Tom deHass (OSU Extension, Lake County) showed pictures during this week's BYGL Zoom Inservice of heavy bark-stripping damage he observed in Pete's Pond Preserve in northeast Ohio. I've also received reports and images of damage occurring elsewhere in Ohio.
There are a number of North American animals that will strip bark, but few can produce damage high-up in tree canopies. Bark-stripping on the upper tree stems in Ohio is most likely caused by eastern gray squirrels (Sciurus carolinensis). If the debarking occurs elsewhere in the U.S., the culprit could be North American porcupine (Erethizon dorsatum). However, it would be highly unlikely to find this fascinating prickly animal in southwest Ohio.
We've been posting BYGL Alerts on squirrels stripping tree bark for a number of years. Of course, squirrels can injure trees in two ways: by stripping bark and lopping off twig tips. The twig pruning most often occurs in the fall and causes no real damage. The "natural pruning" may actually increase canopy density.
However, destructive debarking by squirrels can potentially girdle trees. In fact, gray squirrels are considered a major non-native pest in the United Kingdom (UK) where they've changed their name to grey squirrels.
Eastern grays aren’t the only squirrels that strip bark. The peculiar behavior has been observed in North America with fox squirrels (S. niger) as well as two so-called pine squirrels; the red squirrel (Tamiasciurus hudsonicus) and Douglas pine squirrel (T. douglasii).
Gray squirrels are reported to strip bark on a wide range of deciduous trees. I've most commonly seen the damage on honeylocust (Gleditsia triacanthos) and various maples (Acer spp.); however, I posted a BYGL Alert last spring of damage to dawn redwoods (Metasequoia glyptostroboides) with the bark on the main stem stripped in patches. The damage extended from a few feet above the ground to around 20 – 25' up the tree.
Tree debarking by squirrels remains somewhat rare in North America. This is not the case in the UK. Eastern gray squirrels were introduced to various areas in the UK from the late 1880s through the 1920s. They are now wreaking havoc throughout the UK producing widespread severe debarking of woodland and landscape trees and threatening biodiversity; gray squirrels have caused regional extinctions of their native red squirrel (S. vulgaris).
Reasons posited on both sides of the Atlantic for the odd bark-stripping behavior has ranged from reasonable hypotheses such as feeding on the sugar-rich phloem, searching for a water source, gnawing on trees to wear down ever-growing incisors, to the bizarre such as pregnant female squirrels gnawing bark in response to their pain.
A review of the scientific literature makes it clear that more research is needed to truly pin down the reason(s) for the odd bark-stripping behavior by gray squirrels including the extensive damage observed in the UK. We may become frustrated with hearing, “more research is needed;” however, more times than not, it's the truth of the matter.
A number of studies conducted in the UK on grey squirrels noted two important findings. First, the damage most often occurs in the spring to early summer. Although bark-stripping may be observed later in the season, the vast majority occurs from March through May.
Second, the greatest amount of damage seems to occur after a good mast year supports an elevated population of juvenile squirrels; maybe more accurately juvenile delinquents. Of course, a high population of young squirrels also means there is a high relative population of post-pregnant females.
A study published in 2016 coupled these findings with naturally occurring elevated levels of calcium in tree phloem in the spring to early summer; as much as a 40% increase compared to other times of the year. Of course, it is well documented that gray squirrels will gnaw on calcium-rich sources such as bones, antlers, and even limestone, to get this much-needed mineral.
The authors hypothesized that juvenile gray squirrels strip bark to feed on tree phloem in order to acquire calcium for bone growth. They also posited that post-pregnant females gnaw the phloem to replace calcium lost during pregnancy and nursing.
An open and shut case? Not hardly. Note the authors "hypothesized" which is the beginning of the Scientific Method, not the end. In fact, the same authors published a paper in 2017 that seemed to disprove their hypothesis.
They found the dominant form of calcium in tree phloem is calcium oxalate (CaC2O4) which is commonly abbreviated as CaOx. This is important because the authors noted that many animals cannot utilize this form of Ca for building bone. Calcium carbonate (CaCO3) is a form of Ca known to be readily utilized by animals to build bone.
In a very small scale study, the authors fed grey squirrels a diet with CaOx; a diet with low-Ca; and a control group a diet with calcium carbonate. The researchers found no differences in femur length between squirrels fed three diets. This seemed to refute the phloem calcium hypothesis.
Unfortunately, as the authors recognized, their study was extremely limited. It only included 18 grey squirrels; 10 males and 8 females. Of equal importance, they only had one sub-adult.
The reason for their small sample size highlights that science does not occur in a vacuum. Their study was limited by both money and social concerns. Here is their explanation: “This was an arbitrary sample size chosen for this small-scale study to garner an understanding of the effect of the three custom-made treatment diets, and because it is divisible by three. This number was also restricted by the cost of the treatment diets, and an ethical obligation to keep the total number of individuals involved to no more than scientifically necessary.”
A Call to Arms
As a squirrel hunter in my juvenile (delinquent?) days, I never observed bark-stripping by squirrels in the deep woods of West Virginia. Much of the damage in Ohio seems to occur on landscape trees, trees at the edge of wooded areas, or in limited woodlots including some parks. I've never gotten a report of trees being damage in heavily forested areas.
What do you see? Is the bark-stripping by squirrels an urban phenomenon?
I welcome your own observations. You can get my e-mail address by clicking on my name at the top of this Alert. Your pictures would also be welcome!