Oaks, of the genus Quercus, are very abundant hardy trees of temperate climates, preferring drier soils from the southern edges of the northern forest to the tropics. They frequently are found mixed with pines; and with the absence of major forest fires, which favor pine regrowth, will ultimately replace the pines in natural forest succession.
Oaks have thick, fire-resistant bark. Even when the main stem is killed, the deep root system lives on, sending up successive saplings. Propagation to sites distant from the parent tree is helped by animals' harvesting of acorns, which are then buried and perhaps forgotten, to sprout new seedlings.
Oaks show a great diversity in leaf shape, but are identified by their cupped fruit, the acorn, which is edible. Oaks can be divided into two major groups: white oaks, with rounded leaf lobes; and red oaks with bristle-tipped lobes. Red oaks have bitter acorns, while white oak acorns have less tannin, and are therefore sweeter.
Oaks are monoecious, with both male and female flowers on the same tree, and frequently on the same branch. The female flowers arise from buds on new growth shoots, while the male catkins arise from second-year shoots. Temperate oaks are wind-pollinated, and may be major inciters of pollinosis, during late winter and early spring, frequently accounting for over one-third of the pollen collected in some regions. Tropical oaks and tanbark oaks are primarily insect-pollinated, with only secondary wind dispersal.
In the United States, oaks pollinate from February through June, depending on species and locale. In the southeastern and south-central states, live oaks and white oaks pollinate from February through April, with red oaks somewhat later, March into May. In the northern states, pollen anthesis is usually in May and June.
Oak species show a great deal of hybridization, so there is controversy over exact taxonomy and a difference of opinion over the total number of species. There are 500 species estimated worldwide, 250 in the Western Hemisphere, >150 in Mexico, with approximately 70 species in the United States and Canada.
The southern states have a greater diversity of oaks; Texas has more than 40 species. North American oaks may be further classified into five subgroups based on morphology and distribution: true white oaks, especially prominent in the southeastern states; chestnut oaks, found most frequently in the mid-Atlantic and Ohio River valley regions; true red oaks, common in the eastern half of the continent; willow oaks, prevalent in the southeastern states to central Florida; and live oaks, or evergreens, especially common in California and Arizona, extending up to Pacific Northwest and along the Gulf Coast.
Adapted from: Annals of Allergy, Richard Weber MD,
March 2001.