Time for the next few trees in our Top 15 Trees in the Parks series:
6. Elm (Ulmus americana)
Despite the high mortality caused by Dutch elm disease, Pittsburgh’s parks still feature a significant population of elm trees, although many of them are smaller than the elm’s traditional soaring height (often around 100 feet tall). Because of its strong wood and tolerance of many stress factors like pollution and poor soil quality, the elm is a high-quality tree for an urban park.
Prior to Dutch elm disease’s reign in the 1950s – 1970s, the elm was the most popular street tree in the country thanks to its elegant Y shape, sturdy build, and ample shade. Over 100 million elms were felled by the disease, changing the character of cities, from neighborhoods to forests. But the elm is working its way back into the landscape: to combat the disease, a breeding program has sought to hybridize American elms with Asian elms, which are highly resistant to the disease.
In Pittsburgh, we’re working to become part of the solution. In 2005, Norway maples were removed from the border of the Schenley Park overlook and replaced with 30 elms from six different cultivars: Accolade, Allee, Frontier, Pioneer, Prospector, and Triumph. Students from the Penn State Cooperative Extension are studying the trees to observe their development and keep a careful eye on whether the elms exhibit invasive tendencies in the adjacent natural areas (so far, so good!).
Phil Says: Even though Dutch elm disease swept through Pittsburgh decades ago, elm is such a strong native tree genetically. It’s very well adapted to growing here, and so as these mature trees were sickened, they produced tens of millions of seeds that were dispersed throughout the urban forest. Many of those seedlings grew up, and Dutch elm disease did not find them until they became larger trees. So we still have lots of American elms left—they’re known as climatic escapees. It’s a regenerative population that has survived because the disease is not at the level it once was and the insects that spread it aren’t nearly as abundant. For the most part these newer trees will never attain the height and girth of the elms we were used to seeing in Pittsburgh. To have thousands of large trees is probably not going to happen.
7. Northern Red Oak (Quercus rubra)
(We’re focusing on the Quercus rubra species here, but this section includes all oaks in the red oak family. In this family, the lobes at the ends of the leaves are pointed and acorns ripen over two seasons. The “red” refers to the wood inside the tree, although many of them also have red leaves in fall.)
Red oaks are a fast-growing, fairly tall tree with a mature diameter of 2-3 feet. They are good urban trees because they tolerate pollution and compacted soils. The red oak is a dominant forest tree throughout the state of Pennsylvania.
The leaves contain tannin, which makes them leathery and resistant to decomposition. They are simple, with between 7 and 11 toothed lobes on each leaf. They are dull green throughout the summer and turn a brick red color in fall.
Phil Says: When we mention “red oak,” that refers to a group of oak trees. The trees in the red oak group that are most common in Pittsburgh are red oak, black oak, and pin oak. Oak wilt disease has directly impacted the red oak group, having claimed several hundred trees last year alone in Riverview, Highland, and Frick Parks.
8. Black Locust (Robinia pseudoacacia)
The dark, deeply furrowed bark of this tree and the twisty character of its upper branches make the black locust easy to distinguish. They also have paired thorns that can grow to half an inch long. It has an especially striking winter silhouette. They are usually medium-sized trees but can grow to 100 feet tall.
Black locusts are similar to cottonwood trees, but in summer can be easily distinguished by their compound leaves (cottonwoods have simple leaves). There are between 7 and 19 leaflets on one stem, and all of them are paired except for the leaflet on the end. They are smooth and oval-shaped, and droop at night. Locust fruits are dark brown pods up to four inches long.
Black locusts are the host plants for clouded sulphur butterflies and silver-spotted skippers. They are considered invasive in some parts of the United States, particularly where they have begun to colonize dry prairies and savannas.
Phil Says: Like black cherry, this is an early successional species, so wherever it’s growing, it’s making that site more suitable for other dominant trees—oaks, maples—to move in. It’s a site colonizer. Black locust trees have thorns, even though most people never see them. The thorns occur on new growth, and they are there to protect the tree from being browsed by deer.