Experimental gaps in the Vermont Forest Ecosystem Management Demonstration Project: Effects on stand dynamics, salamanders, understory plants, and fungi

Background/Question/Methods

Management for late-successional forest structure and function is an important element of sustainable forestry in the northern hardwood region.  Canopy gaps are a key characteristic of interest and a variety of silvicultural approaches have been proposed to approximate natural canopy gap dynamics.  The Vermont Forest Ecosystem Management Demonstration Project is assessing long-term responses to different sizes and spatial distributions of silvicultural gaps.  We are testing an approach called “Structural Complexity Enhancement” (SCE) in which variable density horizontal structure, including small gaps (0.02 ha mean), is an explicit objective.  SCE is compared against other modified uneven-aged approaches, including single-tree selection and group selection with legacy tree retention within larger gaps (0.05 ha mean).  The study is replicated at two research areas in Vermont, U.S.A.  Manipulations and controls were applied to 2 ha units and replicated four times per treatment.  Structural objectives for SCE included multi-layered canopies, elevated large snag and downed coarse woody debris (CWD) densities, variable horizontal density (gapiness), and re-allocation of basal area to larger diameter classes.  Accelerated growth in larger trees is also promoted through crown release, achieved by clustering small gaps around selected dominant canopy trees.  On some units, small gaps, root wads, pits, and downed CWD are created simultaneously by pulling or pushing trees over mechanically.  Vegetation data were collected over two years pre-treatment and seven years post-treatment.  Salamander and fungi surveys were conducted several years post-harvest.

Results/Conclusions

Crown release in dominant canopy trees distributed adjacent to or within small gaps is predicted to increase large tree recruitment over the next 50 years based on simulation modeling.  Tree regeneration has been strongly influenced by competition with ferns and shrubs.  All treatments were successful in maintaining overall richness and/or abundance of understory plants, salamanders, and fungi.  Statistical model results show that over time, understory plant responses were strongly affected by overstory treatment and less influenced by soil chemistry and drought stress. However, diversity for sensitive, late-successional herbaceous plants increased significantly in SCE units and decreased significantly in the semi-open canopied conditions within group selection units.  Fungi and salamander responses were strongly associated with microsite characteristics, particularly CWD, and increased significantly (p > 0.05) under SCE, but showed no statistically significant decrease in gaps created by group selection in comparison to controls.  The results suggest that a variety of small gap and group selection with retention techniques can help maintain a range of biodiversity in managed forests.