Fall 2001

The Oaks: family trees

Photo: Willard Clay

For centuries they have been symbols of grandeur, antiquity, and rugged endurance. They have been the subjects of countless proverbs, myths, and poetic allusions. In our region, they are the lords of the land, raising their broad and stout-limbed forms over a varied landscape of moraines, dune ridges, and bottomland.

Still, the more than 400 species that make up the world’s allotment of oaks are a diverse assemblage of woody plants. While many oaks are tall-growing forest and woodland trees of temperate North America and Eurasia, others assume lower, shrubby shapes that populate more challenging regimes – the volcanic highlands of Central America, the sundrenched Mediterranean maquis, the salt-encrusted thickets of Cape Cod.

How far back into mists of geologic time does genus Quercus go? The fossil record suggests the oaks so back at least to the Eocene epoch, 56 to 35 million years ago. Paleobotanists and other collectors have discovered many oak fossils, aged Eocene and younger, from such far-flung locations as the Pacific Northwest and Bulgaria. These remains take the form of either petrified stems, often retaining an almost unbelievable amount of detail down to the cellular level, or leaf compressions, thin carbon films preserved in rock layers that still show the lobes and veins characteristic of long-extinct oak species.

The insects that eat oak leaves are an important food source for many species of songbirds. Photo: Louise K Broman/Root Resources

In modern times, at least, the oaks are found almost exclusively in the Northern Hemisphere; only in Colombia can one find them south of the equator. While oak trees are certainly prominent members of the United States flora – our country boasts about 90 native species – the true locus of the oaks’ genetic diversity is Mexico, where about 125 native species are present. Within the borders of the United States itself, the greatest concentration of species is located in the southeastern states. However, these statistics do little to suggest the true span of oak diversity. Nor do they indicate just how hard it can be, in certain circumstances, to identify which species is which. This is the result of the oaks’ own proclivity for hybridizing even without human interference. As any veteran tree-identification instructor can attest, the best way to adjust the overconfidence of botany students is to have them scrutinize an oak, seemingly so inoffensive at first glance, that combines the traits of at least two different species. In one sense, it’s a frustrating exercise; in another, it is an invaluable lesson in nature’s occasional avoidance of crisply defined categories.

Oaks have provided a variety of products and traditional medicines. These trees have yielded up untold thousands of board-feet of wood for barrel-making, shipbuilding, flooring, fencing, dyes, and tools. Their acorns, though usually much too bitter to eat without a thorough soaking or boiling first, have been an important source of food for centuries. Their bark, famously rich in the acidic compound tannin, has been used extensively in leather making and in a host of traditional cures that make use of its antiseptic and astringent properties. Among the ailments that have been treated with oak-based remedies are skin rashes, sore throat, indigestion, respiratory problems, dysentery, chills, fevers, and even broken bones and cholera.

Nor is this abundance of the oaks appreciated only by our own johnny-come-lately species. Untold multitudes of animals, from such arthropods as insects and spiders to the amphibians and birds that feed on them, are directly dependent on the bounty of the oaks – on their shelter, acorns, foliage, and bark – and have co-evolved over millions of years with them. And in an even grander ecological sense, oaks play a defining role in the continuance of that grand determinant of the Illinois landscape – fire. Where the fallen leaves of such competitor species as maples form a moist, gluey, and fast-decaying mat on the forest floor that discourages fire, oak leaves have a persistent, curly, dried-up texture that does the opposite. The cleansing fire readily spreads through oak leaf litter; and it is probably no coincidence that many oak species have a thick, heat-resistant bark that allows the trees to survive the blaze.

The Chicago Wilderness region has nine species of oak. The types present here are among the most aesthetically impressive and also the most abundant in the food and materials they’ve provided. Modern taxonomists divide the genus Quercus into three subdivisions, or sections: the red or black oaks, the white oaks, and the intermediate or golden oaks. Of these, only the first two sections are represented in Eastern or Midwestern states.

Chinquapin Oak	Bur Oak	White Oak
Photos: Mike Redmer

White Oaks
For the most part, the general characteristics of this group are distinguished easily: its leaves have rounded lobes that are not bristle-tipped; their mature bark tends to be scaly or flat-ridged rather than furrowed; the inner surface of their nutshells is smooth; the meat of their acorns is white and not as bitter as others’. Usually leading their roster is what is now the official state tree of Illinois, the Eastern white oak (Quercus alba). This handsome species, incidentally also the state tree of Connecticut and Maryland, is quite adaptable to different soils and terrains, but is generally most at home in mesic to dry woodlands. Its wood – hard, close-grained, and tan with a faint gray tint – has been prized for centuries. Among its most highly praised virtues is its nonporous nature, the result of tiny, balloonlike swellings in its inner tissue that plug up openings that might otherwise cause leakage.

