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Black Bear (Ursus americanus) foraging on Black Cottonwood (Populus trichocarpa) catkins in Southeast Alaska.

Black Bears (Ursus americanus) in southeastern Alaska forage intensively on seed pods and male and female catkins of Black Cottonwood (Populus trichocarpa), often breaking many branches from the trees. We examined the relationship of tree damage to tree size and sex, and to content of selected nutrients in the catkins. Tree damage was related to tree girth: medium-size trees were more heavily damaged than small or large trees, possibly because more branches can easily be reached from a safe perch near the trunk. Overall, female trees were more heavily damaged than males. Foraging intensity was not clearly related to the content of important elements (N, P, K, Ca, C) in catkins and seed pods. Concentrated use of cottonwoods in the study area may occur because high human density offers some protection of small Black Bears from larger bears, and medium-sized cottonwoods are common in this recently deglaciated, early-successional area.

Key words: Alaska, Black Bear, Black Cottonwood, foraging behavior, Populus trichocarpa, Ursus americanus


Black Bears (Ursus americanus) are excellent tree-climbers. They frequently climb trees to forage on fleshy fruits such as cherries (Prunus spp.), persimmons (Diospyros spp.), and, in southeastern Alaska, Oregon Crabapples (Malus fusca). In these cases, the foraging relationship is mutualistic, because bears benefit the trees by serving as seed dispersal agents (Willson 1993), in contrast to their foraging on seeds of oak (Quercus spp.) or beech (Fagus spp.).

Black Bears also climb trees to forage on flower structures. Bears eat flowering catkins from various trees, including willow (Salix spp.; Raine and Kansas 1990; JDJ, pers. obs.), White Ash (Fraxinus americanus; Kilham and Gray 2002, Michael Pelton, University of Tennessee, Knoxville TN, pers. comm.), and Black Cherry (Prunus serotina; Kilham and Gray 2002). Populus catkins appear regularly in the diet of Black Bears: Quaking Aspen (Populus tremuloides) catkins are eaten by Black Bears in Michigan and Minnesota (Rogers 1987; DeBruyn 1999), Alberta (Pelchat and Ruff 1986), Manitoba (GWP pers. obs.), and Alaska. Kilham and Gray (2002) mention Black Bears eating unspecified poplar catkins in New Hampshire. Black Bears also eat catkins of Big-tooth Aspen (P. grandidentata) in Minnesota (Rogers 1987). Black Bears get Populus catkins by climbing a larger tree or by bending a smaller tree to reach the catkins from ground level. Sometimes leaves are also eaten (for example, Costello and Sage 1994).

We have observed that Black Bears in southeastern Alaska frequently climb Black Cottonwood (Populus trichocarpa) trees to eat flowering catkins and seed pods in spring (Fig. 1). Because they often break branches to retrieve the distal catkins and seed pods, numerous Black Cottonwood trees in certain areas have many missing branches and, in some cases, the entire top of the tree has been reduced to a bare trunk and the tree may not produce flowers again for several years (MFW and others, pers. obs.). Catkins that have fallen to the ground are also eaten. Both male and female Black Cottonwoods are used by foraging Black Bears; male catkins usually are harvested in May as soon as they begin to emerge from buds, and female catkins and seed pods usually are harvested from mid May through early July, before the seed pods open.


We investigated the foraging of Black Bears on Black Cottonwoods. Specifically, we were interested in the following questions: (1) does the level of damage by Black Bears differ between male and female Black Cottonwood trees; (2) does the size of the Black Cottonwood tree affect the level of catkin and seed pod harvest by Black Bears; (3) what is the potential nutrient value of male and female Black Cottonwood catkins and of seed pods; and (4) what is the nutritional composition of Black Cottonwood catkins and seed pods compared to some other potential plant foods that are present in the area in spring, namely male catkins of Sitka Alder (Alnus viridis sinuata), which apparently are not eaten, and stem-bases of Northern Ground Cone (Boschniakia rossi), which are frequently eaten?


Study Area

Our observations took place near the Mendenhall Glacier Visitor Center (UTM: Zone 53, 473457E, 6475259.7N, NAD83) in Juneau, Alaska. The area was most recently deglaciated in <100 y ago, so it is in the early stages of vegetation succession. Small streams and ponds are present in the area, which lies between conifer forest on a mountain ridge and a sizable glacial lake in front of the nearby glacier. In summer, the area is heavily used by tourists, most of who stay on designated trails and viewing platforms.

