Aspens in the United States have been the subject of a number of research studies.  The quaking aspen (Populus  tremuloides) has been used to  study intra-specific variation in a tree species - previous studies have focused on quick growing (herbaceous plants), looking at differences in growth, leaf shape /size, and flowering. the chemistry of the trees’ defences (tannins)  The response to rising carbon dioxide levels - the aspens have grown faster. To add confusion, there is also a European aspen, Populus tremula,  which is known as the quaking aspen, like its American counterpart.  The ‘quaking’ or trembling of the leaves is due to the nature of the leaf stalk or petiole, which has a flattened shape and is very flexible near to the base of the leaf.  The leaves are round with blunt teeth distributed somewhat irregularly around the leaf margin. The leaves of the European Aspen are more coarsely toothed than the American Aspen.  Young leaves have a copper tint before becoming green and then in autumn they turn a vibrant yellow before falling.  Both American and European quaking Aspen species can spread by suckers that form underground away from the parent tree.  The plants so formed are clones, that is they are genetically identical to the parents (ramets). Read more...