Threats to the Urban Forest

December 2, 2010 by Parks Conservancy Staff

Pittsburgh's four regional parks have 372,873 trees, according to the City of Pittsburgh's recently completed Natural Areas Study. These trees represent 53 different species and collectively represent hundreds of millions of dollars in economic value, not to mention their aesthetic and environmental benefits.

EAB emergence holes

But lately we've been hearing a lot about threats to those trees--pests like emerald ash borer and Asian long-horned beetle, and diseases like Dutch elm and oak wilt. The emerald ash borer is in our parks--though we haven't seen the beetle itself, the D-shaped emergence holes are becoming more and more obvious. And oak wilt has forced the clearing of several acres of trees in Frick, Highland, and Riverview Parks to prevent further spread of the disease.

Oak wilt before and after

To add to the problem, many of the park trees are environmentally invasive, making them threats in and of themselves to the biodiversity of our forests--trees like Norway maple, tree of heaven, Siberian elm, and princess tree. It's almost shocking how many of our park trees are invasive. In Schenley Park, it's 32%; in Frick, it's 35%; in Riverview, it's 46%; and in Highland Park, a whopping 70% of the trees are invasive. Highland Park is basically a Norway maple forest, which is why it was so heartbreaking to see the stand of oaks removed recently due to oak wilt. In other parks, we've been working to slowly phase out Norway maple and replant other natives, but in Highland Park many of the hillsides are entirely composed of these invasive trees. Removing them all at once could cause serious erosion problems, so we have to wait until some of the smaller native trees grow large enough to keep the hillsides stable.

These are some of the many reasons that the Parks Conservancy is taking an active role in developing a tree action plan for our parks. We've been talking to experts in government and academia to enlist their aid, as well as local groups like the Pittsburgh Shade Tree Commission and Tree Pittsburgh. Our discussions have ranged from big dreams to practical solutions. One idea being considered is to identify specimen trees--those that are particularly strong, beautiful, characteristic of their species, or definitive to an area--and develop a plan to save them (treating them for diseases if necessary). We can't remove or treat every tree in the parks, because the process would disrupt the surrounding ecosystem. But we could make a dent. The City of Pittsburgh's work to treat oak trees in areas where oak wilt is nearby is a great example of interventions that are being made to save trees.



Funding is, of course, always a concern. Remember that any contribution you make to our Emergency Maintenance Fund helps us preserve trees and respond to threats like these.

You can help out at your home, too. Keep a watchful eye on your own trees, especially ashes and oaks. If you have a particularly magnificent specimen tree and suspect something might be wrong, consult a certified arborist to see whether it can be treated. Research about emerald ash borer is ongoing; in some areas of the country where the infestation was at its peak several years ago, ash trees are beginning to regenerate. We won't know for another 8-10 years whether the bugs will return and eradicate all the new growth, but some scientists say there is hope. So if you have trees you can protect, it's worth a try.

On the other hand, if an infection in one of your trees is obvious, don't wait to remove that tree. The emerald ash borer, for example, will kill a tree within three years, but the pests will remain in the tree until it is completely dead. If you remove the tree while part of it is still alive, you won't be harboring the pests and you'll reduce the population. When you do remove diseased trees from your yard, make sure the brush is chipped or burned and the logs are debarked before being used for firewood. The bugs and the fungus will remain below the bark and can continue to spread.

For more information on these threats, check out this great resource from the University of Maryland Extension. You'll find more information about everything from managing gypsy moth to diagnosing what's wrong with your trees.

Stay tuned for a lot more information--trees will be a primary focus of ours in 2011, with a public symposium planned for February where you can learn more about how to help.

Ecological Restoration, Park Threats, Trees and Forestry, Parks Management