How well do you know the trees that are rooted in your parks?
Recently, we were inspired by this New York Times article to introduce you to just a few of the unique trees that grow in our urban forest. Explore your parks soon to meet them for yourself!
Northern red oaks (Quercus rubra) are fast-growing, fairly tall trees. that are well suited to the pollution and compacted soil of cities. You can find red oaks growing throughout Penn's Woods. Red oak leaves are dull green throughout the summer and turn a brick red in fall.
Red oak leaves contain tannin, which make them leathery and resistant to decomposition (and a favorite choice for squirrel's nests). They are simple, with clearly defined points. You can distinguish types of oak leaves by these points - red oak leaves have sharply defined points (hint: think of flames, which are red), whereas white oak leaves have a rounded edges (hint: think of a snowy hill, which is white).
"Red oak” refers to a group of oak trees. The trees in this group that are most common in Pittsburgh are red oak, black oak, and pin oak. “Red” refers to the wood inside the tree, although many of them also have red leaves in fall.
Far and away the most common tree in Pittsburgh's parks is the invasive Norway maple (Acer platanoides). Covering almost 18% of park woods, it out-competes our native plants through its abundant production of seedlings, the deep shade its leaves make, and the release of phytotoxic chemicals that inhibit the growth of other plants. In crowding out understory plants, Norways contribute to erosion and compacted soil.
Norway maple leaves and twigs ooze a white, milky sap when cut. They are one of the last trees to lose color in the fall, remaining green into November and then turning bright yellow.
To indicate how invasive Norway maples are, our Director of Horticulture and Forestry Phil Gruzska says that well-balanced forests 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, making forest restoration a long-term effort.
For two decades, we've worked with the City of Pittsburgh’s Department of Public Works to remove this invasive tree from park woodlands. However, with other threats to park trees (think: Emerald Ash Borer an other non-native pests), forest management is ever-changing.
Black Cherry (Prunus serotina) is a native tree with a strong presence in all of the regional parks, with its largest concentration in Schenley. The cherries are drupes (fruits derived from flowers where the flesh surrounds a pit, which contains a seed). Straight off the tree, they may taste bitter to us, but birds and other animals love them. The fruits can be used to flavor wine and jelly.
Black cherry trees are easily distinguished by its cracked bark, which is dark gray to black. A mnemonic that we use in our school programs is to associate the bark's appearance with burnt cornflakes (b.c. = black cherry).
Unlike other types of cherry trees, these generally flower well after their leaves have developed, usually in mid-May.
This is an early succession species, so it commonly finds disturbed sites to colonize, making sites ready for other native species like oaks and sugar maples to grow there. Black cherry has the ability to establish itself in places where vines and invasives are choking out other plants, eventually working its way into the canopy. We like to plant this tree when regenerating forest areas.
A native alternative to the Norway maple, sugar maples (Acer saccharum) makes up about 9% of park trees. Although these two trees are not closely related genetically, they are often mistaken for one another. The largest concentration of sugar maples occurs in Frick Park, numbering more than 20,000.
The sugar maple is one of the most shade-tolerant trees, so it makes a good understory plant. When a canopy gap opens and more sunlight comes through, its growth rapidly accelerates. It has been outcompeted by the Norway maple partially because it is less tolerant of urban conditions, such as pollution, acid soil, and road salt.
Sugar maples have interesting fall color, showing a range from bright yellow through red-orange, often on the same tree at the same time.
Sugar maples are strong native trees that have been embraced by arborists to be bred and selected for form, size, and fall color.
Despite suffering high mortality due to Dutch elm disease, our parks still feature a significant population of elm (Ulmus americana). With its strong wood and tolerance to pollution and poor soil quality, the elm is a high-quality tree for an urban park.
Prior to Dutch elm disease’s reign in from the '50's through the '70's, 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.
The elm is working its way back into our city landscape. To combat the disease, a breeding program has sought to hybridize American elms with Asian elms, which are highly resistant to the disease.
Despite Dutch elm disease, 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.
This blog has been adapted from the series Top 15 Trees in the Parks. Read the first part here, and stay tuned for the second half of this list!