Most species of oaks are found in tropical and warm regions. Only a few species grow in Canada and other regions that have short summers and long winters. Oaks thrive in forests throughout the continental United States except for Alaska. They also are important in forests of China, Japan, and central and southern Europe. Oaks vary widely in size and in the way they grow.
Some oaks never become taller than small shrubs.
Many oaks that grow in warm climates do not lose their leaves in the autumn. They are called evergreens. The leaves of many deciduous oaks turn beautiful colors, such as deep red or golden brown, in early autumn. In the spring, oaks produce small, yellowish-green flowers. The male flowers, which form in dangling clusters called catkins, produce large amounts of pollen.