The general identification terms of sapwood and heartwood are used in many aspects of the plant sciences, especially in arboriculture. These terms are used loosely by many practitioners to identify the outer portion of a branch or stem and the innermost part of woody stems. Why do we need to recognize sapwood from heartwood? It’s all wood, right? Well, yes, however, the rationale behind the importance of recognizing these components within the tree is quite important in many of our arboricultural interventions. A couple of common applications include plant health care and pruning.

Image 1: Identification of wood anatomy is important in all aspects of arboriculture. Graphic by Jeff Harris.

Sapwood significance
Let’s begin by talking about each wood type to understand its physiological importance. Sapwood is fundamental to water transport. (Image 1) The sapwood is rich with living parenchyma cells, critical to many tree processes. Sapwood is critical for storage as well as transport, physical support and defense. Trees rely on the strength properties of outer sapwood to remain standing through loading forces such as wind. Exterior to the sapwood is the vascular cambium, which is the thin layer of tissue producing new xylem on the inside and new phloem (inner bark) on the outside. This meristematic growth allows for the continual replacement of sapwood vessels.

Many trees retain from three to 20 sapwood increments; some diffuse-porous hardwoods and conifers have more. The ratio of sapwood volume to heartwood volume reduces with age (meaning there is more heartwood than sapwood by volume), because the number of increments of sapwood remains fairly static. This happens because the amount of sapwood, which is responsible for transporting water and nutrients, remains relatively constant. Meanwhile, heartwood, which is the older, non-living wood, continues to accumulate as the tree grows. The genetically pre-programmed death of sapwood cells in the living tree results in the formation of what we call heartwood.

Heartwood happens
Heartwood is chemically altered wood defined as xylem that does not conduct water and has no living cells. (Photo 1) The formation of heartwood occurs in a transition zone between the sapwood and heartwood. In most hardwood tree species, phenolic compounds, called extractives, are synthesized in the parenchyma and deposited in the walls and spaces between adjacent cells. It is these extractives that give some species’ heartwood its distinctive color and odor.

Various chemicals, such as tannins and phenols in hardwoods and resins and terpenes in conifers, are deposited through rays into the dying sapwood and are responsible for the decay-resistant properties of some tree species. Ray cells grow radially across the grain in small layers that extend across the growth increments of xylem and into the phloem. Rays transport carbohydrates and other compounds into sapwood, store carbohydrates and assist in restricting decay in wood tissue through specialized chemical properties.

When does this transition occur? In temperate-zone areas, heartwood begins formation in summer as the season progresses, ceasing during dormancy. The means of heartwood formation isn’t well understood, and the amount and rate of sapwood and heartwood formation varies greatly with tree species, tree age, rate of growth, environmental conditions and cultural practices.

Heartwood formation usually begins in some species of angiosperms, such as eucalyptus, at five to six years; in several species of pine at 15 to 20 years; in some ash species at 60 to 70 years; and in beech at 80 to 100 years. There are a few species, such as a few Acer species, Betula, Salix and Populus, that may never form heartwood, as living parenchyma cells persist in sapwood even to the center of the stem. This is why defining these wood types in the field is sometimes impossible.

Photo 1: Heartwood and sapwood may be defined by color variations. All photos courtesy of the author.

It’s all about location
The real challenge for arborists is how to locate heartwood and sapwood in a branch or stem. (Photo 2) In the field, the identification of wood types is generic, using visual methods. Typically, we rely on color differential for the demarcation process. Experience and understanding of physical differences such as color and grain pattern may be applied to identify the distinctions between hardwood and sapwood.

There is often a huge variation in the width of sapwood between tree species. For example, just one to two growth rings make up the sapwood in northern catalpa (Catalpa speciosa), while black gum (Nyssa sylvatica) has been identified with 100 growth rings with living cells.

Several coniferous species have been found to contain more than 100 (sapwood) growth rings. The complicating factor in the identification of sapwood and heartwood is that they are not always different in color. Additionally, color difference can occur as the result of wound-induced discoloration (WID). This is wood that is discolored as the result of wounding or other processes, such as infection by decay fungi. WID may resemble heartwood with respect to its position in the tree and its darkened color.

Visually, sapwood is generally light colored, living and located between the heartwood and the bark. In fact, the heartwood is formed because of dying sapwood. Tree age, health and growing conditions can impact the width of sapwood. As mentioned earlier, sapwood becomes heartwood with the death of the parenchyma cells. It is difficult to show the precise zone where this takes place even with the most sophisticated techniques, however.

Sapwood can be distinguished from heartwood using various techniques. The physical method is based on natural color differences between sapwood and heartwood, and is referred to in this text as “color method.” A second method is a chemical technique, which uses a stain to differentiate between the two based on the pH differences in both sapwood and heartwood, and is referred to as the “stain method.” Basically, wood identification is a challenge due to the many variations in species and development. The complicating identification factor is that sapwood and heartwood, if present, are not always different in color, making them difficult for the arborist to distinguish.

Application to arboricultural practices
So how and where does this knowledge apply to arborists? Why does it matter whether we can identify or distinguish the differences between the wood types? Let’s start with a brief application to plant health care (PHC) and more specifically, to trunk injections. Typically, this involves mechanical wounding in the form of a drilled hole for port or feeder tube insertion.

Best practice indicates that to effectively manage a tree pest, manage tree growth or treat a nutrient deficiency, a PHC technician selects the appropriate product and delivery technique. For trunk injections, this often involves drilling into the surface of the trunk directly and into the sapwood for distribution within the tree. (Photo 3) However, drilling depth is a critical part of the process to recognize.

