This study describes the nature and possible causes of compositional variability in forests on dry-mesic to wet-mesic sites in northern lower Michigan. Successional patterns in largetooth and trembling aspen (Populus grandidentata and P. tremuloides) forests are described through an analysis of the population patterns of woody species. Two approaches were employed: 1) an extensive survey of forest stands of different ages, and 2) an analysis of permanent plots which provided a continuous record of change over a 41-year period. Stand ages were determined from increment cores and soil chemical and physical characteristics were determined for the major soil horizons to a depth of 150 cm. Two soil factor complexes were identified using principal components analysis: 1) pH, calcium, and magnesium which indicated differences in parent materials and pedogenetic processes, and 2) bulk density, potassium, and water availability indicating differences in soil texture and parent material. Within each complex, the correlations among individual soil factors were high; these correlation patterns were consistent throughout most soil horizons. Soil factors and stand age were highly correlated with vegetation composition as indicated in detrended correspondence analysis ordinations. Other sources of successional variability included differences in the composition of the predisturbance vegetation, the history of disturbance, seed source availability, and small-scale disturbances. Five general population patterns were expressed through time by the major tree and shrub species. These patterns were correlated with widely-known life-history characteristics of the species. The aspens represented typical early successional dominants. Late successional species sprouted vegetatively and persisted, sprouted and then increased via seedling reproduction, or reproduced from seed only either beginning early or late in succession. Evidence from this study indicates that successional mechanisms differ by site. By relating understory recruitment to aspen thinning curves, patterns of overstory-understory interactions were defined. On mesic sites, tolerant hardwoods exhibit delayed reproduction under the aspen overstory. It is suggested that recruitment and overstory replacement on these sites occur mostly in canopy openings. Reproduction is apparently less keyed to overstory senescence on the dry-mesic sites where pine species reproduce more continuously.