Survival and growth of northern white-cedar and balsam fir seedlings in riparian management zones in northern Minnesota, USA

Abstract

Northern white-cedar (Thuja occidentalis) and balsam fir (Abies balsamea.) are co-occurring species in riparian forests of the western Great Lakes region. Throughout much of the region, northern white-cedar has been experiencing population declines due to herbivory by white-tailed deer (Odocoileus virginianus). Preferential browse on northern white-cedar removes seedlings and saplings, allowing balsam fir, as less-preferred browse species, to recruit into larger size classes and potentially out-compete northern white-cedar. There is great interest in restoring and sustaining northern white-cedar in riparian forests, but the factors that contribute to success or failure of established seedlings are not well understood. We used a riparian harvesting experiment in northern Minnesota, USA to study factors that encourage or deter survival and growth of planted northern white-cedar seedlings in riparian areas, relative to the response of balsam fir. The factors we manipulated experimentally included amount of overstory basal area (uncut or partially harvested), establishment microsite (mound, pit, slash), and browse (protected or not).Browse frequency was significantly higher on northern white-cedar than on balsam fir, particularly in the partially-harvested forest. Northern white cedar survival was similar to balsam fir when unprotected from browsing, but was higher when browsing was excluded. When protected, northern white-cedar survival approached 100% in both uncut and partially-cut forest on mound and slash microsites, whereas survival of balsam fir was significantly lower in uncut compared to partially-cut forest. Both species had low survival in pit microsites. Relative height growth rates were similar between the two species when protected from browsing, and both species had higher growth with partial-harvest. When unprotected from browsing, relative height growth of northern white-cedar was often negative, while height growth of balsam fir was only minimally reduced compared to the protected seedlings. The two species had similar relative diameter growth in the unprotected treatment, and growth of both species was reduced compared to growth when protected. When protected, northern white-cedar had a relative diameter growth advantage over balsam fir, particularly in the partial-harvest treatment. Our results suggest that a strategy to establish northern white-cedar in riparian forests may be to plant seedlings in uncut forest on raised microsites and protect these seedlings from browsing. After these seedlings are well-established, and perhaps balsam fir is intentionally eliminated, a partial-harvest may be used to release the northern white-cedar seedlings, while still maintaining pre-harvest browse protection.