Aspen leaves tremble because the stems are flattened and catch the wind. Aspens and cottonwoods are wind pollinated, whereas willows are insect pollinated. Aspens grow in clones, which may be either male or female. They are declining for unknown reasons in Colorado. If you value your aspens, remove the evergreens that try to grow among them, as the conifers will eventually shade the aspens out, and you will lose them. Aspen clones can be enormous (Pando, a huge clone in Utah, is 106 acres!), and it is speculated that some could even be millions of years old!