“…if you want to save your world, you must save the trees.”

J.R.R. Tolkien

If you’ve ever counted the rings on a tree stump to determine a tree’s age, you have observed how trees grow: they add a new layer of wood in each growing season. The new growth doesn’t come from the center of the tree trunk — it all happens on the outer ring. The living part of a tree trunk is just inside the protective bark.

The living part of the tree is closest to the outside

Immediately under the bark is a layer of spongy tissue called the “phloem” (pronounced “flow-um”). The phloem carries carbohydrates, manufactured by the leaves during photosynthesis, downward to the rest of the tree. The upward flow of water and nutrients from the roots happens in the “xylem” or “sapwood,” just inside the phloem. Sandwiched between these two critical vascular layers is the “cambium,” only a few microscopic cells thick, which is the actively growing part of the trunk. The cambium produces a new layer of phloem on the outer edge, and a new layer of xylem on the inside, each season. 

As a tree ages, the xylem closest to the center of the tree hardens from the inner trunk outward with compounds that make the wood strong. The hard inner layer of the xylem, which is no longer alive and no longer carries water, is called “heartwood.” The older the tree, the more heartwood there is at the center, and that is the part of the tree we use for wood products. 

But the living part of the tree is in those outer layers, just under the bark. Looking at the tough exterior of an old tree, it is easy to forget that the tree’s essential life process is just a few millimeters below that rugged outer surface — and it’s very vulnerable to damage.

Cutting through the living tissue of a tree trunk all the way around the tree is called “girdling” and is a very effective way to kill a tree. Girdling cuts off the essential movement of nutrients up and down the tree, and severs the cambium, preventing growth. Girdling is sometimes used intentionally to kill trees before cutting them down. Unfortunately, it also sometimes happens unintentionally.

Understanding that the outer layer of a tree trunk or branch is the most vulnerable part should make us think more carefully about how we use trees and what we allow to interfere with a tree’s growth.

One common mistake is leaving tree support stakes in place too long. Unless a new tree is very top-heavy, it is usually unnecessary to stake it or use supports at planting. But if it is necessary, care should be taken to remove the supports well before the tree begins adding significant growth at the trunk.

Tree supports, if used at all, should be temporary and not made of material that will cut into bark – rope is not a good choice
A tree girdled by improper staking will not survive
Photo: Guy Pardee
Improper cabling can girdle a tree
Photo: Guy Pardee

Hanging swings, hammocks, and bird feeders from tree branches can also cause damage. Allowing tree trunks or branches to rest against power lines, big rocks, and fences is another hazard.

 A tree swing hangs from a branch on a rope
Power lines often interfere with tree growth and can girdle branches
The branch has grown around the rope, cutting off nutrient flow to the branch
Better to remove the branch before this happens
Tree trunks may be girdled by pavement and rocks that prevent the trunk from continuing outward growth

Another common error is incorrect planting that can result in a tree being girdled by its own roots. If a young tree has been in a nursery container too long, the roots may wrap around the base of the tree within the container. At planting, care should be taken to gently unwrap the roots and extend them away from the trunk in a hole wider than it is deep.

Tree trunk girdled by its own roots

Though trees may survive partial girdling by roots, they will be more vulnerable to storm damage and disease. Consult an experienced certified arborist to evaluate possible remedial action if the tree and girdling roots are in an advanced stage.

Although tree trunks merge with tree roots near the soil line, it is important to recognize that trunks and roots are not the same. Tree trunks “flare” at the base just above the root zone. If the flare is buried beneath the soil line, or by too much mulch, excess moisture can cause the protective outer bark to rot, exposing the living inner layers to mold, fungus, insects, and disease – effectively girdling the trunk at the soil line.

Healthy tree trunks “flare” outward at the base and should be above the soil line
Planting a tree too deep, or allowing mulch to cover the “flare,” can girdle a tree, eventually killing it

Lawnmowers, construction equipment, deer, and invasive vines all can damage trees by wounding the living outer layers of the trunk.

 Lawnmower damage can be prevented by providing a protected area between lawn and tree trunks
Invasive vines, including ivy, should be removed from tree trunks
Wrapping young trees with protective fabric can prevent deer damage, but don’t forget to remove it later!

Trees are strong enough, and adaptable enough, to share their life forces with other living things. They will work around a hole that supplies shelter for an owl or squirrel family. They can survive for decades with a virtually hollow interior because their living and growing happens on the outside. But it’s up to us to protect them from careless damage so they can continue to save the world.

For more on tree care, make sure to read last week’s post,
For the Love of Trees

THIS BLOG IS AUTHORED WEEKLY BY CATHY LUDDEN, CONSERVATIONIST AND NATIVE PLANT EDUCATOR; AND BOARD MEMBER, GREENBURGH NATURE CENTER. FOLLOW CATHY ON INSTAGRAM FOR MORE PHOTOS AND GARDENING TIPS @CATHYLUDDEN.
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Published by Cathy Ludden

7 thoughts on “For the Love of Trees: Part 2”

  1. This article beautifully highlights the many reasons why trees are essential for our planet’s health. Their role in providing clean air, reducing pollution, and creating habitats is truly remarkable.

    Beyond the environmental benefits, I’m also curious about the impact trees have on fostering a sense of community. Do you see community tree planting events or volunteer opportunities related to tree care as a way to build connections and encourage a sense of shared responsibility for the environment? Perhaps you could share some examples of how the Greenburgh Nature Center uses trees to connect people in the community.

    Reply

    1. Yes! Trees provide a sense of place and care that benefits community feeling. In urban areas, tree planting adds much-needed shade and stormwater control to reduce heat and water damage. At the Nature Center, we plant trees annually in our 33 acres as a regular part of land stewardship. Arbor Day and Earth Day celebrations include tree planting. Our summer interns plant native trees every year — sugar maples and paw paws add to our educational programs on maple sugaring and indigenous peoples. We are watching our beech trees suffering from disease and have begun planning to remove diseased trees and reforest with a variety of trees native to our eco-region. It’s all great for community building!

      Reply
  3. Pingback: For the Love of Trees: Part 3 - Greenburgh Nature Center

  4. You’re welcome, Linda. A daily walk around my own neighborhood makes it clear that many people don’t understand how easy it is to accidentally kill our beloved trees. A little more knowledge really helps!

    Reply

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