Top 15 Trees in the Parks - Part 1

March 16, 2011 by Melissa McMasters

We'd like to introduce you to our trees.

In 2010, the City of Pittsburgh completed a Natural Areas Study of Frick, Highland, Riverview, and Schenley Parks. The study included a tree inventory that tallied about 373,000 trees on 900 acres of forested park land. That's a whole lotta shade--and habitat, and air cooling, and soil stabilization, and many other good things.

But we were interested in how the trees broke down by the numbers. We took a look at the tree inventory and decided to introduce you to the top 15 species (by density) that you'll find in the parks. (We're leaving out the sixth item on the list--which was just a catch-all line for dead trees that came in at around 23,000. We're sure those trees are making lots of birds and squirrels happy, but we don't have much else to say about them!)

Along with a general introduction to each tree, I asked our resident Tree Guy, Phil Gruszka, to pass on a few words about each species. We'll bring these to you a few at a time over the next week or so. Enjoy!

#1: Norway Maple (Acer platanoides)

Norway Maple leaves

The runaway winner in the Pittsburgh parks tree sweepstakes is unfortunately the invasive Norway maple, covering almost 18% of the regional parks’ wooded areas. It achieves dominance over native plants through its abundant production of seedlings, the deep shade produced by its canopy, and the release of phytotoxic chemicals that inhibit the growth of other plants. By contributing to the lack of understory plants, Norway maples also help cause erosion and compacted soil. In our restoration work with the City, we’ve tried to turn that on its head by using cut or fallen Norway maples to stabilize hillsides.

Norway maple can be distinguished from native maples by the white, milky sap in its leaves and twigs (the natives have clear sap). It’s also one of the last trees to lose its color in the fall, remaining green into November and then turning bright yellow.

Norway maple fall

To indicate how invasive Norway maple is, Phil says it’s a good rule of thumb that an ideal, balanced forest would contain no more than 10% of any one genus. So all the maples put together should total less than 10%. Instead, this one species alone makes up 18%. And, in fact, the Norway maple accounts for more than half the trees in Highland Park, which means that replacing this species with more appropriate trees will be a long-term effort.

Phil Says: For the past 13 years, the City of Pittsburgh’s Department of Public Works and the Pittsburgh Parks Conservancy have been working towards removing this non-native, environmentally invasive tree from the park woodlands. However, with the advent of EAB and oak wilt disease and other destructive insects and diseases that are making their way to Pittsburgh, we need to be careful about how we approach the Norway maple. Most recently, when oak wilt struck a stand of native red oaks in Highland Park, the understory was made up of Norway maple. There, we ended up leaving the Norway maple tree because it was better to have that tree than no tree at all, because the steep slope needed to have trees on it to prevent soil from running off. The Norway maples will remain until native trees have replenished that site, at which point they will be removed.

#2: White Ash (Fraxinus americana)
White ash treeToday, the most prevalent native species in the parks is white ash, at about 50,000 trees. It is most prominent in Schenley Park, where it makes up 26% of the forest. Its impending, almost total loss to the emerald ash borer beetle will dramatically change the composition of the canopy, especially in Schenley Park.

Before the introduction of EAB, white ash was an excellent tree to plant in the parks because of its size, ease of transplantation, and attractiveness as wildlife habitat. It grows relatively rapidly and has fall colors ranging from yellow to orange to maroon.

White ash leafPhil Says: In sections of park woodlands where we have removed Norway maple, the very first native species to naturally repopulate those sites have been white and green ash. Genetically, this is a strong plant with so many great qualities. It's truly unfortunate that such a great native tree species is being destroyed because of emerald ash borer.

Stay tuned as we continue to reveal more about our parks' most prominent trees!

Park Threats, Trees and Forestry