In many tree species, new bark cells grow beneath the old layer of dead cells which, as the trunk expands, splits apart to form furrows and wrinkles, as in firs or cottonwoods. In contrast, the cells of aspen periderm continually reproduce and expand to keep pace with the tree's increasing diameter. Old cells are sloughed off as a chalky residue, brushing off onto my fingers as a whitish powder. Aspen root systems can remain dormant for centuries, awaiting the right conditions for regrowth. Dense conifer stands frequently shade out aspen, but the aspen roots remain healthy and alive underground until their evergreen neighbors are felled by fire or wind or man.
Sunlight then awakens the aspen's roots and triggers the growth of vertical stems, called ramets.
Aspen are often one of the first plants to revive and revegetate after a fire. Fossil leaves dated fifteen million years ago suggest that at one time two separate species of aspen flourished in slightly different habitats. The modern species is probably a hybrid of its two ancient ancestors. From the moment it emerges from the ground, an aspen begins its interactions with other organisms. Insects are among the first to take advantage of the tree, feeding and burrowing in the leaves and stems.