| This 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.;Chapter 1 investigated the contemporary overstory and understory forest composition of sugar maple communities and successional dynamics across the Upper Great Lakes region. 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.;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.;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. (Abstract shortened by UMI.). |
