Making sure that Texas trees provide shade this summer

Trees are one of Texas' most precious natural resources, especially during the summer when the spreading canopy provides welcome relief from the blistering sun. 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.

Don Smith, associate professor of biological sciences, is saving as many of 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, the single-cell extensions of epidermal cells, located on new roots. 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 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 says. "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 reduced the 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 will begin to thrive again soon after construction ceases, Smith says. However, he says, about 80 percent of post oaks in construction sites will probably die if their root growth is not stimulated.

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 indicate that a post oak is declining, 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, including 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. He says it's important to fertilize healthy post oaks prior to, or soon after, nearby construction begins, in order 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 success rate of more than 90 percent for saving trees. You may e-mail questions about post oak trees to Smith at dsmith@ unt.edu.

"When the tree dies despite my treatment, its energy reserves may have been too low to grow more roots and the treatment actually hastened death by stimulating production of new roots and 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."

BY NANCY KOLSTI nkolsti@unt.edu

Other featured articles in this issue

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• Million-dollar touring bus provides VIPs with star treatment
• Union Gallery welcomes staff arts and crafts show this month
• Hot news: Ideas to conserve energy and to dress to beat the heat this summer
• Staff Council representatives elected
• Faculty Senate elects officers for 2003-04
• Faculty recognized at Honors Day event
• Staff Council working to find more effective ways to communicate with staff

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