Black Bear Use of the Study Area

We observed that most of the bears that frequent the study area in spring are females with year-old cubs and young bears in their 1st full year of independence from their mothers. Females with cubs of the year are seldom seen in the area until later in the summer. Bears that use the area near the Visitor Center become accustomed to the proximity of humans that come to observe them and show little avoidance behavior when humans are present (MFW and others pers. obs.).


To estimate the number of bears using the study area, Alaska Department of Fish and Game calculated a Lincoln-Peterson population estimate (Williams and others 2002) using DNA mark-recapture analysis (for example, Woods and others 1999; Mowat and Strobeck 2000). The data were obtained from single-catch barbed-wire hair snares (Beier and others 2005) set on bear trails in the study area in summer 2008 and checked twice a day for a month. Microsatellite DNA analysis of the hair was done by a commercial lab (Wildlife Genetics International, Nelson, British Columbia) to identify individual bears.

Black Bear Damage to Trees

In 2009 we measured the girth and tagged 96 female and 49 male cottonwoods. We visually estimated the level of damage, and ranked it from 0 to 5, with level 5 representing destruction of much of the canopy (Fig. 2). If part of the canopy was obscured by leaves, we inspected the tree from several vantage points. Whenever possible, 2 observers made independent estimates of damage and the final rank was decided by consensus. Damage levels represent the cumulative effect of several years of bear foraging. In 2010, the original sample was augmented in order to increase the sample size of very small and very large trees (of unknown sex), in order to examine the possible effects of tree size.

We used cumulative logistic regression (Hosmer and Lemeshow 2000; O'Connell 2006), an extension of logistic regression for ordinal responses with more than 2 levels, and a series of ordinary logistic regressions (Hosmer and Lemeshow 2000) to model tree damage level as a function of tree girth. In the ordinary logistic regressions, each damage level was modeled as the probability of a specific damage level versus all other damage levels (for example, damage level 5 vs. 0-4 combined; O'Connell 2006). These 2 types of analyses differ in that cumulative logistic models require responses for the categories to be parallel (on the logit scale), while the ordinary logistic regressions allow separate, non-parallel relationships. For both types of analysis we considered the predictors tree girth and tree girth squared, both separately and in combination; we also fit 'intercept only' models (i.e., no relationship between tree girth and damage). We compared the fit of the models with the various predictors using Akaike's Information Criteria (AIC; Burnham and Anderson 2002) and model-specific AIC weights, which are the relative probability of 1 model versus others considered; the cumulative logistic and series of ordinary logistic models could not be directly compared with AIC. Because of uncertainty in which model is best, we used model averaging (Burnham and Anderson 2002) to estimate final regression equations relating damage level to tree girth. To simplify the presentation of results, we also conducted these same analyses with damage levels 0 and 1 (low damage) and 4 and 5 (high damage) combined.

We used non-parametric analysis of variance (Kruskal-Wallis test) to compare the distributions of bear damage to male and female cottonwood trees because the data were not normally distributed.

Element Analysis of Catkins

Samples of male and female Black Cottonwood catkins and of seed pods were collected from the study area and from other areas around Juneau. We collected Sitka Alder catkins and Northern Ground Cone stem-bases from sites in or near the main study area during the same season of the year as the cottonwood samples. Ground cone stem-bases came from among a group of those plants that had been recently dug up by bears. Basic analyses of several important elements (N, P, K, Ca, C) were conducted by a laboratory (Palmer Research Center, University of Alaska-Fairbanks). Composition was determined from samples ovendried to constant mass at 55[degrees]C for 48 h. For P, K, and Ca, samples were analyzed by atomic absorption spectroscopy. For C and N, samples were analyzed by a TRU Spec CHN Analyzer. Results should be interpreted with caution because of small sample sizes.

Data on element content were not normally distributed, so we used non-parametric (Mann-Whitney) tests to compare element content of Black Cottonwood catkins from different sites and to compare cottonwood catkins with two other potential or actual foods in the study area.


Mark-recapture analyses showed that [greater than or equal to] 11 bears used the study area at the time of sampling in 2008. Because we spent virtually every day for several seasons in the study area, we learned to recognize individual bears by coat color, size, conformation, behavior and, in a few cases, a scar, an ear tag, or a radio collar; at least 15 individually identifiable bears were thought to use the study area in 2011.

