Keystone tree species provide a home to pollinators
Keystone species are native plants that provide the maximum habitat benefit to local wildlife. Research shows that just 5% of a region’s native plants host 70-75% of butterfly and moth species, while also supplying essential pollen to specialist bees. Because these insects form the foundation of the local food web — feeding birds, bats and other wildlife — keystone plants have an outsized ecological impact compared with other native species. Keystone designations are made at the ecoregion level because these relationships shift across the continent, meaning the right species vary by area.
Five of the most beneficial Pennsylvania native trees are oaks, cherries, plums, birches and willows.
The mighty oak leads the list for many reasons. Besides its beauty, this stately tree has historically been valued for building homes and shipbuilding. Its acorns provided vital food for Native Americans and early settlers, as well as many small mammals and birds.
There are three main species of oak trees in Pennsylvania: white oak (Quercus alba), black oak (Q. velutina) and red oak (Q. rubra). The oak tree supports the highest number of native caterpillar species, including the red-spotted purple (Limenitis arthemis), Horace’s duskywing (Erynnis Horatius) and the banded hairstreak (Satyrium calanus).
These trees provide food for 436 caterpillar species that, in turn, feed many of our local bird species. Due to their deep roots, large canopies and abundant production of acorns, they support more wildlife than any other North American tree.
Another tree on the keystone list is the native cherry (Prunus): black cherry (P. serotina), chokecherry (P. virginiana) and American plum (P. americana). The most well known of these is the black cherry, which is highly valued for its lumber and whose fruit can be used to make jam. The inner bark was also used by Native Americans to make a cough-suppressant syrup.
The black cherry is very valuable to many caterpillar species and hosts 340 species of butterflies and moths, including the Eastern tiger swallowtail (Papilio glaucus), viceroy (Limenitis archippus) and the impressive promethea silk moth (Callosamia promethea).
The American plum blooms in late March or early April, making it one of our earliest-blooming trees. The dense clusters of flowers provide pollen and nectar for hungry insects, and the trees form thickets that provide both cover and nesting sites for birds. The fruit of the American plum can be eaten by birds, wildlife and people.
The birch tree, with its attractive peeling bark and beautiful yellow fall display, is a keystone tree that stands out in the landscape and is a valuable tree for nature and people. Three of our native species are: river birch (Betula nigra), sweet birch (B. lenta) and yellow birch (B. alleghaniensis).
The birch tree is valuable for insects and birds, supporting 284 caterpillar species and serving as a host plant for the mourning cloak (Nymphalis antiopa), cecropia moth (Hyalophora cecropia) and Henry's elfin (Callophrys henrici).
River birch can be grown in wet areas of the yard that have poor drainage. This tree is resistant to the bronze birch borer beetle. The sweet and yellow birch trees are aromatic, and the crushed leaves have a wintergreen scent. The twigs of the sweet birch were used by Native Americans as a natural toothbrush.
The willow, black willow (Salix nigra), pussy willow (S. discolor) and prairie willow (S. humilis) are fast-growing trees that thrive in wet areas. They provide erosion control and effectively detoxify soil and water by absorbing metals, toxins and excess nutrients. The willow supports 289 species of caterpillars and 14 species of specialist bees that rely on a single species of tree or plant to feed their offspring.
A few of the caterpillars supported by these trees include the Northern pearly eye (Enodia anthedon), common tortoiseshell (Nymphalis polychloros) and the beautiful luna moth (Actias luna). Because willows bloom early, their catkins provide nectar for early-emerging bees such as mining bees (Andrena spp.), bumblebee queens (Bombus spp.) and mason bees (Osmia spp.).
As forests continue to decline due to land development, homeowners and developers alike have an opportunity to give back to the ecosystem by planting a few of these keystone trees (https://sites.psu.edu/ecologistsnotebook/2019/11/14/signs-of-fall-9-natural-history-of-a-white-oak/) and providing habitat for native insects and birds (https://extension.psu.edu/soft-landings-benefits-insects-and-gardeners).
If you have questions about native plants, pollinators or other gardening practices, telephone the Penn State Extension Master Gardeners of Butler County at the Garden Hotline at 724-287-4761, Ext. 7, or email the Master Gardeners at butlermg@psu.edu.
Diane Walczak is a Penn State Extension Butler County Master Gardener.
