Abstract
We compared the understory communities (herbs, shrubs, and tree seedlings and saplings) of old-growth and second-growth eastern hemlock forests ( Tsuga canadensis) in western Massachusetts, USA. Second-growth hemlock forests originated following clear-cut logging in the late 1800s and were 108–136 years old at the time of sampling. Old-growth hemlock forests contained total ground cover of herbaceous and shrub species that was approximately 4 times greater than in second-growth forests (4.02 ± 0.41%/m 2 versus 1.06 ± 0.47%/m 2) and supported greater overall species richness and diversity. In addition, seedling and sapling densities were greater in old-growth stands compared to second-growth stands and the composition of these layers was positively correlated with overstory species composition (Mantel tests, r > 0.26, P < 0.05) highlighting the strong positive neighborhood effects in these systems. Ordination of study site understory species composition identified a strong gradient in community composition from second-growth to old-growth stands. Vector overlays of environmental and forest structural variables indicated that these gradients were related to differences in overstory tree density, nitrogen availability, and coarse woody debris characteristics among hemlock stands. These relationships suggest that differences in resource availability (e.g., light, moisture, and nutrients) and microhabitat heterogeneity between old-growth and second-growth stands were likely driving these compositional patterns. Interestingly, several common forest understory plants, including Aralia nudicaulis, Dryopteris intermedia, and Viburnum alnifolium, were significant indicator species for old-growth hemlock stands, highlighting the lasting legacy of past land use on the reestablishment and growth of these common species within second-growth areas. The return of old-growth understory conditions to these second-growth areas will largely be dependent on disturbance and self-thinning mediated changes in overstory structure, resource availability, and microhabitat heterogeneity.
Highlights
IntroductionThe understory layer is a critical component of forest ecosystems typically supporting the vast majority of total ecosystem floristic diversity (Halpern and Spies 1995, Gilliam and Roberts2003) and providing habitat elements to associated wildlife species (Carey and Johnson 1995).These communities play a central role in the dynamics and functioning of forest ecosystems by influencing long-term successional patterns (Philips and Murdy 1995, Abrams and Downs1990, Oliver and Larson 1996, McCarthy et al 2001, Royo and Carson 2005, Nyland et al 2006)and contributing to forest nutrient cycles (Chapin 1983, Zak et al 1990, Anderson and Eickmeier2000, Chastain et al 2006)
This study indicates that, despite compositional similarities in overstory trees, differences exist between the understory communities characterizing old-growth and second-growth hemlock forests in western Massachusetts
The understory communities in the old-growth hemlock stands examined in this study are distinctive from those in hemlock dominated stands originating following logging in the late 19th century
Summary
The understory layer is a critical component of forest ecosystems typically supporting the vast majority of total ecosystem floristic diversity (Halpern and Spies 1995, Gilliam and Roberts2003) and providing habitat elements to associated wildlife species (Carey and Johnson 1995).These communities play a central role in the dynamics and functioning of forest ecosystems by influencing long-term successional patterns (Philips and Murdy 1995, Abrams and Downs1990, Oliver and Larson 1996, McCarthy et al 2001, Royo and Carson 2005, Nyland et al 2006)and contributing to forest nutrient cycles (Chapin 1983, Zak et al 1990, Anderson and Eickmeier2000, Chastain et al 2006). 2003) and providing habitat elements to associated wildlife species (Carey and Johnson 1995) These communities play a central role in the dynamics and functioning of forest ecosystems by influencing long-term successional patterns The diversity of understory plants may be low in some old-growth systems (Metzger and Shultz 1984, Scheller and Mladenoff 2002), other studies have indicated that certain taxa may be more abundant or restricted to these forests (Whitney and Foster 1988, Matlack 1994, Halpern and Spies 1995, Moola and Vasseur 2004). The affinity of certain species to old-growth forests has been attributed to several factors, including reproductive characteristics of the plants
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