Our first fall in Harvard was a mast year for oak trees. A bumper crop of acorns enticed the chipmunks to take over the yard. The little creatures must have buried acorns in my potted plants as they summered on the porch because several oak seedlings germinated in my living room window later that winter. While I was making a close acquaintance with the Quercus genus whose groves encircle my property, oaks were growing into minor celebrities of the tree world, featured in native plant expert and entomologist Douglas Tallamy’s latest book, “The Nature of Oaks,” and various other publications.
An older white oak, Quercus alba, shows its characteristic horizontal branches and wide-spreading crown at Bellevue Cemetery, Still River Road. (Courtesy photo)
Although some landscapers may shun native oaks for their acorns and thick leathery leaves (a mature oak can produce more than 700,000 leaves, which take more than two years to decompose), Tallamy asks readers to reconsider their beauty and importance: If you want to feed the birds, grow oaks. That’s because oaks host over 500 species of caterpillars, far more than any other tree genus, and many bird species feed their young only on caterpillars. In fact, the sudden flush of insects in the early spring in temperate climates has co-evolved with bird migration, and migrating birds hang out more on oaks than other trees because they know they will find food there.
The evolution of relationships between plant and animal species over millions of years is what makes native plants—including oak trees—so important. Tallamy estimates that 90% of insect herbivores specialize in eating only a few types of plants because it takes evolutionary time and many fortuitous genetic and behavioral adaptations to gain the ability to overcome plants’ chemical defenses.
A keystone species
Ecologists speculate that oak trees have developed into a keystone species, a linchpin of their ecosystems, for several reasons. Insects are more likely to evolve adaptations to plants they encounter frequently, and the oak genus is exceptionally widespread, with many species overlapping in their ranges. The trees are also long-lived. In addition, tannins, the oaks’ chemical defenses, are easier to overcome through natural selection than more lethal chemicals such as the cyanide produced by cherries.
Even though oaks have a long history in central New England, having readily repopulated after the retreat of the glaciers 13,000 years ago, the ones we see in Harvard are fairly young. In Central New England, after European colonization, forest area shrank until the mid-19th century, at the peak of sheep farming. According to ecologist, author, and Harvard University professor David Foster, after the wool market collapsed, pastures were abandoned and white pines readily filled open fields. With windblown yet robust seeds, white pines germinated and took root in the sod of the former pastures. The resulting successional white pine forests were large enough to log by the 1890s, and were widely cut up to the 1920s.
When the pines were harvested, broadleaf species like oak, red maple, and cherry seized the advantage: They might have been cut along with the pines, but they were capable of sprouting from their stumps (unlike the slowpoke pines which would have to start afresh from seed). Young forests dominated by white pine thus gave way to a next generation of mixed hardwoods, a process common across Massachusetts in the 20th century, and likely the precursor to many of the oaks we see around us.
Notable oaks of Harvard
History and trees go together, a point underscored by the Garden Club of Harvard’s 2002 publication on ancient and notable local trees. Indeed, white oaks can live more than 500 years, although 200 to 300 years is more likely—and these days far fewer, given the way we treat our trees. In Harvard, a very old Quercus alba stands at the intersection of Codman Hill and Stow roads. Surveyors often used white oaks on property deeds—this one marking the boundary between Harvard and Boxborough, first appearing on a Harvard map in 1794—due to their longevity and deep roots. In the early 2000s, when a developer wanted to build a driveway over the Boundary Oak’s roots, community members from Boxborough and Harvard protested: The tree won. If you have white oaks growing on your property, you can identify them by the rounded edges of their leaves and the light grey color of the rough, flaky bark, which may become smoother on old trees like the Boundary Oak.
Quercus bicolor, the swamp white oak, also found locally, is well adapted to nutrient-poor soil and floodplain. Likely the oldest example in Harvard is found in the parking lot at Oxbow National Wildlife Refuge at the end of Still River Depot Road. Sadly, graffiti covers its deeply furrowed dark grey bark. These trees can be identified by their dark bark and their two-toned leaves, dark green on top and silver and slightly furry on the underside. They will grow in a variety of situations, and can be planted in dry conditions, although they may not sprout there naturally.
The tallest oak trees around Harvard are usually the eastern red oaks, Quercus rubra, which have pointy, rather than rounded, leaf lobes and large acorns. Other local oaks with pointy leaves, characteristic of the red oak division, include black and scarlet oaks. Oaks are notoriously hard to identify, and some of them hybridize naturally, which makes it even harder to tell them apart.
Remarkable specimen trees like Harvard’s Boundary Oak are fun to admire; however, from an ecological perspective, fostering small groves of trees makes the most sense. Oaks have male and female flowers on the same plants—the light green catkins hanging from the trees that will appear in April are male—but typically do not self-pollinate, so they need companions. Further, groves will create interlocking roots, providing support in inclement weather and amid changing environmental conditions.
There are probably already plenty of oak seedlings coming up in your yard, so the easiest method is to let some of them grow to maturity: The birds will thank you.
For more information
For an in-depth portrait of the seasonal interactions of oaks with their environments, see “The Nature of Oaks: The Rich Ecology of Our Most Essential Trees” by Douglas Tallamy, 2021.
Catherine Warner is an amateur gardener, freelance writer, and educational consultant who lives in Harvard with her family and their beloved dog Moe.
