Pre-fire forest remnants affect post-fire plant community structure and composition

The vegetation of jack pine (Pinus banksiana Lamb.)-dominated ecosystems in northern Lower Michigan is maintained by large, frequent, stand-replacing fires that often leave unburned forest strips (hereafter called "stringers") which may persist for decades on the post-fire landscape. Regional fire suppression practices have altered the fire regime, however, and structural variability has been further disrupted by extensive plantations designed to provide habitat for the federally-endangered Kirtland's warbler (Setophaga kirtlandii Baird). Jack pine plantations lack stringers and other structural heterogeneity found in fire-regenerated forests, and older live trees that comprise stringers are often harvested for timber. However, ecological effects of stringers on adjacent vegetation patterns and processes are poorly understood. We hypothesized that stringers act as post-fire refugia for plant species and provide a prolonged jack pine seed source for reseeding the burned landscape, and thus should be considered for inclusion in ecosystem-based management plans. We sampled data from four post-fire areas of different times-since-fire to investigate effects of stringers on post-fire plant community composition and stand structure. We evaluated plant community composition and compared stand density and age distributions at increasing distances from stringers to determine stringer effects on post-fire regeneration. In several burned areas, ground cover plant communities farthest from stringers differed from those both adjacent to and within stringers; areas burned relatively recently (12 and 32 years post-fire) showed the most pronounced differences. Structurally, several burned areas displayed broader sapling age distributions closer to stringers, suggesting that mature trees in stringers act as a prolonged seed source to the adjacent burned area. Thus, stringers not only influence immediate post-fire heterogeneity by retaining living trees on the burned landscape, but they also continue to provide inputs to the regenerating post-fire forest. Because the Kirtland's warbler population has recovered, and the warbler's delisting from endangered status is imminent, our results may be useful for guiding future ecosystem-based, adaptive forest management practices and, more broadly, retention patterns in stand replacing fire-prone areas.