Almost everyone can identify an oak by the distinctive pattern of its leaves, but few people can discern which group or species of oak a particular individual belongs to. People of many cultures eat acorns, use oak for construction, or produce charcoal from the wood for recreational and manufacturing processes. These people know some species are better suited for their purposes than others and search them out. The multitude of ways that humans use oaks makes it important to identify the species correctly.
Oaks and Humans
Anthropologists have determined that humans have had a long history of using acorns as food and oak wood to build shelters, baskets, barrels, furniture, and produce charcoal. Some feel that this one genus of tree was alone responsible for the transition from the stone age to the bronze age when people learned to use oak wood to produce shelters, storage containers, and charcoal to smelt metals.
Subdivisions of the Oak Genus
Living on Earth for about 50 million years, oaks are some of the oldest flowering trees known. They are placed in the division of plants known as Fagaceae (Beeches, Fagus spp., Chestnuts, Castanea spp., Oaks, and a few small groups). Within the oak genus, there are three sections:
- White oaks – Quercus section Quercus, with distribution throughout the northern hemisphere
- Red oaks - Quercus section Lobatae, with distribution throughout the northern hemisphere
- Intermediate oaks – Quercus section Protobalanus, found only in Europe, North Africa, and Asia
This paper describes similarities within and differences between the white and red oaks.
Similarities
Both sections of oak have representatives that grow as massive, rounded trees, shrub-like scrub trees with many trunks, and brush-like plants growing in adverse habitats. All oaks produce catkin-like flowers that develop acorns when fertilized.
Although many oaks have deeply lobed leaves, some, often tropical and subtropical oaks, such as the willow oak, live oaks, and water oaks, produce unlobed, narrowly oval leaves. Oak acorns and leaves contain tannins that help reduce the intensity of foraging by insects and herbivorous vertebrates.
Oak wood is dense, easily split when fresh, resists warping, and rots and burns slowly. The wood is greatly desired for making furniture, for construction, and as finished flooring.
Differences
Although the acorns, leaves, and wood of many oaks are superficially similar, close observation demonstrates constant and reliable differences between the red and white sections that indicate which group of oak the plant belongs to.
Acorns
- The most reliable difference between the oaks is that the inner surface of the acorn cap is always hairless in the white oak section and always hairy in the red oak section.
- Scales on the acorn cap are generally fleshy in white oaks (See images) but flat and overlapping in the red oaks.
- White oak acorns mature the year they are formed and are found on the current year's twigs. Red oak acorns mature the year after they are formed and are found on second year twigs.
- Acorns of white oaks have low concentrations of tannin (a growth and digestive inhibitor) and germinate the fall they are produced. Red oak acorns have higher concentrations of tannin and germinate the spring after they fall from the tree.
Leaves
- White oak leaf lobes are normally rounded at the tip (see image of valley oak leaves), while red oak leaves have pointed lobe tips.
- The leaf tips of white oak leaves are not bristled, but the veins of red oak leaves extend past the leaf tips (see image of black oak leaves).
Wood
- Oak wood is composed of long, hollow cells called vessels. Because of generally warmer growing temperatures and more moisture, vessels produced in the spring have larger diameters than vessels produced in summer and autumn. The vessels produced by white oaks in the autumn - but not those of red oak - are filled with resins and the wood resists rot better than does the wood of red oaks.
These differences between white oaks and red oaks generate quite different ecological functions for the plants. These will be described in future articles.
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