Substrate Availability and Regeneration Microsites of Tolerant Conifers in Mixed-species Stands in Maine

This study investigates regeneration in mixed-species stands in the Acadian Forest of Maine. We examined the effect of silvicultural intensity on available regeneration substrates and how seedling-substrate relationships may be impacted by management activities. Silvicultural treatments studied include two replicates each of 5-year selection, 20-year selection, commercial clearcutting (unregulated harvesting), and three replicates of no management (defined as no harvesting for at least 50 years). We focused on red spruce (Picea rubens Sarg.), eastern hemlock (Tsuga canadensis (L.) Carr.), balsam fir (Abies balsamea L. Mill), and red maple (Acer rubrum L.); all four species are common within the Acadian region. Chapter 1 presents analysis of the availability of wood as a regeneration substrate and the role that downed woody material plays in regenerating tolerant conifers and red maple, a frequent competitor. Pieces of wood = Decay Class III and =10 cm on one end were studied. Area of wood substrate was greater in unmanaged stands than commercial clearcuts (p=0.05). Seedlings < 1.3 m in height were analyzed on wood and adjacent (paired) forest floor plots of equal area. Spruce and hemlock seedlings were found at higher densities on wood than forest floor plots, regardless of treatment. Decay Class IV and V wood supported the highest spruce seedling densities; hemlock seedlings were well distributed across all three decay classes studied. Conversely, fir and maple were less abundant on wood than forest floor plots in unmanaged and selection stands, but more abundant (fir) or equally abundant (maple) on wood relative to forest floor plots in commercial clearcuts. These findings suggest that silvicultural treatment affects wood-seedling relationships, and support the conclusion that management for hemlock and spruce in the Acadian Forest should include attention to coarse woody material. This is particularly important in light of the fact that Decay Class III through V wood represented a very small amount of available regeneration substrate. Chapter 2 expands the analysis to include regeneration on other substrates common in the Acadian Forest, in addition to downed wood: mineral and organic soil, leaf litter, and bryophytes. Ages of sampled seedlings (< 0.5 m in height) varied by species and treatment; maximum ages exceeded 30 years for tolerant conifers in unmanaged and selection stands. Unmanaged stands had the greatest area of downed wood substrate (Decay Classes I through V combined) (p < 0.01); this is consistent with Chapter 1 findings for Decay Classes III through V. Unmanaged stands and 20-year selection treatments also had the highest areas of substrate occupied by liverworts, though both selection treatments had more area of bryophytes and greater bryophyte diversity (p < 0.10). Area of hardwood leaf litter was greatest in the commercial clearcut (p < 0.10), and area of conifer litter was greatest in the unmanaged stands (p < 0.01). Regression models and ?2 (chi-square) analysis suggested that bryophyte mats and wood in advanced stages of decay are important factors influencing tolerant conifer regeneration in unmanaged and selection stands. Commercial clearcuts, which appear to have fewer local tolerant conifer seed sources, bryophyte mats, and advanced stages of decayed wood, had very low red spruce and eastern hemlock seedling densities (p < 0.10). Balsam fir and red maple seedlings were present in all treatments, but appear to be favored by conditions associated with greater harvesting intensity. No consistent seedling-substrate relationships were recognized across treatments for balsam fir and red maple. This study indicates that silvicultural intensity affects substrates available for tolerant conifer regeneration within the Acadian region. Treatments resulting in softwood overstory composition, downed wood, and bryophytes are likely to favor spruce and hemlock over competing balsam fir and red maple, though small-scale environmental heterogeneity affects these relationships.