The wind blew 120 mph on the University of Florida campus in Gainesville last week, but the only casualties were 20 live oaks.
One by one, each 8-year-old oak endured three minutes of Category 3 hurricane winds produced by a souped-up airboat motor. Meanwhile, sensors attached to the trunks relayed data to a trailer packed with computers.
After eight straight days of repeating the same procedure until each tree had been tested, University of Florida Professor Ed Gilman concluded that pruned live oaks stand up to hurricane winds better than those that haven’t been pruned.
Last year, hundreds of thousands of Florida trees blew over under the force of Hurricane Wilma, translating into millions of dollars in cleanup costs.
Collier County removed 1 million cubic yards of tree limbs and trees after Wilma ripped through the area, said Margie Hapke, Collier County public utilities spokesman. Lee County removed another 450,000 cubic yards of horticultural debris scattered by the hurricane, said Tracey Cerchie, fiscal manager for Lee County solid waste.
Combined, the counties spent $35.6 million to pick up after the storm, and most of that cost can be traced back to downed trees and snapped branches, Hapke and Cerchie said.
Recognized worldwide as a tree expert, Gilman hopes his research inspires local and regional governments to take better care of trees to reduce wind damage and expensive cleanup.
At least for live oaks, Gilman’s research proved that trimming long branches from the sides of the canopy — a method called reducing — helped the trees best endure strong wind.
Under the force of the wind directed through a 15-foot-tall by 15-foot-long tunnel, the five reduced live oaks bent at a 17-degree incline on average. Trees that received no pruning bent at a 47-degree incline under the same conditions.
Trees pruned from the bottom up — called raising — bent at 35 degrees, and trees thinned throughout their canopies bent at 23 degrees.
Area arborists are not surprised by Gilman’s findings but said the scientific backing will help them promote better pruning practices.
Rick Joyce, an arborist who works with Lee County Environmental Services, said the research will help the county advocate for pruning as a way to help trees survive hurricanes.
“It’s nice when the science is backing what you believe you’ve seen,” Joyce said. “It’s a matter of taking a proactive approach to tree care. It saves money and protects from future damage.”
Doug Caldwell, commercial landscape horticulturist for Collier County, said he sees a lot of over-trimmed trees. Trimming removes branches from the middle of the tree and over time causes the outer limbs to grow too long. Long branches reduce the trees’ ability to distribute weight.
The banyan trees that toppled throughout Naples last year fell over primarily because they had become too top-heavy, Caldwell said.
He said reducing the canopy, as Gilman suggests, would have helped. The trees were also planted too close to the sidewalk and had stunted roots.
Gilman said his research shows the best way to keep trees from bending too far to make sure the outer branches don’t grow too long.
“Our professional arborists have been suspecting it for decades and nobody’s gotten around to testing it until now,” Gilman said. “It’s the first work of its kind in the world as far as we know.”
In 2001, Gilman planted 20 young, genetically identical live oaks on campus to prepare for the research project. He left five trees unpruned and removed about 30 percent of the tree canopy from all the other trees, trimming five each using one of three different methods.
The wind machine, built by an airboat company to specifications intended for hurricane research, has been used primarily by researchers with Florida International University in Miami to stress man-made structures.
“This is a purpose-built machine for blowing buildings apart,” Gilman said. “It will not float.”
The machine, which is still on the Gainesville campus, is so large that each tree had to be cut down after it endured the wind. Once the tree was removed, the machine was situated in front of the next tree and sensors and wires were strapped to the tree’s trunk.
The process took five engineers, some of whom were from Florida International University’s hurricane research team, and seven to eight horticulture experts.
Recognizing a gap in horticulture research, Gilman began intense research on pruning about 10 years ago. In 1997 he published a 330-page book called “Illustrated Guide to Pruning.” A second edition to the reference book was published in 2002.
Gilman’s fascination with pruning for hurricane resistance is a more recent passion.
About three years ago, he did a preliminary study on wind effects using an airboat motor propped on concrete pillars. The results mirrored those of the more-advanced study Gilman completed last week.
Gilman will present his findings this summer at the International Society of Arboriculture conference in Minneapolis. The conference is expected to draw 2,500 arborists from 25 different countries.
With tornadoes zipping through the Midwest fairly frequently and storms bringing gale-force winds to all corners of the nation, Gilman’s work resonates beyond Florida, said Sharon Lilly, director of educational goods and services for the International Society of Arboriculture in Champaign, Ill.
She said arborists will probably start to prune trees for wind resistance in the manner Gilman suggests.
“Of all the things arborists can do to trees, pruning is number one,” Lily said.
Later this year, Gilman will continue his research to document how trees fare in hurricane conditions when planted at different depths and when they have root defects.
“We have a whole lot more trees that we’re planted for different reasons,” Gilman said.
While pruning might be the most intensive activity performed by an arborist, a lot of factors play into whether a tree holds up in strong winds.
Thomas Becker, Lee County extension agent for Florida Yards and Neighborhoods, said saturated ground can make it easier for the tree roots to pop out of the earth. He also said planting trees too close together or too close to a sidewalk can stunt root growth and make a tree less stable.
“Boy, there are just so many tree factors it’s just unimaginable how to sort all that out,” Becker said.
Caldwell said the species of tree makes a huge difference, too. For instance, palms do best when they’re not pruned, he said.
If anyone can sort through all the anecdotal information about how trees withstand a hurricanes and put those observations to the test, it’s Gilman, Joyce said.
“He’s really on the cutting edge of science when it comes to trees,” Joyce said. “Not just to plant them, but to plant them better.”