Injection-hole depth is typically determined by the length of the injection equipment, bark thickness and relative amount of active conducting vessels. Ideally, the drilled portal will be in active xylem. This depth will vary with tree species. During the drilling process, if heartwood or discolored wood is evident in the wood shavings, heartwood or decay-infected wood has been penetrated by drilling, and the hole may be deeper than desirable. Once again, this assumes there is heartwood development and a color differential.

Photo 3: Drilling into the root flare for infusion ports with various wood-shaving colors.

Pruning procedures
Another important application of wood identification includes pruning procedures. This intervention can be serious when removing live tissue by exposing wood tissue that is vulnerable to infection by decay or canker fungi. The functional application of wood-type identification is to determine the maximum pruning-cut size necessary to meet objectives while minimizing decay potential created by pruning wounds. In essence, this exposes the least amount of heartwood possible and facilitates effective compartmentalization of the wound.

In most cases, removing small-diameter branches of 1 to 2 inches is of less consequence. However, pruning through live branches more than about 3 or 4 inches in diameter can cut through nonliving heartwood, which, in general, is less resistant to decay than cuts made only through sapwood. (Photo 4) The lack of active parenchyma cells means there is very little active response to resist the spread of decay-causing organisms into the trunk following any large-branch removal cuts. When heartwood is likely present in a large branch, consider a series of reduction pruning cuts, instead of a large-diameter removal cut to the trunk. Also, heading cuts may be an option to delay the onset of decay entering the main trunk or stem.

Minimize cutting heartwood
When it comes to pruning, smaller wounds are always preferable to larger wounds. Its all about recovery. Smaller pruning cuts reduce the exposed area needing to be occluded through the compartmentalization process. Larger pruning wounds require much more time to fully occlude, remaining as potential sites for decay-fungi colonization for extended periods. These factors should be considered when deciding on pruning locations in the crown or on the trunk, particularly on older, established trees.

It is always preferable (from a tree-health perspective) to have a series of smaller branches removed or larger branches reduced, rather than complete removal of a single large branch on the trunk. During the pruning process, any time discoloration is recognized, consider stopping or backing off on pruning further back into the branch. The discoloration could be either heartwood or wound-induced discoloration.

Photo 4: Smaller cuts are primarily sapwood, while larger branch cuts include heartwood, which is prone to decay.

CODIT characteristics
Also, pruning-wound size requires arborists to be familiar with CODIT (compartmentalization of decay in trees) characteristics of the tree, relative to weak compartmentalizers and effective compartmentalizers. For example, there could be a 3-inch branch cut necessary on Acer saccharum. (Photo 5) They are known to have mostly sapwood and little if any heartwood. However, they are recognized as poor compartmentalizers.

The informed arborist must take both sapwood content and CODIT capabilities into consideration in the pruning-cut decision-making process. If in doubt, always default to the fact that pruning cuts should be made to create the smallest wound possible while meeting pruning objectives.

Mature-tree considerations
Finally, regarding the pruning implications for older, mature trees, be cut cautious. Many older, veteran trees survive with only a thin band of sapwood (living wood), and removing even moderate amounts of live, green tissue will further reduce food production. The theory is that older trees have a low sapwood-to-heartwood ratio, and removing live branches will lower the ratio even more, reducing resources and increasing stress.

In contrast, more live tissue can be removed from a younger tree to accomplish certain objectives, because of their higher sapwood-to-heartwood ratio. Because decay can enter these removed branches, consider implementing more reduction cuts or, if necessary, heading cuts. For older, mature trees, remove as few live branches as practical to accomplish objectives and maintain pruning cuts as small as possible to minimize decay.

Photo 5: This 3-inch Acer branch contains sapwood to the center. The darkened areas are likely wound-induced discoloration.

Conclusion
The identification of wood components is necessary to guide our arboricultural practices, such as tree removals, pruning and PHC. The ability to define the locations of heartwood, sapwood and wound-induced discoloration is important for several arboricultural purposes – and it is a challenge to be sure! This skill takes practice, experience and research to recognize each, as well as knowing tree-species’ propensities for development of these wood types. There is much more research needed to help with the process. Also, consistency in terminology found in our industry literature is necessary to be clearer in our communications as we continue to advance arboricultural science.

Citations and references
Thanks to Chris Luley for his contributions to this article.

Luley, Christopher J. 2022. Wood decay fungi common to urban living trees in the Northeast and Central United States. Second edition. Urban Forest Diagnostics LLC, Naples, NY.
Purcell, Lindsey. 2024. Arboricultural Practices; A science-based approach. Waveland Publishing.
Hirons, Andrew & Thomas, Peter. (2017). Applied Tree Biology. 10.1002/9781118296387.
Hillis, W.E., 1987. Heartwood and Tree Exudates. Berlin, Springer Verlag: 268.
Hillis, W.E. Distribution, properties and formation of some wood extractives. Wood Science and Technology 5, 272–289 (1971).
Kozlowski, Theodore T. and Pallardy, Stephen G. Physiology of Woody Plants (Second Edition), 1997.
Wiedenhoeft, Alex & Miller, Regis. (2005). Structure and Function of Wood. Handbook of Wood Chemistry and Wood Composites.
Gilman, E. F. (2011). An illustrated guide to pruning (3rd ed.). Delmar Cengage Learning.

Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) Registered Consulting Arborist (RCA) and principal with Lp Consulting Group LLC in Cortez, Florida. He spent many years as an urban-forestry specialist and teacher in the Department of Forestry and Natural Resources at Purdue University and serves as the executive director of the Indiana Chapter of the International Society of Arboriculture.