Sapling regeneration and recruitment in small canopy gaps formed by the death of dominant trees is an important process affecting forest structure and composition in temperate mesic forests. Although these gaps form naturally due to mature tree mortality, in many forests, tree removal from selection harvesting has become the primary mode by which these gaps are created. A multitude of factors can influence the density and composition of seedlings (<1 m tall) and saplings (1-7m tall) regenerating in these harvest gaps. Our study explores some of these important gap- to landscape-scale variables in 317 five to fifteen year old selective harvest gaps located in 55 northern hardwood stands across a 400,000 ha managed forested area in the Western Upper Peninsula of Michigan. We quantified gap-scale light availability and competition from understory vegetation, gap- to stand-scale seed source availability, stand-scale habitat type (proxy for site moisture and nutrient conditions), stand- to landscape-scale winter white-tailed deer (Odocoileus virginianus) density (estimated with pellet counts), and landscape-scale snow depth for its potential impact on deer densities.
Results/Conclusions
We observed a complete lack of Acer saccharum saplings, a shade tolerant species preferred by deer, in 48% of 154 m2 plots centered in harvest gaps. Although the direct relationships between the density and composition of saplings and stand-level winter deer density generally showed high variance, browse pressure was evident across our study area with 92% of Tsuga canadensis seedlings browsed in their first year following planting. At the landscape scale, A. saccharum sapling density increased and deer density decreased with latitude, whereas the deer non-preferred browse species Ostrya virginiana showed no trend with latitude. Confounding these spatial trends is the geographic distribution of habitat types. Higher sapling densities occurred on lower-fertility more northern habitat types compared to higher-fertility more southern habitat types. Competing vegetation cover was negatively related to sapling density, and also varied between habitat types. Counterintuitively, our results show that gap size was not related to sapling density for either all species or shade intolerant species alone. Our results suggest that landscape-scale variation in habitat types and deer browse pressure, regardless of gap size, affects the density of regenerating saplings following selective harvesting.
