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Saving Nature's Shaders
Shade trees are an oasis to many Texans during June, July and August. There's nothing quite like sitting under a leafy umbrella when the temperature is more than 90 degrees.
But one of the state's most dominant shaders, the post oak, may die from insufficient water absorption at any time of the year, not just during dry summers.
University of North Texas biology professor Don Smith is saving as many as the trees as possible with his homemade treatment.
"I grew up in West Texas without many trees. I don't take them for granted," he says.
The post oak, or Quercus stellata, thrives in warm climates from Texas to Georgia. Post oaks may grow up to 50 feet tall with trunks 2 feet wide, and may live 100 years or more.
Yet most of these rugged trees are hypersensitive, reacting to any root system disturbance by slowing down production of root hairs.
These single-cell extensions of epidermal cells are located on new roots smaller than a little finger. Root hairs live less than a month, but absorb more than 95 percent of a tree's water and minerals, Smith says.
"Smaller roots do not necessarily absorb more water than larger roots, but they make up a large fraction of the total surface area available for absorption," he says. "Very little absorption occurs in roots much more than an inch from the growing tip, so a tree must constantly produce new root hairs to thrive."
Smith says he first became aware of post oaks' hypersensitivity in the early 1990s through his tree-trimming business.
"I noticed that many trees located in new construction sites died within a few years. It was obvious that insufficient water reached the tips of limbs. People asked me why this happened, and I got tired of answering 'I don't know,'" he said. "I decided to find an answer."
After ruling out irrigation system problems, Smith tried to discover if a parasite was destroying the trees' roots. But roots in even dying trees appeared healthy.
He soon discovered that any disturbance to most post oaks' root areas, such as a bulldozer driving across nearby ground or a new sidewalk being paved, resulted in slowed growth of new roots and, in turn, reduced numbers of root hairs.
"Botanists groan and say post oaks 'don't do well with civilization,' which is accurate," Smith says.
Almost any species of tree will become somewhat distressed in construction areas, but begin to thrive again soon after construction ceases, Smith says. However, approximately 80 percent of post oaks in construction sites will probably die if their root growth is not stimulated, he adds.
Smith says builders preserve these trees to provide highly desired shade in landscapes.
"Prospective homebuilders pay thousands of extra dollars for lots with large, 50- to 100-year-old trees," he says.
He adds these trees are impossible to replace in a lifetime.
"You never see them in nurseries because they can't be transplanted without usually dying," he says. "You cannot replace post oaks unless you start with an acorn. Sadly, while trying to preserve and take advantage of the beauty and utility of these lovely trees by building close to them, we are killing significant numbers each year."
Several signs point to a declining post oak, including thinning leaves. When a tree is in full foliage in the summer, "you should be able to stand under it, look up and not see any sky," Smith says.
The presence of water sprouts -- twigs that grow out of the bark of a larger limb rather than from a joint on another twig -- and less than 6 inches of annual growth on the tree's twigs also indicate an unhealthy tree.
Smith stimulates root growth in ailing post oaks through a homemade mixture of chemicals, included powdered fertilizer. He has applied for a patent for the mixture.
To apply the treatment, Smith drills 8-inch-deep holes around the perimeter of the tree's canopy and places the treatment at the bottom of the holes. Usually, only one application is needed to restore root growth rate, Smith says.
Although early spring is the ideal time for fertilizing, Smith says it's more important to fertilize healthy post oaks prior to, or soon after, nearby construction starts to ward off future slowdown of root growth. Unhealthy post oaks should be treated as soon as possible, he adds.
"I actually get more calls during the summer because it's then that people notice thinning leaves," he says.
Since developing the treatment, Smith has treated more than 2,000 trees. He has a more than 90 percent success rate for saving trees.
"When the tree dies despite my treatment, its energy reserves may have been too low to grow more roots and that the treatment, by stimulating production of new roots, hastened death by exhausting the meager food supply," Smith says. "But in those few cases when I have applied the treatment before or soon after the root area is disturbed, the trees almost never go into decline."
Smith says he hopes his research will ultimately result in post oaks existing and thriving despite new construction.
"Oaks are easy to maintain, and they're part of our culture. You see many references to the strength of oaks in literature," he says. "We should do everything possible to preserve them."
UNT News Service Phone Number: (940) 565-2108
Contact: Nancy Kolsti (940) 565-3509
Email: nkolsti@unt.edu
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