The other august member of this section that could rightly dispute the title of Illinois’ state tree is Q. macrocarpa, the bur oak, a species of unsurpassed shape, impact, and character. Take a trip on a windy and well-lit day to our region’s savannas or farmland, and you’ll sense at once the great extent to which bur oaks still define the landscape. Their distinctly two-toned leaves (darkest forest green above and pale white below) are clustered toward the stem-tips. That, and the sparseness of internal branching, produce a certain airiness in this otherwise most stolid plant: breezes and the light of day pass almost unimpeded through the crown. At some distance, the bur oak is easily mistaken for the eastern white, but in the warm months its fiddle-shaped leaves, and its acorns with bristly cups almost wholly enclosing the nut, reveal its real identity. This species is the most sun- and cold-tolerant of all North American oaks. Its natural habitats include prairies, savannas, and open woodlands.

The two other native species of this section are less well known to the nonbotanist. They, too, can be confused with the Eastern white oak, even though their leaves usually bear rounded teeth rather than more deeply incised lobes. The swamp white oak, Q. bicolor, lives up to its common name by inhabiting floodplain forests where water stands at the surface for parts of the year. In contrast, the chinquapin oak, Q. muhlenbergii, prefers stony uplands where our area’s calcareous glacial till and bedrock engender alkaline soils.

Red Oaks
If distinguishing its individual species is often frustrating, at least it’s no great challenge to recognize this section as a whole. Its trees have leaves armed with bristle-tipped lobes or tips; its acorns are especially bitter; the nutshells have hairy or fuzzy interiors; the mature bark is deeply and vertically furrowed. By far the most prevalent representative is Q. rubra, the Northern red oak. It, with the Eastern white oak, is co-regent of our rich-soiled, mesic forests. Originally considered quite inferior to the white oak because of its porous wood – those who injudiciously made rum casks of red oak lost much of their cargo en route – this tree is now the apple of the lumberjack’s eye. The manufacture of modern, pressure-treated lumber requires woods that absorb preservative compounds well; here the red oak’s former detriment is transformed into a distinct asset – if not for the tree itself, for the ever-ravenous lumber industry.

Perhaps the northern red oak’s closest look-alike is Q. velutina, the black oak. Fortunately for budding tree-identification experts, this species tends to grow in the wild in distinctly different conditions. The best places to get a hint of the once-extensive tracts of Chicagoland’s black-oak community are the magnificent lakeshore savannas of Miller Woods, on the Gary side of Indiana Dunes National Lakeshore, and Illinois Beach State Park, just south of the Wisconsin border. These sites, originally formed by surf- and wind-deposited sand, show a different aspect of the oak genus. Instead of towering individual trees you’ll see a sun-dappled expanse of smaller forms, often distinctly shrublike. Still, though they do not compete with the bur oak’s majesty, the black oaks somehow impart the same sense of venerableness. On a more practical level, the bark of these trees, called quercitron, yields the highest concentration of dye and tannin, and was much sought after in earlier times.

Among the remaining species of the red oak section is one easily identified standout, the shingle oak (Q. imbricaria). The margins of its elliptical leaves lack both lobes and teeth – a unique trait among this region’s oaks. This species takes both its scientific and common names from the fact its wood was once used extensively for house shingles. It prefers dry woodland habitats and is not a particularly frequent sight in this area. The other species, all of which have deeply lobed leaves, are the all-too-similar pin, scarlet, and Hill’s oaks (Q. palustris, Q. coccinea, and Q. ellipsoidalis, respectively).

The pin oak, widely if not always successfully planted as a street tree, in this region is found primarily in sand savannas and sand flatwoods and is best identified by its distinctive branches: drooping at the bottom of the crown, horizontal in the middle, and upward-pointing above. In contrast, the scarlet and Hill’s oaks are trees of well-drained high ground. So minute are the distinctions between these two that some naturalists, have been tempted to combine them into a single species. In this way, they are rather emblematic of the oaks as a whole: even if they sometimes cannot be defined with perfect clarity, they are among the great definers of our region’s landscape.

– Raymond Wiggers