Overstory and Understory Characteristics Following Partial Harvesting in Mixedwood Stands of Western Maine

This study investigates the effects of partial harvest treatments on regeneration within mixedwood forest stands in the Acadian Forest region. Chapter 1 assesses composition, density, growing environment and relative abundance of seedlings and saplings. Trail corridors (which occupied 38.3% of the study stands) had lower canopy closure and higher shrub and graminoid cover than between-trail areas (matrix). Canopy closure increased as distance from trail increased. Regeneration density and stocking varied by species, height class, and proximity to trails. Softwood species decreased in stocking and density near the trails, and in the tallest height classes. Seedling stocking was inversely related to canopy openness and graminoid abundance with the exception of aspen-birch. Density and stocking of noncommercial species (striped maple, elderberry, and pin cherry) were greater than any other species group in the tallest height classes. Density of all saplings was greater in the matrix for all species, except red maple. Chapter 2 investigates the growth dynamics of understory balsam fir (Abies balsamea (L.) P. Mill.) in partial harvests. Stocking of fir seedlings was significantly greater in the partially harvested matrix (between skid trails) than in the trail. Radial growth increased after treatment, and continued to increase for 8-9 years after the treatment. Seedling height and the previous year’s height increment were significant predictors (p \u3c 0.0001) of the annual height growth, highlighting the potential of selecting understory trees for release based on current size and growth rate. Release was observed in 14% of sampled seedlings. Conifer litter, canopy openness, and shrub competition were significant predictors of seedling size and establishment time, though none were useful in directly modeling these seedling characteristics. Seedlings nearest the trail responded to a greater extent after the treatment. In chapter 3, we examine the response of the non-tree understory relative to the harvest trails. Implications of the trails on diversity and presence of plants is discussed. Trails possessed an elevated coverage of graminoids, and an increase in total species richness and richness of graminoid species. Multidimensional scaling techniques revealed that interior forest species tended to be more prevalent in the matrix, while the trails possessed mostly opportunistic, early-successional species. Trails contained greater species richness and less evenness than matrix. Richness was greatest for herb and tree species groups. Richness of graminoids was greater in trails than the matrix, as was richness of facultative wetland species. The latter suggests altered surface water conditions in areas trafficked by machinery. Nonnative invasive species were rare, occurring on only 3% of plots and not apparently impacting tree regeneration. None of the measured environmental variables significantly improved prediction of percent cover of species \u3c 30.4 cm in height. Highest percent cover of commercially valuable hardwood species was found within 2 to 4 m of trails and in intermediate levels of canopy openness, suggesting that areas untracked by machinery but benefiting from side lighting were most suitable for their regeneration. This research suggests that mechanical operations associated with partial harvesting in Acadian mixedwoods results in residual stands in which composition, diversity, and abundance of understory plants, including tree regeneration, differs between trail corridors and residual matrix. Specifically, the increased graminoid and shrub cover in the trail, lower stocking and density of softwood species in and near the trail, greater graminoid and facultative wetland species richness in the trail, and the presence of commercial tree species at intermediate levels of canopy openness, fir sapling abundance, and proximity to trail were significant observations of this study. These findings suggest the width of trails and the distance between trails could be manipulated to achieve desired species composition. Routing trails through dense softwood thickets could decrease the presence of softwood species, while re-using trails could maintain the softwood matrix