By Mark Schleifstein
New Orleans, LA (February 28, 2012)- Just two weeks after New Orleans’ Parkway Partners celebrated the planting of its 10,000th tree after Hurricane Katrina, a new U.S. Forest Service study tagged the city’s urban forest as having the greatest decline since 2005 — a finding that, in New Orleans’ case, can be directly attributed to the storm.
The federal survey of urban forests in 20 of the nation’s largest cities found that the amount of New Orleans land covered by trees and shrubs dropped by 9.6 percentage points between 2005 and 2009, far outdistancing Houston, which showed a 3 percentage point decline between 2004 and 2009.
The area of the city covered by trees dropped from 32.9 percent in 2005 to 23.3 percent in 2009, a 29 percent reduction. The study, published recently in Urban Forestry & Urban Greening magazine, found that the nation’s cities are, on average, losing about 0.25 percent of their tree cover a year. That works out to about 30.5 square miles of lost canopy per year, or 4 million lost trees.
In New Orleans, the cause of the decline was definitely Katrina, said study co-author David Nowak, a research forester in the service’s Northern Research Station in Syracuse, N.Y. The city’s inclusion in the study was aimed at pointing to one of the myriad ways in which the nation’s urban forests are being challenged. The study also focused on Detroit, which ranked 14th in loss of trees, with a 0.7 percent decline fueled by significant damage to its trees by the Emerald ash borer, an invasive insect.
Insects, tree diseases, storms and human intrusion — the uprooting of trees to add streets, buildings and other infrastructure — are the leading causes of the losses, Nowak said. Cities with the greatest annual increase in “impervious cover,” meaning new pavement and buildings, were Los Angeles, Houston and Albuquerque, N.M.
The survey also found that Syracuse, N.Y., was the only city surveyed to measure an increase in tree and shrub cover. But that was the result of the rapid spread of an invasive species, the European buckthorn. The plant, which acts like a shrub but grows to 25 feet tall, is thorny and its black, fruity seeds are easily spread by birds.
The amount of tree cover in those cities today ranges from a high of 53.9 percent, in Atlanta, to only 9.6 percent, in Denver. The study is aimed at giving city planners ammunition in their efforts to expand their tree canopies, Nowak said.
“Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces,” said Forest Service Chief Tom Tidwell. “Photo-interpretation” of digital images, used in the study, offers a relatively quick and low-cost way to measure changes among various cover types. The Forest Service offers a free tool, i-Tree Canopy, to allow users to photo-interpret a city using Google images. It’s available on the Web at www.itreetools.org.
Trees provide a variety of benefits in urban areas, including improved air and water quality, conserving energy use by shading buildings, reducing ultraviolet radiation exposure, soaking up stormwater runoff and even helping to cool local temperatures.
This survey found that despite active tree-planting campaigns in a number of cities, the urban canopy remains in decline. Monitoring changes in the tree canopy can provide local leaders with the background information they need to determine what trees to add to their communities and where, Novak said. “Those decisions have to be made locally, but they need information to make that decision,” he said.
Parkway Partners executive director Jean Fahr agrees.“When you see it in black and white, it hits home again how bad the loss is,” she said of the survey’s New Orleans results. She said the city lost more than 100,000 trees during and after Katrina. Her group’s work in post-Katrina New Orleans has found that just about every tree and shrub species in the city suffered losses during the storm. The worst hit were large magnolia grandiflora trees, such as those along Elysian Fields Avenue, which were most likely killed by the salt content in floodwater.
For the city’s normally hardy live oaks, the storm and its aftermath delivered repeated waves of stress, she said. The first attack was the flooding combined with loss of leaves from Katrina’s winds. But the new leaves the trees sprouted out in the storm’s aftermath became targets for an unusually large buckmoth caterpillar season in March 2006, a time when spraying for the leaf-eating pests dropped off dramatically.
Energy used by the trees to try again to sprout replacement leaves killed many of them, including some of the city’s oldest trees along City Park Avenue and Canal Boulevard.
Formosan termites also took advantage of trees wounded by the storm, she said.The push to replace many of the lost trees has been led by Parkway Partners. The city’s Department of Parks and Parkways has used federal grant money to plant about 3,000 trees, and a variety of local nonprofit groups, like Hike for KaTREEna, are adding to that number.
The Hike for KaTREEna group was begun by New Orleans resident Monique PiliÃ©, who hiked the 2,175 miles of the Appalachian Trail between Georgia and Maine in 2006, raising $10,000 in seed money for its planting effort. The organization has planted almost 13,000 trees. But Fahr said much of those efforts are aimed at public rights-of-way. Private landowners, including homeowners and businesses, are also joining the planting effort, she said.