Eastern White Pine (Pinus strobus) Growth Response to Partial Hardwood Overstory Release

We measured the response of white pine (Pinus strobus L.) saplings after partial release of a hardwood overstory on three sites in central Minnesota. Both height and diameter growth increased quickly after release compared to prerelease growth. Diameter growth response was related to prerelease diameter growth, but not to initial size of the sapling (diameter at time of release). On the other hand, height growth response was strongly influenced by the initial height of all trees, but not related to prerelease height growth in the largest saplings (initial heights > 8.76 m). Increased release intensity (i.e., difference between prerelease and postrelease overstory densities) resulted in higher diameter and height growth responses. The height/ diameter ratios decreased after release, indicating that stability as well as growth increased after the release. North. J. Appl. For. 17(3):89–94. Over the last decade, land managers in the Lake States have increasingly emphasized retention of residuals after harvesting to maintain more forest structure and diversity across the landscape (Puettmann and Ek 1999). On many sites, the structure and density of residual overstory is not necessarily determined by the regeneration ecology of the desired species [e.g., for frost or sun protection (Buckley et al. 1998, Valigura and Messina 1994)], but more and more by other objectives, like visual quality, wildlife habitat, or pest management concerns (Puettmann and Ek 1999). One result of this trend is that regeneration may have to be attempted under conditions that are not optimal for a species. This requires understanding how seedlings and saplings respond under a wide range of competitive environments, especially in regard to optimal intensity and timing of overstory removal (i.e., liberation or overstory release) above advanced regeneration. While many studies have investigated growth responses to reduced competition when the competing trees are approximately of the same size, as in a thinning (e.g., Oliver 1985, Gillespie and Hocker 1986), few studies have documented the effects of liberation or overstory release on seedlings or saplings. Unfortunately, most of these only investigated seedlings’ responses to total removal of the overstory (e.g., Seidel 1980, Ferguson and Adams 1980, Helms and Standiford 1985). These studies, while providing information about responses to removal cuts in traditional shelterwood systems, are not applicable to conditions with partial overstory retention for many years after the initial release (Carlson and Schmidt 1989). The optimal regeneration conditions for eastern white pine (Pinus strobus L.) in Minnesota are determined by a combination of climate and soil conditions, competing vegetation, and pest management concerns. For example, retention of at least 20–50% overstory cover discourages attacks by white pine weevil (Pissoides strobi Peck) and reduces infection rates by white pine blister rust (Cronartium ribicola Fisher) (Lancaster and Leak 1978, Katovich and Morse 1992). Since land managers have targeted white pine for restoration throughout its range in Minnesota (WPRSWG 1996), successful regeneration is often attempted on sites where overstory residuals remain for a variety of reasons. However, the best timing and intensity of subsequent liberation cuts to promote understory white pine growth is relatively unknown. With these issues in mind, this study’s objectives were to: (1) determine if there was a growth response of understory white pine to partial overstory release; and (2) determine how release timing (as represented by sapling size at the time of the release) and release intensity (i.e., percent trees removed) influences postrelease growth response patterns. NOTE: Klaus J. Puettmann—Phone: (612) 625-3733; Fax: (612) 625-5212; E-mail: kpuettma@forestry.umn.edu. Mike R. Saunders—Phone: (612) 6245317; Fax: (612) 625-5212; E-mail: saund011@tc.umn.edu. The authors thank Martin J. Skoglund of the Training Site Environmental Office at the Camp Ripley Military Reservation for access to the sites. They gratefully acknowledge John Gerlach, Mike Counte, and Matt Duvall for help with design layout and fieldwork., and the three anonymous individuals who reviewed this manuscript for their comments. This work was funded by the Legislative Commission for Minnesota Resources, the Iron Range Resources and Rehabilitation Board, the Minnesota DNR, and the St. Louis County Land Department.