The Aspens that are seen across Canada and in the north-east United States are a remarkable species. What seems to be individual trees are likely to be parts of one larger organism, each growing from "suckers" emerging above ground from a vast root system. One example covers 80 hectares (over 160 acres) forming over a thousand trees. The age of the grove is not known precisely but is thought to go back to the time at the end of the Pleistocene when the ice age's thick sheet withdrew from North America. Consequently it is likely to be the largest and oldest living thing on earth.
The root system is particularly robust, an observation from Russia has an Aspen grove springing up in a former ranch from suckers after some 300 years of cattle had prevented them from growing to any appreciable height.
In the Rockies, were this image was made, elk grazing on the bark affects the trees. They all appear to be uniformly darkened for a little over a metre from the ground -- as in the following picture.
The "Trembling" in the name comes from the leaf motion in a breeze. The stalk holding the leaves are very long causing them to move in even a very gentle wind.