Cloven-hoofed game increasingly harms younger spruce stands by browsing and peeling off the bark
In recent decades, the population of wild ungulates has increased across Europe, including the Czech Republic. This rise is evident not only in native species, such as wild boar and red deer, but also in introduced species like sika deer, fallow deer, and mouflon. This trend has significantly negative impacts on forest ecosystems.
Photo: On the left, there is older damage to a young Norway spruce, which, if severe enough, can lead to the death of the tree’s terminal trunk. On the right, there is newly formed damage to the tree caused by barking. Author: Jan Cukor
The growing influence of cervids is diminishing the species diversity within forest ecosystems, particularly by eliminating rare and desirable tree species. The extensive cutting of both artificial and natural regeneration in forests, shrubs, and herbs fundamentally alters the structure and composition of vegetation, ultimately slowing the growth and development of forest stands.
Damage caused by browsing and peeling, primarily by red deer, sika deer, and mouflon, poses significant challenges to the stability of forest stands. This issue is particularly evident in spruce trees, which suffer from a reduction in mechanical stability due to the secondary spread of fungal pathogens. The peeling and browsing of trees often lead to subsequent decay, especially with the development of the bleeding conifer crust (Stereum sanguinolentum) and the basidiomycete fungus Heterobasidion annosum. These fungi primarily affect the most valuable basal part of the stem, further compromising the health of the forest.
Damage to trees can lead to several consequences. One of the main effects is a reduced growth in bark thickness for trees that have been barked or gnawed. Additionally, damaged trees tend to be more vulnerable to extreme climate conditions, particularly during repeated dry spells and periods of high temperatures. These climate extremes are becoming more common due to ongoing climate change.
Photo: Norway spruce stands that were severely damaged during their early years are now mature. The lower parts of their trunks have suffered from rot, making the trees more susceptible to breaking, which significantly reduces the mechanical stability of the stand. Author: Jan Řezáč
Damage caused by game not only reduces the resistance of trees to biotic and abiotic factors but also leads to significant economic losses. For instance, in the Ore Mountains, spruce trees that were heavily damaged by browsing and bark peeling from red deer exhibited a 50% reduction in stem volume. Likewise, in the Pilsen region, damage from red deer resulted in a total production loss of 71% compared to healthy tree stands.
The effects of browsing and bark peeling are closely related to when the young spruce stands were initially damaged. To investigate this, scientists from VÚLHM, v. v. i., and ČZU in Prague conducted research to assess the extent of damage in young spruce stands across four key locations in the Ore and Lusatian Mountains (Krušné and Lužické hory).
They later published their findings in an article titled “Bark stripping damage in Norway spruce stands: confirmation of shift to younger stands” (Škody ohryzem a loupáním ve smrkových porostech: potvrzení posunu do mladších porostů), which appeared in the journal Forest Research Reports, issue 4 of 2024. This research was conducted as part of the project “Determination of the Impact of Wildlife and the Degree of Damage to Forest Stands Based on Growth Analyses of Main Commercial Tree Species,” Stanovení vlivu zvěře a míry poškození na lesní porosty na základě růstových analýz hlavních hospodářských dřevin) supported by the Grant Service of the Forests of the Czech Republic (GS LČR).
Photo: Heavy damaged young spruce tree is protected against rot. Ore Mountains, author Jan Řezáč
Scientists evaluated the damage to young spruce stands (aged 7–26 years) in two distinct locations within the Sudetes system, each located in two consecutive hunting grounds.
The first area that was assessed is located in the Lusatian Mountains, specifically near Česká Kamenice and the village of Chřibská. This area falls under the jurisdiction of the Rumburk Local Government Area (LČR, s. p.). The second assessed area is situated in the Krušné Mountains, managed by the Klášterec nad Ohří Local Government Area (LČR, s. p.), in the vicinity of the village of Kalek and further around Vejprt. In the surveyed areas, the primary cloven-hoofed game species include red deer, roe deer, and wild boar. Other species of cervids are relatively rare, and mouflon do not inhabit this region.
In Česká Kamenice (LS Rumburk), scientists evaluated a total of 206 Norway spruce trees and found damage in 69 of them, which represents 33.5% of the sample. The damaged trees had an average thickness of 4.6 cm for old damage, while new damage was observed in trees with an average thickness of 4.0 cm. In contrast, the average thickness of undamaged trees was only 2.0 cm. This indicates that the damaged trees had reached sizes that are appealing to red deer, which cause damage through barking and gnawing. The circumferential damage to the spruces varied between 26.2% and 32.1%, with an average damage length ranging from 52.6 cm to 56.3 cm.
