Abstract
Forest composition within a local landscape is influenced by physical site characteristics and prevailing disturbance regime. In many areas of eastern North America, the natural disturbance regimes that influenced presettlement forest composition have been altered by human activities associated with European settlement. These alterations have led to substantial changes in composition and the development of successional pathways markedly different from presettlement conditions. In this study we examined presettlement and present-day forest composition of two bigtooth aspen-dominated landscapes in northern Lower Michigan. Our objectives were to: (1) reconstruct and relate presettlement forest composition to the potential natural disturbance regime of each landscape; (2) compare presettlement and present-day forest composition of each landscape; and (3) assess the influence of post-settlement disturbance history on the development of the current forests and the potential successional pathways of the two landscapes. Presettlement forest composition, reconstructed using General Land Office survey records, differed substantially between the two landscapes. Landscape 1 was dominated by fire-sensitive eastern hemlock and American beech, while landscape 2 was dominated by fire-dependent red pine, white pine and jack pine. Compositional differences may have been related to differences in presettlement fire frequency, or to differences in physical site characteristics or regional climates. The present-day overstories of both landscapes were dominated by bigtooth aspen, red oak and red maple species that were of minor importance in the presettlement forests of the study areas. Compositional convergence was attributed to the similar influence of postsettlement disturbance history on each landscape. Logging and wildfires eliminated advanced regeneration and many remnant seed sources of hemlock, beech and pines and favored the development of forests dominated by bigtooth aspen, red oak and red maple. Individuals of the latter species survived the disturbance and proliferated vegetatively in the postsettlement landscape. Differences in seed source availability in the present-day forests have led to marked differences in species recruitment in the understories of the two landscapes. Changes in seed rain and fire exclusion in the two landscapes are apparently leading to development of forest types markedly different from the presettlement conditions. The results illustrate how human-induced changes in disturbance regimes can have important, long-lasting effects on forest composition.