Forests composed of deciduous species—ones that periodically shed their leaves—grow in well-watered parts of the temperate zone in which a long, warm growing season alternates with a cold winter. Precipitation, as either rain or snow, tends to fall throughout the year.
In the middle latitudes, the prevailing winds carry moisture-laden air masses over the west coasts of the continents, which receive substantial rainfall in consequence. In much the same manner, the subtropical east coasts of the continents are kept wet by the trade winds. In these regions, the combination of ample precipitation and comparatively mild temperatures produces dense forests in which evergreen species, both broadleaf and coniferous, preponderate.
Some of these forests are sufficiently wet to be called rainforests. As in the more familiar rainforests of the tropics, many of the trees reach enormous size, and epiphytes are common. Because more light penetrates the canopy, however, the vegetation of the understorey and forest floor is better developed than in the tropics.