Spatiotemporal dynamics of natural tree regeneration in unmanaged subalpine conifer forests with high wild ungulate densities

Increasing wild ungulate populations in the northern hemisphere led to extensive research on their influence on forest regeneration. According to the results of different studies, high ungulate densities may alter forest structure and composition, as well as inhibit forest regeneration. However, little is known about the long-term impact of large herbivores on forest dynamics at the landscape scale. Therefore, we studied forest regeneration in conifer forests of the highly protected Swiss National Park. We assessed sapling densities and apical shoot loss by repeated systematic sampling in 2003 and in 2011-2012 in two valleys featuring exceptionally high densities of wild ungulates of up to 86 individuals per vegetation-covered square kilometre during summer and 28 individuals per vegetation-covered square kilometre during winter. Mean apical shoot loss ranged between 2.9% and 10.6% depending on study site and assessment year. In addition to the increasing sapling densities, these results suggest that ungulates had little impact on tree establishment in the studied subalpine conifer forests. Additionally, sapling diversity was higher on plots repeatedly influenced by ungulates (apical shoot loss), suggesting rather positive effects of ungulates on sapling diversity. Habitat containing a mixture of forest and grassland without livestock or human disturbance provides a high-quality environment for ungulates, which likely explains the low impact of ungulates on forest regeneration in our study area.