When looking at your customers’ trees, what do you see? Leaves. Branches. A trunk with bark. Flowers and fruit. Sometimes, surface roots, especially if you’re looking at a baldcypress or silver maple.

But, what don’t you see?

On average, you don’t see at least half of the tree. What’s missing from the visual? In most cases, lots of tree tissue isn’t visible — sapwood, heartwood, most of the roots, cambium, soft inner tissues of unopened flowers and leaf buds — yet, just as important as the structures that you can see.

Why focus on the unseen? Several reasons. First, new tree workers usually don’t know much about the inner tree tissues and certainly not how they function and why they’re critical to overall tree health. Secondly, your customers usually know even less than new tree workers. Third, it’s key to the success of an arborist’s business to explain their significance to both groups and perhaps get a refresher, as well.

Red stain indicating movement in the normally unseen sapwood.

Name the parts

A highlight of one of the educational activities I’m involved with targets fifth-grade students, teaching them about the importance of trees in urban landscapes and natural areas. One of the core concepts is the parts of a tree and their function. I really enjoy quizzing the kids on this, holding up wood slices and pointing to various tree tissues to encourage discussion.

I use the discussion to point out the importance of bark and what’s underneath … using the opportunity to tell them not to run a lawn mower into the base of a tree, telling them to stop when they start driving over mulch; before, not after they hit the trunk. Most of the fifth graders can relate to this concept, as this seems to be the time that kids start mowing lawns these days.

Because it’s hard for fifth graders to name too many parts, I usually limit it to just four: bark, cambium, sapwood and heartwood. In many cases, curiosity gets the better of them, and they start asking about other structures, tree planting and even uses for the wood when trees die.

I’ve found after 30 years of doing this, one thing has stayed the same: No matter what part of the city/country the kids are from, how much money their parents make, how many other family members they have or any other demographic, the more their teacher works with them and the more mutual respect the student and teacher have for one another, the better the learning and more behaved the students is. So, hats off to good teachers out there!

Moving parts

Some tree parts move and some don’t. The time-honored tradition of carving initials in a tree when it’s young and expecting them to move upwards over time as the tree grows taller is an apt description of those that don’t.

So, which ones do? Actually, most of them, both visible and nonvisible. Many tree parts move over time, such as roots and shoots growing longer and thicker.

The effects of storm damage often reveals the sight of heartwood, which is usually out of sight.

This is one of the relatable elements to my presentation to the fifth graders. When I’m pointing to the wood slice, inevitably, one of the kids gets a pondering look on his or her face and then enthusiastically raises their hand and asks how old the tree is and, if I don’t know, offers to help count the number of annual rings to find out.

Thankfully, Mother Nature made most tree parts with flexibility to move in rain and windstorms. It’s amazing to watch the canopy of a tree whip and bend back and forth as the forces of nature rail upon it.

Of course, in some cases, the force of the storm exceeds the capacity of the tree to recover, and roots become loosened and branches break off.

Short of a root observation chamber, washed away soils provide the best insights into “the other half” of a tree’s parts.

While these occurrences may seem tragic, they can also be a profit center for tree care companies to inspect for loosened roots, leaning potential and hangers in the canopy. As well, damage repair can often be provided for clients, with appropriate associated charges.

In addition to the tree parts moving themselves, water and nutrients move in the tree – in soft tissues – from roots to shoots, flowers and fruit. As with the inner tissues themselves, it’s pretty hard to see the movement, unless you happen to cut a branch off and the sap flow is strong at that time of year. In order to illustrate the movement of injectable fertilizers and pesticides, university researchers and product manufacturers sometimes hold workshops and demonstrations to show how the equipment and products work. In order to make the invisible visible, colored dye is mixed with water to replicate movement of the actual injectable product and tracks the movement inside a tree from initial injection to the likely endpoint.

Most products tend to be translocated more thoroughly on warm, dry days in spring when evapotranspiration is high and soil moisture is adequate.

When can you see them?

Seeing the dye stain in the sapwood and cambium above the injection point is great on the day of the demonstration, but what other times or locations are the nonvisible tree parts available to inspect? The most common and accessible is the aforementioned opportunity of the aftermath of a storm. In extreme cases, all inner tissues are exposed, including the inner workings of the buttress roots and root hairs. When restorative work is being accomplished, and you have a little time to rest, direct your attention to identifying various inner tree parts.

Four less-than-obvious opportunities are when smoking meat (my personal favorite), cutting firewood, during woodworking and after a baseball bat is broken. Maybe you didn’t think of it this way, as the primary focus of these activities and objects are for other purposes, but anytime you get the chance to see the unseeable, it should be taken advantage of. Splitting logs open and cutting them to size is a big help in getting the glimpse inside. If working with another person, challenge each other to identify various parts and the relative strength associated with each one.

Another less-than-common opportunity to look at tree parts that normally aren’t seen is in root observation chambers at research field days. Actually, there are more such facilities than you might think. I first became acquainted with the concept in graduate school with observation of turfgrass roots after various herbicide, plant growth regulator or fertilizer treatments were applied. If you want to know the full effect of these products on plants, it’s wise to look at all parts, over time, without disturbing them. One of the more famous tree root observation chambers is in Houghton, Michigan, at a U.S. Forest Service facility. A variety of root expansion and development experiments are conducted at this laboratory and others in Florida, Minnesota, Vermont, Maryland, Canada and Europe.

Finally, the unfortunate occurrences of erosion and trenching exposes roots and the base of trees to those willing to pay a little attention. With a little patience and an inquisitive mind, information about what these structures look like in the real world and how they compare to the drawings in textbooks can be of present and future use.