Successional Changes in a Mature Aspen Forest in Northern Lower Michigan: 1974-1981

The Wells Plot, a 1-ha area with over 1800 marked trees in northern lower Michigan, showed many changes from 1974 to 1981 anticipated for the succession from aspen to northern hardwoods. Early successional species (bigtooth aspen, trembling aspen and paper birch) became established immediately after logging and fire about 65 years ago because of rapid growth of root sprouts and stump sprouts and are now in the canopy. Since 1974 these species have experienced mortality with no recruitment. By 1981, there were large numbers of small white ash and red maple, and more shade-tolerant, late successional species such as beech, sugar maple, hop hornbeam and striped maple. The diameter distribution curve for aspen was bell-shaped, moving into larger size classes during this 7-year period because of continued diameter growth, no recruitment, and mortality in the smaller size classes. Red maple, sugar maple and beech had distributions skewed toward the smaller size classes. Red maple maintained its high importance value, although with increasing reliance on sexual reproduction, to maintain the population. Although a third of the current red maple population was presumably of stump sprout origin, almost all of the recent recruitment was of seed origin and over half of the red maple mortality occurred among sprout stems. Sugar maple recruitment was evenly divided between sprouts and seeds, and mortality occurred in both small sprouts and seedlings. Beech increased in density because of modest recruitment coupled with low mortality. Because the trees are mapped, and mortality, recruitment and growth of individual trees can be followed, these baseline data will be of value to future studies on the relative importance of seed vs. sprout reproduction, on spatial pattern and its effect on competition and reproduction, and on long-term tests of theories of plant succession and population dynamics. INTRODUCTION Stands of early successional bigtooth aspen (Populus grandidentata) and trembling aspen (Populus tremuloides) dominated large areas of northern lower Michigan following logging and subsequent burning of mature white pine (Pinus strobus), red pine (P. resinosa) and northern hardwood forests between 1850 and 1920 (Gates, 1930; Kilburn, 1960a,b). The aspen overstories in many stands are now approaching senescence, providing ideal conditions for studying successional changes. The University of Michigan Biological Station (UMBS), established in 1909, has long been the center for many studies of plant succession in these aspen-dominated areas (Gates, 1930; Kilburn, 1957; Farmer, 1958; Benninghoff and Cramer, 1963; Barnes, 1967, 1969; Cooper, 1981; Scheiner and Teeri, 1981; Scheiner, 1983; others cited in Roberts, 1983). Initial studies and projections of successional patterns in this region were based on observations of many areas between 1919-1929 (Gates, 1930), as well as on historical records and comparisons of stands of different ages at one point in time (Kilburn, 1957). Several permanent plots and controlled burn areas have also been established for long-term studies of plant succession. I Present address: Department of Biology, University of Chicago, 940 E. 57th Street, Chicago, Illinois 60637. 2 Present address: Faculty of Forestry, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2.