Is this tree sick? That may be a natural question to ask when looking at the striking difference in the bark pattern on this white oak. In fact, it is an infection, but not a harmful one to the survival of the tree. This is called smooth patch and is caused by a fungus that infects the outer bark of oak and some other tree species. There are several species of fungus that may cause this appearance, but the most common is Aleurodiscus oaksii. The fungus gradually decomposes the outer layers of bark, producing the flattened and smooth surfaces which contrast with the rough bark that is uninfected. If you look closely during moist periods, you may see the small cup-shaped fruiting bodies of the fungus on the bark. It is not a threat to the health or integrity of the tree as only the outer bark is impacted.

This isn’t the same species of tree, is it? Yes, it is! Bark pattern differences in some species may reflect the growth rate or relative vigor of the tree. These photos show black walnut bark from two very different growing sites. The blocky, alligator-hide bark pattern is often an indication of slow growth and may result from a tree growing on a very difficult site, like the dry hilltop where this tree is located, or severe competition from surrounding trees. The strongly interlacing ridges of the bark in the second photo are from a black walnut growing in a plantation on a creek bench with deep, moist and fertile soils, resulting in more vigorous growth.

What happened here? The pattern in a horizontal line across these trees looks too perfect to be natural, but it was not produced by a human. This is the feeding activity of the yellow-bellied sapsucker, a species of woodpecker that migrates through Indiana. The sapsucker, true to its name, uses its sharp bill to create holes through the bark that ooze sap. The sapsucker feeds on the sap and may also feed on insects drawn to the sap. They tend to create straight lines of holes that leave this diagnostic pattern behind, even long after the birds have ceased their activity. This activity is usually not fatal to the tree, but does cause some damage to the wood beneath the bark, so trees that have been used by sapsuckers may have discolored spots or other blemishes in the wood revealed when processed for wood products. To see images and learn more about sapsuckers visit American Bird Conservatory Website.

Humans also may leave a history of our presence on trees. This American beech tree in a recreation area has recorded the comings and goings of visitors, with some apparent editing as well! I do not endorse this activity as it can be harmful to the tree by introducing avenues for decay and disease, and is generally prohibited on public property. Instead, enjoy reading the nature stories found in bark.