Bark stripping damage on Norway spruce (Picea abies L. Karst.)
Many European Cervidaes are known to strip bark. Bark stripping damage is reported for red deer (Cervus elavus L.), moose (Alces alces L.) (Vasiliauskas 1998; Arhipova et al. 2015), sika deer (Cervus nippon L.) (Welch et al., 1988), and fallow deer (Dama dama L.) (Gill, 1992). Roe deer (Capreolus capreolus L.) and reindeer (Rangifer tarandus L.) do not strip bark in winter, but forage small trees and grass or lichens, respectively (Gill, 1992).
The red deer is the most widespread bark stripping cervid in Europe, resident in all countries, absent only in northern Fenno-Scandia (Lovari, et al., 2019). Similarly, the moose is common, but is restricted to the northern part of Europe (Hundertmark, 2016). Sika deer and follow deer have been introduced from Japan and Turkey, respectively (European Mammal Assessment Team, 2007), and the sika deer is now resident in Great Britain, Czech Republic, and other Central European countries. Populations of all Cervidaes, native or introduced, are stable or increasing in Europe (European Mammal Assessment Team, 2007; Harris, 2015). In particular, populations of red deer (Côté et al., 2004; Burneviča et al., 2016; Candaele et al., 2021) and sika deer (Vacek et al., 2020) have been reported to increase rapidly in many areas, causing severe damage to forest stands (Čermák & Strejček, 2007; Vacek et al., 2020), with sika deer causing more severe damage than the native red deer in some areas (Vacek et al., 2020).
Moose is the largest of the Cervidaes with a shoulder height of 150–220 cm; shoulder height for red deer varies between 84 and 110 cm and is considerably smaller than that of moose. The other cervids only grow up to 100 cm (sika deer: 65–95 cm; fallow deer: 80–100 cm) (Flamm et al., 1992). Accordingly, the wound height, width, and length inflicted by different cervid species also differ significantly with wound heights above ground and sizes being higher and larger for the larger animals (Gill, 1992; Månsson and Jarnemo, 2013; Arhipova et al., 2015; Vacek et al., 2020). Bark stripping wounds by red deer were found at a height above ground of 97–110 cm (Arhipova et al., 2015), corresponding to red deer shoulder height. In scientific literature, wound direction was reported to be arbitrary (Welch et al., 1988) however, in steep terrain wounds, seem to be mostly found on the uphill side of the stem, but wound position in steep terrain has not been documented before.
Preferred species are Norway spruce (Picea abies (L.) Karst.) (Čermák et al., 2004; Vospernik, 2006; Cukor et al., 2019a, b, Vacek et al., 2020; Candaele et al., 2021), lodge pole pine (Pinus contorta Douglas ex Loudon) (Arhipova et al., 2015), Douglas fir (Pseudotsuga menziesii), (Candele et al., 2021), European ash (Fraxinus excelsior L.), (Vasiliauskas & Stenlid, 1998; Vospernik, 2006; Candaele et al., 2021), linden (Tilia spp.) (Fehér et al., 2016), maple (Acer spp.) (Féher et al., 2016), sweet chestnut (Castanea sativa Mill.) (Vospernik, 2006), and Sorbus spp. (Vospernik, 2006), whereas thick-barked species are often avoided (Gill, 1992; Vospernik, 2006; Candaele et al., 2021). Open wounds exposed 4–4355 cm2 of sapwood (Čermák & Strejček, 2007; Arhipova et al., 2015), and repeated assessment or dendrochronological analysis showed that the same tree was debarked several times (Gill, 1992; Arhipova et al., 2015; White, 2019; Nagaike, 2020), with an average of 1.8–2 wounds per tree (Arhipova et al., 2015). Wounds were inflicted in stands aged 8–10 to 40–60 years (Čermák et al., 2004; Fehér et al., 2016; Cukor et al., 2019a, b; Vacek et al., 2020; Candaele et al., 2021) depending on site productivity and deer abundance (Candaele et al., 2021), with the age of vulnerability corresponding to mean diameters of 6–24 cm for thin-barked species (Vasiliauskas & Stenlid, 1998; Arhipova et al., 2015; Vacek et al., 2020) and a considerably shorter period of sensitivity and smaller diameters for thick-barked species (Gill, 1992; Vospernik, 2006; Candaele et al., 2021).Wound size increases with diameter at breast height (DBH) (Arhipova et al., 2015; Vacek et al., 2020), and wounds inflicted in summer are reported to be larger than winter wounds (Månsson & Jarnemo, 2013; Candaele et al., 2021).
Food scarcity in winter is thought to be the main reason for winter bark stripping (Gill, 1992) The phloem of trees provides a nutrient-rich food that lacks toxins and feeding deterrents, and bark stripping often occurs in late winter and early spring, when the phloem is more nutrient rich (White, 2019). Reasons for summer bark peeling are less clear. Summer bark peeling has a beneficial effect for the digestion in the rumen (Månsson & Jarnemo, 2013) and has also been related to water scarcity (König, 1968). The two types of wounds can be easily distinguished, since winter bark stripping wounds show teeth marks. In winter, the bark is firmly attached to the stem, whereas in summer, the bark is more loosely attached, and thus, large stripes of bark can be removed from the trees (Gill, 1992; Candaele et al., 2021).
