Jack pine-dominated ecosystems in northern Lower Michigan have been facilitated and maintained by frequent, large, stand-replacing wildfires. These fires often leave unburned forest strips, or “stringers,” representing legacies of the pre-fire ecosystem. Stringers may persist for years on the recovering post-fire landscape, but the potential effects of their presence on vegetation processes and patterns are poorly understood. For example, stringers may act as post-fire refugia for plant species, or may provide a prolonged jack pine seed source for reseeding the burned landscape. Fire suppression has altered the disturbance regime of forests in the region, and structural variability has been disrupted by plantation plantings designed to provide habitat for the federally-endangered Kirtland’s warbler. Jack pine plantations lack the structural variability, including stringers, found in wildfire-regenerated ecosystems, and the living, more mature trees that comprise stringers are often targeted by managers for clear-cuts. This field-based study investigates the effects of stringers on post-fire plant community composition and stand structure using four wildfires representing a temporal, successional gradient. We used non-metric multidimensional scaling to evaluate plant communities, and compared stand density and age distributions at increasing distances from the stringer to determine structural effects of stringers on post-fire regeneration.
Results/Conclusions:
Preliminary results suggest that plant community diversity and the age of post-fire jack pine regeneration are influenced by stringers. Plant communities farthest from the stringer were found to differ from those both adjacent to and within the stringer; areas burned relatively recently (12-13 years) showed the most pronounced differences. Average ages of post-fire jack pine saplings were significantly different with respect to distance from the stringer; saplings in plots adjacent to the stringer were significantly younger than in plots farthest from the stringer, again with the most marked affects in intermediate-aged areas. Stand density was not significantly affected by stringers. These findings suggest that stringers influence post-fire regeneration and establishment, and that this is most pronounced in the ~15-year-old burn. This is particularly important given the almost-singular regional management focus on producing young stands for Kirtland’s warbler habitat. Stands <20 years old are appropriate nesting habitat, and therefore plantation management targets precisely the age range when we found stringers to have the most pronounced effects. Results from this study have the potential to influence forest management practices to more closely resemble naturally-regenerated ecosystems. More broadly, these findings may be useful for guiding future retention patterns in stand-replacing fire-prone areas.