Substrate limitations of Tsuga canadensis and Betula allegheniensis seedling establishment

In this dissertation, I provide evidence that the distribution of hemlock (Tsuga canadensis), yellow birch (Betula allegheniensis ), and sugar maple (Acer saccharum) decaying wood maintains two patterns of tree distribution in Upper Michigan: the eastern hemlock-northern hardwood patch structure and the hemlock/yellow birch spatial association. Patches (3-30 ha) of hemlock with scattered yellow birch have remained hemlock-dominated and the same size for over 3000 years, even when adjacent to patches of northern hardwood forest usually dominated by sugar maple. Across both patch types, hemlock are most closely spatially associated with yellow birch, an association that makes little sense from a life history perspective, since yellow birch is a gap-phase hardwood and hemlock is a late-successional often slow-growing conifer. However, both hemlock and yellow birch seedlings are most abundant on wood and, I demonstrate here, in particular on hemlock wood. I show that hemlock wood is the most favorable substrate for hemlock and yellow birch seedling establishment (seedling density = 0.42 hemlocks/m 2, 0.60 birches/m2), followed by yellow birch wood (0.21, 0.15), and that sugar maple wood (0.08, 0.10) and undisturbed soil (0.01, 0.01) are less suitable and support few to no hemlock and yellow birch seedlings older than three years. Sugar maple seedlings, in contrast, do not establish on any species of decaying wood (sugar maple seedling density = 0.03 to 0.09/m2 across wood species). Hemlock and yellow birch wood are rare everywhere, but are most abundant in hemlock patches where they cover 2.8% of the forest floor, reinforcing the hemlock-northern hardwood patch structure and the spatial association between hemlock and yellow birch.; I combine field studies of seedling demographics, wood distribution, seed rain, and decaying wood properties in three field sites in Upper Michigan, USA with greenhouse studies of seedling growth, ectomycorrhizal colonization, and nutrient content to determine why hemlock wood and to a lesser extent yellow birch wood support higher densities of hemlock and yellow birch seedlings than either sugar maple wood or soil. Hemlock logs are more favorable for hemlock and yellow birch seedling establishment for several reasons, among them lower pH, sufficient nitrogen and phosphorus supply, a tendency to decay more slowly than hardwood logs and to be attacked by brown rot rather than white rot decay fungi, and a tendency to lose bark cover and develop moss cover. A greater ability to provide ectomycorrhizal inoculum to seedlings and the relative absence of sugar maple seedlings on hemlock logs may also contribute to the higher survival rates of hemlock and birch seedlings. The full text of this dissertation is available free of charge until at least 2010 at www.lauramarx.net.