Factors influencing tree species diversity and Betula alleghaniensis establishment in silvicultural openings

JOSHUA M. SHIELDS 0 1 CHRISTOPHER R. WEBSTER 0 1 LINDA M. NAGEL 0 1 0 Institute of Chartered Foresters , 2007. All rights reserved 1 School of Forest Resources and Environmental Science, Michigan Technological University , 1400 Townsend Drive, Houghton, MI 49931-1295, USA We examined the short-term effects of group-selection harvesting with seed tree retention on the diversity, abundance and establishment of tree seedlings in a northern hardwood forest in the Upper Peninsula of Michigan (49 openings, 20 closed canopy reference sites). Three opening sizes were examined - opening radius 0.5 canopy height (267 62 m2, n = 16), 0.75 canopy height (642 85 m2, n = 17) and 1.0 canopy height (1192 155 m2, n = 16) (canopy height = 22 m). A single yellow birch (Betula alleghaniensis Britt.) seed tree was retained in the centre of each opening. Tree seedling density was significantly higher in the largest group-selection opening than at the closed canopy reference sites (P < 0.05), the main factor for this was the increased proportion of yellow birch, red maple (Acer rubrum L.) and other minor species. Nevertheless, yellow birch was still a minor component of the developing gap cohort, comprising 5.9 per cent of the seedlings and 1.1 per cent of the saplings. Within openings, microsite variables, such as per cent covers of bare soil and coarse woody debris, were the best predictors of yellow birch occurrence and density. Our results suggest that microsite limitations and competing vegetation may greatly reduce the efficacy of openings for ensuring the maintenance of mid-tolerants. Summary Introduction It is widely acknowledged that single-tree selection often leads to a decline in tree species diversity because the small and often ephemeral canopy openings created by this harvesting method favour the recruitment of shade-tolerant species at the expense of shade-intolerant and mid-tolerant species (Metzger and Tubbs, 1971; Leak and Filip, 1977; Johnson, 1984; Jenkins and Parker, 2001; Crow et al., 2002; Leak and Sendak, 2002; Schuler, 2004; Angers et al., 2005). To counter these declines, practitioners have increasingly relied on gap-based silvicultural techniques, such as group selection and/or modified patch clear-cutting (Coates and Burton, 1997; Leak, 1999; Kelty et al., 2003; Webster and Lorimer, 2005). Gap-based approaches have been particularly successful in northern hardwood forests of the north-eastern United States. For example, 61-year results presented by Leak (1999) from New Hampshire suggest that group/patch selection (employing a range of opening sizes from 0.04 to 0.89 ha) can maintain viable components of intolerant and mid-tolerant species in northern hardwood forests dominated by shade-tolerant species. Additionally, gap-based approaches have shown utility as a framework for understanding ecosystem responses to harvesting (Dobrowolska, 2006). Nevertheless, both natural and silvicultural disturbances can also accelerate succession, especially in stands with understories dominated by shade-tolerant species (Abrams and Scott, 1989; Webb and Scanga, 2001). In oak (Quercus spp.) dominated forests of Pennsylvania, Zaczek (2002) found that mechanical treatment of competing vegetation was necessary to inhibit successional momentum towards shade-tolerant species. Similarly, De Grandpr et al. (2000) found that clear-cutting in balsam firyellow birch (Abies balsamea (L.) Mill.-Betula alleghaniensis Britt.) forests in Quebec resulted in a rapid conversion to shade-tolerant species. In some temperate forests, shade-tolerant shrubs may also reduce recruitment opportunities for trees in canopy gaps (Yamamoto et al., 1995; Beckage et al., 2000). Conversely, in some instances large openings are rapidly occupied by dense colonies of early seral shrubs and graminoids (Kirby, 1990; Schumann et al., 2003) which inhibit tree establishment (Marquis, 1965; Tabari et al., 2005). Eyre and Zillgitt (1953) postulated that smaller opening sizes might prevent the establishment of competing early seral vegetation while still providing adequate light environments for mid-tolerant tree species. In an early study at the Upper Peninsula Experimental Forest, Eyre and Zillgitt (1953) found 23 years following harvest that small group-selection openings (<405 m2) contained more yellow birch the principle mid-tolerant in northern hardwood forests of the Lake States and fewer competing shrubs than larger openings. While researchers have long advocated special cutting practices for less tolerant species in uneven-aged stands (Eyre and Zillgitt, 1953; Marquis, 1965; Leak and Filip, 1977; Miller and Kochenderfer, 1998), these methods often have not been widely applied. Consequently, many selection stands in eastern North America have moved towards increasing dominance by shade-tolerant species (Schuler, 2004; Neuendorff et al., in press). For example, in the northern Lake States, yellow birch appears to be declining in both old- (...truncated)