The Big White Tree with the Peeling Bark

March 3, 2010 | Greg Dodge

am sycamore

American Sycamore

I was recently asked whether or not I knew why the bark on American Sycamore drops off the tree in large thin flakes. Coincidentally, while walking around the Outdoor Exhibits this winter with camera in hand, I’ve been taking photos of various trees to use on this blog in a series of informal, mini-field guides. My intention was to start with some of the more readily identifiable winter trees, trees without leaves. Although winter’s nearly over, this is a good time to squeeze in at least one of those mini-guides.

One of the easiest of trees to identify is the American Sycamore. It’s white upper bark standing off against the winter sky, and other trees in the forest, make it hard to miss and appreciate.

Sycamore is a tree of the bottomlands. Just about any stream or river in our area has it’s share of sycamores growing along its banks, often alongside River Birch. Here at the Museum it grows next to the Wetlands as well as in the swamp between Catch the Wind and Explore the Wild. This handsome tree is often planted along city streets but its roots are near the water.

Many trees have a different type of bark at the base of the trunk than they do at the younger, upper portion of the tree. The sycamore takes this to the extreme.

Take a look at the tree on the right. Scroll up and follow the bark up to the top. It starts out as “typical” gray bark. As you scroll up you’ll notice that the darker bark begins to flake off until finally it becomes nearly all white at the top of the tree.

Towards the middle of the three, starting at about a quarter to a third-way up the tree, the bark often appears “camouflaged” with various gray, brown, and green hues.

Below are some close-ups of the bark.

lower bark

The lower portion of the trunk has rough gray-brown bark.

middle of tree

The bark becomes scaly and flakes off about 1/4 the way up the trunk. It often appears as if camouflaged.

Towards the top of mature trees the bark may be nearly all white.

But, why do sycamores shed their bark? Why does the bark peel off in large thin flakes all season long? I’ve sometimes wondered that myself, but apparently not long or hard enough to actually find out…until now.

Trees have bark to help protect them from losing moisture and drying out, protect themselves from insects (although there are many insects that get past this defense), birds (woodpeckers go in after the insects), and disease. And, all trees have bark specific to the trees themselves. Flowering Dogwood has scaly “alligator” bark. Loblolly Pine has furrowed, segmented bark which bares a resemblance to the loblollies on a dried lake bed. Shagbark Hickory has a shaggy appearance with big, gray pieces of bark sticking out from the trunk. Sycamore has bark that peels off and becomes white near the upper portion of the tree.

What evolutionary advantage is it for a tree to shed it’s bark the way sycamores do? Everything from the tree’s favored habitat to photosynthesis has been suggested by people who should know, people who study trees. Most of the theories that I’ve read seem like sound reasons for this bark shedding habit. Apparently though, there is no definitive answer to the question of why this tree looks and behaves the way it does.

Instead of me quoting or paraphrasing the various suggested answers to this question, from another source, it may be easier for you to read them yourself. The linked article is from the Daily Plant out of NYC, NY and tries to answer the question of why sycamore sheds its bark through an interview with Dr. Marc Abrams, Professor of Forest Ecology at Penn State University.

Oh yeah, back to the identification of the tree. If the bark doesn’t nail down the ID of this tree for you, perhaps the many fruit balls that typically dangle from the branches will do it.

Sycamore has fruit balls on its branches all winter.

The fruit hang from the tree throughout the winter, are about an inch to an inch and a quarter in diameter and, unlike the spiky seed balls of Sweetgum, are soft to the touch.

On your next walk through Explore the Wild stop at the platform midway down the boardwalk. If you stand facing the Animal Footprints Exhibit and look about 40 degrees or so to your left you’ll see an excellent example of an American Sycamore with all of the features described above. Another is about the same amount of degrees to your right. If you look about 90 degrees to your right you’ll see a slim, straight sycamore which is nearly all green, all the way to the crown. Why is this sycamore green and not white? That one will have to wait for another day, maybe.

Enjoy your walk.

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