Résumé
1. Relationships between vegetation, climate and disturbance are likely to be altered in the near future as a result of changes in both climate and human impacts on ecosystems. These changes could trigger species losses and distribution shifts in sensitive (e.g. mountainous) ecosystems. Models project a 2.5-3 degrees C increase in global temperatures by the end of the 21st century, and combined with effects of current land-use abandonment (leading to the build-up of fuel material); these increases could cause future ecosystems to become similar to ecosystems of the first half of the Holocene when human impact was negligible.
2. A high-resolution macroremain record from a small subalpine lake in Italy allowed us to examine 8000 years of stand vegetation dynamics. Linkages between disturbance, vegetation and climate were deciphered by reconstructing local fire occurrence patterns from charcoal remains, characterizing plant species richness by rarefaction analysis, and regional climatic reconstructions in the light of the human archaeological context.
3. Before 5100 cal. year bp, forest ecosystems had fluctuating richness and an intermediate mean fire frequency. The climate was warmer and drier than today. From c. 5100 to 2200 cal. year bp when the climate became wetter, a higher fire frequency promoted the establishment of mixed ecosystems with several tree species, a herbaceous understorey and higher richness. The last 2200 years were characterized by a decrease in richness, a lower fire frequency, and lower temperatures and precipitation. Over the last 8000 years, the probability of fire has increased with time since the last fire, probably as a result of increased fuel-load or composition, which has assisted the spread of fire.
4. Synthesis. The fire frequency was higher when forest richness was higher, during periods of wetter climate. Temperatures had no correlation with the fire frequency and the ecosystem dynamics in the study region.