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Forests are a complex ecological system in which trees are the dominant life-form. Tree-dominated forests can occur wherever the temperatures rise above 10 °C (50 °F) in the warmest months and the annual precipitation is more than 200 mm (8 inches). They can develop under a variety of conditions within these climatic limits, and the kind of soil, plant, and animal life differs according to the extremes of environmental influences.

In cool, high-latitude subpolar regions, forests are dominated by hardy conifers like pines, spruces, and larches. These taiga (boreal) forests have prolonged winters and between 250 and 500 mm (10 and 20 inches) of rainfall annually.

In more temperate high-latitude climates, mixed forests of both conifers and broad-leaved deciduous trees predominate. Broad-leaved deciduous forests develop in middle-latitude climates, where there is an average temperature above 10 °C (50 °F) for at least six months every year and annual precipitation is above 400 mm (16 inches). A growing period of 100 to 200 days allows deciduous forests to be dominated by oaks, elms, birches, maples, beeches, and aspens.

In the humid climates of the equatorial belt, tropical rainforests develop. There heavy rainfall supports evergreens that have broad leaves instead of needle leaves, as in cooler forests. In the lower latitudes of the Southern Hemisphere, the temperate deciduous forest reappears. 

Britannica, 2015