Black Bear foraging in the Black Cottonwoods in our study area was clearly focused on male catkins, either just emerging from the bud covers or fully expanded, and female catkins and immature seed pods. Both observation of foraging bears and inspection of fallen branches showed that leaf buds and leaves were generally ignored.

We evaluated damage to 195 cottonwood trees. Tree girth ranged from 28 to 229 cm. Most trees had little or no damage: level 0 = 34.9%, level I = 29.7%, level 2 = 15.4%, level 3 = 8.2%, level 4 = 7.2%, level 5 = 4.6%. Overall, male trees were larger than female trees (male mean = 101.9 [+ or -] 4.2; female mean = 87.4 [+ or -] 2.5; H = 8.1; df = 1,143; P = 0.004) and females had greater damage than males (male mean = 0.94 [+ or -] 0.2; female mean = 1.86 [+ or -] 0.2; H = 10.06; df = 1, 143; P = 0.002). However, damage was not independent of tree size. Preliminary analyses showed that small male trees were more damaged than small females, and large females were more damaged than large males, but more sophisticated analyses revealed more complex interactions.

Because the patterns in the logistic regression analyses were largely unaffected by combining damage levels for low (0-1) and high (4-5) damage, for clarity we present only the results from these simpler analyses (Fig. 3). For the cumulative logistic model, the model with both linear and quadratic predictors is overwhelmingly supported over the other models, indicating a strong, non-linear relationship between damage and tree girth (Table 1). Examination of the individual-level predicted response probabilities shows large differences among the response patterns among the various damage levels, which led us to choose the individual-level models over the cumulative logistic model. In the individual-level models, models with a quadratic term, with or without the linear term, are highly supported for damage levels 0-1, 3, and 4-5 (Table 1). For damage level 2, the AIC weights among models are relatively equal, indicating greater uncertainty In the choice of model, but the model with the highest weight is the intercept-only model. The predicted probability of little damage (levels 0-1) is highest for the smallest and largest trees, dropping to a minimum at 112 cm (Fig. 3); note that there are few large trees in the dataset, so the pattern for large trees should be viewed cautiously. The probability of damage level 2 has no relationship to tree size. The patterns for moderate (level 3) and high (levels 4-5) damage have the highest probabilities of damage predicted for intermediate-size trees, but the peaks of the probability curves are shifted for each damage level (Fig. 3). The peak predicted probability for damage level 3 occurs at tree girth 132 cm, whereas the peak damage probability for levels 4-5 is at 94 cm. In summary, trees with little or no damage occur across the range of tree sizes, but small or large trees had the highest probability of little or no damage. Intermediate-size trees were most likely to receive the higher levels of damage, with the probability of the highest levels of damage peaking for smaller, intermediate size trees.


Some of the low levels of damage we recorded were done by Porcupines (Erethizon dorsatum), which also eat Black Cottonwood catkins and occasionally break small branches on cottonwoods in the study area. However, Porcupines are not capable of producing the heavy damage observed on many trees in this study area.

Female cottonwood catkins near the Visitor Center had significantly higher levels of K and Ca than those from other sites (U=1; P=0.03 in both cases), but male catkins from different sites did not differ (Table 2). There was no difference between male and female catkins at the Visitor Center, but in samples from other sites, male catkins had higher levels of K than females (U=0; P<0.05). Young pods had significantly less N than female catkins (Table 2; U [less than or equal to] 2; P=0.03). Because it appeared that there were probably differences among sites, we did not compare young and mature pods statistically, but the medians suggest possibly lower values for N, P, and Ca in mature pods (Table 2).

Neither Sitka Alder catkins nor Northern Ground Cone stem-bases provided as much P, K, or Ca as cottonwood catkins, although the N value for catkins of alder and cottonwood were similar (Table 2). Although ground cone was markedly low in N, P, K, and Ca (no overlap with cottonwood values), it is eaten frequently by Black Bears in the Juneau area (MFW and others, pers. obs.).


Much of the variation in levels of Black Bear damage to Black Cottonwoods in our area was accounted for by tree size: both small and large trees were less damaged than medium-size trees. We suggest that this may be related to tree reproductive effort and architecture: small trees offer relatively fewer catkins or seed pods than large trees, and large trees have sprawling limbs that make the catkins and seed pods less accessible. In contrast, the medium-size trees offer good catkin and seed pod crops within easy reach of the tree trunk, where the bears' weight is safely supported. From a perch near the trunk, a bear can reach out and pull in or break off branches in order to eat the catkins or seed pods, especially near the top of the tree, where the branches often curve upward and are reachable from the trunk.

