Understory plant response to alternative forestry practices in northern hardwood-conifer forests

The understory layer encompasses the majority of plant species diversity in forested ecosystems and may be sensitive to timber harvest disturbance. We hypothesize that i) uneven-aged, low-intensity silviculture can maintain understory plant diversity and lifeboat late-successional species in northern hardwood-conifer forests, and ii) retaining and enhancing stand structural complexity can increase understory plant diversity. Experimental treatments include two conventional uneven-aged prescriptions modified to increase structural retention, single-tree selection and group selection, and structural complexity enhancement (SCE), a technique designed to promote late-successional forest structure and function. Four replications of each treatment were applied to 2 ha management units at three sites in Vermont and New York, U.S.A. Understory vegetation was monitored over 2 years pre- and 4 years post-treatment. We used a linear mixed effects model to evaluate the effects of treatment, soil chemistry, and drought stress on understory composition and diversity. Model results show that over time, understory responses were strongly affected by overstory treatment and less influenced by soil chemistry and drought stress. All treatments were successful in maintaining overall richness and diversity. However, late-successional diversity increased significantly in SCE units and decreased significantly in group selection units. These results indicate that while conventional uneven-aged systems are capable of maintaining understory plant diversity, variations that retain or enhance structural complexity may be more efficient at lifeboating late-successional species. Increased microsite heterogeneity as a result of these techniques may also increase understory plant diversity, at least during initial post-harvest years.