I once asked a nursery co-worker I respected for his knowledge and years of experience: “Why do trees reach a certain height and then stop growing taller?”
His answer stopped me cold. “It is in their genes.”
Reflecting on this, I now see that answer as facile. Well, of course it must have something to do with genetics, but what? And what about environment?
Questions like this serve as a small door into a very large space.
Reviewing literature, I found references showing genetics and environment both influence tree size.
Coast redwoods: the current champion, Hyperion, measured 379 feet tall when it was discovered have inherent anatomy that enables them to grow tall by lifting essential water to lofty heights, but they achieve their greatest potential only where the climate is right.
Hand-in-glove with the genetically determined structure of their internal hydraulic system is the moderate and foggy climate where the redwoods are able to grow so tall.
Historically in prime redwood country, temperatures have ranged from lows rarely below 40 degrees Fahrenheit to highs rarely much above 70 degrees F. That means that their hydraulic system is unlikely to be interrupted by “cavitation,” breaks in the continuous internal water column, which result from freezing and heat stress.
In addition, a daily bath of ocean fog reduces water stress.
Even so, there are limits to tree height. An article in the International Journal of Science, “Limits to Tree Height,” considers water transport constraints as the best current hypothesis, and gives a theoretical maximum tree height of 426.5 feet.
At great heights availability of water and minerals decrease, and the tree responds accordingly. In some instances, tops die, lose vigor, and in some cases the foliage becomes smaller.
I have seen this in some of the upper branchlets blown out of tall redwoods in storms. They have compact, prickly foliage, very much like that of giant Sequoia (the Sierra redwoods), in contrast to the broader, flatter needles on lower branches.
How trees raise water to great heights is explained in “cohesion theory.” It may not be the final word but it explains a lot. Water molecules tend to stick together. Check it out by putting a drop of water on a glassy surface then try to pull it apart with a pin or knifepoint. It does not readily separate into two drops. Instead, it stretches out.
In trees, water evaporating from leaves causes internal tension, a stretching of the continuous water column, which extends from the soil to the leaf surfaces in segments: “vessels” or “tracheids” depending on the type of tree. Water moves in from below, equalizing the tension. When the stream is interrupted — cavitated — trees need to respond or begin to lose upper portions.
Consider large old valley oaks. It is quite common for them to show reduced growth and thinning of foliage in the upper canopy while lower branches continue to show good vigor.
The most likely cause is reduced availability of water and minerals, which can result from a long history of droughts, wounds, root disease, and root zone damage, all of which can cause cavitation.
In response, trees shed parts they can no longer sustain. Redwood treetops die. Over-mature valley oaks “retrench”.
That latter term, defined as a reduction in spending in response to difficulty, has taken on a new meaning in some circles of tree care.
Though “retrenchment” is not an official system of pruning, many arborists mimic the process by reducing the size of large, old trees. The ANSI Standard pruning system that can be used to imitate retrenchment is “reduction pruning”.
For older trees, especially those that tend to shed large limbs, reduction can be a wise choice, but it must be employed with good reason and thoughtful consideration of tree species, risk management and tree health.