Yellow area: subgenus Euquercus; pink area: subgenus Cyclobalanopsis

The genus Quercus spreads all over the northern hemisphere in Asia, North America, Europe and Africa (Axelrod, 1983), down to the Equator. While most text books report between 300 and 600 species belonging to the genus Quercus, more recent inventories (Valencia (2004) and Nixon (1993) for America; Menitsky (2005) for Asia and Flora Europea for Europe, Schwarz (1964)) amount to 320 to 354 species for the subgenus Euquercus and 76 species for the subgenus Cyclobalanopsis. Over the whole genus, there are about as many species in America (200 to 234) than Eurasia (196). However as sub genus Cyclobalanopsis is only present in south east Asia, the subgenus Euquercus is less diverse in Eurasia (120 species) than in North America (200 to 234). Europe comprises only 22 species and Asia 98. Diversity is highest between 15° and 30° Northern latitude in central America (particularly Mexico) and South Asia (China and Yunnan Province in particular).

In Eurasia, the northern limit of distribution reaches 62-63° Northern latitude, in Scandinavia and the southern limits extends a few degrees south of the Equator (up to the Sunda platform) (Menitsky, 2005). In Europe, oaks grow from Northern Africa (Q. suber, Q. ilex, Q. afares) across the Mediterranean region up to southern Norway, Sweden and Finland, which are only reached by Q. robur and Q. petraea. Most European species grow in the Mediterranean region (about 20 species). In eastern Asia, oaks of the subgenus Euquercus extend from Manchuria down to Myanmar and Thailand, where they grow in mountainous areas (mainly Q. semecarpifolia, Q. lanata, Q. kingiana, Q. griffithii and Q. acutissima). Species of the subgenus Cyclobalanopsis extend further south in lowland tropical forests. According to Menitsky (2005), there are 19 oak species growing in the equatorial Asia all belonging to subgenus Cyclobalanopsis, which are evenly distributed in Malacca, Sumatra and Kalimantan. The isle of Java is less diverse and comprises only 5 species of the 19 equatorial species. In central Asia, oaks spread from central China to the western border of the Himalaya mountains, from Bhutan, Nepal up to Northern Pakistan and Afghanistan (Negi and Naithani, 1995). Here again oaks grow in altitude (Q. baloot and Q. dilatata on the western part up to 2200 meters, Q. semecarpifolia up to 3900 meters in Nepal). In western Asia, oaks grow in Iran and Irak in the mountains of Zagros (Q. brantii, Q. libani) and Elbruz (Q. castaneifolia, Q. macranthera). Widespread European species extend at their eastern limit in Caucasus (Q. petraea, Q. robur and Q. pubescens) where they meet eastern Asian species (Q. macranthera, Q. hartwissiana, Q. infectoria). Lebanon and Palestine are at the extreme Asian edge of the Quercus distribution (Q. aegilops, Q. infectoria).

In America, the northern limit of distribution reaches 50° Northern latitude, and the southern limit is in Colombia. There are only a few oak species extending up to Canada (Q. garryana, Q. rubra, Q. alba and Q. macrocrapa). USA has 34 red oak species (section Erythrobalanus), and 49 white oaks (section Lepidobalanus) (Nixon, 1993). Most of the oak species are present in the Eastern part of the country, up to the Mississippi river. In the western part of the USA, Miller and Lamb (1985) identified three groups: the west Texas oaks (9 species), the southwestren oaks (16 species) and the pacific coast oaks (11 species) (Pavlik et al., 1991). In Mexico oaks occupy all the major mountain ranges, including the Chihuahan desert range. There are 30 oak species in Central America and only on in South America (Q. Humboldtii).

Oak species grow from the sea level up to 4000 meters in the Himalaya mountains (Wang, 1961; Menitsky, 2005). Throughout its natural distribution, the genus has differentiated in numerous species adapted to extremely variable habitats from swamps to deserts, and from lowland to high altitudes Oaks also exhibit a very wide diversity of forms from shrubs to massive trees. The range of distribution of the species can vary from very local to wide inter continental scales. Typical examples of extremely wide distribution are Q. rubra and Q. alba in North America, Q. acutissima and Q. mongolica in Asia, Q. robur and Q. petraea in Europe.

Worldwide distribution of oaks

Oak species in America
Oak species in Europe
Oak species in Asia

AxelrodD.I. 1983. Biogeography of oaks in the Arcto-Tertiary province. Ann. Missouri Bot. Gard. 70: 629-657
MenitskyY.L. 2005 Oaks of Asia Science Publishers, Enfield (NH) USA, 549 pages (original book in Russian, 1984)
Miller H., Lamb S.H., 1985. Oaks of North America. Naturegraph Plublishers, 325 pages
NegiS. S., Naithany H.B., 1995. Oaks of India, Nepal and Bhutan. International Book Distributors, New Dehli, 266 pages

Flora of China (1999)

Nixon K.C., 1993. The genus Quercus in Mexico. In Ramammoorthy T.P., Bye R., Lot A., Fa J. (eds) . Biological diversity of Mexico: origins and distribution pp 447-458, Oxford University Press
PavlikB.M., Muick P.C., Johnson S.G., Popper M., 1991. Oaks of California. Cachuma press, 184 pages
Schwarz, O. 1964. Quercus L. In: Tutin, T. G., Heywood, V. H., Burges, N. A., Valentine, D. H., Walters, S. M. and Webb, D. A. (eds) Flora Europaea, vol. 1: Lycopodiaceae to Platanaceae, pp. 61-64. Cambridge University Press, Cambridge
Valencia S.A., 2004 Diversidad del genero Quercus (Fagaceae) en Mexico. Bol. Soc. Bot. Mex. 75:33-54

Flora europeae (1999)