Resistance of mixed subalpine forest to fire frequency changes: the ecological function of dwarf pine (Pinus mugo ssp. mugo)

摘要

The availability of fuel and climate are major factors responsible for forest fire activity over time. Here, we tested the hypothesis that forest ecosystems containing a high shrub biomass, which constitutes a fuel load, and affected by a warmer climate, which is associated with drier conditions and a longer fire season, are more prone to fire. Fire occurrence and woody vegetation histories were reconstructed for a subalpine site (Lago di Colbricon Inferiore) in the Dolomites, part of the eastern Italian Alps, for the past 13,000 years. The modern wet climate prevents fire in this area, in spite of the warm summers and an abundant biomass of dwarf pine (Pinus mugo) and three other conifer tree species (Pinus cembra, Picea abies, and Larix decidua). Past fire history reconstructed from sedimentary charcoal showed a median fire return interval of 140 years (30-735 yr fire-1), with a high variability (SD ± 170 years) throughout the Holocene, suggesting that the past environment was more favourable to fire than the modern one, probably due to a drier climate or to different fuel availability. The subalpine community containing P. mugo remained stable for the past 9000 years, despite the variability of the fire return interval. Interestingly, the fire frequency is higher at Lago di Colbricon than at sites in the western Alps that lack P. mugo, suggesting that this species tolerates fire disturbance. In fact, it probably favours the spread of fire due to its flammable biomass, prostrated form, and dense layering canopy, thus offsetting the influence of the wet climate. Since the 19th century, the removal of dwarf pine to promote subalpine grasslands may have suppressed fires in this region.