Other factors may also be involved. For example, the concentration of defensive chemicals in Quaking Aspen leaves and catkins varies with plant genetics, age, and environment, and several vertebrate herbivores (Ruffed Grouse, Bonasa umbellus; Porcupine; Beaver, Castor canadensis; hares, Lepus spp.) forage selectively on aspen trees in response to this variation (Jakubas and others 1989, 1993; Lindroth 2000; Osier and Lindroth 2006; Donaldson and others 2006; Diner and others 2009). Catkins have lower levels of the principal defensive compound than leaves (Jakubas and others 1989). It is possible that some of the variation we observed in levels of Black Cottonwood use by Black Bears is a reflection of such variation in defensive chemistry.

Differences in the median composition of the 5 elements we examined cannot account for differences in the intensity of Black Bear use of 3 potential spring foods. Male and female catkins did not differ in any of the assessed nutrients at the principal study site. Young pods had less N than female catkins, although they were harvested heavily. Black Bears did not use Sitka Alder catkins, despite a composition largely similar to that of cottonwood catkins, although Kilham and Gray (2002) noted bears eating catkins of another, unspecified, alder species in New Hampshire. Black Bears used Northern Ground Cone extensively (MFW and others, pers. obs.), despite its low level of 4 of the 5 elements we analyzed.

The extensive, localized damage to cottonwoods near the Mendenhall Glacier Visitor Center remains to be explained. We have not observed this extent of damage in other local stands of cottonwood, although similarly concentrated damage on a smaller scale was observed in Skagway, about 140 km north of Juneau (R Scott, Alaska Department of Fish and Game, Douglas, AK, pers. comm.). Whether the Black Cottonwood catkins offer needed micronutrients is not known; although there was some evidence that female catkins in this area were somewhat higher in K and Ca than elsewhere, it is not known if these elements are in short supply.

We suggest several possible nonexclusive factors that might help explain the local concentration of cottonwood damage. The density of Black Bears may be higher near the Visitor Center than elsewhere, because females and yearlings take advantage of the density of humans there as protection from large male Black Bears and the occasional Brown Bear (Ursus arctos). Large cubs and young Black Bears recently independent of their mothers often seek refuge in trees when threatened, and cottonwoods offer a handy food source while avoiding threats. In similar situations, human activity has been reported to provide a temporary refuge from males for females and young Brown Bears (Nevin and Gilbert 2005).

Furthermore, Black Cottonwood trees of a size that Black Bears readily exploit are common in the area. This study site was recently deglaciated after the Little Ice Age; the Mendenhall Glacier covered the area less than a hundred years ago. The vegetation is therefore in relatively early stages of succession, in which deciduous trees are prevalent and conifers are still few and small. In addition, Black Bears may be attracted to the study area because a wide variety of foods is available from spring through fall, including berries, ground cone, greenery, and fish.

There also may be cultural transmission of foraging habits from mother bears to their cubs, because cubs learn to identify suitable foods by observing and smelling what their mother is eating or has eaten (DeBruyn 1999; Kilham and Gray 2002). So once a few females begin to use the area, the tendency could be passed on to their offspring, and thence to the next generation.


The statements in this paper do not necessarily represent the views of the Forest Service or the United States. Personal funds paid for the nutrient analyses. We are grateful to the Alaska Department of Fish and Game for use of the hair snares and the Lincoln-Peterson estimate of the bear population in the study area. Thanks, as always, to the librarians of the University of Alaska-Southeast. We thank R Hebert and R Maraj for helpful comments on the manuscript.


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5230 Terrace Place, Juneau, Alaska 99801


Alaska Department of Fish and Game, 802 3rd Street, Douglas, Alaska, 99824


U.S. Forest Service, Juneau Ranger District, 8510 Mendenhall Loop Road, Juneau, Alaska, 99801

Submitted 16 September 2011, accepted 10 April 2012. Corresponding Editor: Thomas Jung.
TABLE 1. AIC values and model weights for cumulative logistic
models and individual level logistic models of tree damage
predicted by tree girth. AIC values and weights only are
comparable within analysis type and level; weights sum to
1 within analysis type and level.

