Regeneration responses to gap size and coarse woody debris within natural disturbance-based silvicultural systems in northeastern Minnesota, USA
Highlights
► We examine effects of natural disturbance-based gaps on composition and structure. ► Gap creation increased shade tolerant seedling and sapling abundance. ► Recruitment of Betula allegheniensis was strongly linked to conifer coarse woody debris. ► Harvest gaps did not restore tree diversity presumably due to minimal soil disturbance.
Introduction
There is increasing global concern regarding the loss of native biodiversity from forest systems (Klenner et al., 2009), particularly within managed forests (Hunter, 1999). In many cases, past management practices have led to a simplification of forest structure, resulting in a loss of habitat for a diversity of organisms (Latty et al., 2006). One proposed management approach for restoring and maintaining native biodiversity and accelerating forest succession toward old-growth characteristics within managed forests is natural disturbance-based silviculture (Fries et al., 1997, Seymour et al., 2002). This approach bases the intensity and patterns of harvest entries upon natural disturbances occurring throughout the landscape (Perera et al., 2004). Despite the endorsement of this approach as a means to accomplish biodiversity-related goals in managed forests (Long, 2009), there have been few formal tests of the response of forest systems to harvesting regimes patterned after natural disturbances relative to more traditional approaches.
Natural disturbances within northern hardwood systems in the north temperate region of North America are primarily driven by wind, insects, and disease (Frelich and Lorimer, 1991, Frelich, 2002). These disturbances create canopy gaps, ranging from small to large in size (0.0004–0.1 ha; Seymour et al., 2002) that typically close by border tree encroachment and understory tree ascension (Hibbs, 1982, Runkle, 1982). Generally, small canopy gaps maintain shade tolerant species, such as Acer saccharum Marshall (sugar maple) (Canham, 1988), whereas large canopy gaps allow for the recruitment of shade mid-tolerant species (hereafter “mid-tolerant”), including Betula alleghaniensis Britton (yellow birch) (McClure et al., 2000, Webster and Lorimer, 2005), due to higher light availability within larger canopy gaps.
Historically, a variety of gap sizes have been created by natural disturbances in the region. In contrast, management of northern hardwood forests in the upper Great Lakes region has emphasized single-tree selection (Arbogast, 1957, Crow et al., 2002, Angers et al., 2005). Canopy openings created through this approach generally favor the ascension of shade tolerant species, such as A. saccharum, to the canopy (Neuendorff et al., 2007). As a result, B. alleghaniensis and other mid-tolerant species have declined throughout the region while shade tolerant species are increasingly abundant (Webster and Lorimer, 2005, Shields et al., 2007, Webster and Jensen, 2007). Similar increases in shade tolerant species at the expense of more mid-tolerant components have been documented in other regions of the globe in which single-tree selection systems are employed (Yoshida et al., 2006). The use of natural disturbance-based systems that create a diversity of gap sizes, including larger canopy gaps, may provide one potential means to restore this mid-tolerant component. Nonetheless, several studies in other regions of the globe have indicated that even larger gap sizes are often dominated by shade-tolerant species when pre-disturbance communities contain high levels of advance regeneration (Kwit and Platt, 2003, Nagel et al., 2006, Collet et al., 2008, Madsen and Hahn, 2008).
Beyond gap size, other factors, including the presence of suitable microhabitat conditions, such as downed coarse woody debris (CWD), tip-up mounds, and exposed mineral soil, strongly influence the recruitment of a given species within gaps (Grubb, 1977, Nakashizuka, 1989, Gray and Spies, 1998). Within northern hardwood systems, several genera primarily establish on CWD or exposed mineral soil seedbeds relative to the undisturbed forest floor (e.g., Betula, Picea, and Thuja) (Cornett et al., 2001, Caspersen and Saprunoff, 2005, Shields et al., 2007, Marx and Walters, 2008). These species generally have small wind-dispersed seeds with low nutrient reserves for germination and establishment (McGee and Birmingham, 1997) and large-diameter CWD and exposed mineral soil represent microhabitats with relatively stable moisture environments and less competition for early establishment (Harmon et al., 1986, Franklin et al., 1987, Nakashizuka, 1989).
