Understanding the geomorphic imprint of the northern pocket gopher on the subalpine zone of the Colorado Front Range
PhD: University of Colorado Boulder, 2017.
To continue bridging the gap between biology and geomorphology, we documented activity of the northern pocket gopher (Thomomys talpoides) in two subalpine meadows in the Colorado Front Range. To investigate the plan-view annual cycle of gopher activity, we mapped surface mound and infilled tunnel data on a weekly basis. We find that 1) they subsist strictly within the meadow and not the forest, 2) they reside at the upwind forest meadow edge during the winter and disperse throughout the meadow during the remainder of the year, 3) they bring 1 mm of material to the surface per year, averaged over their entire meadow habitat, and 4) they are responsible for a landscape diffusivity of 0.008 m2y-1. To investigate their vertical geomorphic signature, we 1) probed the subsurface for the depth to large stones, and 2) collected 137Cs and 210Pbex concentration profiles meant to constrain soil mixing rates. Our probing surveys suggest stonelines are present at ~ 15 cm depth. We crafted a numerical soil-mixing model to interpret these observations and contrast results from this model with results from a commonly applied advection-diffusion model. Our Gopher Model incorporates rules that emulate how the gopher operates at our field site. While both models capture the essence of both isotope concentration profiles, only the Gopher Model can track grain size and therefore mimic evolution of both stone lines and the overlying fine-grained material. To investigate what drives the gophers to the upwind meadow edge during the winter months, we collected time series of subsurface and air temperature, snow depth, and solar irradiance. Snow accumulates in a wedge with maximum depth of 1.5 m near the upwind forest-meadow edge that tapers into the meadow center. The many-month long insulation provided by the thick layer of snow prevents significant freezing of the soil while the meadow center temperatures drop well below 0°C. The warmer meadow edge is more attractive to the gophers, who generate their infilled tunnels there during the winter months. Our thermal subsurface data can be adequately modeled by a conductive thermal model using reasonable values for thermal conductivity and surface albedo.