Ecological restoration of upland northern white -cedar forests on the Lake Superior highlands.

摘要

In cold-temperate ecosystems of the upper Great Lakes Region, conifer-dominated forests were once common. As a result of past management practices, early-successional deciduous forests now dominate the landscape. Embedded in this matrix are stands of shade-tolerant conifers, including Thuja occidentalis. For the past several decades, large-scale T. occidentalis regeneration in remnant T. occidentalis and adjacent Betula papyrifera forests has not occurred. An investigation of the barriers to regeneration of T. occidentalis was conducted producing recommendations for the ecological restoration of T. occidentalis forests. Among the primary barriers investigated were herbivory and canopy-seedbed feedback relationships.;Experimental plantings of T. occidentalis seedlings under different herbivory protection treatments and different canopy conditions were conducted. Extremely high browsing and subsequently high mortality rates indicated that planting is impractical under current levels of browsing by Odocoileus virginianus. Simulation models and vegetation surveys demonstrated that the best recruitment rates occurred for seedlings planted under Betula canopy with low browsing severity, but no recruitment occurred under high browsing pressure under either canopy type. The current level of browsing in these forests has the potential to alter the future canopy composition through sustained prevention of T. occidentalis recruitment.;A combination of field and greenhouse work was used to examine the role of safe sites in T. occidentalis regeneration. From field experiments, it was concluded that T. occidentalis regeneration is more limited in Betula forest by seed and seedbed availability, while in Thuja forest other canopy conditions, such as light availability, are more limiting. Greenhouse work indicated that good moisture retention is an important attribute of safe sites for the early stages of T. occidentalis regeneration.;The ecological restoration of forests dominated by T. occidentalis faces a number of challenges on the Lake Superior Highlands. Any measures to restore T. occidentalis forest cover, including planting, seedbed manipulation, or areas conserved for natural succession, must be accompanied by expensive protection from browsing. Mimicking appropriate safe sites for T. occidentalis regeneration has produced mixed results, and conservation of areas allowing for the natural deposition of decayed wood safe sites is alternatively recommended.