Any of three tree species of the genus Populus , of the willow family: P. tremula (the common European aspen), P. tremuloides (the American quaking, or trembling, aspen), and P. grandidentata (the American big-tooth aspen).
Native to the Northern Hemisphere, aspens are known for the fluttering of their leaves in the slightest breeze. Aspens grow farther north and higher up the mountains than other Populus species. All aspens have a smooth, gray-green bark, random branching, rich green leaves that turn brilliant yellow in fall, and catkin s that appear before the leaves in spring.