By Amy Schaarsmith, Pittsburgh Post-Gazette

Pittsburgh, PA (January 4, 2015) — Newly planted urban forests can easily succumb to disease. A rich genetic composition, however, is key to giving trees the resources to withstand a variety of environmental challenges. A Pittsburgh Post-Gazette feature talked to horticulturalists on what their research says about how to cultivate hardy urban trees.

canopy_schenleyfarms_8In Pittsburgh, London planetrees are survivors, braving diseases, pests, air pollution and other hardships. According to horticulturists, that success is thanks, in part, to a rich genetic composition that gives them the resources to overcome a variety of environmental challenges. They are cultivars — plants produced in cultivation through selective breeding.

New trees usually planted in cities, however, are clones of a smaller number of “parent trees,” placing them at greater risk of death from insects, infection or other stressors because they have such a limited range of genes with which to respond, according to Cynthia Morton, associate curator of botany at the Carnegie Museum of Natural History.

“Trees are great for the environment, but you have to plant something that will resist the next disease that comes through,” said Morton, who discovered the genetic disparity while researching the Schenley Plaza planetrees for a study published in 2008. “If we plant something genetically diverse, it will naturally have more defense mechanisms.”

Even genetically diverse trees with an ample arsenal of defenses can succumb to an especially tough enemy. The nation’s American chestnuts, Dutch elms, many hemlocks and most recently, ash trees, have fallen victim to diseases and pests, even though they were genetically wealthy.

london planetreesThat means the shallow gene pool of most nursery-grown hardwoods commonly planted in parks and along streets — planetrees, maples, lindens and ornamental pear trees are the most popular — makes them even more vulnerable, Morton said. In the case of planetrees, she said, nurseries breed cultivars that are resistant to anthracnose, a fungal disease that withers leaves and causes them to drop off early, but does little more damage.

Approximately 90 percent of nursery-grown trees are cloned. “Disease is spreading, but it is spreading like wildfire through cities with all these cloned trees,” she said.

Losing trees and having to replant can quickly become expensive, with the average cost of a single hardwood sapling $350 to $450, not counting the cost of the labor to cut down and remove the dead tree and replant the new one, Morton said. Conversely, a healthy, mature planetree in a Pittsburgh business district can save the city and local businesses $240 a year in reduced need for heating and cooling, reduced storm water runoff, improved air quality and increased property value, according to the National Tree Benefit Calculator, available at treebenefits.org.

The discovery of reduced genetic diversity in the nation’s urban tree population is “alarming,” said Phil Gruszka, an urban arborist at ACTrees Member Organization Pittsburgh Parks Conservancy and a co-author of the Schenley Plaza planetree study. The number of invasive species and diseases is increasing, even as the trees’ ability to resist them weakens.

Some local groups, such as ACTrees Member Organization Tree Pittsburgh, have begun growing genetically diverse trees from seed, but a much larger effort will be required to change the availability of such trees on a scale needed to supply the entire country, he said.

Morton has sought funding from the U.S. Forest Service for a project that would share cuttings and seeds from genetically diverse hardwoods with nurseries for the development of new and hardier cultivars. Several large nursery owners have written letters of support to the Forest Service, but so far no funding has emerged.

Tree nurseryAlthough at least some nursery managers understand the importance of creating more genetically diverse cultivars, the practical points of their development can be daunting, according to Debbie Lonnee, who manages the acquisition of new genetic material and development of new plant and tree cultivars for Bailey Nurseries Inc. in St. Paul, Minn.

Developing new trees requires a huge investment including land, labor, chemicals and water, among other needs.

Bare root trees typically are pulled out and sold within three years, so they don’t get very large and can be planted close together, Lonnee said. But developing a new cultivar requires growing it for 10 years or more to allow the company to observe its characteristics, which means such “test” trees must be allotted much more space to accommodate their larger canopies and root systems.

And that ties up land that otherwise could be used to raise trees that could be sold. “In a nursery setting, it’s a huge commitment of space and resources,” Lonnee said.

Until approximately 25 years ago, land grant universities’ horticultural programs developed new cultivars because they had the space to grow them and available students to take care of them.

But Lonnee said pressures including eroding public funding, higher costs, greater interest in profitable patents on medical inventions and pharmaceuticals, and reduced interest in agriculture among college students mean horticulture departments and their development of new cultivars are shrinking.

To Morton and Gruszka, the unmet need for new and hardier cultivars presents a business opportunity of growing trees from cuttings or seeds — a process that includes genetics from two trees for the greatest possible diversity — that Pittsburgh should find a way to fill.

Such a project, which potentially could make use of what are now abandoned properties around the city, also could reduce storm water runoff as part of meeting a U.S. Environmental Protection Agency mandate to reduce combined sewer discharges into area rivers, Gruszka said.

In one day, a single tree can lift up to 100 gallons of water out of the ground and discharge it into the air, according to studies conducted by North Carolina State University. And for every 5 percent of tree cover added to a community, storm water runoff is reduced by approximately 2 percent, researchers have found.

“Those lots could represent far more to us environmentally than just a place to grow trees,” said Gruszka.

Read the full article: Amy Schaarsmith, “Newly planted urban forests can easily succumb to disease, ”Pittsburgh Post-Gazette

This article reprinted in Tree Link as it originally appeared the Treebune News, the official newsletter for the Alliance for Community Trees.