Old-Growth Southern Boreal Forest Stability and Response to a Stand-Replacing Wildfire

摘要

Half of a 400-year-old forest in northeastern Minnesota, USA, burned in autumn 2006 providing a unique opportunity to examine stability and response to stand-replacing fire in a region where the historic mean fire return interval was about 100 y. We installed 18 permanent 50-m transects in 1977 and 18 more in 1997. In 2008, 18 additional transects were surveyed along with the initial transects; half were in the burned portion of the forest and half remained unburned. In 2010 and 2014, all transects were again inventoried. Before the fire, eastern white pine (Pinus strobus), historically the dominant tree, represented less than 4% of the canopy in which northern white cedar (Thuja occidentalis), balsam fir (Abies balsamea), and paper birch (Betula papyrifera) had become dominants. In 37 y of observation, few significant changes occurred in the unburned forest. In the burned forest, however, the intense fire killed virtually all trees on the upland, and reduced tree cover from near 90% to just over 10%, with survival primarily in wet draws. There was no nearby seed source for jack pine (Pinus banksiana) or quaking (Populus tremuloides) or big-toothed (P. grandidentata) aspen, the most frequent post-fire pioneers. Seedlings of aspen and paper birch established dominance in the small-tree layer within eight years, and dense white cedar established in mesic microsites near surviving seed sources. Stand-level diversity of vegetation after the fire was about the same as before the fire, but primarily as a result of species that survived in wet draws. Raspberry (Rubus idaeus), fire-weed (Epilobium angustifolium), and several sedges (Carex spp.) became temporarily abundant, peaking six years after fire. There was no post-fire white pine regeneration. Results indicate that in the long-term absence of fire there was a gradual shift in canopy composition with relatively little corresponding change in the understory. Results also underscore the importance of microsites in buffering change and driving landscape scale diversity, and how a shift in fire regime can initiate a different successional trajectory.