In Chico, California (north state), I have been pruning Japanese Zelkova trees for dead wood and severely damaged branches due largely to squirrels. The trees are probably about 45 to 50 years old.
There is evidence that gray squirrels sometimes visit the same wound sites yearly, gnawing at the bark of successive layers of wound wood.
Often branches of all sizes (up to about 6 or 8 inches) have been chewed beyond the cambium into wood. And the circumferential damage is typically about 50%, but is sometimes less or up to 100%.
Damage is most common on the top surface of horizontal or arching branches. Underside or side bark-gnawing is more common near branch unions.
It seems to me that the trees with the heaviest squirrel damage are ones that have been heavily pruned for overhead line clearance. I am speculating that the stress on these trees may have something to do with greater susceptibility of the trees to squirrel attack.
Could it be that these trees taste better because they produce less of something that might normally make them taste bad to squirrels? The only other trees around here that get significant squirrel damage are Liquidambar and some maples.
Is this problem common for this species elsewhere?

Reply to post by Mark Goodwin, on April 03, 2003 at 15:26:53:

Mark,

Interesting question, in Oregon and Washington I have seen squirrel damage in additional species e.g. Oregon White Oak, Alder, Douglas-fir, Western Red Cedar.

On one particular site, the conifers and oaks seemed to have equal damage. The health conditions were sap rots, and tree decline due to micro-climate which seem to be influencing leaf fungus(s) not common to these plants.

Sounds like the "need for further research".

Reply to post by Stephen Wiley, on April 03, 2003 at 15:26:53:

Gray squirrels busy here too. Upper sides of branches, some die outright or break later. Red maples and willow oaks are most common and most commonly attacked. NOt seen in conifers but then I haven't really been looking for it.

Also twigs are severed, resulting in defoliation and stress. One lightning-struck willow oak chewed on a lot, its unstruck neighbor much less, adding credence to the Goodwinian theory that damaged trees don't make enough repellant compounds.

Reply to post by Guy, on April 09, 2003 at 00:26:38:

The web site linked below has some interesting notes on squirrel damage to trees in England.
Incidentally, I accidently knocked a squirrel nest with three babies out of one tree. They were in good condition and pretty well developed, with fur, open eyes, and able to cling to the bark. They were taken by a Friends of Animals group.
I'm not sure, but I suspect these squirrels are not the natives, but rather the exotic species, as are the trees.

Reply to post by Mark Goodwin, on April 03, 2003 at 15:26:53:

Observations: In Portland Oregon, squirrels have killed the tops of coast redwood, and also giant Sequoia. In western Oregon, killed the tops of Lawson's cypress. In Portland, again, they are the main crown destroyer of bigleaf maple. This damage always occurs, in my experience, with verticillium fungus in the maple tree. I've never seen otherwise. Whether there is a causal relationship there or not, I don't know because I have not made any studies. Observations, however, are the original basis for science. Chinese empress trees and apple trees are getting hit in my yard this year. It is always the top surfaces of branches first: bowed branches exposed to the sun, and thin diameter, high in the tree, are preferred. They might chew the diameter and leave a thin river of bark on the very underside. I don't remember seeing squirrel damage back in New England states, but that was in the 1980's. Also, I do not know which species are do'in the chew'in.
Speculation: I believe squirrels chew bark for the sheer joy of having different flavors in their mouths, and to exercise their teeth and gums. Don't suppose that squirrels are very different from humans. Watch human behavior. Figure out why squirrels behave in a similar fashion. I have it on good authority that squirrels and humans eat nuts, so you can do the math.

Reply to post by Mark Goodwin, on April 03, 2003 at 15:26:53:

Update:
The squirrels have been very active during the last week (end of April) stripping pieces of bark off the Japanese Zelkovas.

I have extensive damage to my maple (think it is a sugar maple) Damage is more in the notches They are killing this tree very quickly with bark removed 50 ot 90% around 6 inch dia branches and for as much as a foot in length!!
The tree looks extremely distressed and I expect this has been their knawing tree for years. In the spring they eat the early leaf buds and early summer they also chew off large clumps of leaves generally letting them fall to the ground.
ANYONE HAVE A SOLUTION to the bark eating,that does not involve some sort of projectile??
I am considering sraying the whole tree with either a soluition of tobacco juice (chopped up cigars) or hot pepper-- or both.