Long-term study of damage to trees by brown bears Ursus arctos in Poland: Increasing trends with insignificant effects on forest management

Highlights

• Six tree species: fir, larch, spruce, pine, beech, and ash – were damaged by bears.

• Conifers tree species were more frequently damaged than deciduous.

• Mean forage ratio was higher for larch than for fir, spruce and pine.

• The improvement in fir condition and the bear numbers had influence on damage scale.

• Tree damage by bears is not a substantial problem for forest management in Poland.

Abstract

We present a long-term quantitative analysis of forest damage caused by the brown bear (Ursus arctos) in the biggest refuge of this species in Poland. Based on questionnaires distributed to the relevant authorities we estimated the number of trees damaged by bears in 1991–2013, changes in the tree species composition and large-scale factors potentially affecting the extent of damage. We also discuss the importance of bear tree damage to forest management. Throughout the 23 years of the study we recorded 6937 trees damaged by bears: a clearly increasing trend and distinct fluctuations in tree numbers and species composition were discernible. Conifers (91.7% – fir 70.0%, larch 11.3%, spruce 9.5%, pine 0.9%) were more frequently damaged than deciduous species (2.9%). Larch and spruce were preferentially affected during the whole study period, and the preference for larch was distinct when collated with its availability in forest stands – a forage ratio of 0.50 compared to 0.35 for fir, 0.17 for spruce and 0.13 for pine. In 2003, however, bears suddenly switched to fir and it is this species that now predominates among the damaged trees, reaching 96.5% in 2013.

Two models based on minimum AIC_c values were best explaining damage to trees. The most parsimonious model contained one explanatory variable: brown bear population size. The second best model included both bear population size and the average fir tree-ring width. Neither fluctuations in daily temperatures nor the number of days with snow cover had any influence on the scale of damage. Our findings suggest that damage caused by bears is not to be regarded as a serious problem by forest management in Poland and it is unlikely to reach a level of economic significance in the short term.

Introduction

Biotic and abiotic factors can affect forest health and productivity. Damage to a tree trunk can disrupt its mechanical structure and hence the transport pathways linking the elevated and underground parts of the tree (Vasiliauskas, 2001). The life processes of a wide variety of organisms from fungi to insects and mammals may cause the death or reduce the growth of individual trees or their stands (Bobiec et al., 2005, Hegland et al., 2013, Jędrzejewski and Sidorovicz, 2010, Vospernik, 2006). Wild ungulates are most commonly identified with damage to forest stands in Europe and elsewhere (Bergqvist et al., 2014, Gill, 1992, Jędrzejewski and Sidorovicz, 2010). Moreover, in some European forests, animals with a high conservation priority, such as European bison Bison bonasus, Eurasian beaver Castor fiber or brown bear Ursus arctos, may be responsible for damage. In view of the rarity of these species and the difficulties in studying tree damage over large areas, this phenomenon is still poorly understood, even though tree damage may significantly affect forest health and productivity (Flowers et al., 2012, Lowell et al., 2010, Manning and Baltzer, 2011).

This study discusses the characteristics of tree damage caused by the brown bear (U. arctos). Tree damage is observed throughout the distribution range of this species and other Ursidae, and is done for marking, feeding, and play. Marking behavior relates to single trees and is thought to be associated with body care (Meyer-Holzapfel, 1968) or social communication (Green and Mattson, 2003). Tree marking may be repeated by bears over the years and generally does not lead to tree death (Jakubiec, 2001, Sato et al., 2013). Foraging behavior usually relates to many trees and consists in stripping the bark from the trunk and then feeding on the tree cambium (Ziegltrum and Nolte, 2001, Zyśk-Gorczyńska and Jakubiec, 2010). This behavior has been reported mainly from North America in black bears (Ursus americanus) (Flowers et al., 2012, Nolte and Dykzeul, 2002, Ziegltrum, 2005), but also from Asia in Japanese black bears (Ursus thibetanus japonicas) (Watanabe, 1980, Watanabe et al., 1970, Yamada and Fujioka, 2010) and from Europe in brown bears (Jakubiec, 2001, Kunovac et al., 2008, Krapinec et al., 2011a, Krapinec et al., 2011b, Zyśk-Gorczyńska and Jakubiec, 2010, Zyśk-Gorczyńska and Jakubiec, 2014). Bark stripping by bears can result in the complete girdling of the stem, which normally leads to tree death. Nonetheless, wounds are usually small and the damaged trees live on in the ecosystem until their natural death, providing a range of ecological niches for different organisms (Kanaskie et al., 1990, Ziegltrum and Nolte, 2001, Zyśk-Gorczyńska et al., 2015).

