The function of surface fires in the dynamics and structure of a formerly grazed old subalpine forest

Abstract

Changes in the frequency and intensity of disturbances are expected to occur during the coming decades as a consequence of climatic changes. Mountain forests are sensitive to climate variability, disturbances and changes in human activities; this is particularly true for subalpine forests located close to the limits of tree-growth. Here we test the role of surface ?res on the structure and the dynamics of a subalpine forest compared to a control stand not affected by ?re events for at least two centuries. 2. The ?re events are deduced from ?re scars, the age-structure from tree-ring counting, regeneration from sapling and seedling counts, necromass from the volume of woody debris, and the understorey structure from shrub and herb cover, height and biomass. Land-use history is assessed from livestock and human inventories. 3. Four surface ?res occurred during the last 200 years in the burned stand. Tree density is greater in the burned stand, and, even 50 years after the last ?re, burning could still be having a positive effect on regeneration density. Land-use history, mainly grazing by cattle and sheep, explains the modern forest structure through its promotion of Larix decidua. The understorey composition is the same within the two stands, suggesting a rapid recovery of dwarf shrubs (Rhododendron ferrugineum and Vaccinium myrtillus) after the latest ?re, which coincided with the ?nal change in land-use. The exception is V. vitis-idaea cover, which is higher in the control stand. The removal of livestock in the 1940s indirectly triggered a decline in L. decidua regeneration, which was formerly promoted by grazing and trampling. Conversely, Pinus cembra seedlings have begun to establish during the last 70 years. 4. Synthesis. Expansion of forests dominated by P. cembra is expected during the 21st century, unless current global warming leads to a signi?cant and lethal increase in ?re frequency, that is, a reduction of ?re-return intervals, which would reduce the number of sexually mature trees. The re-introduction of grazing could maintain the mixed subalpine forests, whereas surface ?res would alter the woody debris load, promoting greater regeneration of L. decidua and P. cembra. © 2009 British Ecological Society.