An obvious but nonetheless important proviso is that the mature fruit produced by any given species varies with location. Size, sugar content, and digestibility all vary with site conditions, as well as with annual weather and seasonal maturation (see Temporal patterns).
The figure at left is illustrative, featuring data collected by Patricia Martin for huckleberry as part of her Master's project at the University of Montana. Sugar content of these berries tended to increase with elevation, peaking on lower subalpine sites between 5250 and 5750 feet. I haven't run across similar data for other species, but I suspect that the general pattern is for fruits to be largest, most abundant, and most nutritious in environments typifying the core of each species' niche.
Contingencies of Location
Vaccinium scoparium (Whortleberry, Grouse whortleberry): Whortleberry is a dwarf spreading shrub typical of high-elevation forests in the northern US Rocky Mountains. Its berries are quite sweet but very small. Although birds voraciously consume whortleberry, it is rarely an important food for bears. Even so, I include it here because it is widespread in current grizzly bear range and a consistent, albeit minor, feature of bear diets.
Vaccinium membranaceum (Huckleberry): Huckleberry is a medium-statured shrub that produces medium-sized berries interspersed with foliage. When abundant, berries can be relatively dense and easily harvested. Huckleberry is most abundant at mid-elevations in areas with ample precipitation, non-calcareous substrates, and open forest conditions following forest fires. The sweet fruits are consistently one of the most heavily consumed by bears in the northern US Rocky Mountains and adjacent Canada west of the Continental Divide.
Sorbus scopulina & sitchensis (Mountain ash): Mountain ash is typically a large erect shrub that produces robust dense clusters of fruit at the ends of branches, often in sufficient abundance to bend the branches towards the ground. The fruits are pomes, typical of the rose family to which mountain ash belongs. The late-ripening fruits are at first bitter, but freezing ameliorates the bitterness and results in a tendency for bears to consume these fruits late in their active season, sometimes as late as November. Mountain ash, which grows in mesic forests, is rarely an important source of fruit for bears at mid-latitudes.
Amelanchier alnifolia (Serviceberry, Juneberry, Saskatoonberry): Serviceberry is a medium to large shrub that produces clusters of large fruits borne on the end of twigs in a manner that allows easy access by a foraging bear. The fruits are pomes typical of the rose family to which serviceberry belongs. Serviceberry grows on diverse sites, including high-elevation sidehill parks, mid-elevation conifer forests and aspen groves, and lower-elevation riparian habitats. Where abundant, it is an important food for both black and grizzly bears.
The fruits consumed by grizzly bears in the northern US Rocky Mountains vary widely in characteristics that affect their digestibility, nutritional benefit, and ease of acquisition. This page serves as an introduction to key species, as well as basic traits of relevance to bear foraging (for more on foraging, as such, see this entry). All but one of the species featured below belong to the rose (Rosaceae) or heath (Ericaceae) families and all but two sport fruit that are either pomes (fleshy with a fibrous interior receptacle containing small seeds) typical of the rose family or berries (predominantly fleshy encasing small seeds) typical of the heath family.
Arctostaphylos uva-ursi (Kinnikinnick, Bearberry): Kinnikinnick is a dwarf spreading shrub that produces single relatively small fruits at the ends of twigs, sometimes in sufficient abundance to appear clustered. The fruits are berries, typical of the heath family to which kinnikinnick belongs. When abundant, kinnikinnick can be easily harvested by both people and bears. Kinnikinnick grows at mid- to lower-elevations, typically in drier open forests and adjacent openings. It is commonly consumed by bears, but rarely an important dietary item.
Crataegus chrysocarpa & douglasii (Hawthorn): Hawthorn is a medium to large shrub that produces clusters of medium-sized fruit borne on the end of twigs, typically towards the top of individual plants. Like serviceberry, hawthorn fruits are pomes typical of the rose family to which hawthorns belong. Hawthorn usually grows in riparian habitats at middle and lower elevations, including along streams in grasslands bordering mountain foothills. Where abundant, hawthorn fruits are often an important bear food.
Prunus virginiana (Chokecherry): Chokecherry is a medium to large shrub that produces dense elongate clusters of medium-sized fruit borne on the end of twigs that droop towards the ground when heavily laden. The fruits are drupes, meaning that the center of each is occupied by a comparatively large stony seed that compromises food quality. Chokecherry is a lower-elevation species that occupies moist habitats such as aspen groves, riparian shrub communities, and open forests. Where abundant, it can be an important source of fruit.
Shepherdia canadensis (Buffaloberry, Soopolallie, Soapberry): In temperate regions buffaloberry is a medium-statured shrub that produces scattered to clustered small fruit that are born admixed with foliage, which complicates harvest when fruit are not abundant. These drupe-like fruit are fleshy, slightly bitter, and contain several medium-sized seeds. Unlike all the other species featured here, buffaloberry is dioecious, which results in fruit being produced only by female plants. Despite these potentially compromising traits, buffaloberry is one of the most widespread fruits heavily consumed by bears. Buffaloberry is typically most abundant in open and recently-burned forests at low to mid elevations.
The fruits commonly consumed by grizzly bears in the northern US Rocky Mountains vary widely in sugar content, digestibility, and size, all of which affects their comparative nutritional value for bears. Other traits, such as protein and energy content, are relatively consistent.
The bar (and dot) graphs at left summarize information that I compiled from a number of different sources about key traits of fruits eaten by Rocky Mountain grizzly bears. The top graph shows average protein content of each as a dot, sugar content as the front lighter bar, and energy content as the back darker bar. The lower graph shows average weight of individual fruits as a dot bounded by lines denoting 1 standard deviation above and below, and average digestibility by bears as bars. The wider darker colored bars in the bottom graph are based on data from laboratory trials. The light-gray narrower bars are estimates that I extrapolated from the strong relationship between sugar content and digestibility.
In brief, all fruits have a low protein but high energy content, which requires that bears admix high-protein content foods (e.g., meat) to balance energy and nutrient requirements when heavily consuming fruit (see this entry). Aside from this commonality, the two Vaccinium species plus buffaloberry (Shepherdia) produce fruits with the highest sugar content and digestibilities of all, although fruit size of each is also comparatively small. In fact, the very small size of whortleberry fruit debars it from being an important bear food pretty much anywhere. Serviceberry (Amelanchier) is of intermediate digestibility and sweetness, but benefits from being the largest of all fruits featured here, in addition to being born in easily harvested clusters.
At the other end of the spectrum, mountain ash (Sorbus), kinnikinnick (Arctostaphylos), and chokecherry (Prunus) produce fruits that are comparatively the least digestible and rich in sugars. As a result, both mountain ash and kinnikinnick tend to be consumed most heavily by bears during spring and late fall, after frost or over-wintering conditions have increased concentrations of sugar. This nutritional downside is only partly mitigated by the comparatively large size and dense clustering of mountain ash pomes and chokecherry drupes. When viewed as a totality, these traits readily explain why, when abundant, huckleberry and serviceberry are often dietary mainstays for grizzly bears.
Grizzly bear eat fruit from a number of other species in the northern US Rocky Mountains, but consumption is incidental or, when more than that, highly localized in time and space. Among these less-important species are redosier dogwood (Cornus sericea), buckthorn (Rhamnus alnifolia), various gooseberries and currants (Ribes sp.), wild strawberries (Fragaria sp.), raspberry (Rubus idaeus), and thimbleberry (Rubus parviflorus).