Oak trees (genus Quercus) are native to the Northern Hemisphere (about 600 species) There are 43 kinds of oaks in the eastern United States! Many oak trees appear very similar and they can and do hybridize which makes identification even more difficult. The leaves of different species can look very similar or very different or even like leaves of other trees and there can be variation in the leaves from the same tree! Oaks are grouped into two broad categories: red oaks and white oaks.
Red oaks have pointed leaf lobes (example on left in picture) and include the northern red oak (Quercus rubra), the scarlet oak (Quercus coccinea), the northern pin oak (Quercus ellipsoidalis), and many others. The white oaks have rounded leaf lobes (example on right in picture) and include the white oak (Quercus alba), the bur oak (Quercus macrocarpa), the chestnut oak (Quercus montana), and many others.
Oak trees can be deciduous (shed leaves annually) or evergreen. Some of the deciduous species do not drop their dead leaves until spring. All oaks bear fruit, which is a nut called an acorn (aka oak nut). Acorns sit in a cup-like structure called a cupule and contain one seed each, rarely more. The leaves and acorns contain tannic acid and the amount varies with species.
Oak tree wood has many uses including lumber, flooring, furniture, railroad ties, paneling, interior trim, and fence posts. Even more important is the fact that oak trees are the most important source of hard mast in North American hardwood forests. Mast refers to the seeds, nuts, buds or fruits of trees and shrubs that wildlife and humans eat. Soft mast refers to berries and fruits such as blueberries or pears. Hard mast comprises hard nuts and seeds such as acorns or walnuts. Hard and soft mast are both important food sources, but hard mast has a higher energy content and is especially important in winter.
Oak trees are considered keystone species in their ecosystems. National Geographic defines a keystone species as one that “helps define an entire ecosystem. Without its keystone species, an ecosystem would be dramatically different or cease to exist.” Over 5000 species of insects and over 100 species of birds depend on oak trees for habitat and/or food. More than 100 vertebrate species eat acorns, including humans. Acorn-eating animals include deer, chipmunks, wild turkeys, bears, rabbits, opossums, raccoons and mice. One squirrel or one scrub jay can gather and hide thousands of acorns in a year, but will find only about half – so in addition to feeding on acorns, they also are planting more oak trees! Another bird, the acorn woodpecker, stores acorns in “granaries” (dead tree trunks or telephone poles into which they have drilled holes). These stored acorns provide food for them year round.
Acorns have a very interesting history. Long before the beginning of the cultivation of wheat and other grains, people throughout the entire northern hemisphere relied on them for food. They are high in fat, protein, carbohydrates, antioxidants, Vitamin B6 and manganese. Oak trees and acorns were found in most of California and sometimes made up 50% of the Native American diet there. Entire villages engaged in acorn harvesting and could gather enough acorns in two or three weeks to feed their families for a couple of years. After drying, acorns could be stored for years in baskets. John Muir learned to make acorn cakes from the northern California natives living in the Yosemite Valley. He called them “the most compact and strength-giving food.” The cakes were very nutritious and lasted for months without spoiling. What caused the transition from eating acorns to cultivating grains? One hypothesis is that oak trees were lost to grazing animals that ate too many seedlings and to the cutting down of too many oak trees for heating and building.
For humans to eat acorns, a little preparation is necessary. The acorns must be ripe, that is, brown in color and the tannins must be leached out, because tannins are bitter and cause intestinal problems. The ripe acorns are cracked open and the nuts hulled and dried. The whole nuts are put into a jar of water. To speed up the process, the nuts can be ground first and the ground nuts put into a muslin bag for leaching. When the water turns brown, it is changed and this continues until the water stays clear. When the nuts do not taste bitter, leaching of the tannins is accomplished. If whole nuts were used, they are then ground into flour.
Another interesting fact about acorns is the phenomenon of oak tree masting (years when bumper crops of acorns are produced). And masting occurs with all the oak trees in a region, not just with one tree. You have probably noticed this with hundreds of acorns on the sidewalks and lawn or hitting you when they are falling off the tree. Mast years occur every 2 to 5 years. It is not definitely known what causes masting.
To end, I would like to mention 3 oak species that you can find in the park. The first is the bur oak (Quercus macrocarpa) which has fantastic looking acorns. Hint: you can find one near the Visitor Center. Another is the chestnut oak (Quercus prinus), whose leaves resemble the leaves of the chestnut tree. One lives near the Visitor Center along the path from the Shannon mansion. And last, the northern red oak (Quercus rubra) that has beautiful red leaves in autumn (picture at top of article).