An aspen tree is the most widely distributed tree species in North America, ranging from Alaska to Newfoundland and down the Rocky Mountains to Mexico. Interestingly, Utah and Colorado is home to the largest portion of the natural acreage of aspen in the World. Aspen trees are described as an all important and community dependant "keystone species" within its natural range. Aspen trees are the most visible of western North American hardwoods providing understory biodiversity, wildlife habitat, livestock forage, specialty forest products, and highly desirable scenery. I would recommend reviewing this audio slide presentation by Dr. Wayne D. Shepperd, USFS Rocky Mountain Research Station for an exhaustive report: Aspen Ecology in the Western United States
Start with the Tree Finder if you are not sure what kind of tree you have!
Map of Aspen Tree/Birch Type - USFS
The Western Fir Aspen Tree Life Zone