Photo: Norway spruce stands that were heavily damaged in their early years are now mature. The lower parts of the stems have rotting damage, making the trees prone to breakage. As a result, the mechanical stability of the stand is significantly reduced. Author: Jan Řezáč
Scientists observed similar results in the nearby Chřibská locality. Out of 150 trees, 36 Norway spruce individuals (23.4%) were damaged, which is the lowest percentage among all the studied localities. The undamaged trees had the smallest average listed thickness at 3.7 cm. New damage was found on trees with an average thickness of 5.4 cm, while old damage occurred in trees with a thicker average of 6.1 cm. The circumferential damage to spruce trees ranged from 27.3% to 31.4%, and the average length of the damage was shorter compared to the Česká Kamenice locality, measuring between 32.2 cm and 41.0 cm.
At the Kalek site, the average diameter of freshly damaged spruce trees was 5.7 cm, while older damage was observed on trees with an average diameter of 6.7 cm. Undamaged trees had the lowest average diameter at 2.9 cm. Out of a total of 228 spruces, 119 trees, representing 52.2%, were found to be damaged. This site also recorded the highest level of circumferential damage across all monitored hunting grounds, with new gnawing and peeling affecting 45.1% of the trees. Additionally, the highest point of damage on the trunks was noted to be 108.2 cm from the ground. The length of the damage ranged from 44.0 cm to 56.3 cm.
Photo: Norway spruce stands that were heavily damaged in their youth are now mature. The lower parts of the stems have rotting damage, making the trees more prone to breakage and significantly reducing the mechanical stability of the stand. Author: Jan Řezáč
At the Vejprty site, the highest level of damage recorded was 58.3%, with 116 out of 199 trees affected. In contrast, the extent of peripheral damage was relatively low, ranging from 20.2% to 24.0%. Additionally, the centre of the damage was located at a height of 74.0 cm to 90.0 cm above the ground. New damage was observed in trees with an average thickness of 5.5 cm, while older damage occurred in trees with an average thickness of 5.7 cm. Trees considered healthy at this site had an average thickness of 3.5 cm.
At the Česká Kamenice site, Norway spruce individuals exhibited newly observed damage starting from the thickness class of 1–2 cm. The smallest damaged specimen measured only 1.7 m in height and had a thickness of 1.5 cm. The highest frequency of new damage in this area was found in the third thickness class, which ranges from 2 to 3 cm. In contrast, at the other sites, new damage due to gnawing and peeling was noted beginning at the thicknesses of 2 cm and 3 cm. At the Chřibská and Vejprty sites, the most frequently damaged thickness class was 5–6 cm, while at the Kalek site, the most commonly damaged class was 4–5 cm.
As thickness increases, the probability of damage rises significantly. On average, only 22.5% of undamaged spruce individuals are found in thickness classes above 7 cm across the monitored areas.
Photo: Heavy damaged young spruce trees are protected against rot. Ore Mountains, author Jan Řezáč
The analysis of the results revealed that the damage caused by browsing and peeling is increasingly affecting younger stands, averaging around ten years in age. This corresponds to the lower threshold of the stands’ susceptibility to the negative impacts of deer, particularly regarding the risk of such damage.
Damage by browsing and peeling ceased with a total thickness of over 9 cm. This fact plays a crucial role in terms of forest protection, possible fencing of stands and their security. Therefore, after the removal of the fence, individual protection of the spruce tree skeleton appears to be necessary from a cultivation point of view.
A secondary risk associated with damaged trees is the colonization by fungal pathogens, which can attack the wood and lead to stem deformation. For instance, the bleeding heart fungus often invades tree stems within the first year following bark removal, and a damage area of just 50 cm² is sufficient for the entry of these pathogens.
Damage to younger trees increases the risk of rot, which is concerning given the ongoing development of these stands. The temperature range that is ideal for infection by fungal pathogens is between -8.3 and 5.0 °C. This range aligns with the average temperature in the Czech Republic during the non-vegetation period, which is 1.7 °C. Consequently, the presence of winter bark browsing is especially troubling.
Repeated damage to young spruce stands can lead to stem rot in affected individuals, allowing them to reach heights of up to 6 meters by the age of around 40 years. This damage, caused by browsing and peeling, has been recorded at rates between 23.4% and 58.3% in the monitored areas. Therefore, protecting these forest stands is critical.
Effective hunting management and regulating the number of ungulates to an ecologically sustainable level are essential strategies for achieving this protection.
The paper “Bark stripping damage in Norway spruce stands: confirmation of shift to younger stands” (Škody ohryzem a loupáním ve smrkových porostech: potvrzení posunu do mladších porostů) can be downloaded here.
Authors: Jan Cukor, Stanislav Večeřa, Richard Ševčík, Kateřina Brynychová, Vlastimil Skoták, František Havránek, Forestry and Game Management Research Institute (VÚLHM – Výzkumný ústav lesního hospodářství a myslivosti, v. v. i.); Zdeněk Vacek, Pavel Brabec, Stanislav Vacek, Czech University of Life Sciences Prague (ČZU – Česká zemědělská univerzita v Praze), e-mail: cukor@vulhm.cz
Prepared by Jan Řezáč based on the original, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz