DISSERTATION RESEARCH: Substrate Limitations to Tsuga Canadensis and Betula Allegheniensis Seedling Establishment

在密歇根州北部和威斯康星州，东部铁杉通常生长在靠近糖枫森林的密林中。这些边界非常稳定，有时数千年都不会改变。由于铁杉树荫浓密，糖枫无法入侵铁杉林。但为什么铁杉不能扩展到糖枫森林呢？部分原因是铁杉幼苗不能在森林地面上存活超过几年。较老的铁杉幼苗(和黄桦幼苗)通常生长在腐烂的原木和树桩上，这些树桩比周围的土壤保持湿润，也会脱落落叶。这些研究人员之前的工作发现，铁杉和黄桦幼苗比糖枫和其他落叶树种更喜欢铁杉原木。这可能不仅解释了为什么铁杉不能“入侵”糖枫森林，也解释了为什么铁杉和黄桦(一种生长缓慢的针叶树和一种生长较快的落叶树)经常被发现如此紧密地联系在一起，以至于它们的根相互交织在一起。在对原木和森林地面上的幼苗进行了计数和测量后，调查人员现在正在测量原木的水分、光照和营养水平，以及木材腐烂率，以解释为什么铁杉和黄桦树的幼苗更喜欢铁杉原木。这项研究最终可能被用来帮助恢复被铁杉毛虫摧毁的铁杉森林，铁杉毛虫是一种来自亚洲的害虫，已经入侵东海岸。这项研究的初步结果目前正被用于改善密歇根州古老的铁杉森林的管理。

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.