Type                  Model            K     AIC     DAIC   Weight

Cumulative        intercept only       1   615.37   15.19   <0.001
                   int. + girth        2   609.92    9.74    0.008
               int + [girth.sup.2]     2   615.49   15.32   <0.001
             int + girth + girth (2)   3   600.18    0.00    0.991
  0-1             intercept only       1   255.42   17.17   <0.001
                   int. + girth        2   252.56   14.31    0.001
               [int + girth.sup.2]     2   252.21   18.96    0.001
             int + girth + girth (2)   3   238.25    0.00    0.998
  2               intercept only       1   169.94    0.60    0.357
                   int. + girth        2   170.54    1.30    0.265
                   int + girth_        2   171.23    1.25    0.187
             int + girth + girth (2)   3   169.94    0.00    0.191
  3               intercept only       1   112.66    6.04    0.041
                   int. + girth        2   110.72    4.10    0.108
                   int + girth_        2   114.46    7.84    0.017
             int + girth + girth (2)   3   106.63    0.00    0.834
  4-5             intercept only       1   139.43   18.27   <0.001
                   int. + girth        2   140.96   19.80   <0.001
                   int + girth_        2   125.41    4.25    0.107
             int + girth + girth (2)   3   121.16    0.00    0.893

TABLE 2. Median content of selected elements in cottonwood
catkins and two other potential spring foods of bears in the
study area, based on dry weights of samples. Values for
cottonwoods in the same column with the same superscript
are significantly different (P [less than or equal to] 0.05;
Mann-Whitney U test; ranges of values in parentheses).

Food                     Site                     %N

Black Cottonwood
  Female catkins    Visitor Center    4.58 (dh) (4.30-5.02)
  Female catkins    Various others     4.55 (g) (3.9111.67)
  Young pods        Various others    3.62 (gh) (2.8611.44)
  Mature pods       Visitor Center     2.26 (d) (1.71-2.85)
  Male catkins      Visitor Center         3.35 (2.47-5.08)
  Male catkins      Various others         3.83 (2.58-4.95)

Sitka Alder
  Male catkins                             3.11 (3.07-3.56)

N. Ground Cone
  Stem-bases                               1.87 (1.50-1.96)

Food                           %P

Black Cottonwood
  Female catkins    0.61 (e) (0.57-0.67)
  Female catkins        0.54 (0.50-0.63)
  Young pods            0.51 (0.37-0.65)
  Mature pods       0.32 (e) (0.23-0.42)
  Male catkins          0.44 (0.35-0.73)
  Male catkins          0.62 (0.36-0.68)

Sitka Alder
  Male catkins          0.28 (0.27-0.28)

N. Ground Cone
  Stem-bases            0.09 (0.08-.015)

Food                            %K

Black Cottonwood
  Female catkins     2.04 (a) (1.82-2.59)
  Female catkins   1.71 (a,e) (1.66-1.89)
  Young pods             2.03 (1.92-2.20)
  Mature pods            2.09 (1.82-2.21)
  Male catkins           2.56 (1.93-2.80)
  Male catkins       2.41 (c) (2.00-3.24)

Sitka Alder
  Male catkins           1.10 (0.90-1.20)

N. Ground Cone
  Stem-bases             1.03 (0.88-1.60)

Food                            %Ca                      %C

Black Cottonwood
  Female catkins   0.58 (b,f) (0.43-0.70)       45.83 (44.59-46.95)
  Female catkins     0.36 (b) (0.22-0.52)       46.48 (46.09-47.28)
  Young pods            0.62 (0.48-0.63)        44.25 (41.59-45.91)
  Mature pods         0.36 (f)(0.17-0.38)       46.11 (45.49-46.37)
  Male catkins           0.64 (0.20-0.93)       45.80 (44.22-46.69)
  Male catkins           0.50 (0.27-.080)       47.28 (46.09-48.55)

Sitka Alder
  Male catkins           0.28 (0.21-0.32)       49.94 (49.56-50.36)

N. Ground Cone
  Stem-bases             0.13 (0.10-0.18)       49.64 (47.73-50.29)

Food                 n

Black Cottonwood
  Female catkins     5
  Female catkins     4
  Young pods         5
  Mature pods        5
  Male catkins       9
  Male catkins      11

Sitka Alder
  Male catkins       3

N. Ground Cone
  Stem-bases         4
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Author:Willson, Mary F.; Pendleton, Grey W.; Jones, J. Douglas; Craig, Laurie F.; Sherwin, Amy E.
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Article Type:Report
Geographic Code:1USA
Date:Dec 22, 2012
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