Once upon a time biomass burning in the western Alps: Nesting effects of climate and local drivers on long-term subalpine fires

The present article questions the relative importance of local- and large-scale processes on the long-term dynamics of fire in the subalpine belt in the western Alps. The study is based on soil charcoal dating and identification, several study sites in contrasting environmental conditions, and sampling of soil charcoal along the elevation gradient of each site. Based on local differences in biomass combustion, we hypothesize that local-scale or landscape-scale processes have driven the fire history, while combustion homogeneity supports the hypothesis of the importance of large-scale or macro-ecological processes, especially climate.

Biomass burning during the Holocene resulted from the nesting effects of climate, land use, and altitude, but was little influenced by slope exposure (north versus south), soil (dryness, pH, depth), and vegetation. The mid-Holocene (6500–2700 cal BP) was an important period for climate-driven biomass burning in the subalpine ecosystems of the western Alps, while fires over the last 2500 years appear much more episodic, prompting us to speculate that human activity has played a vital role in their occurrence.

Our working hypothesis that the strength of local drivers should offset the effects of regional climate is not validated. The homogeneity of the fire regime between sites thus underscores that climate was the main driver during the Holocene of the western Alps. Long-term subalpine fires are controlled by climate at the millennial scale. Local conditions matter for little in determining variability at the century scale. The mid-Holocene was a chief period for climatic biomass burning in the subalpine zone, while fires during the late Holocene appear much more episodic, suggesting that social drivers has exercised key function on their control.