Regional variation in recruitment of hemlock seedlings and saplings in theupper Great Lakes, USA

Mature eastern hemlock-northern hardwood forest cover decreased drastically in the upper Midwest following European settlement and has yet to rebound substantially. Previous studies show that stands retaining substantial heml ock canopy coverage have low hemlock seedling and sapling densities. Result s from various geographically restricted studies suggest several possible m echanisms that could cause low seedling or sapling density. We examined the relative importance of these proposed mechanisms in the Southern Superior Uplands Section of the Laurentian Forest Province. We surveyed 294-m(2) plo ts in 100 hemlock stands in northern Wisconsin and western upper Michigan t o assess how these proposed mechanisms affect the number of hemlock seedlin gs and saplings in four sequential size classes, Seedling and sapling abund ance increases with greater light availability and differs significantly am ong geographically distinct ecological units (ecological subsections). In c ontrast, the abundance of medium and large saplings (30-99 and 100-300 cm t all) appears unrelated to light and geographic factors but declines as deer browsing increases, diminishing recruitment of larger saplings: The abunda nce of seedlings and saplings in each size class also depends strongly on t he number of seedlings or saplings in the next smaller size class at the sa me site, reflecting demographic inertia. Path analysis integrates and separ ates these effects, explaining 24% of the regional variation in seedling ab undance, 63% of small sapling abundance, and similar to 80% of medium and l arge sapling abundance. Light and ecological subsection directly affect the number of seedlings and small saplings, whereas deer browsing directly aff ects the number of medium saplings. Demographic inertia remains important f or all larger size classes. These results imply that restoring hemlock popu lations requires both appropriate microsites for seedling establishment and sanctuary from excessive browse for successful recruitment.