Initial effects of structural complexity restoration treatments and deer browsing on ground-layer community composition and tree regeneration in northern hardwood forests

Abstract

As researchers realize the value of old-growth forests and their unique attributes and dynamics, managers have begun developing management regimes aimed at restoring old-growth characteristics in forests managed for wood products. However, changes in these forests since European settlement, especially increased population of white-tailed deer (<italic>Odocoileus virginianus<italic> Zimmerman) may have important implications for forest composition and regeneration. The objective of this research was to determine the initial (three- and four-year) effects of several old-growth structural restoration treatments and browsing by white-tailed deer on tree regeneration and understory community composition in northern hardwood forests in northern Wisconsin, USA. 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 overstory gap treatments designed to emulate patterns of natural gap and mesoscale canopy disturbance: small gaps (10.7m diameter), large gaps (18.3 and 24.4m diameter), and a mesoscale wind disturbance treatment consisting of 0.4 and 1.2 ha shelterwoods. All treatments included multiple small deer exclosures to examine the impact of deer herbivory. Assessment of cover by herbaceous plants and seedlings indicated that overstory treatments had a 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 gap treatment and lowest in the control stands, possibly reflecting the increased dominance by a few ruderal and exotic species in treatments with increased canopy openness. Richness was also significantly lower inside exclosures than outside in some treatments, although browse-sensitive <italic>Trillium<italic> spp. were largely restricted to exclosures. Post-harvest seedling density increased with increasing overstory removal, with the mesoscale wind disturbance treatment containing the highest seedling density after three years. While browse protection generally did not have a significant effect on overall density of regeneration, several species, especially <italic>Betula alleghaniensis<italic> benefited from protection from deer browse. These initial results underscore the utility of natural disturbance-based treatments at increasing the complexity of second-growth communities and the importance of accounting for herbivory impacts on treatment responses.