Harvest block aggregation as a driver of intensive moose browsing pressure on hardwood regeneration in a temperate forest

Abstract

In many regions of the world, browsing pressure by ungulates has increased over the last century, often reaching unsustainable levels. Overbrowsing by ungulates is threatening biodiversity and causing delays in the establishment of forest regeneration. Moose (Alces alces) is considered as an ecological engineer, as it can cause severe impacts on stand structure, composition, quality, and succession through overbrowsing. In conifer-dominated stands, moose browsing pressure has been shown to vary with the distribution of forage and cover patches across the landscape, but such spatial relationships have not been explored in hardwood-dominated landscapes, where intensive forest management can create clustered patches of good-quality forage and cover, ideal conditions for wintering moose. We investigated the influence of local and landscape variables on browsing by North American moose (Alces alces americana) on commercial and non-commercial woody regeneration in an intensively managed forest landscape of northwestern New Brunswick, Canada. We predicted that browsing pressure at the local scale (vegetation sampling plot) would increase with moose winter counts in the surrounding landscape (1 to 3 km radius). We modeled moose counts as a function of environmental features and road density, using a quasi-Poisson regression. According to an aerial survey (25 km2 sampling plots), moose counts increased with the proportion of forage patches (disturbed stands < 20 years old), whereas they decreased with the proportion of mixed mature stands > 80 years old, which offer both forage and cover. We extrapolated this model to a 2770 km2 study area and used predicted counts as an independent variable to assess its influence on browsing pressure. In 720 plots of 4.15 m2, browsing pressure was quantified using occurrence (i.e. proportion of seedlings/saplings with at least one twig browsed during the previous winter) and intensity (i.e. mean proportion of twigs browsed per stem) on commercial and non-commercial tree seedlings and saplings. We explained variation in occurrence and intensity using zero-inflated models with snow precipitation and local vegetation variables as covariates. Browsing pressure increased with predicted moose counts within a 3 km radius, with snow precipitation and with the proportion of coniferous trees in the canopy, whereas it decreased with sapling density. These results highlight the importance of seedlings and saplings in moose winter diet and confirm the attraction of moose toward recent (<20-year-old) harvest blocks during winter. Therefore, we should avoid aggregating harvest blocks to decrease their attractiveness to moose and browsing pressure on natural hardwood regeneration.