1. Biomes are biotic communities considered on a global or at least a continental scale. Major biomes of North America include tundra, taiga, temperate deciduous forests, grasslands, deserts, mountain forests, and tropical rain forests.
2. Tundra is found primarily above the Arctic Circle. It includes few trees and many lichens and grasses and is characterized by the presence of permafrost below the surface. Alpine tundra occurs above timberline in mountains below the Arctic Circle.
3. Taiga is dominated by coniferous trees, with birch, aspen, and willow in the wetter areas. Many perennials and few annuals occur in taiga.
4. Temperate deciduous forests are dominated by trees such as sugar maple, American basswood, beech, oak, and hickory, with evergreens such as hemlock and eastern white pine toward the northern and southeastern borders.
5. Grasslands are found toward the interiors of continental masses. Those located in Mediterranean climatic zones usually include vernal pools with unique annual floras. Many grasslands have been converted to agricultural use.
6. Deserts are characterized by low annual precipitation and wide daily temperature ranges, with plants adapted both in form and metabolism to the environment.
7. Mountain and coastal forests occupy much of the Pacific Northwest and extend south along the Rocky Mountains and California mountain ranges. The primarily coniferous tree species tend to be in zones determined by altitude. These forests have mostly dry summers, and some of the species associated with them have thick bark that protects the trees from fires that occur frequently in this type of climate. Other species (e.g., knobcone pine) depend on fires for normal distribution and germination of seeds.
8. The tropical rain forests constitute nearly half of all forest land and contain more species of plants and animals than all the other biomes combined. Numerous woody plants and vines form multilayered canopies that permit very little light to reach the floor. The soils are poor, with nutrients released during decomposition being rapidly recycled. The biome is being destroyed so rapidly that it will disappear within 20 years if the destruction is not halted.
