PARENT SESSION
Oral Session # 69: Forest Ecology IV: Communities.
Presiding: D Gavin
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 200.
Decaying wood species composition and distribution limit eastern hemlock and yellow birch seedling establishment.
Marx, Laura*,1, Walters, Michael1, 1 Michigan State University, East Lansing, MI
ABSTRACT- Eastern hemlock (Tsuga canadensis) in Upper Michigan is usually found in distinct patches of either pure hemlock or hemlock in association with yellow birch (Betula allegheniensis). These patches often border areas of northern hardwood forest. The distribution and species composition of coarse woody debris (decaying logs, stumps, and large branches) help maintain the eastern hemlock-northern hardwood patch structure and the hemlock/yellow birch association. In four old growth hemlock-hardwood forests, we found 93% of hemlock and yellow birch seedlings growing on coarse woody debris, though this substrate covered only 3% of the forest floor. Of the seedlings found on coarse wood, 62% were on conifer wood (which made up 43% of total wood area), and average seedling density on conifer wood was 3.1 ± 0.67 s.e. seedlings/m2 while hardwood wood supported only 0.84 ± 0.29 seedlings/m2. Conifer coarse wood was more abundant in hemlock patches than in mixed hardwood patches (paired t-test, N = 7 paired forest patches, p = 0.0492). The lack of suitable coarse wood for establishment of hemlock seedlings in northern hardwood stands helps to maintain the hemlock patch structure. Similarly, the restriction of hemlock and yellow birch seedlings to the same rare substrate -- conifer coarse wood -- maintains the strong association between adult hemlock and yellow birch trees. Differences in wood chemistry may partially explain why conifer coarse wood is more suitable than hardwood coarse wood for seedling establishment. Conifer coarse wood has modestly higher supply rates of ammonium (paired t-test, n = 13 field plots, p < 0.10) and lower pH (p < 0.01) than hardwood coarse wood.
Key words: Betula, Tsuga, coarse woody debris, old growth