From mid-September through October, the leaves on deciduous trees and shrubs along the canyons and creeks of the High Sierra—aspens, cottonwoods, alders and willows—turn gold and orange and red. "The intensity of color is determined by the plant’s response to complex gradients of temperature and moisture," writes Anne Halford, a botanist with the BLM. "Conditions favoring formation of brilliant autumn color are warm, sunny days followed by cool nights with temperatures below 45º F." Colors tend to appear at the highest elevations first and work their way downward as the season progresses.