news & information center

Newsletter Excerpt

Tree Bears and Bare Trees by Tony Hacking

With the warmth of Spring, the mother Black Bear and her first cub had just emerged from their secure winter den in a cavity half way up a large Douglas-fir tree near Burrill Peak in the Bluff Creek drainage. Although the cub had nursed from its mother throughout their winter torpor, Momma bear hadn’t had much to eat since feasting on spawned-out salmon in Bluff Creek several months before. She was hungry, but the energy-laden berries that her body craved weren’t ripe yet, and neither were the nutritious acorns. It was going to be tough re-gaining her weight on just the new spring shoots. With some of the highest concentrations of black bears anywhere, competition for food was going to be keen. With her vulnerable cub, Momma didn’t want to challenge the numerous larger and often cannibalistic male bears for winter-kill, wasp larvae, or other of the concentrated food sources they were also seeking out.

Deep in her memory, the mother bear remembered what her mother had taught her while growing up in her mother’s territory across the Klamath River on the Hoopa Reservation. “Look for a young tree, about 2 paws widths through (8-12 inches diameter). Claw off a piece of bark, and taste the tree beneath. If it tastes sweet, that’s what to look for. Claw off more and use your front teeth to scrape up the slimy material under the bark”.

After sampling a number of trees, ones that had been specifically planted by (human) reforestation experts, in part due to their genetic predisposition to grow rapidly, the mother bear and her cub found several trees that tasted sweet and soon had stripped the bark and scraped off the cambial-zone vascular tissue completely girdling, and eventually killing those trees. By the end of May, by which time other, easier to glean food sources became available, the sow bear and her cub had girdled a daily average of over 50 conifer plantation trees. In one 20 year-old Douglas-fir plantation that had been pre-commercially thinned to an average of about 400 vigorously growing trees per acre, this industrious pair of bruins had stripped over 70 trees in one day. In a nearby plantation where the lower limbs of the trees had been pruned to reduce fuel ladders, the taste and the sweetness just weren’t as satisfying and after sampling a few, the bears left the rest alone.

By June, the smell of ripening thimble berries along with the enticing smells from the campers at Fish Lake made the bears forget about the arduous labor of stripping trees, for now at least.

With this fictional story I’ve tried to present some of the factors affecting a bear phenomenon that has been spreading across the Hoopa Reservation, and into the Bluff Creek area, but is almost nonexistent east of the GO Road. Cambium stripping seems to be a learned behavior passed on from mother to cub, done to provide carbohydrates, in the form of simple sugars, to bears emerging from winter torpor when other food sources are in short supply. Not all bears do this, and it does not appear to provide enough sustenance for the effort to make it worthwhile for the larger male bears.

It turns out that in May, the phloem of a Doug-fir can contain up to 5% sugar, and the energy content can be twice as digestible as that of grasses and forbs. Bears typically select the most vigorous trees, from a variety of species, within the most productive stands, because rapid tree growth results in the greatest sugar content. This is why you can often see bear damage to trees growing by the side of a road, where they are receiving unobstructed sunlight, and why plantations that have just been thinned (released for growth) or fertilized, receive a disproportionate amount of bear damage. On the other hand, studies have shown that trees that have had the lower limbs pruned, thereby improving growth form and resistance to ground fire, but reducing yearly growth, tend to be passed up by a factor of 4 to 1 compared to un-pruned trees. Through the summer, the ratio of sugars to terpenes (think turpentine) in the cambium changes, and the palatability of the cambial tissues decreases. More cambium stripping occurs in seasons when other more typical food sources are in short supply.

Supplemental feeding of bears has been used to reduce the amount of bear damage in some parts of Washington State, while biologists on the Hoopa Reservation have developed a method of collecting hair samples at stripping sites for genetic analysis and then trapping suspect bears to look for a genetic match to the chronic tree strippers. From doing this it became apparent that the tree eating bears have more tooth decay and characteristic dental stains that have allowed the culprits to be identified when trapped, anesthetized and examined.

As the social phenomenon of cambium stripping by bears expands into our area, it is likely to have an interesting effect on our efforts to manage fuels, timber and wildlife.