Q. What is brown maple? We seem oftentimes to see cracks in this colored wood and wonder why? Is it drying or storage?

A: Let me answer your great question by giving you and the readers some background information. When cells are first formed in the tree -- on the outer circumference of the tree right under the bark -- their main role is to transport water from the roots to the leaves. As a result these cells are fairly wide open and are white in color. They are called sapwood.  After a few years, these sapwood cells age and convert to heartwood. This aging process often includes the deposition of various chemicals that give wood its characteristic odor, color, decay resistance, and so on. In some species, these chemicals fill the hollow space in a cell so well that the wood is nearly impervious to liquid water...that is why white oak is used for wine barrels without leaking.

These age-altered sapwood cells, now called heartwood, no longer can conduct liquids from the roots to the leaves. Oftentimes, these cells lose moisture as they age. In some species, conversion to heartwood might begin in three years and in others, like maple, in 50 to75 years. This is why many large maple logs are all white wood; the formation of the brown heartwood has not started. On the other hand, within 5 to 10 years, oak, walnut and cherry begin the conversion.

Now, let's assume that a maple tree gets injured (wind or ice storm, insects or woodpeckers, taps for sugar, etc.) Bacteria, viruses, or fungi can enter the wood and cause damage to the living tree, potentially killing the tree. The tree, not having antibodies like we do, has developed a system that encapsulates the infected area, closing this area off to liquids and access to the rest of the tree. Sapwood quickly converts to heartwood in this area; this heartwood can be brown or grey or even greenish. Because this is what we might call "unnatural" or "unusual" heartwood, as it is not age related, we often call this wood "pathological heartwood," because it its formation is triggered by a pathog[1] en.

The wood in this enclosed area is often subject to severe weakening, due to the action of the pathogens, and may even dry out somewhat. This heartwood may even have some cracks develop in the living tree. Now, when the tree is sawn into lumber, we will see this pathological heartwood as brown, often dark brown, areas. After drying, even under normal drying conditions, the pathological heartwood may have open cracks that result because the damage existed in the living tree or because the wood was so weak before drying began.

In short, cracking, often called checking, is inherent in brown maple.

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.

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