Recently, an increase in the intensity of tree damage by brown bears has been observed in different parts of their European distribution (Krapinec et al., 2011b). Clearly, the phenomenon is dynamic and ought to be better understood. Unfortunately, however, long-term trends in the extent of damage, its causes, ecological characteristics and significance for forest management, have been largely neglected in Europe. For instance, a wide variety of tree species are known to be damaged, although conifers are definitely preferred (Barnes and Engeman, 1995, Kanaskie et al., 1990, Krapinec et al., 2011a, Krapinec et al., 2011b, Sullivan, 1993, Witmer et al., 2000, Zyśk-Gorczyńska and Jakubiec, 2014). In the USA the black bear readily forages on Pseudotsuga menziesii, Pinus contorta, Larix occidentalis (Barnes and Engeman, 1995, Sullivan, 2009, Witmer et al., 2000), Japanese black bears regularly damage Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa) (Watanabe, 1980, Yamada and Fujioka, 2010), while brown bears in Europe mostly forage on fir (Abies alba), larch (Larix decidua) and spruce (Picea abies) (Jakubiec, 2001, Krapinec et al., 2011a, Krapinec et al., 2011b, Zyśk-Gorczyńska and Jakubiec, 2014). However, data from Poland indicate that these preferences have changed in recent decades (Jakubiec et al., 1993, Jakubiec, 2001, Zyśk-Gorczyńska and Jakubiec, 2014), possibly as a result of long-term changes in the condition of particular tree species (=nutritional value of cambium) (Kimball et al., 1998a, Kimball et al., 1998b, Radwan, 1969), changes in alternative food supply in forest stands (Noble and Meslow, 1998), or in bear population size (Krapinec et al., 2011b, Nolte and Dykzeul, 2002). However, no quantitative analyses have been carried out to verify these conjectures.

In North America, the foraging of black bears on tree cambium is known to have a substantial impact on the forest economy (Flowers et al., 2012, Nolte and Dykzeul, 2002). For instance, in NW Oregon the area of damaged stands is estimated at approximately 1400 ha per year (corresponding to a loss of $11.5 million). The related high economic losses are generated by the necessity of removing the damaged trees, by the preferences of bears for trees in good condition, subject to earlier costly treatments (Flowers et al., 2012, Kanaskie et al., 1990, Sullivan, 2009, Witmer et al., 2000), by reduced tree growth following bear damage (Lowell et al., 2010, Manning and Baltzer, 2011), and also by indirect costs associated with damage prevention.

In Europe the damage caused by bears is incomparably smaller than in North America, although in Croatia financial losses due to bear damage have increased (Krapinec et al., 2011b). Similar trends also pertain to Poland, but they are not considered a serious problem by forest management. Bear damage is not even assessed by forest administrations, in contrast to damage caused by other large mammals (Zyśk-Gorczyńska and Jakubiec, 2014, Zyśk-Gorczyńska et al., 2015). In Poland damage caused by bears is usually identified as domestic damage, comparable with the killing of farm animals or damage to apiaries (Jakubiec, 2001).

The objectives of this study were to evaluate the long-term dynamics of tree damage caused by brown bears in their most important refuge in Poland, and to determine which factors might be affecting the scale of damage and the choice of damaged tree species. We expected that the total number of trees damaged by bears would be directly affected by the size of the brown bear population. However, we also hypothesized that factors related to climate and tree condition could affect on the extent of damage. Finally, we discuss the role of brown bears in forest management. In particular, we ask whether this species might be regarded as a pest in Polish forest stands, given its present numbers and the current and potential future extent of damage to trees.