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Tree squirrels (Sciuridae household) remove bark, specifically in winter season and early spring, when food is limited. Squirrels can remove bark from fairly large parts of a tree; nevertheless, this rodent is not likely big enough to achieve such significant tree damage.
They also chew on bark at the base of the tree, but in deep snow the damage happens higher on the tree.
Moose (Alces alces) focus on wood (deciduous) saplings and smaller sized trees; although, they will eat the needles and branches of conifer trees when no deciduous plants (e.g., willow, pin cherry) are easily offered. Bears will climb up higher in the tree where the bark is thinner and peeling is simpler.
Every spring my other half and I anticipate hiking in the mountains, often to an alpine lake for a little high-altitude fly-fishing. As a wildlife biologist I am always on the lookout for animals and spring is a fantastic time to observe a range of activity.
More frequently than not, I see wildlife sign– scat, tracks, and occasionally an evergreen (conifer) tree with the bark peeled off and laying on the ground or hanging in long ribbons in the branches.
I inspect the trunk for vertical impressions that look suspiciously like teeth marks when I see one of these curiously embellished trees.
Wildlife CSI: Ruling Out the Usual Suspects
Trees are among the numerous plant foods in natures lunchbox that supply nourishment for wildlife.
I typically come across trees with a specific sort of damage. These marks were made by something with teeth, that can climb up, and is bigger than a breadbox.
What animal triggered these marks? © Mary Terra-Berns
They burrow under the snow to the tree and begin feeding on the bark. Considering the proof mice and voles are not suspects.
Pocket gophers (Geomyidae, spp.) are subterranean experts that choose open areas with shorter plant life and less woody plants. Huge trees are not their jam.
Ive had pocket gophers tunnel into my garden and pull my three-foot tall generously flowering tomato plants into their tunnel. Most likely smiling all the method to their food storage cache. Tree seedlings can suffer the exact same death. This suspect is excused.
Tree squirrels (Sciuridae family) remove bark, especially in winter and early spring, when food is limited. Squirrels, in this case the pine squirrel, normally gnaw on horizontal branches and in some cases the trunk targeting the inner bark. Squirrels can remove bark from fairly big portions of a tree; however, this rodent is unlikely large enough to accomplish such considerable tree damage.
A porcupine left these marks on a sugar maple. © Cephas/ Wikimedia Commons
Snowshoe hares (Lepus americanus) prefer smaller trees. Much to the aggravation of lots of foresters, they will skillfully work their way through a stand of freshly planted seedlings nipping off tops and some branches. They also chew on bark at the base of the tree, but in deep snow the damage occurs greater on the tree.
They appear to have an internal radar for identifying recently planted seedlings. To these ungulates it is like discovering candy scattered around an open area of ground.
Moose (Alces alces) concentrate on wood (deciduous) saplings and smaller sized trees; although, they will eat the needles and branches of conifer trees when no deciduous plants (e.g., willow, pin cherry) are easily available. They do strip bark, however not on conifer trees and they cant climb. Excused.
Woodpeckers (Dryocopus spp.) are unlikely transgressors. They target trees bursting with bugs like carpenter ants and beetle larva. Woodpeckers benefit from this delicious food source by drilling into the tree to feast on the bugs. Woodpeckers do not have teeth and the proof clearly suggests something toothy.
A likely candidate is the porcupine (Erethizon dorsatum). Long claws on both back and front feet make them effective climbers. They eat a range of plant life during the snow totally free months, however in the winter, they focus on needles and bark. They can shred an outhouse, so why not a big tree? About the size of a breadbox? Yes. Incisor impressions left on the tree? Yes. Wait, a porcupine scrapes in a crosshatched, horizontal pattern. Our subject tree has a vertical scrape pattern following the length of the truck and the teeth impressions are bigger.
The offender that left these fine-looking incisor impressions was a bear, and although near my Idaho house it could be a grizzly, it is most likely a black bear.