Much of the timber harvesting within northern hardwood systems occurs during winter months under snow-covered, frozen soil conditions, thus limiting the levels of exposed mineral soil within harvest gaps (Shields et al., 2007). As such, CWD may represent the only suitable microhabitat for several species within gap-based silvicultural prescriptions that do not include provisions for deliberate soil scarification. Numerous studies have demonstrated that the abundance of CWD is lower in managed northern hardwood forests relative to old-growth systems (Goodburn and Lorimer, 1998, Hura and Crow, 2004, Vanderwel et al., 2008). Lower levels of CWD in managed forest systems result primarily from the removal of larger trees for timber products that otherwise would have served as inputs of CWD (Fridman and Walheim, 2000, Siitonen et al., 2000, Lorimer et al., 2001). As a result, there is often a lower availability of suitable microhabitats for species, such as B. alleghaniensis, within managed northern hardwood forests compared to pre-European settlement conditions.
This study investigated the influence of harvest techniques that emulate some aspects of natural canopy gap openings, namely size, on tree composition and stand structure 6- and 7-years post-harvest within second-growth northern hardwood systems in the upper Great Lakes region. The objective for this work was to develop an understanding of how effectively natural disturbance-based harvest gaps maintained and restored tree species diversity within second-growth northern hardwood forests in the region. To achieve this objective, we examined the response of tree regeneration and forest structure (e.g., seedling densities, CWD) to a range of gap sizes patterned after natural disturbances for the region. We hypothesized that (i) large harvest gaps will enhance seedling diversity, including the presence of mid-tolerant species, relative to smaller canopy gaps; (ii) advance regeneration will dominate small gap succession; and (iii) B. alleghaniensis establishment will be strongly correlated with the availability of CWD substrates.
Section snippets
Study sites
Study sites were located along the northern shore of Lake Superior in northeastern Minnesota, USA (Table 1). Elevations within this area range from 381 to 472 m and soils are loams derived from glacial tills (Hobbs and Goebel, 1982). Mean annual precipitation is 739 mm and mean annual temperatures range from −8.5 °C in January to 18.7 °C in July. Forests within the study area are dominated by A. saccharum, with lesser amounts of B. alleghaniensis, Fraxinus nigra Marshall (black ash), Betula
Seedling and sapling response to gap treatments
A total of nine tree species were encountered in both regeneration layers across the study sites (Table 2). Of these species, eight were found in both the seedling and sapling layers. A. saccharum dominated seedling and sapling layers, with the highest relative density among the combined stem densities across all sites (37% and 82%, respectively). Densities generally increased as gap size increased (Fig. 2). In particular, density of seedlings and saplings was greater in large gaps (>0.02 ha)
Tree response to natural disturbance-based harvest gaps
The findings of this work suggest that solely applying harvest gaps approximating the natural range of gap sizes documented for the upper Great Lakes region may not be sufficient for increasing the diversity of tree species within second-growth stands. In the present study, size-class diversity increased following silvicultural treatments through the release of existing advance regeneration; however, few changes in tree species composition were detected within the regeneration layer. Despite
Management implications
Forest management goals increasingly incorporate biodiversity objectives in response to global concerns regarding the ecological sustainability of traditional forest management regimes. Approaches based on natural disturbance patterns provide significant opportunities to harvest wood products, while also achieving biodiversity-related goals. Nevertheless, these approaches are currently in their experimental phases and the findings of this work highlight that simply creating harvest openings
Acknowledgements
We are grateful to The Nature Conservancy in Minnesota, Lake County Land Department, and the Manitou Collaborative for initiating the project. Funding for the project was provided by the Cox Family Fund for Science and Research and the Minnesota Agricultural Experiment Station. We thank Eric Zenner, Meredith Cornett, Julia Burton, Chris Dunham, Lee Frelich and Mark White for their efforts in designing and overseeing treatment implementation. In addition, we acknowledge Matt Tyler, John Segari,
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