What is it eating? I first imagined it might be feeding on the epiphytic microbes on the trunk (and it might be)--but then I read a bit more and it turns out that the green color on the trunk is due to the presence of chloroplasts from the tree! There are more chloroplasts per unit of tree trunk than in the leaves. Looking back at the photo it is easy to see why one might miss the greenish hues, which are covered up by that white powder. Thanks to the snail for lifting the veil of my ignorance regarding aspen trunks!
For more information on photosynthesis in aspen bark, see this study: http://cedarcreek.umn.edu/biblio/fulltext/Amer%20Jou%20Botany%20Vol%2045%201958%20Pearson.pdf
Aspens are remarkable in other ways--the oldest stand ("Pando") in Utah is probably tens of thousands of years old--the trees are clonal for the most part, sending up new trunks from underground stems. They reproduce sexually after fires or other stressful conditions. The clonal nature of the Pando stand was confirmed using molecular markers: http://naldc.nal.usda.gov/download/27665/PDF
This brings up another question--what is an individual in the case of clonal trees? Do somatic mutations that occur in some stems mean that they are genetically distinct enough from other stems to be individuals?
The snail's radula is a funky thing. A chitinous structure studded with teeth, it allows the snail to scrape food from the smooth skin of the aspen. Nearly all molluscs have a radula, except bivalves.
Bivalves can also live a long time, although not as old as an apsen clone. One, called a quahog, was found off the coast of Iceland, and the years of its life were catalogs by counting the rings of growth in its shell. It was 405 years old! See: http://news.nationalgeographic.com/news/2007/10/071029-oldest-clam.html. The rings of trees can be used to estimate age as well.
We are quite proud of our legendary tree ring lab (The Laboratory of Tree-Ring Research: http://ltrr.arizona.edu/), established as a founding institution at the University of Arizona in 1937. By chatting with my colleague, Dr. Michael Nachman, however, I learned that creosote bushses, which are just a stone's throw from where I am typing this, can be quite old. Creosote, like aspens, grows in a colony and one has been estimated to be around 12,000 years old! Justly called the "King Clone" these plants, in addition to being relics, send volatile compounds that fill the heavy desert air of August with a heavenly scent that to me, is the smell of rain:
This is a photo of a creosote bush near the Arizona Inn, in my neighborhood.