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.

Soil conditions are distinguished according to depth, fertility, and the presence of perennial roots. Soil depth is important because it determines the extent to which roots can penetrate into the earth and, therefore, the amount of water and nutrients available to the trees. The soil in the taiga is sandy and drains quickly. Deciduous forests have brown soil, richer than sand in nutrients, and less porous. Rainforests and savanna woodlands often have a soil layer rich in iron or aluminum, which give the soils either a reddish or yellowish cast. Given the vast amounts of rain they receive, the soil is often poor in tropical rainforests, as the nutrients are quickly leached away.