Bill Pramuk

Bill Pramuk: Trees and People

Last week, watching a documentary on the life of Dr. Marian Diamond, a UC Berkeley professor of anatomy, it occurred to me that her work on brain anatomy evokes some comparisons with tree anatomy.

Diamond’s research on the brains of rats led to a revolution in our understanding of brain development and anatomical change in response to environment now commonly called “neuroplasticity.”

Before her research came out, it was considered dogmatic that the structure of the brain was predetermined by genetics: fixed and unchangeable. She showed either enriched or impoverished environments, at any age from prenatal to old age, can alter the structure of the brain.

In her rats, an enriched environment with toys and companions changed the anatomy of rats’ brains.

The cerebral cortex of “enriched rats” was thicker than that of “impoverished rats,” allowing for greater learning capacity.

This concept has all kinds of implications. Personally, I expect mental challenges, such as writing arborist reports, learning new music, even doing the daily “Jumble” word puzzle in the newspaper, help to maintain my brain function and build cerebral cortex-mental muscle.

So far, so good.

Now, with respect to trees, I avoid anthropomorphizing. After all, trees and people occupy separate kingdoms in the “Tree of Life.”

A tree has no localized brain nor central nervous system, yet it has the capacity to physically change in response to environment.

For example, consider leaves. One might expect the leaves of a particular tree species to be fairly uniform. In fact, as stated in “Physiology of Woody Plants” (Kramer and Koslowski): “Generally there are important differences in leaf morphology and the photosynthetic machinery between sun and shade leaves.

Sun leaves are generally smaller and thicker and have greater volume and more chlorophyll per unit of leaf area than shade-grown leaves.” Light intensity influences leaf size, thickness, and function.

So do wind and gravity. Think of windswept trees and old, heavy trees. They show structural characteristics resulting from environmental conditions.

In a process called “thigmomorphogenesis,” trees and other plants respond to mechanical disturbance.

Heavy limbs respond to weight and wind forces by adding wood where needed for support. It is called “response growth” resulting in “reaction wood”, visible in the muscular appearance of old trees.

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Another good example comes from the work of Dr. Richard Harris at UC Davis in 1971. Studying the effects of trunk movement and staking of young trees, he showed “When the upper stems of greenhouse sweet gum (Liquidambar trees), and corn are moved as little as 30 seconds a day, height growth is reduced by 25 percent or more.”

(Local landscape contractor Steve Clerici once told me one of his assignments, taking classes from Dr. Harris, was to shake the Liquidambars for 30 seconds a day.)

Conversely, tightly spaced nursery-grown trees experience little movement, resulting in tall slender trunks. Such trees are less able to stand on their own when subjected to wind.

The same principle applies to trees that grow up sheltered in a forest or cluster. When adjacent trees are removed, they lack the structural strength to resist new wind forces.

Effects like this are also seen in tree limbs that cross or rest on adjacent limbs. They add wood only where it is needed for support. If the supporting branch is broken or pruned away, the weak resting branch tends to fail.

In his interesting book “The Hidden Life of Trees”, Peter Wohlleben refers to response growth in forest trees exposed to new wind forces after forest thinning as “learning stability”. He says: “Conceivably, (the root system) is where the tree equivalent of a brain is located.”

Trees do not have brains as we think of brains, but they seem pretty smart. It is something to think about.

Bill Pramuk is a registered consulting arborist. Visit his website,, email questions to or call him at 707-226-2884.