Factors affecting the regeneration of northern white cedar in lowland forests of the Upper Great Lakes region, USA

Regeneration of northern white cedar (Thuja occidentalis) has been poor in the Upper Great Lakes region for decades. To understand why, we used a spatially extensive approach to examine which factors limit regeneration in lowland swamp forests at a regional scale. We investigated patterns of seedling establishment and sapling recruitment in 1990-1991 and 1996 among 77 lowland stands distributed across northern Wisconsin and the eastern Upper Peninsula of Michigan. These stands differed in ownership, local site conditions, and ambient deer densities, allowing us to evaluate several factors potentially limiting regeneration. Lowland cedar seedlings in this region typically require 10 years to grow to 30 cm. and 30 years to attain 3 m. Regression and path analyses demonstrate that initial seedling establishment increases in areas with greater seed input (as inferred from cedar basal area) and (in 1996) higher light levels. Subsequent recruitment to saplings +30 cm. tall, however, depends far more on demographic inertia (the number of smaller seedlings) and escape from deer browsing. Surprisingly, estimated deer browse was as significant a factor as demographic inertia, reflecting cedar's palatability, slow growth, and chronic high browsing pressure. Efforts to regenerate this species seem unlikely to succeed without significant and sustained reductions in deer density or extensive efforts to mechanically protect saplings from browsing. (C) 2002 Elsevier Science B.V. All rights reserved.