A black bear lounging in a tree. © Scott Suriano/TNC Photo Contest 2019
Bear Trees or Measuring Trees
Ernest Thompson Seton (1860-1946) was an author, wildlife artist, and early conservationist. His eight-volume set, The Lives of Game Animals (1909, 1926) goes over the nature of many mammal types. I have the full eight-volume set and refer them regularly.
Seton recommendations various studies, publications, and reports and I find these references interesting, and often entertaining.
In Volume II– Part I, there is a discussion about “Bear Trees or Measuring Trees.” Descriptions of bark-stripped trees are spot-on to what Ive experienced. The thoughts of the time were that this was a method of bear intercommunication or an annual height determining stick.
According to a few recommendations, a bear would base on its hind legs and reach as high as possible to rip the bark. There was a concern regarding whether the same bear came every year or if another bear reached higher as if to state, “Im bigger than you.”
Furthermore, only “he-bears” would bite trees along their trails; the one that bites the highest is the one that owns the road.
© Dave Shaffer/TNC Photo Contest 2021
What Is Really Going On?
Bears are extremely smart, incredibly strong, incredibly curious, and, at times, entertaining. They are opportunistic omnivores and their memory, resourcefulness, and intelligence are particularly apparent when it concerns finding food.
They come out of their dens, shake off the fog of hibernation, and start trying to find fuel. Food might be limited so they often continue to lose weight as they feed off their staying fat reserves. Any food that offers nutrition and energy is necessary and trees are easily available.
Under the tree bark there is a thin layer of living cells referred to as the phloem, which carries nutrients to the cambium, another layer that produces bark cells on the exterior and wood cells in the interior of the tree.
As winter recedes into spring, starch that has been kept in the tree roots is transformed to sugar, and with the addition of water creates sap, which rises through the cambium to nurture new growth. This is when the sap from maple trees is “tapped” to produce maple syrup.
Proof of bears eating trees. © Mary Terra-Berns
A Bit of History
In the 1940s, near Hoquiam, Washington, foresters saw unusual damage at the base of trees in recently thinned stands. Long strips of bark lay on the ground around the tree, and vertical teeth marks ran the length of the debarked section.
The damage was more comprehensive than what a 12-to-35-pound porcupine might do. Foresters correctly thought the damage was caused by black bears.
Black bear claw marks on a tree in Oregon. © Jon Cox/ Flickr
Damaged trees are usually 15 to 30 years old and 18 to 30 inches in diameter. Many of the time the damage is on the bottom 4 to 8 feet of the truck; however, trees can be peeled as high up as 30 feet.
The wood market unintentionally created a springtime food source for bears by thinning timber stands. Thinning minimizes intra-tree competition and provides additional wetness and nutrients to the remaining trees, which permits more energetic growth. A direct relationship between thinned lumber stands and bear damage has been documented in several places.
Bears are not specific about the trees they remove; damage to a variety of types has actually been recorded. Bears tear the bark off using their teeth and brief, curved claws, then scrape the sapwood off with their incisors.
In some circumstances, a tree will be entirely girdled and eliminated. Bears will climb up greater in the tree where the bark is thinner and peeling is much easier. For smaller or more youthful bears this is an advantage.
Plants have a natural defense reaction to reduce animal feeding– producing chemical substances called terpenes. When you walk through the forest, as you brush aside branches you typically smell “pine.” You are smelling terpenes, the fragrant, highly unpredictable carbon substances that produce aroma.
Several studies have revealed that thinning timber stands increases the sugar concentration and tissue mass while terpene concentrations do not increase.
Bears prefer inner bark with strong concentrations of sugar and weak concentrations of terpenes, which might discuss the preference for trees in thinned stands.
Bears may target managed wood stands, however I have actually seen removed trees along hiking routes in random stands of timber. Next time you are out taking pleasure in the forest and you come across strips of bark at the base of a tree and obvious teeth marks on the tree trunk, keep an eye out: a bear could be nearby.
