Evidence for biologic response to pedogenesis along the Merced River chronosequence, Central Valley, California

Long-term soil weathering results in accumulations of clay and reduced hydraulic conductivity. How biology responds to these changes in the physical environment and how the response, in turn, influences landscape development are crucial questions in the effort to elucidate the links between the biologic and physical earth surface domains. Mima mounds are small, circular half-domes of soil that are suspected of being formed by burrowing rodents, as an adaption to saturated soil conditions. In the swales between the mounds, ephemeral wetlands called vernal pools, support a suite of endemic and endangered plant and animal species. Mima mounds, then, may provide a useful model by which to examine the complex feedbacks between landscape and life. In this study, changes in mound characteristics and in biological activity (pocket gopher, Thomomys bottae) are investigated across the Merced River chronosequence, a series of alluvial terraces which have been shown to exhibit an increasing degree of soil and hardpan development with landform age. Mound morphology (size, slope, curvature, concentration, elongation, dispersion) and relation to environmental parameters were analyzed using airborne LIDAR (light detection and ranging) imagery of the mounds. The high-resolution (1m) LIDAR surveys (conducted in 2006 and 2010) cover 65km2 and comprise seven different-aged landforms, ranging from several hundred years to several million years. Minimal filtering was performed on the dataset given the absence of large vegetation or other obstructions. A 20x20m moving window filter was used to smooth out the low frequency signals and accentuate mounded features. To test how and whether the subterranean mammals modify their burrowing habits in response to soil age, biotic sediment transport was measured monthly on 0.01, 0.5, and 2 m.y.o. terraces using RFID (radio frequency identification) technology. Half-liter portions of soil containing five RFID tags were implanted in active gopher tunnels on several size classes of mounds and at three positions on each mound (base, mid, and top). Within two weeks of insertion, tags were located using an RFID reader, and displacement was measured using a differential GPS. The surveys show distinct differences in mound and burrow form and biotic activity across the terraces. These results enhance our understanding of the rates at which landscapes are biotically transformed, and, when applied to sediment transport models, provide further evidence that Mima mounds are likely created and maintained by pocket gophers.