GOPHERS, TREES AND UNGULATES ON THE MONTANE HILLSLOPES OF THE COLORADO FRONT RANGE: A LINKED GEOMORPHIC AND ECOLOGIC SYSTEM

Pocket gophers have been shown to be significant geomorphic agents in several settings. Here we explore their roles in montane hillslopes of the Colorado Front Range. We argue that in this landscape the efficiency of hillslope sediment transport and vertical mixing accomplished by gophers is highly non-uniform, with greatest efficiency in meadows, and reflects a strong interaction between gophers, seedling establishment and ungulate trampling. The feedbacks in this system may govern not only the spatially averaged transport by gophers, but forest-meadow dynamics. The key is that gophers cannot be viewed in isolation from either their ecological or geomorphic settings.

We report preliminary findings from a 54 m x 32 m experimental plot spanning a small meadow and adjacent forested fringe of aspen and spruce. The substrate is glacial ground moraine with a wide range of grain sizes, up to 1 m boulders. We planted 200+ seedlings in late spring at 2 m spacing along 8 contour transects. We monitor gopher mound activity, subsequent evolution of the mounds, and where tree seedling roots have been consumed by active gophers. Preliminary results from detailed mound mapping demonstrate that gophers avoid the forest, avoid coarse rocky portions of the meadow, and preferably dig in proximity to the forest edge. The volume of mound material brought to the surface is equivalent to a 1.3 mm layer of soil in one gopher-digging season (~3-4 months); within a century the top decimeter of soil will have been well stirred by such activity. Time-lapse photography and monitoring of artificial colored-stone lines on mounds reveals that subsequent motion of mound material is dominated by ungulate trampling and rain splash. Field observations suggest deer, elk and moose move through these meadows daily. Sediment motion is significantly curtailed within a single season by vegetation growth that serves to tack down mound material.

These observations inspire a numerical model of the geomorphic-ecological system in which we will explore the feedbacks that result in meadow hotspots of activity.