GOPHERS AS GEOMORPHIC AGENTS IN THE COLORADO FRONT RANGE

Incorporating biology into landscape evolution models is an enduring challenge due to the highly variable temporal and spatial dynamics of biological agents. We studied the northern pocket gopher (Thomomys talpoides) in the subalpine zone of the Colorado Front Range (CFR) for three field seasons to gain insight into the effects of these creatures, which are the primary geomorphic agents in this landscape, imparting both a horizontal and a vertical signature on the land. Gophers inhabit meadows and not forests. Patterns of mounds that surface over the summer and fall reflect expansion of subsurface tunneling. On average the volume of mounds and infilled tunnels suggests that gophers spread ~ 1 mm of material per year over a meadow surface, implying a soil turnover time of order 1-2 centuries. Horizontal transport represents a landscape diffusivity of 0.008 m²yr^-1. Their winter digging patterns appear to be governed by subsurface temperatures, which in turn are regulated by snow depth: high accumulation at the upwind meadow edge, and wind scoured low depths in meadow centers. In the vertical, gopher activity has generated a distinct subsurface stoneline at ~ 15 cm depth. We use this pattern and profiles of atmospherically derived ¹³⁷Cs and ²¹⁰Pb_ex to constrain a numerical model of vertical gopher bioturbation. Our model diverges from more traditional advection-diffusion models in that it both allows “shuffling” of material from subsurface tunnel depths to the surface, and honors the threshold nature of gopher digging. That gophers cannot hoist clasts greater than ~ 3 cm to the surface is represented by the conditional rule that dictates whether the clasts encountered while digging are small enough to relocate to the surface. Vertical shuffling leads to the development of stonelines near the base of gopher digging depths, and to the development of a layer of fine material above this. The vertical distribution of ¹³⁷Cs and ²¹⁰Pb_ex in soils that we measured can be explained by a model accounting for monthly atmospheric fluxes, radioactive decay, and gopher digging. We combine these model results with gopher mound and infilled tunnel data, and local forest fire history to characterize the long-term geomorphic impacts of gophers on the subalpine zone of the CFR.