Forest communities of the upper Great Lakes region are changing in response to multiple biotic and abiotic factors. However, long-term, permanent research plots for studying the drivers of these changes at the landscape-level are rare. We used a large array of plots first censused over 25 years ago to investigate how plant community composition has changed in a mid-successional northern hardwood forest in northern lower Michigan, USA. This landscape consists of distinct glacial landforms; moraine, high-level outwash plain, and glacial lake basin, which differ in their abiotic characteristics, including nutrient availability and soil moisture content. We hypothesized that successional change will occur at different rates between and within these glacial landforms and that rates of change will vary between vegetation strata (i.e. overstory, sapling, and groundcover layers). 104, 15 x 30 meter plots were established at the University of Michigan Biological Station and first censused in 1989/90. At that time, glacial landforms were defined based on soil and other geologic data. In 2015, we re-censused vegetation in these plots, surveying the forest overstory (>9cm DBH), sapling layer (1.5-9cm DBH), and groundcover (% cover). Temporal shifts in communities were assessed using PERMANOVA and Bray-Curtis distances. Species-specific abundance shifts were compared using pairwise t-tests.
As we expected, vegetation composition in all three strata changed significantly both across time and within each landform. In the overstory, Betula papyrifera, an early successional species, decreased in overall basal area across the landscape. Populus grandidentata, another early successional species, declined on the moraine but has increased in basal area in the other landforms. Fagus grandifolia has become widespread primarily in the high-level outwash plain. In the sapling layer, Quercus rubra and Pinus strobus, two mid-successional species, showed significant increases across all landforms. We saw the greatest landform-specific change in community composition in the groundcover layer, where the moraine showed the most change, followed by the high-level outwash plain and the glacial lake basin. This rank order in community change parallels the relative fertility of these landforms. Overall, while this landscape has shown significant successional change in 25 years, landform-mediated changes were only evident in the groundcover layer. Our analysis provides an important picture of the forces shaping current and future community composition in the rapidly changing mixed deciduous forests of north central North America.