"...why would something as large as an elk be interested in something as barren-looking as aspen trunks?"
Lots of shrubby plants or trees get attention from critters who are looking for something to eat. Beavers, of course, will clear-cut whole stands of various trees to get at the tasty upper branches. Wild and domestic fruit trees attract birds, skunks, deer and other critters including bears with their enticing, fruity bounty. Deer and rabbits are notorious browsers (meaning they eat shrubby, twiggy things), as many a homeowner can attest.
But why would something as large as an elk be interested in something as barren-looking as aspen trunks? The secret is just under the surface.
Aspen (Populous tremuloides) is a widespread species that is best adapted to northern climes. Like all deciduous trees, they leaf out in springtime and the green leaves do a fine job of capturing the available sunlight to power the tree's summer growth. Leaves turn a beautiful golden hue in autumn, then fall to the ground.
But the smooth trunks of aspen (which attract so much admiration from artists) represent a kind of extended childhood: a juvenile bark to serve an important purpose. Just beneath that smooth surface lies a thin, bright layer of cells which contains the same intense green chlorophyll that the leaves would, seldom noticed because the cells die so quickly when exposed. The semi-translucent bark permits sunlight to pass into that layer of cells. So the aspen is able to utilize sunlight even in long northern winters when the leaves are absent.
Of course, it's this layer that the elk, deer and beaver enjoy—especially in wintertime.
Click on the photos below for a closer look at an elk-chewed trunk, beaver workings and a slice through the bark of a recently beaver-felled stump.