Oaks are trees and shrubs in the Fagaceae family, which family also includes beeches, chestnuts, and chinquapins. The genus of oaks, with the scientific name Quercus, is broadly distributed in the Northern Hemisphere, and has a pronounced diversity and endemism in North America, particularly in Mexico. Oaks are wind pollinated, and may occur as evergreen, winter deciduous, or drought deciduous morphologies. Many oaks occur as top level canopy species, but an equally large number are shrubs or sub-canopy level associates.
There is a strong association between human civilizations and oaks, beginning at least 30,000 years before present (b.p.) in Europe and Asia, and 14,000 years b.p. in North America. Oaks have been a significant source of building materials as well as foodstocks for humans since prehistory, and remain a significant resource of many modern economies.
Oaks are usually divided into two subgenera: Quercus and Cyclobalanopsis; however, some taxonomies consider these two divisions are distinct genera. Taxa within Cyclobalnopsis occur in China and Southeast Asia and consist of about 150 different species.
Leaf blades present a leather-like or thin appearance, with margins entire, toothed or awn-toothed. Secondary veins are either unbranched or generally parallel, most of the time extending to margins. Leaf stipules are deciduous and inconspicuous. Terminal buds exhibit as spheric to ovoid, terete or angled, with all of the scales imbricate.
Inflorescences are unisexual, in the axils of leaves or bud scales, but normally clustered at base of new growth. Staminate inflorescences are lax, although pistillate inflorescences are typically rigid, with a terminal cupule and occasionally one or more sessile lateral cupules. Staminate flowers have connate sepals, with a circumferential tuft of silken hairs. Pistillate flowers manifest as one per cupule, with connate sepals. Fruits may mature in the first or second year. Acorn cups come in a variety of shapes (cup, saucer, bowl or goblet geometry), with no manifestation of valves; this cup (or cap) encases the nut base (only rarely the entire acorn); acorn caps are composed of scales that are imbricate or reduced to tubercles, but never hook-shaped.
Based on molecular genetics analysis, the genus Quercus is estimated to have separated from Castanea about 60 million years ago. Oaks first appear in the fossil record in North America during the Paleogene between 55 to 50 million years b.p. Most interspecific separations occurred within the Quercus species between 22 and three million years ago. During this period, oaks became the most dominant tree type in the Fagaceae family.
In mesic environments oaks are frequently canopy dominants, but in dry, rocky, high elevation or edaphically extreme settings, oaks are often shrub forms or even dwarfed. Edaphic extremes may include ultramafic soils, sandy barrens or inundated lands. The greatest concentrations of oaks are betweeen subtropical and middle-temperate climate regimes. Further north than this, conifer species typically become dominant; further south, oaks cannot successfully compete with taller trees of tropical rainforests with respect to sunlight, and in some cases due to intolerance to high rainfall combined with high temperatures.
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