Some of the most beautiful trees in winter are sycamore, birch, crape myrtle, and other species with exfoliating bark. Thin layers of bark peel off the trunks and branches of these species, which gives them an interesting, flaky texture like the top layers of an oven-crisped spanikopita. Before they blow away, the layers come in a range of browns, reds, oranges, and grays. Adding to the chromatic richness, the smooth new bark underneath is often multicolored, sometimes even green.
Botanists still aren’t sure why certain trees have evolved exfoliating outer bark. Outer bark is made of dead tissue. Non-exfoliating trees like oaks have trunks and branches that develop furrow-like gaps in their outer bark as they grow in diameter. Each spring, living, inner bark tissue forms a new protective coat for the tree. For exfoliating trees, which have smooth outer bark, the peeling––and the formation new layers––may allow the tree to safely expand.
Another possibility is that trees with thin bark evolved exfoliation to deal with pests. Thick bark confounds insects, bacteria, and fungi that might otherwise tap the sugars that flow beneath the bark. Exfoliating trees, by shedding their bark, also shed themselves of perilous hangers-on. The scientific jury’s out, but the verdict from my one-woman aesthetic jury? Thumbs up on exfoliating bark.