Spatial variability of fire history in subalpine forests: From natural to cultural regimes

Abstract

The goal of this study was to determine the effect of local and large-scale processes on fire frequency during the postglacial period in a subalpine ecosystem (Alps, France). Large-scale processes should produce homogeneous distribution of fire-free intervals and synchronicity of fire series, and dominance of local-scale processes, such as those triggered by differences in relief, slope aspect, human history, etc. should create heterogeneous fire regimes. Four ponds and peat were sampled at different elevations and exposures. Sedimentary charcoal was used as a fire proxy, and plant macroremains were used as a vegetation proxy. Synchronicity analysis was based on a transformed Ripley's K-function. Similar fire-free intervals during the early Holocene suggest that fire regimes were controlled at that time by large-scale natural processes such as climate and vegetation patterns and establishment. No fire was reconstructed before 9000 y cal BP Infrequent fires occurred following establishment of the subalpine bio-climate belt. However, local-scale processes have dominated the pattern of fire intervals during the late Holocene, with more fires at lower elevation and on south-facing slopes. Although altitude, topography, and slope aspect certainly drove between-site differences during the early Holocene, these differences disappeared during the late Holocene, when fire frequency was related not to ecological features of the natural landscape but likely to human population density and activities, e.g., need for pastures (woody fuel suppression). Fires were certainly controlled at first by climate and vegetation (Pinus cembra), but human practices have affected the fire regime for centuries. A new fire epoch might result from both the current global warming and on-going land-use abandonment, which has led to a significant fuel build-up in the Alps.