Sugar maple (Acer saccharum) forest community dynamics across an environmental gradient from the prairie-forest border to interior forest biome

Abstract

Forest ecosystems are dynamic entities that are subject to a variety of biotic and abiotic environmental changes. Invariably, climate is one of the principal factors controlling the distribution of ecosystems and past fluctuations in climate are known to have shaped the Midwestern United States forests. The Upper Great Lakes region of North America includes Minnesota, Wisconsin, and Michigan, and is characterized by a gradual south to north climatic gradient that defines the eastern deciduous forests (oak-hickory) to the south, the northern mixed hardwood forests (maple, hemlock, and beech) in northern Michigan and Wisconsin, and the sub-boreal forest (spruce-fir) in the far northern parts of the region. Additionally, the Upper Great Lakes region lies at the intersection of three major contrasting air masses: the cold, dry, polar continental air mass descending from the north; the dry, continental westerlies; and the warm, moist, tropical maritime air mass coming from the Gulf of Mexico. Interactions among these three air masses have created a southwest to northeast climatic gradient across the region resulting in a drier and warmer environment in southwest Minnesota as opposed to wetter and cooler conditions in Upper Michigan. It is in this context that the Upper Great Lakes region ecosystems were formed. They include the prairie-forest border (Zone 1), a transition zone between the tall grass prairies and the northern forests in Minnesota; the forest interior (Zone 2), which extends beyond the prairie-forest border into northern Wisconsin; and the deep forest (Zone 3), which lies in Michigan's Upper Peninsula. In recent years, numerous concerns about global environmental changes and their impact on the Upper Great Lakes northern hardwood forests have emerged. Increases of temperature caused by ongoing climate change, along with the proliferation of white-tailed deer (Odocoileus virginianus) populations and invasive European earthworms are predicted to drastically change the overstory and understory of northern hardwood forests. Sugar maple (Acer saccharum) is a common late successional species in the Great Lakes region and is widespread in dry mesic to mesic northern temperate forests. Sugar maple forests provide habitat for many wildlife species while trees are valued for their products (e.g. timber and syrup). This PhD dissertation aims to advance the understanding of the Upper Great Lakes northern hardwood forests by studying sugar maple forest community dynamics across an environmental gradient from prairie-forest border to interior forest biome and discuss their future in a context of global environmental changes. Rather than adopting a field-based approach, 3515 plots including tree and seedling data from the Forest Inventory and Analysis (FIA) program were studied, with 378 plots in Zone 1, 1823 plots in Zone 2, and 1314 plots in Zone 3. Analyses incorporated a combination of ordinations (Bray-Curtis and successional vector overlay), Ordinary Least Squares (OLS) regression models, and Aikaike's Information Criteria (AIC) provided a means for model selection. Chapter 1 investigated the contemporary overstory and understory forest composition of sugar maple communities and successional dynamics across the Upper Great Lakes region, and considered three hypotheses: (1) hardwood forests form a series of distinct communities via association of sugar maple with other tree species in the overstory across the Upper Great Lakes region; (2) sugar maple dominates the understory and succession to sugar maple is occurring in all of the communities--therefore the overstory communities identified are not stable; and (3) mesophication (which was used as a broad concept to include increasing maple proportion within mixed maple-oak forests) in sugar maple-red oak communities is progressing towards greater dominance of sugar maple. Sugar maple dominated forests were identified in association with red oak, bur oak-aspen, and basswood at the prairie-forest border, red oak-red maple, quaking aspen, and basswood in the forest interior, and quaking aspen, red maple-balsam fir, and hemlock-yellow birch-white cedar in the deep forest of Upper Michigan. Mesophication is occurring in most sugar maple-red oak communities of the prairie-forest border and sugar maple regeneration dominates in combination with white ash, ironwood, and bitternut hickory (Zone 1), red maple, balsam fir, and ironwood (Zone 2), and balsam fir (Zone 3), indicating that the distinct overstory communities may not be stable and that sites are trending towards relative homogeneity. However, despite the regeneration success of sugar maple, some stands had no sugar maple regeneration and we predict a decline in future sugar maple abundance resulting from the long term effects of deer browsing, earthworm invasion, and increased drought effects due to global environmental change.</DISS_para> <DISS_para> In Chapter 2, tree and seedling richness-site productivity relationships were examined in sugar maple forests of the Upper Great Lakes region. First, the form of the species richness-site productivity relationship of the overstory and understory of sugar maple forests was investigated on three data sets (i.e. whole data set, upper 90th quantile subset, and random sample subset) by testing the null hypothesis that the species richness-site productivity is flat. The alternative hypotheses were that the richness-site productivity relationship is 1) hump-shaped, 2) positive monotonic, 3) negative monotonic, and 4) U-shaped. Second, after noticing that sugar maple abundance approached 100% on some plots, the existence of threshold effects of sugar maple abundance on species richness was investigated by testing the null hypothesis that no threshold effect exists (i.e. species richness decreases linearly with increase sugar maple abundance) against that alternative that there is a threshold effect of sugar maple abundance (i.e. species richness display threshold responses to increase basal area). Results varied across zones and data sets, indicating that sample size might be influencing the results. Overall, there was a significant positive relationship between tree richness and site productivity but a flat seedling richness-site productivity relationship. The addition of sugar maple basal area to the models greatly improved the results. There was no apparent threshold effect but sugar maple abundance had very strong negative effect on species richness, which appeared to increase from the prairie-forest border towards Upper Michigan. Chapter 3 focused on the ecological niche of sugar maple seedlings from the prairie-forest border to the interior of the forest biome. Sugar maple seedling sensitivity to current forest structure and composition (sugar maple basal area and stand age), as well as site level environmental conditions (sand proportion, soil depth, slope, and TRASP - an index related to aspect) was assessed under the hypothesis that sugar maple seedlings respond differently to environment variables across the region and have a broader environmental niche in the Upper Peninsula of Michigan than at the prairie-forest border. As expected, basal area of sugar maple was generally positively related to sugar maple seedling density, while the effects of % sand and soil depth varied across the three zones. TRASP, an index related to aspect, had a strong negative influence on seedling abundance at the prairie-forest border and interior zones (Zones 1 and 2, respectively), but had no influence in the deep interior zone (Zone 3). The overall interpretation of the models and patterns across the climate gradient indicated that sugar maple seedling abundance is currently insensitive to environmental variables (i.e. has a very broad environmental niche) in Upper Michigan, with many stands currently growing on sites with relatively high percent sand content, shallow soils, and southerly slopes. The expected shift in future climate would make the climate of Upper Michigan like that of the prairie-forest border by late 21st century, thereby reducing the probability of seedling establishment on many sites currently with high dominance of sugar maple. Additionally, high deer populations and earthworm invasion will narrow the niche of sugar maple in Upper Michigan even more so than changing climate alone.