Abstract:
Species richness and abundance of forest floor bryophytes, including epiphytes from incorporated litterfall, were assessed at two sites in western Oregon. Bryophyte diversity, abundance, and community composition were compared between sites, and between young forest stands ([approximately] 55 yrs.) and old-growth stands (400+ yrs.) within each site. Relationships of stand structural features to diversity and community composition were assessed by stratifying sampling between "diversity" plots placed in areas of greater structural diversity, such as hardwood openings and remnant old-growth trees, and "matrix" plots situated within the remaining more homogeneous conifer-dominated forest matrix. Bryophyte relationships to substrate and stand age were quantified using the method of Dufrene and Legendre, which combines a species' relative abundance and relative frequency to calculate that species' importance in relation to environmental variables. The resulting "indicator value" describes a species' reliability for indicating the given environmental parameter. Ninety-three bryophyte species were found. Thirty-nine were indicative of either humus, a decay class of coarse woody debris, or stand age. A suite of bryophytes indicated old-growth forest. These were mainly either epiphytes associated with older conifers or liverworts associated with coarse woody debris. Hardwood-associated epiphytes mainly indicated young stands. Richness, particularly for liverworts, was significantly higher in old-growth than young stands, and the two ages differed significantly in community composition Substrate (ground versus coarse woody debris) and overstory (conifers versus hardwoods) were most strongly correlated with variation in bryophyte community composition. Composition changed along the continuum of coarse woody debris decomposition from recently fallen trees with intact bark to forest floor humus. Relatively open hardwood-dominated diversity plots differed in composition from matrix plots. Bryophyte abundance was lower in denser stands and plots, and positively correlated with canopy gaps, percentage of hardwoods, and incident solar radiation. The generality of inferences derived from results is limited by the number of stands studied. Landscape level factors such as topography and prevailing slope and aspect may influence results between sites and ages. However, results suggest that 1) availability of light may limit bryophyte productivity in these stands, 2) bryophyte richness increases with stand continuity, 3) older conifers, hardwoods, and coarse woody debris foster habitat complexity and diversity of bryophytes, and 4) diversity of forest floor bryophytes will be enhanced when a full range of coarse woody debris decay classes is present. Implementation of strategies to protect these biological legacies when thinning managed stands is consistent with an ecosystem approach to forest management.
