Tree of the Month, January 2022, Oak (Quercus sp)

by Martin (Mort) Schmidt for Simply Living

Oaks, the trees of the genus Quercus, are among the most important hardwood trees. There are approximately 500 species of oaks worldwide and a little under 100 in the United States, but they are found primarily in the northern hemisphere. Oaks provide a huge percentage of the edible mast for wildlife, and of course oak is tremendously useful as lumber. A few oak species are shrubby, but most are mid-sized trees, reaching heights of 80 to 120 feet at maturity. They are among the longest lived trees and can survive for four or five centuries or more in favorable conditions.

Left – Pin Oak in winter. Upper right – Alternate versus opposite twig and leaf arrangement. Lower right – Pinnate versus palmate veining (and leaflet arrangement in compound leaves). Photos by Mort Schmidt.

We generally think of oaks as having lobed leaves. Most do, but this is a secondary trait. Oaks, and only oaks, produce acorns — depending on the classification scheme, some closely-related trees bear fruits similar to, but arguably not exactly acorns). Oak leaves are alternately arranged and simple. That is, the leaves do not occur directly across from each other like maples with their opposite leaves, nor do they occur in more or less constant numbers and share a stem with other leaflets, such as Shagbark Hickory with its five leaflets per stem. Oak leaves are pinnately veined, i.e. there is one primary vein in the center of the leaf with secondary veins branching out at different points. Most oak leaves have lobes, but Live Oak, Willow Oak, Sawtooth Oak, and Ohio’s native Shingle Oak do not.

Some trees are dioecious, having separate male and female trees, but oaks are monoecious, so male and female flowers, and subsequently acorns, potentially occur on every tree. Oak trees are usually divided into two groups — white oaks and red oaks. Here, the terms “white oak” and “red oak” are used in the general sense, but within these groups are the species White Oak (Quercus alba) and Red Oak (Quercus rubra). When discussing oaks, it’s helpful to specify whether you’re using the terms “white oak” or “red oak” as a group or a species.

White oak acorns mature in one year and germinate soon after falling. Red oak acorns lie dormant over winter and send out roots after approximately 40 days of cold. White-oak acorns have less tannins than red-oak acorns, and are sweeter. Perhaps white oaks germinate sooner to make them less susceptible to foraging. Oak trees might be decades old before producing acorns, and like most trees, the fruits, in this case, acorns, are absent most of year. Additionally, most white oaks produce acorns only four years out of ten, and red oaks produce acorns only six or seven years out of ten. The productive years are known as “mast” years. Other nut crops, including hickory and walnut, also have mast years. The population of birds, squirrels, deer, and other acorn consumers is limited by the availability of mast in lean years. Consequently, during mast years, the trees produce more than the animals can eat, giving nuts a greater chance of becoming trees. The odds of an acorn growing to maturity are slim. Of the countless thousands of acorns an oak produces in its long life, only one must replace the parent tree to maintain a stable population.

Although acorns are the defining feature of oak trees, they’re absent most of the time, and not always very distinctive, so we use secondary features, primarily leaf shape, to identify species. In general, leaves from the white oak group have rounded tips. It might help to remember that snowballs are white and round. Leaves from the red oak group have pointed tips, or at least a small bristle at the tip or tips. In winter, oaks can be distinguished by the presence of multiple terminal buds. Like the leaves, white oak buds are rounded and red oak buds are pointed, but the difference can be subtle. Also in winter, oaks and the related beeches tend to retain their leaves longer than other trees. Mature oaks can usually be recognized by their bark. Most white oak bark consists of overlapping shingle-like plates. Red oak bark is usually furrowed with long smooth ridges between ridges.

Upper left – Pointed leaf tips from red oak group versus rounded leaf tips from white oak group. Upper and center right – Rounded terminal buds from white oak group versus pointed terminal buds from red oak group. Bottom right and center – Plate-like bark from white oak group versus smooth-ridged bark from red oak group. Lower left – Ring porosity characteristic of oak, ash, and hickory. Photos by Mort Schmidt.

Oak wood is famously hard. I put together a table of 68 Ohio trees for which I found lumber-hardness data. All of the oaks are harder than the median, walnut, and white oaks are generally harder than red oaks. Because white oak pores are filled with a foamy looking substance called “tyloses” that keep out water, white oak lumber is more durable (rot resistant) than red oak. The pores in oak, like those of ash and hickory, are lined up along the annual rings, and these ring-porous woods split readily along their length. This might appear to be a weakness, but it makes oak ideal for long, thin wooden parts, as long as the grain is parallel to the piece’s length. In earlier times, woodworkers made barrel staves, chair spindles, wheel spokes, etc., perfectly parallel to grain by splitting the wood with an ax or froe. (Splitting was also faster than cutting boards along their length). You might have noticed that chair spindles, rake handles, and other slender pieces of wood usually break where the grain runs at an angle to the piece’s length. Breakage seldom happens if the wood is hand-split instead of machine-sawn, because the piece’s edges are exactly parallel to grain.

Oak’s strength, wear resistance, and abundance make it well suited for furniture and flooring. Its strength, ease in splitting, and flexibility make it ideal for barrel staves. Since tyloses block fluid flow, white oak barrels are used for liquids (tight cooperage), while red oak barrels are, or used to be used for solids, such as flour or nails (slack cooperage). Oak supplies not only barrels for aging wine and whiskey, but the bark of Quercus suber provides the cork with which to bottle them.

Like most hard, dense woods, oak has a high fuel value, which, with its ease in splitting, make it an excellent firewood. Oak bark is high in tannins, which were once extracted for tanning leather. Tannins from oak bark and oak galls were also used as mordants for fixing dyes and inks. Acorns are eaten not just by wildlife, but also by people. Native Americans and Europeans ate, and some Asians today eat acorns after prolonged soaking to leach out tannins. White oaks are less acidic and sweeter, and some California acorns were so sweet that they could be eaten without leaching.

Oaks play a huge role in human culture. In the book Oak, The Frame of Civilization, William Bryant Logan lists a number of family names derived from words for oak, including Aikman, Acton, Akroyd, Cash, Chaney, Encinas, Oakley, Rove, and others. Logan’s book, and Rebecca Rupp’s Red Oaks & Black Birches discuss the tendency of oaks to attract lightning, which may explain why the ancients dedicated oak trees to the Greek god, Zeus and the Nordic god, Thor, the thunder god. Acorns provided sustenance to Europeans in years of crop failures, and oaks were so valued that cutting one down without the proper rituals and priestly approval was punishable by death.

Of course, oak is a symbol of strength and toughness. Oak leaves are the insignia of Majors and Lieutenant Colonels in the U.S. Army (and the equivalents, O-4 and O-5, in other service branches). And Rupp points out that the Latin species name for English Oak, Quercus robur, has the same root as the word “robust.” [I had to admire the cultural importance of trees when I noticed my use of “branch” and “root” in the two preceding sentences]!

I’ll discuss individual oak species in later articles, but in the meantime, an excellent source for identifying oaks is USDA’s free, downloadable, Field Guide to Native Oak Species of Eastern North America.