This time of the year—late October—most of the deciduous trees have dropped their leaves. Prominent exceptions are members of the family Fagaceae, which includes the American beech (Fagus grandifolia) and all the oaks (Quercus). They hold on to their leaves through the winter, a phenomenon called “marcescence.” The oaks are among the more diverse genera in the northeastern forest. They are dominant species in the “oak-heath” and “oak-hickory” communities that cover the drier, warmer, south-facing slopes of temperate eastern forests. There are about 90 species in the U.S., and many of them are important economically, mostly for building interiors and for furniture, and more esoterically, for boat building.
The oaks are divided into five “sections.” The first has the same name as the genus, Quercus (synonyms Lepidobalanus and Leucobalanus), which is also referred to more colloquially as the “white oak group.” Another is the Lobatae (synonym Erythrobalanus), which is usually called the “red oak group.” There is another North American section, the Protobalanus, which includes the canyon live oak and its relatives in Mexico, and two Eurasian sections, the Mesobalanus, the Hungarian oak and its relatives, and the Cerris, the Turkey oak and its relatives.
The Quercus section is Holarctic; its species are distributed in both the Old and New World in the Northern Hemisphere, but the Erythrobalanus species are confined to the New World north of northern South America.
The “white oak group” is distinguished from the “red oak group” by leaves with rounded lobes that lack a terminal bristle on each lobe. The acorns of the white oaks ripen in one year and tend to be sweet and quite edible. Red oak species have leaves with angular lobes and a small terminal bristle. Their acorns take 18 months to ripen, and they are full of tannins, which give them a bitter taste. Squirrels and birds that collect the red oak acorns will bury them and let the tannins leach out of them for weeks or months to make them more palatable.
Each group has an eponymous species that demonstrates the characteristics of the classification clearly. The white oak (Quercus alba) can get to be an enormous spreading tree when it grows in the open. The crown can be wider than the height of the tree, which can approach 85 feet. It tends grow taller in the forest, but is still a massive tree. It doesn’t begin to produce acorns until it is at least 20 years old, and after it reaches 50 years, it produces more every year.
The xylem cells of white oak have tyloses, obstructions that cause the lumber to be very water-tight. Consequently, white oak is used for boat building and the construction of barrels, notably those for storing wine and bourbon whiskey. Quarter-sawn white oak was the signature material of Mission-style furniture made by Gustav Stickley.
The northern red oak (Quercus rubra) is large tree, like the white oak, but it tends to be less massive and when grown in the open forms a narrow round-topped crown. While white oak bark is gray with irregular raised plates separated by narrow furrows, red oak bark is darker, with narrower ridges and broader furrows. The ridges of the northern red oak appear to have shiny stripes down the center.
Red oak acorns are larger than those of the white oak. The inside of the cupule that covers the nut is also fuzzy in red oaks and smooth in white oaks. The red oak acorns display epigeal dormancy; they have to be exposed to three months of temperature below 40°F before the germinate. They over-winter on the tree and fall during the following growing season.
Red oak is used for building, but the wood lacks the tyloses of the white oak, which makes it much more permeable. Most hardwood floors made of oak, however, are made from trees of the red oak group. Q. rubra is the highest quality wood in the group, but flooring sold as “red oak” may include pin oak (Q. palustris), black oak (Q. velutina), scarlet oak (Q. coccinea) and other “red oak” species.