Sapwood is the living, outermost portion of a woody stem or branch, while heartwood is the dead, inner wood, which often comprises the majority of a stem’s cross-section. You can usually distinguish sapwood from heartwood by its lighter color.
But, color in wood can be very misleading; not all heartwood is dark and not all dark-colored wood is heartwood. And, the relative amounts of sapwood and heartwood in any stem can vary greatly among individuals, species, and growing conditions. So, for a more accurate – and less specious – distinction, we need a more complete understanding of what wood is and how both sapwood and heartwood form. This won’t hurt.
All wood starts as sapwood. It is formed just under the bark by a thin layer of living cells known as the cambium, which produces bark cells to the outside and wood cells to the inside. Tree stems increase in girth during each year of growth because a new layer of wood cells is added inside the cambium. In good growing years, this new layer of wood can be many cells thick, and in poor years, it is relatively thin. Regardless of thickness, when any such growth occurs, the cambium moves outward to accommodate the new layers of wood forming inside. Sapwood – this newly formed, outermost region of wood – contains a variety of cell types, most of which are living and physiologically active. This sapwood is where water and dissolved minerals are transported between the roots and the crown of the tree and, to a lesser extent, where energy reserves are stored.
In young trees and young parts of older trees, all of the wood in the stem is sapwood. But as the tree gets older and its trunk increases in diameter, things change. No longer is the entire cross-section of the trunk needed for conducting sap. This, combined with an increased need for structural support, causes significant changes in the wood. The cells nearest the center of the trunk die, but they remain mostly intact. As these older sapwood cells age and die, they become heartwood. That is, they are altered to accommodate a shift in function. As residues of the once-living cells and additional chemical compounds from elsewhere in the plant accumulate in the heartwood, those cells cease to transport water or store energy reserves.
These compounds (including resins, phenols, and terpenes, sometimes referred to as extractives) not only help make heartwood more resistant to attack by insects and decay organisms but also tend to give this inner portion of the stem a distinctive darker color. For example, the famous dark brown of black walnut lumber and the striking red hues of black cherry boards occur only in the heartwoods of these trees, and both owe their characteristic colors to these chemicals.
Such woods are highly prized largely because of their colored heartwoods, but it is important to remember that color alone is not the sole distinction between sapwood and heartwood, regardless of species. Indeed, wood can be colored for reasons unrelated to heartwood. There are many discolorations associated with injury or fungal infection of wood, for example, and some heartwoods – including that in most spruces, fir, cottonwood, and basswood – are naturally very light colored. Then again, if these light-colored heartwoods are injured, they often do become darkened by discoloration. So, in summary, sapwood, which is nearly always light colored, results from new wood formation. Heartwood, which is often – but not always – dark colored, results from the natural aging process of the tree. But both can be discolored by many other causes.
Typically there is less sapwood than heartwood in any given stem. The exception, of course, is in young trees and the youngest portions of stems and branches on older trees which – because they are young – are naturally dominated by sapwood. The proportion of heartwood to sapwood in the main stem does vary with species. Black locust, for example, usually has a very narrow band – often less than an inch – of functioning sapwood, whereas maple stems often can have many inches of sapwood and relatively narrow cores of heartwood. In general, more vigorously growing trees tend to have wider bands of sapwood.
This sapwood-heartwood distinction has important implications for woodworkers beyond the obvious implications of color. Because sapwood contains the sap-conducting cells of the tree, it tends to have a relatively high moisture content. This is good for the living tree but it is not so good for the woodworker, because sapwood tends to shrink and move considerably when dried, and it is much more susceptible to decay and staining by fungi.