Saturday, 30 July 2016

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

Volume 366, 15 April 2016, Pages 53–64


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.


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 AICc 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.


  • Damage to forests;
  • Long-term trends;
  • Tree damage;
  • Foraging behavior;
  • Forest economy;
  • Carpathians
Corresponding author.