An oak is a tree or shrub in the genus Quercus having 600 extant species native to the Northern Hemisphere, and includes deciduous and evergreen species extending from cool temperate to tropical latitudes in Asia and the Americas. Oaks have spirally arranged leaves which may have lobed margins, serrated leaves or entire leaves with smooth margins. The flowers are catkins, produced in spring and the fruit is a nut called an acorn.br/br/Many species of oaks are under threat of extinction in the wild, due to land use changes, livestock grazing and unsustainable harvesting. Over the past 200 years, large areas of oak forest in the highlands of Mexico, Central America and the northern Andes have been cleared for coffee plantations and cattle ranching. Today there is the continuing threat from exploitation for the timber.br/br/The oak is a common symbol of strength and endurance and has been chosen as the national tree of many countries.