Whitebark Pine

 

Whitebark pine--Pinus albicaulis--is one of several stone pines, an evolutionarily loose-knit group of species of the genus Pinus. Other stone pines include Swiss, Siberian, Dwarf, and Korean pines--P. cembra, P. sibirica, P. pumila, and P. koraiensis, respectively. All have comparatively large fat-rich seeds lacking wings, and depend upon animals such as avian nutcrackers (Nucifragus sp.) rather than wind for dissemination. The origin and meaning of "stone pine" is not clear, but it could either refer to the nature of the seeds or, more likely, the often harsh and unforgiving habitats occupied by most of these species. Stone pines used to be confidently included in a subsection of the genus Pinus called Cembrae, included in the subgenus Strobus, which contains almost all of the "soft" pines with needles contained in bundles of five, as in the case of whitebark pine. Recent genetic work has called this confidence into question, and placed P. cembra and P. sibirica closely together, perhaps also with P. pumila and more distantly P. koraiensis. By contrast, whitebark pine has been placed in a bin with species such as the Himalayan P. parviflora and Sierra Nevadan P. lambertiana, AKA sugar pine. Phylogeny aside, all of the stone pines share an ecological niche and distinct morphologic similarities. They are also all a rich source of food for numerous animals.

Stone Pine Seeds

Stone pine seeds are much larger than seeds of most other conifers, and comparable in size to those of pinyon pines (e.g., P. monophylla and P. edulis of the Pinus subection Cembroides). The graph at far left shows, at bottom, a box-and-whisker plot of seed mass (in milligrams) for 20 species of conifers occurring in western North America. The median is around 7 mg, and the largest around 49. Above this "normal" distribution, I've plotted seed mass for pinyon pine, whitebark pine, and another large-seeded white pine called limber pine (P. flexilis). Whitebark pine seeds average around 175 mg, fresh weight, more than 20 times the median for most other conifers. Moreover, research by Frank Forcella and Tad Weaver suggests that this large mass of individual seeds results into a unit area availability (i.e., grams per square meter per year) that exceeds that of other conifers by 10-30 fold, despite the fact that whitebark pine produces fewer cones than most other conifers. Moreover, whitebark pine seeds, like those of other stone pines, are rich in fat. Multiple studies show fat content of whitebark pine seeds averaging roughly 22 to 30%, which makes this food one of the fattiest in the northern Rocky Mountains, along with army cutworm moths and elk and bison at the end of summer. 

Stone Pines & Bears in the Northern Hemisphere

Bears eat stone pine seeds pretty much everywhere they have access to them. This includes brown and black bears in Asia (Ursus arctos and U. thibetanus), as well as grizzly and black bears in North America (U. arctos again, and U. americanus; see this link for more on distributions of these species). The scale of pine seed consumption by bears is far-and-away greatest in Asia, where the overlap between distributions of bears and stone pines is huge.

 

The map at right is illustrative. Shades of green represent stone pines of one species or another; the brown line and lightly shaded tan area within represents the historical distribution of Ursus arctos--brown and grizzly bears. Dark green corresponds to the distribution of Siberian stone pine. Other light green areas in Asia correspond with the distributions of Dwarf and Korean stone pines. Pretty much everywhere within the joint overlap of bears and stone pines in Asia that has been examined by naturalists, bears eat pine seeds, sometimes in enormous quantities. The patchy distribution of stone pines in Europe is comprised of Swiss stone pine, a denizen of the Alps and Carpathian Mountains. Primarily because brown bears have been long-gone from the Alps--albeit recently returned--there are no records of bears consuming pine seeds in Europe. 

The map at right provides more detail of whitebark pine distribution in North America. In addition, the various shades of green and yellow denote the extent to which bears are known to eat pine seeds in a given region, with darkest-green corresponding to areas where pine seeds are a major food, and yellow where consumption of pine seeds is trivial.

 

The point here is obvious. Even though bears will eat pine seeds where available, pine seeds are not an equally important bear food everywhere there is overlap. Regional differences seem to be driven by differences in abundance of whitebark pine, finer-scale distributions, and the availability of alternative foods. Farthest north, in Canada, whitebark pine is rarely abundant and typically present only as a fringe near timberline, and hence rarely exploited by bears. By contrast, greatest levels of exploitation typically occur in areas farther east with continental (versus maritime) climates, where whitebark pine tends to be more abundant, distributed across a wider range of habitats, and in areas with fewer berries and spawning salmon.

 

Parenthetically, the red lines at right delineate the current distribution of grizzly bears in the contiguous United States, including the isolated Yellowstone ecoregion farthest south. Yellowstone and the East Front of the Rocky Mountains to the north have long been centers of pine seed consumption by bears in North America. In fact, pine seeds, along with meat from large herbivores, are such singular centerpieces of the Yellowstone grizzly bear diet as to make it unique in all of North America, and most closely related to that of brown bears in Siberia.

The Squirrel Connection

Siberian chipmunk
Red squirrel

One interesting facet of the relationship between Ursus arctos and stone pines is the role of sciurids--squirrels and chipmunks. As it turns out, sciurids are important if not critical intermediaries for bear feeding on pine seeds, especially bears such as brown and grizzly bears that do not readily climb trees (see this link for more on morphology of U. arctos).

 

Cones of stone pines do not spontaneously fall out of treetops with seeds intact. In other words, they are indehiscent. Instead they remain in the canopies until naturally disintegrating, unless a seed predator breaks them open or drops them to the ground to cache the cones or seeds for later consumption. And some of the most important of these seed/cone harvesters are squirrels and chipmunks. In fact, North American red squirrels can make huge caches of cones in repeatedly-used sites called middens. Chipmunks, especially Siberian chipmunks (Tamias sibiricus), make caches of seeds as large as 9-11 pounds.

The bears, then, need only find these caches of cones or seeds and pilfer them. The chipmunks and squirrels will have done all the hard work of getting the provender out of tree tops, and concentrating it in larders. Grizzlies typically home in on red squirrel chatter, which often brings them to a squirrel perched over its midden. The role of squirrels is so central that 93% of all documented instances where a grizzly bear consumed pine seeds in the Yellowstone ecosystem involved bears raiding squirrel caches. In Siberia, the Siberian chipmunk seems to play much the same role for brown bears consuming seeds of Siberian and Korean stone pines.

The maps at left show the distributions of Siberian chipmunks (top) and North American red squirrels (Tamiasciurus hundsonicus; bottom) relative to the distributions of stone pines on each continent, shown in green. Chipmunks encompass nearly all of stone pine range in Siberia, and red squirrels the main part of whitebark pine distribution in North America, with the potential in both instances of playing a critical role in bear foraging wherever bears and stone pines coexist.