Assessing the efficacy of natural disturbance-based harvest gaps at restoring old-growth composition and structure

Background/Question/Methods

Over the past two decades, an increasing emphasis has been placed on developing forest management prescriptions that are modeled after the severity and frequency of natural disturbances for a given region.  These approaches have been recommended as a coarse-filter strategy for maintaining native biodiversity in managed landscapes, as well as a potential tool for restoring old-growth structural and compositional attributes to the simplified second-growth forests that predominate over most areas. The objective of this research was to determine the effects of several natural disturbance-based restoration treatments and browsing by white-tailed deer on tree regeneration, understory community composition, and structure in second-growth northern hardwood forests in northern Wisconsin. Community composition and tree regeneration were measured in stands with six different silvicultural treatments replicated across three large study areas (> 50 ha). Treatments consisted of a combination of two levels of coarse woody debris and three canopy gap treatments designed to emulate patterns of natural canopy disturbance: small gaps (10.7m diameter), medium gaps (18.3 and 24.4m diameter), and a meso-scale wind disturbance treatment consisting of 0.4 and 1.2 ha shelterwoods. All treatments included multiple small (25 m²) deer exclosures to examine the impacts of deer herbivory.

Results/Conclusions

Assessment of cover by herbaceous plants and seedlings indicated that canopy gap treatments had a much larger effect on understory community composition than browsing by deer, whereas there was no effect of coarse wood levels. Species richness was highest in the small gaps treatment and lowest in the control stands, possibly reflecting the dominance of larger gap treatments by grasses and Rubus spp. Seedling density increased with increasing canopy removal and large canopy gaps and the meso-scale wind disturbance treatment served to recruit historically important mid-tolerant species, including Betula alleghaniensis.  Sapling dynamics followed a similar pattern; however, herbivory impacts on sapling composition and structure were much more pronounced than in the seedling and understory layers.  In particular, sapling densities of mid-tolerant tree species were significantly greater in exclosures illustrating the pervasive effects sustained deer browsing may have on limiting opportunities for restoring compositional diversity to the canopy layer of second-growth systems. These results underscore the utility of natural disturbance-based treatments at increasing the complexity of second-growth tree regeneration and understory communities and the importance of accounting for herbivory impacts on treatment responses.