Frequent wildfires erode tree persistence and alter stand structure and initial composition of a fire-tolerant sub-alpine forest

Question: Frequent severe wildfires have the potential to alter the structure and composition of forests in temperate biomes. While temperate forests dominated by resprouting trees are thought to be largely invulnerable to more frequent wildfires, empirical data to support this assumption are lacking. Does frequent fire erode tree persistence by increasing mortality and reducing regeneration, and what are the broader impacts on forest structure and understorey composition? Location: Sub-alpine open Eucalyptus pauciflora forests, Australian Alps, Victoria, Australia. Methods: We examined tree persistence and understorey composition of E. pauciflora open forests that were unburned, burned once, twice or three times by high-severity wildfires between 2003 and 2013. At each of 20 sites (five per fire frequency class) we assessed extent of top-kill and mortality of eucalypt clumps, spatial configuration of surviving and dead clumps, densities of new and ligno-tuberous eucalypt seedlings, and shrub and grass cover. Results: At least 2 yr after the last wildfire, proportions of top-killed E. pauciflora stems were significantly higher, and densities of live basal resprouts significantly lower, at sites burned two or three times compared to once burned or unburned sites. Clump death increased to 50% of individuals at sites burned by three short-interval wildfires, which led to changes in live tree patchiness, as indicated by nearest-neighbour indices. Increased tree mortality was not offset by seedling recruitment, which was significantly lower at the twice-and thrice-burned sites relative to single-burn sites - although seedling recruitment was also influenced by topography and coarse woody debris. In addition to changes in the tree layer, the prominence of understorey shrubs was substantially reduced, and the frequency of grasses markedly increased, after two, and particularly three wildfires. Conclusions: Our study provides strong empirical evidence of ecologically significant change in E. pauciflora forests after short-interval severe wildfires, namely, erosion of the persistence niche of resprouting trees, and a shift in understorey dominance from shrubs to grasses. Our findings highlight the need to consider the impacts of compounded perturbation on forests under changing climates, including testing assumptions of long-term persistence of resprouter-dominated communities.