Comparing the Importance of Seedbed and Canopy Type in the Restoration of Upland Thuja occidentalis Forests of Northeastern Minnesota

In cold-temperate ecosystems of the upper Great Lakes Region, evergreen 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. Using a combination of restoration experiments and field surveys at three study sites on the Lake Superior Highlands, Minnesota, U.S.A., we examined safe sites for T. occidentalis regeneration under both Thuja and Betula canopy types. This study focused on the colonization and establishment phases of regeneration, differentiating among safe-site components for T. occidentalis . Seedbed type determined colonization success, with higher rates on conifer logs than on B. papyrifera logs, mounds, or pits. Mortality rates for seedlings on natural decayed wood seedbeds were higher under Thuja canopy than under Betula canopy, but the reverse was true for seedlings on manipulated seedbeds, suggesting that seedbed type was more important than the dominant canopy type. Growth rates for seedlings on moved log segments were greater under the Betula than the Thuja canopy type, but seedlings on natural decayed wood seedbeds did not exhibit this difference. Results indicated that T. occidentalis regeneration was more limited in Betula forest by seedbed availability, while in Thuja forest canopy conditions were more limiting.