Ungulate Species and Abundance as well as Environmental Factors Determine the Probability of Terminal Shoot Browsing on Temperate Forest Trees

Abstract

Ungulate browsing is a major factor influencing tree regeneration. However, it is unclear if the observed increase in ungulate abundance in Central Europe implies increased browsing, and which other factors influence the incidence of browsing. We investigated the impact of forty variables (site, climate, forest and ungulates) on the probability of leader shoot browsing of six tree species which are frequent in Switzerland. The analysis was based on a large dataset including 49 monitoring areas, each containing 25–64 circular plots, in which 10 to 130 cm tall seedlings were repeatedly assessed. Browsing probability was estimated for each plot and year by mixed effects logistic regression and used as a response in random forests to disentangle the influence of the explanatory variables. Browsing probability was positively correlated with ungulate density measures (number culled by hunting or found dead) for all six tree species. Where beyond roe deer, some red deer and/or chamois were present, the browsing probability was higher. Small timber tree stands had less browsing than young growth and thicket stands. Seedlings tended to be more frequently browsed in stands with >80% canopy shading. Browsing increased with increasing understory cover, independent of vegetation category. In conclusion, browsing is a multifactorial phenomenon and ungulate density estimates alone do not explain the whole browsing probability.

Light can modify density‐dependent seedling mortality in a temperate forest

“…However, we cannot rule out the possibility that the growth rather than the survival of tall seedlings is reduced by conspecific inhibition (Brown et al, 2020), especially over the relatively short two-year period of our study. In contrast to our initial expectations, the survival of tall seedlings appears to be inhibited by conspecific and heterospecific tree neighbours, suggesting that size-asymmetric competition with trees and/ or density-responsive generalist enemies becomes important once seedlings are taller (Dickie et al, 2005;Kupferschmid et al, 2020;Wulantuya et al, 2020). As the survival of tall seedlings is also reduced under low-light conditions irrespective of tree neighbours, our findings support variation in density-dependent effects and abiotic niche partitioning across plant life stages (Lasky et al, 2015;Zambrano et al, 2019;Yao et al, 2020).…”

Assessment of ungulate effects on trees in the canton of Vaud: comparison of data from the Swiss NFI2 and NFI4 on ungulate browsing, fraying and bark stripping

“…Other species with a high browsing intensity in Switzerland are maple (Acer spp. ), rowanberry (Sorbus aucuparia) and silver fir (Abies alba) [15], for which segregation has been found in some regions using data sources other than NFIs, such as indicator areas [12]. In the western Plateau, which includes part of the canton of Vaud, the browsing intensity on silver fir was lower than in all other economic regions during the NFI1 (1982)(1983)(1984)(1985)(1986)) and NFI2 (1993-1995) periods (Figure 1) [18].…”

“…Since a correlation between ungulate population size and browsing frequency has been observed in many studies [12], an increase in browsing in the last decades most likely also occurred in the canton of Vaud. The Swiss National Forest Inventory (NFI) is the only dataset including assessments of browsing, fraying and bark stripping that is available from the last decades for the whole canton of Vaud.…”

“…In both NFIs the large seedlings (height class 40-129 cm) had significantly higher browsing probabilities than the small seedlings (height class 10-39 cm; Figure 43). Browsing by ungulates has previously been observed to mostly occur at an intermediate height between the ground and the full reach of red deer in other regions in Switzerland [12] and in other countries [40,41]. This is probably caused by the higher visibility of the large seedlings, as they stand out from the herbaceous vegetation.…”

“…In the Plain (hunting zone) or Plateau (production region), the browsing intensity increased over time for silver fir and oak. Nevertheless, silver fir still (see Figure 1 and [18]) seems to be browsed less in the canton of Vaud than in many other regions in Switzerland [12,15]. Overall, oak and rowan berry were browsed the most, with a browsing intensity in NFI4 of around 35%, followed by ash and maple, then silver fir, and finally spruce and beech (about 1-3.3%).…”