There are more species of plants and animals in tropical rain forests than in all the rest of the world's ecosystems combined. About 70 percent of all plant species in these forests are trees. Although temperate grasslands may have more than 50 species of vascular plants, and tropical grasslands more than 200, two or three species of grasses usually dominate upward of 60 percent of the biomass above ground.
Grasslands are natural, seminatural, and cultivated.
Natural grasslands occupy, or once occupied, large areas of continental landmasses, including the prairies and plains of North America, the pampas and paramos of South America, the veld of South Africa, and the steppes of Eurasia. Because of a lack of moisture in the soil and low humidity in the atmosphere, most of the sunlight penetrates to the ground. At night the desert floor radiates heat back to the atmosphere, and the temperature can drop to near freezing. Deserts are caused by a combination of climate patterns and geological features. Northern coniferous forests form a worldwide belt in subarctic and alpine regions of the northern hemisphere.