Published: Nov. 8, 2010

In the drought year 1879, the Lime Creek fire consumed 26,000 acres, allowing large stands of aspen to spring up. Photo by Jeff Mitton.

By Jeff Mitton

On a leisurely drive from Ouray to Durango, I was puzzled by the lack of trees on Molas Pass and around Little Molas Lake but dazzled by the extensive stands of aspen farther south and lower, around Lime Creek.

A historic sign explained that the treeless area and the extensive stands of aspen were both consequences of the Lime Creek fire of 1879.

The Lime Creek fire burned 26,000 acres (the Fourmile and Hayman fires consumed 6,400 acres and 138,114 acres, respectively) but it was not the only large fire that year. Cumbres Pass also had a fire that was estimated to be 26,000 acres, and a large portion of the Uncompahgre Plateau burned that year as well. In addition, smaller fires were reported in the Routt and White River national forests. Long-term precipitation records for Las Animas, starting in 1868, reveal that 1879 was the fourth-driest year in the last 141 years (2002, 1894 and 1890 were drier) with less than half of the normal annual rainfall.

It is intuitively obvious that large fires would occur during drought years, but why did one portion of the Lime Creek burn area fill in with aspen, while the highest portion hosts grasses and wildflowers but has not yet been recolonized by trees?

It is important to appreciate that fire has different consequences for conifers and aspen. A cool fire will burn the litter dropped by conifers and will kill conifer seedlings and small trees, but leave large trees unharmed. A hot fire will kill all of the conifers, large and small. The clonal growth habit of aspen predisposes it to thrive and dominate in areas of frequent fires. The fire scorches the bark, killing all of the portions above ground, but leaves the extensive root system intact. After the fire, numerous new stems pop up from the root system and grow quickly, supplied by a mature root system and released from the shade of the canopy cover.

In the San Juan National Forest, in the area of the Lime Creek fire, stands of aspen can be divided into two groups that fall out with elevation but are caused by different frequencies of fires. Pure stands of aspen contain virtually no conifers and appear to be climax successional communities. That is, they are the stable endpoints of long-term competition among plant species in those sites and will persist until they are destroyed by a major disturbance. Pure stands predominate below 8,000 feet. Above 8,000 feet, successional stands of aspen are intermixed with Engelmann spruce and subalpine fir and are slowly eliminated by the conifers. In these environments, the stable endpoint of forest succession is the spruce--fir forest.

At lower elevations, aspen is frequently intermixed with or adjacent to ponderosa pine. These mixed stands have frequent fires, with fires recurring on 15- to 30-year cycles. The relatively rare hot fire clears an area for aspen to establish a pure stand, and the frequent cool fires remove ponderosa seedlings and saplings, giving aspen the competitive edge.

At higher elevations, such as the area between Molas Pass and Lime Creek, fires are far less frequent, with return frequencies of about 70 years or longer. Long periods without a fire (frequently more than 100 years) allow the Engelmann spruce and subalpine fir to competitively exclude aspen. When a hot fire runs through a spruce-fir forest, virtually all of the trees are killed. If the fire clears a large swath, it takes centuries for spruce and fir seeds to reach the site and to become established in the meadows lushly vegetated with grasses and wildflowers.

Jeff Mitton ( is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

November 2010