What has not been mentioned in this thread (unless I missed it) is that the entire area around Cumbres Pass burned in forest fires that ravaged much of southwestern Colorado in 1879. Many people mistakenly assume that Cumbres is near the natural timberline--it is not. The areas burned in that fire simply have not completely regrown yet--not uncommon for it to take a couple of centuries to do so at that altitude. The highest reaches of the area around Cumbres Pass are also at the upper altitudinal limit for aspen trees, so they will not colonize burned over areas formerly occupied by conifers at that elevation. That also really slows down reforestation. Below Cumbres, as others have noted, aspen will vigorously colonize areas where conifers have been removed by fire, disease, or logging.
Comparing the forests of the Cumbres Pass area with areas of northern Colorado (and farther north) is also not applicable. Those more northern areas are often heavily forested by lodgepole pine in mid- to high elevation--the lodgepole a colonizing tree itself, and one very susceptible to disease and fire (as is happening now). A forester friend of mine commented that there are only two kinds of lodgepole forests--the kind that are going to burn, and the kind that are burning. Lodgepole forests reach their southern extent along a line that roughly parallels US Highway 50 through the Rockies. south of there, lodgepoles become increasingly uncommon, and are nearly absent by the time one reaches the Colorado/New Mexico border.
As to brush along the ROW, the D&RGW for years regularly used long-acting soil sterilants along their ROW's. I personally observed the section crew spreading it in the Animas Canyon along the ROW in the 1960's. Some of the types used, like Ureabor, will sterilize the soil for several years. I'm sure the granola-crunchers would go ape over that now, but it did reduce the fire danger along the tracks, and reduce the amount of moisture-retaining vegetation between the ties that promoted tie rot.
