The effect of fire frequency on local cembra pine populations.

It has been predicted that global climate change will lead to increasing drought in the Alps during the 21st century, as well as an increased fire risk, fires being currently rare in these mountains. Herein we describe fire frequency reconstruction using high-resolution analyses of macroscopic sedimentary charcoal, pollen, and plant macrofossils. Sediments were sampled from a subalpine pond within the dry western French Alps. Results show that the early-Holocene expansion of Pinus cembra (7200 calibrated years BP) occurred in Acer/Alnus incana/Betula woodlands, which were affected by fires with moderate mean fire-free intervals (MFFI = 173 +/- 61 yr [mean +/- SE]). Superposed Epoch Analyses show that the abundance of P. cembra macroremains decreased significantly after burning, although they never disappeared entirely. Statistics suggest that fires spread through cembra pine communities; they were not stand-replacing fires but mainly surface fires, probably killing nonreproductive pines. An increase in fire frequency occurred 6740 years ago, when four fires appear to have occurred within 140 years. These fires may have been associated with a regional drought and could have affected the composition of the subalpine forest by depleting the local P. cembra population in the short-term. The predicted increase in drought in the future could, therefore, affect the cembra pine ecosystem in the Alps if fire frequency is reduced to intervals of less than 80 years.