Trees and shrubs are divided into two main groups, gymnosperms and angiosperms. Gymnosperms produce seeds which are not enclosed, often borne in cones, where as angiosperms produce seeds enclosed in a fruit. Examples of gymnosperms are conifers like spruce and pines. Deciduous trees such as birch and mountain ash are angiosperms. Here they are divided in three categories, trees with inconspicuous flowers, flowering shrubs and trees, and flowering evergreen shrubs.
Conifers are often more drought and cold tolerant than deciduous trees, because their leaves are needle-like, with a small surface area and often covered in wax to prevent water loss. They are usually evergreen. The conifer forests replace deciduous forests in the northern hemisphere and reach far north to the arctic tundras.
Most angiosperm trees and shrubs are deciduous, but there are some evergreen species with thick, leathery leaves that stay green throughout the winter.