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1998.10--293-298.pdf | Download |
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Damage to trees by black bears (Ursus americanus) is an ongoing problem in west-coastal North America. We studied damage to Douglas-fir (Pseudotsuga menziesii) during spring by comparing food habits of bears between an area with high damage (north, x_N = 29.4 trees damaged/ha, SE = 6.9) and an area with low damage (south, x_S = 2.7 trees damaged/ha, SE = 1.6). We surveyed 40 forested stands in each area to measure and describe bear damage and to determine if site factors were related to damage levels. Analysis of scats revealed differences in spring diets that included a higher frequency of berry-producing shrubs scats from the south area (P < 0.01) and a higher frequency of forbs (P < 0.01) in the north area. Site characteristics differed between stands with and without bear damage (P < 0.01). Forest stands with bear damage (n = 33) had a lower density of trees >40 cm dbh (P < 0.01), lower total basal area (P < 0.01), occurred on less steep slopes (P < 0.01), and differed by aspect (cosine[aspect]: P < 0.01) compared to forest stands without bear damage (n = 47). Most damage occurs during spring, a season frequently associated with nutritionally poor foods for bears. This is also when carbohydrate production peaks in conifers and cambial zones have the most mass. Stands with prominent damage resemble continuous patchy habitats. Clustered food items appear to be efficiently located and exploited in continuous patchy habitats, even if foragers can only poorly estimate resource distribution. Cambium-feeding may be an energetically viable option for some bears. We recommend altering forest structure in stands vulnerable to bear damage and providing patches of nutritious bear foods as a test to decrease bear damage by reducing the foraging efficiency of bears feeding on cambium.