Northern white-cedar (Thuja occidentalis L.), a long-lived conifer closely associated with many riparian boreal forests of Minnesota, is an important species to riparian ecosystem health and to the forest products industry. Northern white-cedar is currently experiencing a decline in recruitment throughout much of its natural range, due primarily to herbivory by white-tailed deer (Odocoileus virginianus). Herbivory on northern white-cedar removes seedling and sapling size classes, and allows less browsed species (notably balsam fir (Abies balsamea (L.) Mill) to further displace the northern white-cedar forest component. This study examined practices that could encourage advance regeneration of northern white-cedar in the riparian setting. The role of partial overstory removal and forest floor microsite was tested on the development of planted
3-0 seedlings of northern white-cedar and the potentially competitive species of balsam fir. Seedlings were planted in mound, pit, and slash microsites (important to seedling germination and establishment) in partially harvested and unharvested riparian areas, and assessed for survival and growth over a four-year period. The test was duplicated inside and outside of fenced plots to evaluate the impact of deer browse and to more fully understand the current dynamic between species.
Protected from herbivory, results indicated that partial harvest of the overstory contributed to significant seedling growth with both species demonstrating potential to recruit into taller height and larger basal diameter classes. Basal diameter growth rates were greater in northern white-cedar than balsam fir. Microsites of mound, pit, and slash did not contribute significantly to growth. Percent survival of northern white-cedar was greater than balsam fir, which experienced lower survival in controls. Due to seasonal flooding, pits had a significant negative effect on survival of both species. When protected from herbivory, planted northern white-cedar seedlings were shown to be good candidates for outplanting in a variety of riparian settings, particularly partial harvest areas, and were competitive with balsam fir seedlings for up to four years in the field.
Percent browse frequency on unfenced seedlings was greater in harvest treatments and on mound microsites. Unfenced northern white-cedar seedlings were browsed at a significantly higher rate than balsam fir. Northern white-cedar showed significant mortality and no height increase in any treatments, with browse on seedlings overwhelming the benefits of overstory harvests. However, seedlings continued carbon allocation to basal diameter. In addition, northern white-cedar displayed survival resiliency, although reduced vitality, for the four years subjected to herbivory. Basal diameter increase and survival of browsed northern white-cedar seedlings are most likely due to their ability to replace browsed foliage in the current year. Unfenced balsam fir seedlings maintained growth responses to harvest treatments and did not experience significant mortality due to browse. Balsam fir seedlings exhibited the potential to recruit successfully into the overstory even under browse pressure, while northern white-cedar did not. Management activities that increase light availability while retaining some overstory structure, and selection of planting sites that limit or exclude browse, will result in the best chance for northern white-cedar seedlings to recruit into the sapling stage.
University of Minnesota M.S. thesis November 2011. Major: Natural resources science & management. Advisors: Dr. Andrew J. David, Dr. Charles R. Blinn. 1 computer file (PDF); ix, 210 pages.
Haworth, Brooke Karen.
Response of Thuja occidentalis and Abies balsamea seedlings to stand manipulations in Northern Minnesota riparian forests..
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