Alliance for Community Trees News

Genetic Diversity Low In Pittsburgh’s Urban Forest

By Conni Kunzler | September 18, 2017

Source: Don Hopey, “Study: Genetic diversity low in Pittsburgh’s urban forest,” Pittsburgh Post-Gazette

Pittsburgh, PA (September 17, 2017) – A recent study of Pittsburgh’s urban forest found that genetic diversity is much greater among wild red maples that grew from seeds, and much lower among the younger maples in city and county parks that were propagated in nurseries through cloning. It found a similar lack of genetic diversity among maples in Canadian nurseries.

“The cloned nursery trees are genetically the same and that’s a potential problem,” said Cynthia Morton, a biologist with the Pittsburgh Parks Conservancy, who was formerly with the Carnegie Museum of Natural History and is an author of the study. “Because they are all genetically identical, the trees do not have the diversification and resistance needed to survive diseases and pests that may attack them.”

And that lack of resilience could be costly. According to the study, America’s urban forests contain about 3.8 billion trees, a $2.4 trillion asset that federal, state and local agencies and private homeowners spend billions of dollars to maintain and replenish.

“Low genetic diversity means that the trees being planted from nurseries are at huge risk for climatic and environmental diseases,” the study states, citing as an example the emerald ash borer, which has killed millions of ash trees, from Michigan, across the Midwest and through Pennsylvania and New York, in recent years.

“While cloning trees is in itself a benign practice, doing so on a mass scale without a proper understanding of the implications of drastically reducing the genetic diversity of urban forests is ill-advised and potentially creating an area for natural disaster.”

Genetic diversity refers to the variability of genetic traits within a species. For trees it’s important because trees have long life spans during which they can experience a wide variety of pests and pathogens and changes to their environment and even climate.

So if trees produced by cloning a very limited array of cuttings all have the same genetics and are bred to have the same shape and color, they are also more uniformly vulnerable to the same diseases or insect pests. Trees with differing genetics, especially those that have a history of surviving diseases and pests, have a much better chance of doing the same in the future.

Ms. Morton said the use of mass-produced, widely cloned trees in urban, landscape and park settings began in the 1970s. Big wholesale nurseries in the Pacific northwest with national, even international, reach, could clone and grow the uniformly genetic cultivars more quickly, cheaply and dependably from cuttings taken from just a few trees than from different seed sources. And urban planners, landscape architects, foresters and private homeowners liked, and still like, planting the cloned trees, cultivated to have the same aesthetically pleasing shapes and colors, along streets, in yards and in parks.

“It was all about money and uniformity, which is just crazy because how unform can trees really be given they are exposed to all manner of weather,” Ms. Morton said.

While the recent study focused on red maples — the most abundant trees in Pittsburgh and many urban areas — it also found that big wholesale nurseries in Washington and Oregon that supply most regional nurseries used cloning to grow a handful of other tree species popular in urban forests.

The genetic diversity issue first surfaced in a 2008 study by Ms. Morton and Philip Gruszka, director of horticulture and forestry for the Pittsburgh Parks Conservancy, that compared 100-year-old London plane trees (a cross between the American sycamore and the Asian plane tree that dates to the 1600s) growing in Schenley Park to the same tree species from three nurseries. That study found the nursery trees exhibited “dangerously low genetic diversity.”

“The problem that study showed was that as we lose our old trees, we will have no chance of getting nursery trees with the resilient genetic makeup,” Mr. Gruszka said. “But there are old, wild seed trees with strong genetics all over the place and that’s what we want to plant. We don’t need the uniformity.”

The Garden Club of Allegheny County was an early financial supporter of the genetic research done by Ms. Morton and Mr. Gruszka on the plane trees and the red maples because of its importance for long-term urban forest planning, said Cindy Tilson, a long-time club member and former grants chair.

She said a good example of the bad things that can happen when cloned trees are planted can be seen along Waterfront Drive in Homestead, where 75 London plane trees, each 40 feet tall, were planted. All are dead, she said, killed by an insect pest.

“Those are mature trees, planted when the mall was built, and they’re all dead,” Ms. Tilson said. “That’s exactly what can happen. And if you think about what it will cost to dig up and replant those trees it’s going to be tens of thousands of dollars. That’s why it’s foolish to do big plantings of trees with identical genetics.”

David Nowak, senior scientist at the U.S. Department of Agriculture’s northern research station in Syracuse, N.Y., said the studies of genetic diversity done in Pittsburgh are breaking new ground in urban forest management.

“We’re starting to monitor urban forests for species diversity, so we know the different kinds of trees they contain, but we don’t do tree genetics and I don’t know of anyone else who does,” said Mr. Nowak. “So we don’t really know enough to say how big a problem this is. But if there are a lot of cultivars planted along streets, in parks, that could be an issue.”

Ms. Morton said the genetic diversity problem is fixable, and some nurseries and professional horticultural and nursery trade organizations have recognized the problem and are working on it.

Mr. Gruszka said the Pittsburgh Parks Conservancy is talking with Tree Pittsburgh, a community forestry nonprofit, to develop a new $2.5 million tree nursery to breed genetically diverse trees from multiple sourced “heritage seeds.” Development of a viable seed collection and seedling propagation program could eventually be replicated in other urban areas.

“I’m not condemning what urban foresters have done,” Mr. Gruszka said. “I’m just saying there’s a better way. We’ve got to put an end to the cycle.”

Read the full article.

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