Bark stripping greatly damages saplings and trees by destroying water conductivity and increasing fungal infection (Nagaike, 2020; Vacek et al., 2020). Wounds with more than 65% of stem circumference resulted in increased tree mortality (Nagaike, 2020), and recent studies have also evidenced a significantly decreased productivity and increased sensitivity of stripped trees to drought and heatwaves (Cukor et al., 2019a, b; Vacek et al., 2020). Another important consequence of bark stripping is fungal infection and subsequent wood decay (Vasiliauskas, 1998; Vasiliauskas & Stenlid, 1998; Čermák et al., 2004; Čermák & Strejček, 2007; Butin, 2011; Vasaitis et al., 2012; Rönnberg et al., 2013; Arhipova et al., 2015; Burneviča et al., 2016).
Infection spreads vertically along the tree (Arhipova et al., 2015; Burneviča et al., 2016) and is linearly correlated with wound length, whereas the spread of wound decay beyond the wound margin is limited (Vasiliauskas & Stenlid, 1998; Mäkinen et al., 2007; Arhipova et al., 2015). Decay in the Norway spruce was reported to range from 2.7–62.5 cm·year−1, finally reaching an extent of 1.5–6 m (Čermák et al., 2004; Čermák & Strejček, 2007; Rönnberg et al., 2013; Vacek, et al., 2020) depending on tree age and the age of the wounds (Čermák & Strejček, 2007). Further, decay was reported to increase with site fertility (Vasaitis et al., 2012; Burneviča et al., 2016). Even though wound sizes for different tree species vary a little, pathogenic consequences vary between species, since fungi colonizing wounds are tree species specific (Mäkinen et al., 2007; Metzler et al., 2012) and have different spread rates (Vasiliauskas, 1998; Mäkinen et al., 2007; Metzler et al., 2012).
For example, Mäkinen et al. (2007) isolated different fungi from the Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) and Metzler et al. (2012) from the Norway spruce and silver fir (Abies alba Mill.). In the Scots pine, a lower proportion of trees were decayed than in the Norway spruce (Mäkinen et al., 2007). Similarly, infection rate for the Norway spruce with pathogens was 4.6 times higher than for silver fir (Metzler et al., 2012). Fungal infection and limited water uptake further increase stand sensitivity to wind and snow damage (Vasiliauskas, 1998; Čermák et al., 2004; Čermák & Strejček, 2007; Burneviča et al., 2016; Snepsts et al., 2020). Damaged stands had a 1.68 times higher probability for wind damage (Snepsts et al., 2020), and pulling experiments evidenced a lower resistance of bark-stripped trees to pulling force (Krisans et al., 2020). However, contrary to expectations, most trees in the pulling experiment were uprooted, and only one bark-stripped tree broke at the wound (Krisans et al., 2020).
In Austria, bark stripping by red deer prevails, whereas wounds inflicted by other cervids are rare (Völk, 1997). The tree species most affected by bark stripping are the Norway spruce, European ash (Fraxinus excelsior L.), sweet chestnut (Castanea sativa Mill.), and Sorbus spp. (Vospernik, 2006). Of these tree species, bark stripping damage of the Norway spruce is considered most important, because of its high economic importance for Austrian forestry, since it constitutes 60.4% of the standing timber volume (BFW, 2021). While the Austrian National Forest Inventory (BFW, 2021) records the presence/absence of bark stripping wounds, wound size, position, and the spatial distribution of damage within stands is not assessed. In particular, within stand spatial patterns have been little investigated in detail before, although bark stripping damage has been reported to be clustered (Gill, 1992).
Further, bark stripping is more common in areas with higher agriculture (Månsson & Jarnemo, 2013; Sun et al., 2020) and increases with the distance to forest roads (Kiffner et al., 2008). Moreover, a substantial increase in damage is found around diversionary feeding stations, constructed to protect forestry and natural habitats (Putman & Staines, 2004).
Deer animals and bark stripping wounds are a frequent source of debate between wildlife managers and forest managers (Reimoser & Gossow, 1996). On the one hand, bark stripping causes important economic damage and seriously devaluates timber (Čermák et al., 2004; Čermák & Strejček, 2007; Vacek et al., 2020; Candaele et al., 2021), and forest managers have a high interest in controlling red deer populations (Kiffner et al., 2008; Candele et al., 2021). On the other hand, deer animals are a part of the forest ecosystem and are highly valued for hunting and aesthetic reasons (Ehrhart et al., 2022). An objective assessment of bark stripping damage is the basis to settle these differences. For damage assessment, between-stand and within-stand variation of bark stripping wounds is important. Wound size is an important factor for the economic evaluation since the decay extent is closely linked to it (Arhipova et al., 2015; Vacek et al., 2020).
Hypotheses and research questions analysed in this paper
The aim of this study was to analyse wound size, position, and its spatial patterns. Specifically, the following hypotheses were analysed:
Wound size is larger in summer than in winter.
Winter bark stripping is more common than summer bark stripping.
Wounds are larger on larger trees.
Wounds are located at the uphill side of the trees.
Wound height is animal specific and is therefore at a constant height above ground.
Wound sizes are larger on trees close to winter feedings and forest roads.
Wound sizes are spatially clustered.