About > Classification of Oaks

By Allen Coombes, lately botanist at the Sir Harold Hillier Gardens, Hampshire, UK.

 

The oaks are a genus of some 450-500 species of deciduous and evergreen trees and shrubs. They are widely distributed across the northern hemisphere but absent from the colder areas of the far north, and extend south to Colombia in South America and Indonesia, where they just enter the southern hemisphere. Oaks definitely prefer warm climates and the greatest diversity of species occurs in warm temperate, subtropical and tropical regions. The further south an oak grows, the more likely it is to be restricted to mountain regions.

Within the genus, the species are extremely varied ranging from large trees of 40 m (130 ft) or more to small, sometimes suckering shrubs of 1 m or less. While most will identify an oak from its leaf, this is perhaps the most variable aspect, ranging from large with rounded or pointed lobes, to saw-toothed, spiny or entire (without lobes or teeth). What does remain constant is that they all have separate male and female flowers, the males in pendulous catkins (thus distinguishing them from Castanea and Lithocarpus). All have an acorn for the fruit which can vary a great deal in size and shape, as can the scaly cup in which it is borne.

The genus is divided into two major groups, or subgenera. Most of the familiar species are in subgenus Quercus. The other group, subgenus Cyclobalanospis, is still treated by some as a genus in its own right but as Professor Jensen has pointed out in this book, current molecular work confirms its placement within the genus Quercus. Members of subgenus Cyclobalanopsis, sometimes referred to as the ring-cup oaks, are restricted to Asia, occurring from the western Himalaya in Pakistan to East and South East Asia. They are evergreen, or sometimes briefly deciduous, trees and shrubs with entire or shallowly toothed leaved. The scales on the acorn cup are united into concentric rings. There about 150 species, many in tropical and subtropical regions. The most familiar species in gardens are Q. glauca and Q. myrsinifolia.

Subgenus Quercus, which contains the remainder of the species, is further subdivided in to sections. Section Quercus, the white oaks, consists of deciduous and evergreen trees and shrubs widely distributed in North and Central America, Europe, North Africa and Asia. The normally have leaves with rounded lobes or with short, blunt teeth, occasionally entire. The acorns ripen the first year (and so are found on the shoot amongst the leaves) and germinate as soon as they fall. Examples of this section are English oak, (Q. robur), and white oak (Q. alba).

Section Lobatae, the red oaks, also consists of deciduous and evergreen trees and shrubs but these are confined to North and Central America. While there are more technical differences between these and the white oaks, they often have leaves with pointed lobes ending in bristle-like teeth. The acorns usually ripen the second year (and so are found on mature, leafless shoots) and they normally germinate the following year. Well-known examples of this section are red oak (Q. rubra) and pin oak (Q. palustris).

Section Cerris consists of evergreen and deciduous trees and shrubs found only in Europe, North Africa and Asia. These oaks often have conspicuous stipules, their acorns normally ripen the second year, and the cups often have bristly scales. Although these oaks have often been included in Section Quercus, Professor Jensen points out that recent work shows that they are in fact very distinct and are more closely related to oaks of subgenus Cyclobalanopsis than to the other oaks. Examples of this section are Turkey oak (Q. cerris) and sawtooth oak (Q. acutissima).

Section Protobalanus, the intermediate oaks, consists of only five species of evergreen trees and shrubs from the south west United States and north west Mexico. They have leathery leaves with spiny teeth, at least when juvenile, and acorns that ripen the second year. The best known species is golden cup oak (Q. chrysolepis).

Hybridisation between different species in the genus, both in the wild and in cultivation, broadly confirms these divisions. Hybrids within subgenus Cyclobalanopsis are reported from Japan but no hybrids have been found between any member of this subgenus and subgenus Quercus. Members of section Quercus and section Lobatae hybridise very frequently, both in the wild and in cultivation, but only within their sections. Some are known in section Protobalanus but only within that section. Curiously, it is section Cerris that is the odd one out here. Certainly, hybrids are known within this section but one hybrid with a member of section Lobatae is known: Q. ×kewensis (Q. cerris × Q. wislizeni), and a small number of hybrids also occur with members of section Quercus, such as Q. ×turneri (Q. ilex × Q. robur).