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The Ribbon Forest on Buffalo Pass

Jeff Mitton

Natural Selections (appeared April 9, 2000 in the Daily Camera)

            High in the Rocky Mountains, in places where winter winds are strong and snowpack is deep, the wind, snow, and snow molds sculpt stands of Engelmann spruce,  Picea engelmannii, and subalpine fir, Abies lasiocarpa, into ribbon forests. Here the trees grow in narrow, parallel rows perpendicular to the prevailing winter winds. Well-developed ribbon forests are found on West Flattop Mountain in Glacier National Park (MT), at Libby Flats in the Medicine Bow Mountains of WY, and on Buffalo Pass at the southern edge of the Mount Zirkle Wilderness Area in CO. Smaller ribbon forests are found near tree line on Mount Audubon, Cone Mountain near Empire, Rollins Pass, and Niwot Ridge. On Buffalo Pass, spruce and fir 25 meters tall grow in ribbon 5 to 10 meters thick, but 20 meters to several kilometers long. Much of our understanding of ribbon forests was provided by David L. Buckner, who received his doctoral degree in 1977 from CU for his study of the ribbon forest on Buffalo Pass.

            Before a ribbon forest is established, the site was a wind-swept meadow, packed deep with snow in winter. A low ridge of rock or a subtle promontory would be the first site colonized by the trees, for the snow would have been shallower there, and hence the growing season would have been longer. As the trees grew, they would have modified both the wind and the snow, just as a snow fence does. The first line of trees would anchor the pattern of the ribbon forest, and cause the ribbons to propagate across the meadow.

            The area immediately upwind of a line of trees is often scoured clean of snow by wind, which makes the site inimical to establishment of trees. Seedlings and young trees that are not afforded the protection of a blanket of snow are dessicated by winter winds and abraded by wind-blown ice crystals.

            The snowpack is deepest immediately downwind of a line of trees, and deep snow modifies the site so that trees cannot colonize it. First of all, the deep snow lingers well into July, soaking the soil with meltwater and making the growing season unusually short and harsh. In addition, pocket gophers are attracted to these sites, and their digging and browsing kills seedlings and young trees. Finally, snow molds (genus Lyophillum) proliferate in snow banks that linger long into summer, turning the snow from white to pink. Snow molds invade and destroy the photosynthetic tissues, killing the trees.

            The large trees within a ribbon modify their immediate environment, creating an ideal nursery for seedlings. The trees partially block the winds, mitigating their impacts, and maintaining the drifting snow at moderate depths. Furthermore, the shade of adult trees maximizes the survival of seedlings, for bright sunlight on a warm summer day can damage seedling needle tissue.

            A ribbon forest is in a dynamic, stable equilibrium produced by the interaction of wind, sun, snow, and snow molds. An established ribbon sustains itself, enhancing the establishment and growth of seedlings by providing moderate amounts of wind and snow.  But trees cannot colonize either the wind-scoured areas upwind of a ribbon or the deeply drifted snow glades in the lee of a ribbon. The influence of a single ribbon diminishes with distance, permitting another ribbon at a safe distance downwind, and then another, creating a ribbon forest that can extend for kilometers.

A trail in the Mount Zirkel Wilderness Area winds through the ribbon forest

 

Photo by Jeff Mitton

Following are photos of the ribbon forest on Buffalo Pass, adjacent to the Mount Zirkel Wilderness Area.

Photo by David Buckner

Photo by David Buckner

Photo by David Buckner