Without fires, forests grow dense with trees competing for nutrients, sunlight, and space, and that competition and stress leave them more susceptible to disease, insects, and fire. Many plants on the forest floor die while competing for nutrients, adding to fallen trees and the buildup of burnable fuels. The United States Forest Service uses prescribed fires to clean up the forest floor and create habitat for wildlife, as small fires sweep up dead plants and help clear out insects and disease. Afterward, grasses, shrubs, and wildflowers grow in the enriched soil, attracting deer, elk, and bighorn sheep, and aspen trees are the first to return, providing homes for many birds and animals. In Colorado, altitude, rain, snow, and the amount of sunlight determine where plants grow. Ponderosa pine forests grow on sun-dried south-facing slopes, are adapted to surviving small, warm ground fires every 15 years, have thick bark that protects them from fire, and are vulnerable to the mountain pine beetle. Spruce-fir forests grow on cool, damp, shady hillsides on north-facing slopes, rarely burn, but when they do the fires are hot and burn the tops of the trees, and they are vulnerable to the spruce budworm.