From tree-lined streets to our beautiful parks, Pittsburgh has its share of leafy shade. In fact, in 2013, we were cited by National Geographic for the city’s impressive amount of urban tree cover, coming out ahead of cities including Portland, Austin and New York.
But a disturbing report from Tree Pittsburgh shows that the tree canopy in our region is suffering.
Allegheny County lost 10,000 acres of its tree canopy between 2011-2015, according to the report. A two percent change from 2010 (56 percent canopy coverage) to 2015 (54 percent) may not sound like much, and yet it is. In more vivid terms, our county lost the equivalent of more than 7,500 football fields of trees.
The city alone lost six percent of its canopy coverage. Some trees were lost to the emerald ash borer, a non-native pest that arrived in Pittsburgh around 2009. Others died in an oak wilt fungus outbreak that spread in Schenley Park. Around 3 to 5 percent was due to the removal of trees that were naturally aging and dying.
But much of the loss is man-made.
“We’re seeing a lot of development throughout Allegheny County, and then some commercial residential development, that’s impacting the canopy,” says Matt Erb, director of urban forestry at Tree Pittsburgh. “We’re also seeing canopy loss in peoples’ yards, for whatever reason. Whether the tree is healthy or unhealthy we don’t know, but a lot of people are removing trees.”
According to Tree Pittsburgh, communities that experienced some of the biggest losses (Aspinwall, Whitehall and Cheswick) are also experiencing new real estate development.
Beyond their beauty, Erb says trees offer a host of other benefits, including stormwater management, help with air pollution and even human health benefits. One study published in the American Journal of Preventive Medicine suggests that the significant number of trees lost to the emerald ash borer “increased mortality related to cardiovascular and lower respiratory tract illness” in certain communities.
“The impact of that canopy loss has a lot of consequences that we can’t fully quantify yet, but obviously trees provide a lot of pollution reduction, and without it, we’re more susceptible to respiratory illnesses and more strain on cardiovascular systems,” says Erb. “We’re losing out on millions of dollars of benefits from the loss of this canopy.”
Fortunately, he says, this study does offer detailed information to help us examine the problem.
“Allegheny County might be the only county in the country that has this complete analysis,” he says. “We’re still kind of learning what this information means and how to present it accurately without making too many assumptions. It’s a continuing process.”
Diversity dilemma
So how can Pittsburgh tackle its tree problem? A decade-long study done by experts at the Pittsburgh Parks Conservancy has explored one possible solution — increasing genetic diversity.
“It’s all about resiliency,” says Phil Gruszka, Director of Horticulture and Forestry at Pittsburgh Parks Conservancy. “The more we can diversify the genetic makeup of trees being planted in the urban forest the better off we’re going to be long-term.”
Since 2008, Gruszka and his study co-author, biologist Dr. Cynthia Morton, have been calling attention to the potential problems posed by a lack of genetic diversity. It started with a study published in the Journal of Horticultural Science and Biotechnology that found that tree species taken from three different nurseries had a “dangerously low genetic diversity” compared to that of 100-year-old London plane trees growing in Schenley Park.
“That was the alarm bell for us,” says Gruszka.
The issue stems from the widespread practice of nurseries propagating trees through cloning, where a certain type of tree is mass produced from an array of cuttings as opposed to being grown from seeds. This leads to identical trees that, while pleasing to the eye as decoration along driveways or city streets, leaves them vulnerable to diseases, pests and environmental shifts caused by climate change. In other words, if one cloned tree becomes infested, so will the rest.
“They’re literally replicating one plant tens of thousands of times over,” says Gruszka. “The genetic makeup of those trees doesn’t change.”
This could mean devastating consequences for forests or parks replanted with genetically similar trees. As an example, the study cites the millions of ash trees lost to the emerald ash borer in recent years, which spread from Michigan and up through Pennsylvania and New York.
Even more worrying: Because cloning is so common, genetic diversity isn’t just a Pittsburgh problem — it’s a global problem, as proven when Gruszka and Dr. Morton presented their findings at an international conference on tree species and genetics in Australia.
“It was amazing how many people from around the world came up to us and said they never even thought about the fact that their urban forest was heading in a negative direction,” he says, adding that North American tree propagators send their cloned plant material abroad.