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By Helen Fosgate
UGA forestry researchers may have discovered why some pines grow straight and tall while others are twisted and bent. A new study, funded by the Georgia Forestry Commission and the USDA Forest Service, shows the culprit could be a bent or J-shaped taproot. Researchers found  trees with bent taproots are more than twice as likely to exhibit above-ground deformities like wavy trunks and branches.
“Seedlings are often planted with the taproot bent into an ‘L’ or ‘J’ shape,” says Tim Harrington, a forestry researcher in the Warnell School of Forest Resources. “Once planted, the root tends to grow in this same configuration for at least 10 years.”
Stem “sinuosity” is serious business since crooked trunks drastically reduce the value of pine trees. The condition relegates trees to the pulpwood rather than the higher-priced saw-timber market. This study, the first in North America to show such a relationship, was published in the Southern Journal of Applied Forestry in the fall, and the Georgia Forestry Commission prepared a report on the work for landowners.
Previous research addressing the survival and growth of seedlings planted with bent taproots was generally inconclusive. But long-time Georgia Forestry Commission entomologist Terry Price pushed for further analysis. His observations in pine plantations across the state led him to suspect a relationship, and he contacted Harrington. Research coordinator Jason Gatch and Forest Service scientist Boyd Edwards worked on the study with Harrington at 48 sites across Georgia’s Piedmont and Coastal Plain regions.
“For years, we believed this was a genetic problem,” says Price. “But I had formed an opinion that it was an environmental condition caused by a bent taproot that had either hit a soil hardpan or had been J-rooted at planting.”
In the study, researchers paired loblolly pines--one with a straight stem to one with a crooked or wavy stem--in plantations of trees three to 10 years old. They dug down as much as two feet to excavate the taproot. They found that 77 percent of the trees with bent taproots had medium to high levels of stem sinuosity, while 71 percent of the trees with straight taproots exhibited low levels of stem sinuosity. Trees with bent taproots were also 7 percent shorter in height and 9 percent smaller in diameter than their straight-trunked neighbors.
And it’s not just trunks that exhibit sinuosity. Harrington says the phenomenon also affects pine branches and upper stems, a fact that led researchers to question the biological mechanism behind the deformities as well as the cause.
“Because sinuosity is expressed throughout the tree, we believe the mechanism may be hormonal,” he says. “We know from other studies that bending the stem causes an increase in ethylene production in the tree, and that, in turn, stimulates production of denser compression wood. The same response could stimulate the development of stem sinuosity.”
The first study looked at sinuosity in loblolly pines. Now the researchers are analyzing a new set of data collected on slash pines. Price and Harrington suspect that wavy trees are also more susceptible to attack from tip moths and pine bark beetles. To test this theory, they have planted trees with and without bent taproots at six sites across the state and are currently monitoring insect damage.
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