Effects of variation in ecosystem carryover on biodiversity and community structure of forest floor bryophytes and understory vascular plants: a retrospective approach

National Forest managers in the Pacific Northwest are faced withthe task of simultaneously harvesting forest products and maintainingecosystem integrity and biological diversity. Consequently, forestmanagers are applying "ecosystem management" techniques which seek tomanage stands for both ecosystem integrity and productivity. Ecosystemmanagement approaches include leaving standing green trees and snags, aswell as coarse woody debris, as would be left by natural disturbances(e.g. fire). However, these techniques are being implemented withlittle data on long term effects. This study uses a retrospectiveapproach to examine effects of green tree retention on biodiversity andcommunity structure of understory vascular plants and forest floorbryophytes. Fourteen natural two-storied stands of mature residualtrees over well-stocked rotation-aged regeneration (60-100 years old) inthe Tsuga heterophylla (Raf.) Sarg. zone of the central Cascades inOregon were selected for study sites. Within each site, a pair ofnested, concentric fixed radius plots (one with residual trees,analogous to green tree retention, and one without residual trees) weresampled. Data were collected on residual trees, tree regeneration,vascular plants and forest floor bryophytes. Residual trees werepositively correlated with several structural features including snagmass and percentages of western hemlock in the regeneration, andnegatively correlated with percentages of Douglas-fir in regeneration
and overall conifer volume. While differences in undergrowth speciesrichness and cover between plots with and without residuals were notapparent on a pairwise basis, analyses across all sites suggested thatresiduals did influence undergrowth communities. Shrub communitycomposition was correlated with residual tree number, volume, crown areaand basal area and with regeneration conifer volume, abundance andspecies composition (e.g., percentages of Douglas-fir verses westernhemlock). Herb communities were most strongly correlated with speciescomposition of tree regeneration, which was correlated with residualtree abundance. Vascular plant species richness was higher inrelatively open drier, Douglas-fir-dominated forests, which tended tohave fewer residual trees. Similarly, species richness for late-successional species decreased as the percentage of western hemlock inthe regeneration and canopy cover increased. Occurrence of some late-successional species was positively correlated with the abundance ofcoarse woody debris. Bryophyte communities were apparently influencedby snag mass, residual tree volume and live basal area, and hardwoodvolume.