Influence of Tree Species and Size on Bark Browsing by Large Wild Herbivores

Abstract

Although an important part of the ecosystem, large wild herbivores (LWH), especially red deer (Cervus elaphus L.), cause significant damage to economically valuable timber in forests of Central Europe. Recent work has demonstrated that less valuable softwood broadleaved trees can act as “biological control” that helps reduce bark browsing on more valuable trees in a mixed stand. To better understand the factors that influence how much bark area and mass are removed by LWH from these broadleaved trees, we took advantage of a novel “natural” experiment that occurred after a breach in a herbivory exclosure surrounding a 10-year old mixed broadleaved/conifer stand in the Western Carpathians in north-western Slovakia. We measured the area of old (up to 2 years previously) and new browsed patches on stems of common aspen (Populus tremula L.), common rowan (Sorbus aucuparia L.) and goat willow (Salix caprea L.), and their position along the vertical profile of the stem. The browsed bark area (cm2) was then converted to the bark mass (g) removed and the proportion of browsed bark to total bark (%) using conversion equations. Our models demonstrated that the amount of bark removed was influenced by tree species, stem diameter, age of browsing (old vs. new), and stem section along the vertical profile. LWH removed the most bark area from willow but the most bark mass from aspen because aspen had thicker bark than the other tree species. Bark browsing was greater on trees > 6 cm basal diameter. The distribution of bark browsing along the vertical profile was symmetrical (unimodal) with maximum intensity at 101–125 cm from the ground, which corresponds with the height most optimal for feeding by red deer. However, previous browsing in 2019 and 2020 caused new browsing on willow in 2021 to be focused in stem sections lower (51–75 cm) and higher (126–150 cm) than that optima. By quantifying browsing patterns and the amount of bark that is accessible to LWH for forage on the most attractive softwood broadleaved trees, our work will contribute to developing better methods for protecting commercially important species such as European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L. Karst.) in areas of Central Europe that are greatly affected by increasing population density of LWH, especially red deer.

From the simulation of forest plantation dynamics to the quantification of bark-stripping damage by ungulates

“…We assumed that very few bark-stripping damage occurred in plantations younger than 8 years old (the simulations started in stands 8 years old) in line with several studies (Vospernik, 2006;Jerina et al, 2008;Gill et al, 2000;Konôpka et al, 2022). At 8 years old, the mean tree diameter ranged between 3.2 cm (SI = 21m) and 6.0 cm (SI = 33m), and so it seems reasonable to assume that bark-stripping damage seldom occurs on such small trees because their stem will not be stiff or accessible enough (Gill et al, 2000).…”

“…In addition, we assumed that only 5% of total bark stripping damage occurred in stands younger than 8 years and older than 36 years. This assumption seemed reasonable because most damage has been observed, in different independent studies, in stands 8-36 years old (Vospernik, 2006;Jerina et al, 2008;Gill et al, 2000;Konôpka et al, 2022). With these assumptions (distribution modes, proportion of damage in stands 8-36 years old), we computed the parameters of the two corresponding probability density functions ( f summer and f winter in Eq.…”

“…However, such plantations have been increasingly affected by climate change, warmer temperatures, bark beetle outbreaks (Jönsson et al, 2009) and droughts (Lévesque et al, 2013). Nevertheless, the extent of such damage could be reduced if spruce is progressively replaced by other tree species and if winters become milder (Ligot et al, 2013;Candaele et al, 2021;Konôpka et al, 2022). The combined effects of climate changes and changes in forest structure and composition need to be addressed and clarified in future studies.…”

“…This rate is correlated to deer abundance but the relationship saturates beyond a certain ungulate density (Ligot et al, 2013;Candaele et al, 2021). The other factors that can affect the frequency of bark-stripping damage are winter harshness, forest composition, distance from roads and urban areas, canopy opening, availability of other food resources such as beechnuts and acorns, and topographic position (Ligot et al, 2013;Candaele et al, 2021;Vospernik, 2006;Widén et al, 2022;Konôpka et al, 2022). The damage might occur both in winter and summer (Gill, 1992b;Widén et al, 2022).…”

Bark Browsing and Recovery: A Comparative Study between Douglas Fir and Silver Fir Species in the Western Carpathians

“…This bark browsing occurred over three winter seasons: 2019/2020, 2020/2021, and 2021/2022. Previously, we conducted a similar study at the same site for common aspen (Populus tremula L.), common rowan (Sorbus aucuparia L.), and goat willow (Salix caprea L.; see Konôpka et al [31]). We found that the most intensive BATA es was in goat willow (14.5%), and the least intensive was in common rowan (5.1%).…”