How much does it cost to maintain a city's trees?

"More than you think," said Gene Hyde, city forester for Chattanooga, Tenn. "People take trees for granted in a city, forgetting that they not only have to be purchased and planted, but they have to be properly maintained."

Hyde should know. He and his team in Chattanooga's Urban Forestry Division spent 7,367 hours pruning and maintaining nearly 4,500 trees in 2002.

Since then, Hyde has been on a mission to add technology to Chattanooga's tree maintenance process as part of an effort to document maintenance costs for the city's urban forest. The division is using GPS and GIS to map tree locations, and track the type and size of every tree along city streets and in downtown parks.

"We're raising the red flag and saying, 'You can't just plant trees and walk away,'" Hyde said. "Every one of these trees has a post-planting cost associated with it. Our budget has to keep up to maintain them all."

Working the Numbers

It took Hyde's team four months to inventory the trees in Chattanooga's expanded central business district, an area that covers about 200 square blocks. Members of his team hiked through downtown carrying backpack GPS units, entering data on each tree's location, species and size of its planting pit. They also noted the type of covering in each planting pit (monkey grass, turf, concrete, etc.) and whether or not the tree was irrigated.

Once the data was collected, the Urban Forestry Division created five categories based on diameter (0-6 inches, 7-12 inches, 13-24 inches, 25-36 inches and more than 36 inches) and assigned each tree to its appropriate category. "Obviously the smaller the tree, the less time it takes to prune," said Hyde. "Classifying the trees helps us determine the number of hours required to maintain them."

Chattanooga was already using Tree Manager software from Ohio-based ACRT Inc., which maintains a tree inventory and generates user-defined summary reports, listings and work orders. Tree Manager is designed for integration with GIS, so combining the data was easy. ACRT then helped Chattanooga determine a per-tree pruning time for each category, and Hyde made projections on how large each tree would eventually grow.

"We figured we are going to have a huge baby boom of what are going to become very large trees," Hyde said. "We estimated that if they require 7,300 hours today, 10 years from now it will take 9,500 hours. Twenty years from now it will take 13,500 hours."

That doesn't take into account the 600 or so new trees the city plants each year on average. With one four-man crew in house and a three-man crew on contract to cover 1,200 miles of city streets, 50 miles of alleys and 150 city parks and recreation sites, Hyde knew it was critical to alert city hall to the adjustments needed to keep city trees healthy in the future.

What's Out There?

In addition to maintenance projections, the GIS tree inventory map helps Hyde and his team in other ways.

Because the map has the power of a database behind it, Urban Forestry personnel can query by tree height, condition, pests, maintenance needs -- whatever information is in the database. For example, overloading on one tree species could be disastrous should a pathogen, insect or disease attack that species, so experts suggest cities have no more than 5 percent of one species in their overall mix. By conducting a query on existing tree species, Chattanooga can assure they are staying within that guideline.

Urban Forestry Division personnel also can query how many trees are in poor condition. The trees in question will show in color on the map, making it easier to see which city areas need more attention.

Justine Brown  |  Contributing Writer