In northern Michigan and Wisconsin, eastern hemlock trees often grow in dense stands bordering sugar maple forests. These borders are remarkably stable, sometimes not changing for thousands of years. Because hemlock trees cast such dense shade, sugar maple is unable to invade hemlock stands. But why can't hemlock expand into sugar maple forests? Part of the reason is that hemlock seedlings cannot survive more than a few years on the forest floor. Older hemlock seedlings (and yellow birch seedlings) are usually found growing on rotting logs and stumps, which stay moister than the surrounding soil and also shed leaf litter. Previous work by these investigators has found that both hemlock and yellow birch seedlings prefer hemlock logs to those of sugar maple and other deciduous species. This may explain not only why hemlock trees are unable to "invade" sugar maple forests, but also why hemlock and yellow birch (a slow-growing conifer and a faster-growing deciduous tree) are commonly found so close together that their roots are intertwined. Having already counted and measured seedlings on logs and the forest floor, the investigators are now measuring water, light, and nutrient levels of logs, along with wood decay rate, to explain why hemlock and yellow birch seedlings prefer hemlock logs. This research may eventually be used to help restore hemlock forests destroyed by hemlock woolly adelgid, a pest from Asia that has invaded the east coast. The preliminary results of this study are currently being used to improve management of old-growth hemlock forests in Michigan.
