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Internal defects topple towering live oaks

Earlier this month, the trunk of a big live oak split and fell near where children play ball at the Dunedin Academy. Within two weeks, another live oak, 60 feet tall and a century old, flattened three cars outside a Clearwater hotel.

First responders puzzled over the cause of the falls. The trees had collapsed without any major wind or rain. Neither oak showed blight or damage to signal it was in danger. Their bark looked strong, healthy, flawless. The only clue to the trees' demise: their inner wood had rotted to mush.

Alan Mayberry believed he knew what had happened. He had worked as a city arborist for Clearwater and Dunedin for 25 years, tracking this type of fall whenever he could. Lately it had kept him busier than usual.

Eight trees in north Pinellas County had collapsed like this in the first half of the year; four more had fallen over the last few weeks of summer. One smashed through a fence; another, through the roof of a house.

The defect that caused the falls had begun hidden beneath the bark. But the real problem, he said, was deeper, more systemic and intrinsic to how our neighborhoods have grown for decades.

Preventing the falls, he said, has proved difficult due to the strangeness of the problem:

The trees, locked in an internal power struggle, had essentially killed themselves.

• • •

In 1996, an 11-year-old Lakeland girl was playing in her back yard when a falling live oak crushed her to death. There had been no rain or noticeable wind. The tree's trunk had, a Lakeland Ledger story stated, "forked into several sections" that led to its collapse.

It's a tragedy, Mayberry said, that could reappear. Defective trees can be anywhere, from south Tampa's old oaks to tropical hardwoods along the coast, their core flaws hidden to untrained eyes.

"You drive around and some, they just scream at you," he said. "How the hell do you tackle a problem so vast? Where do you start?"

The first step, Mayberry said, is understanding the problem.

• • •

Trees crave light. It feeds them, strengthens them and directs how they grow.

In a primitive "old-growth" forest, a sapling's only chance for survival is to grow straight up through the canopy of neighbor trees and into the sunlight, where its crown of leaves can feed.

Shorter trees or offshoot branches below die off from too much shade. The competition among the living trees keeps them sturdy, each with an arrow-straight trunk known as the "central leader."

Plant those same kinds of trees in an urban landscape, alone as accents on a suburban lawn or tucked in small clumps to shade a playground, and the canopy that had served as nature's pruner disappears. There's no longer any need for the trees to grow straight and tall. Awash in sunlight, the trees grow outward instead of upward. Thick sprouting stems begin to undermine the central trunk.

Most trees are sturdy enough to support these "co-dominant stems," where the trunk splits in two. But on occasion, the stems grow into each other, crushing each other's insides as they expand further into the light. Experts call this "included bark": the outer wood keeps growing, while the inner wood decays.

"It's kind of like leaving a teenage kid at home with a Wii and Twinkies and Mr. Pibb," said Richard Cervi, a certified arborist with Cervi and Associates in St. Petersburg. "It'll live. But it won't be pretty."

The problem of "included bark" seems closely tied to genetics, specifically how the tree connects its stems — U-shaped joints are stronger than V-shaped joints. Ironically, the co-dominant trees with the best growth are the most at danger of falling — robust wood and dense leaves often prove too heavy for the tree's rotted core.

The only dead giveaways to this kind of defect are dark gashes etched in the bark, where the stems meet, or swelling bulges known as "elephant ears" where the tree is attempting to isolate the decaying joint.

The defect, arborists said, is often only discovered when the tree collapses. In 2005, a great oak towering over Zephyr Park in Zephyrhills suddenly snapped off and pinned a woman to a picnic bench, killing her.

There had been no rain or wind, a Times article stated. The Zephyrhills police chief said the oak had appeared to be in "good shape," mostly green but for a dead zone in the center. A witness said the fall was so sudden it seemed unexplainable.

• • •

John Whitney, a forester with Progress Energy who covers from Brooksville to St. Petersburg, said the falls have become a "fairly widespread problem this year."

"That doesn't mean they're falling down everywhere, but they're falling quite a bit more than usual," he said. He recalls 20 falls through January, about double last year's count. "If I've seen that many, there have got to be lots more."

The recent falls, he said, were likely hastened by the recent wet weather. Weakened trees that developed cracks in their wood during dry spells were suddenly swelled with water, adding unbearable weight.

The best way to prevent such sudden and destructive falls, arborists said, is to begin pruning trees regularly at an early age. Even as saplings, trees need training to establish a strong central leader and keep competing stems from growing to a critical size.

"Urban foresters have been talking it up for decades: less planting, more maintenance. And all that really amounts to is plant less, prune more," said Ed Gilman, an urban tree professor with the University of Florida. Included bark, he said, is "without question" one of the most problematic causes of tree falls. "It's real. It happens all over the place, all over Florida, all over the U.S."

Cities and counties conduct inventories of problem trees, though they rarely set out solely in search of elephant ears. Municipal arborists and commercial tree services often tend to problematic trees by pruning early or bracing them with steel rods.

A lack of "structural pruning," Mayberry said, is what led to the spread of problematic trees. Laurel and live oaks planted en masse during the building boom of the late 1970s have reached maturity without much competition or maintenance.

"Nature had it all worked out," he said. "We're the ones who are screwing it up."

Times researcher Shirl Kennedy contributed to this report.

Internal defects topple towering live oaks 09/24/10 [Last modified: Friday, September 24, 2010 10:24pm]
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