Different facets of tree sapling diversity influence browsing intensity by deer dependent on spatial scale

Abstract

Browsing of tree saplings by deer hampers forest regeneration in mixed forests across Europe and North America. It is well known that tree species are differentially affected by deer browsing, but little is known about how different facets of diversity, such as species richness, identity, and composition, affect browsing intensity at different spatial scales. Using forest inventory data from the Hainich National Park, a mixed deciduous forest in central Germany, we applied a hierarchical approach to model the browsing probability of patches (regional scale) as well as the species‐specific proportion of saplings browsed within patches (patch scale). We found that, at the regional scale, the probability that a patch was browsed increased with certain species composition, namely with low abundance of European beech (Fagus sylvatica L.) and high abundance of European ash (Fraxinus excelsior L.), whereas at the patch scale, the proportion of saplings browsed per species was mainly determined by the species’ identity, providing a “preference ranking” of the 11 tree species under study. Interestingly, at the regional scale, species‐rich patches were more likely to be browsed; however, at the patch scale, species‐rich patches showed a lower proportion of saplings per species browsed. Presumably, diverse patches attract deer, but satisfy nutritional needs faster, such that fewer saplings need to be browsed. Some forest stand parameters, such as more open canopies, increased the browsing intensity at either scale. By showing the effects that various facets of diversity, as well as environmental parameters, exerted on browsing intensity at the regional as well as patch scale, our study advances the understanding of mammalian herbivore–plant interactions across scales. Our results also indicate which regeneration patches and species are (least) prone to browsing and show the importance of different facets of diversity for the prediction and management of browsing intensity and regeneration dynamics.

Stability of species and provenance performance when translocated into different community assemblages

“…The treatment differences in susceptibility of the focal eucalypt species to deer damage were mainly due to greater susceptibility in the presence of the shrub C. aculeata (which was still well‐established 2 years after planting). Species‐specific browsing of plants by deer is well documented (Kirby ; Bee et al ) and there is evidence for indirect, associational damage to less susceptible species (Côté et al ; Ohse et al ). While not formally assessed, C. aculeata was observed to be highly browsed by deer which likely led to its marked decline by 6 years, and the increased presence of deer in these plots could have caused collateral damage to the focal eucalypt species.…”

How do trees respond to species mixing in experimental compared to observational studies?

“…In the inventory, and even more in the exploratory approach, the juvenile trees are exposed to pressure by game species, which are known to be affected by tree species richness (Milligan & Koricheva, 2013;Ohse, Seele, Holzwarth, & Wirth, 2017). Except for the Satakunta site, all tree diversity experiments were fenced to exclude game species and safeguard the successful establishment of all planted trees.…”

“…Except for the Satakunta site, all tree diversity experiments were fenced to exclude game species and safeguard the successful establishment of all planted trees. In the inventory, and even more in the exploratory approach, the juvenile trees are exposed to pressure by game species, which are known to be affected by tree species richness (Milligan & Koricheva, 2013;Ohse, Seele, Holzwarth, & Wirth, 2017).…”

Exclusion of large herbivores affects understorey shrub vegetation more than herb vegetation across 147 forest sites in three German regions

“…The absence of browsing damage allowed for unhindered growth of the shrubs that were already present and increased the Shannon diversity of shrubs. This most likely indicates plant-specific preferences of herbivores, most likely roe deer, which has been shown to have more selective browsing habits than other large herbivores [21,49]. Such selective browsing leads to specific shrub species suffering more than others and those browsing-sensitive shrubs increase in cover when herbivores are excluded.…”