Autumn aspen trees near Guardsman Pass, Utah, United States

What makes a tree 'quake?' Take a closer look at an aspen leaf.

Its flattened stalk catches even a light breeze, making the leaves flutter and giving the tree its name: quaking aspen.

Near Guardsman Pass in Utah, United States, these trees form striking groves across the higher mountain slopes, where cooler, moist conditions suit them well.