Multiple successional pathways on recently disturbed jack pine sites in Michigan

Abstract

Jack pine communities in northern lower Michigan recently disturbed by clearcutting, deliberate burning, or wildfire were studied over three growing seasons, and were compared to undisturbed jack pine stands. Newly disturbed sites generally had more vascular plant species than mature forests. Many of these species did not persist, especially on burned sites, and species richness declined sharply the second year after fire. In several cases annual and biennial species dominated first-year burns but were unimportant thereafter. Several pathways of early successional development were evident on the disturbed sites, which was facilitated by jack pine regeneration failure on all but one of the disturbed sites. Unburned clearcuts rapidly developed into Carex meadows, with few other species of any importance. This pathway was also followed on some deliberately-burned and wildfire sites. Other burned areas developed a stratified canopy of shrubs and early successional hardwoods, and were rich in species and high in cover. Stand replacement by jack pine occurred on one mature jack pine site burned by wildfire. Jack pine regeneration failure has been attributed to numerous factors involving: seed loss due to fire and predation, and low germination and seedling survival due to inadequate seedbed, drought, competition, pathogens, grazing and high surface temperatures. Index of similarity comparisons showed that different sites of the same post-disturbance age were significantly ( P < 0.01) less similar in species composition than a single site sampled in 2 or 3 consecutive years. Post-disturbance age was not a particularly strong indicator of species composition on these sites, because each site had a different assemblage of species (30–40% dissimilarity among sites of the same disturbance class and post-disturbance age). Therefore, the individualistic nature of each site, rather than age following disturbance, is the dominant aspect in understanding successional relationships in these communities.