Study: Genetic Diversity Low in Urban Forest Trees Planted from Seed More Likely to Survive Than Clones

By Hopey, Don | Pittsburgh Post-Gazette (Pittsburgh, PA), September 17, 2017 | Go to article overview

Study: Genetic Diversity Low in Urban Forest Trees Planted from Seed More Likely to Survive Than Clones


Hopey, Don, Pittsburgh Post-Gazette (Pittsburgh, PA)


There's a big red maple that's about 75 years old standing just inside of the Beechwood Boulevard Gatehouse entrance to Frick Park.

It's old enough that its genetic makeup is unique, old enough to have survived years of Pittsburgh's pollution. Old enough too to wear a quarter-sized aluminum medallion, stamped with the number "434," that marks it as part of a recent study that found a stunning and significant lack of genetic diversity among younger red maples, those under 50 years old, in the city's urban landscape.

The study found that genetic diversity, a desirable trait, 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. It cites 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 uniform can trees really be given they are exposed to all manner of weather?" Ms. Morton said.

Looking for solutions

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. …

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