Pedologic-biologic feedbacks on the Merced River chronosequence: The role of pocket gophers (Thomomys bottae) in Mima mound-vernal pool ecosystems of the San Joaquin Valley

Distinctive Mima mound topography, found on all continents except Antarctica, has long inspired intense interest, both because of the uncertainty surrounding its origin, as well as the rich biodiversity often found in the adjacent depressional vernal pool wetlands. Emerging from the array of explanations proposed, a biological mechanism for mound formation has steadily gained strength. According to this model, subterranean animals such as pocket gophers build the small hillocks as a response to changes in soil thickness and permeability caused by pedogenesis (soil formation). However, the precise mechanics of the hypothesis -- how soil weathering leads to changes in gopher soil movement and how the modified soil movement leads to the widespread Mima mound features -- are not understood. To address this issue, this thesis presents three related efforts to document and explain Mima mound morphology and to elucidate the linkages between soil formation processes, mound form and distribution, and biotic sediment transport. The research targets the effects of pedogenesis by focusing on a set of alluvial terraces in the San Joaquin Valley of California, a chronosequence which ranges in age from a few thousand to a few million years old. In the first portion of the study, I designed a custom algorithm which automatically identifies Mima mounds from LIDAR (light detection and ranging) data in order to analyze the morphology and spatial patterns of the microtopography across the soil age gradient. I demonstrate that mound size changes systematically with soil age and with depth to a restrictive layer, consistent with the predictions of the biologic model. Further, I show that the nearly quarter-million mounds in the study are distributed in a highly regular pattern, on scales consistent with the home range of pocket gophers, adding further support to the proposal that gophers created the topography. In the second part of this dissertation, I developed a method to use radio frequency identification tags (as a proxy for soil particles) and periodic aboveground mapping of gopher soil movement to quantify gopher sediment transport vectors on each chronosequence surface. I found that gophers on Mima mounds move soil uphill and towards the center of mounds, and I show that this moundward tendency increases with soil age and the environmental pressure caused by soil formation over long time periods. Third, I combined the morphometric measurements with the the sediment transport data from the first two parts of the study and developed a sediment transport model which estimates mound erosion and swale deposition rates. If the Mima mounds are steady-state landscape features, erosion must be balanced by a restorative upslope transport. I show that the erosion rates estimated for this study are largely counteracted by the observed rates of sediment mounding via pocket gopher burrowing, supporting the notion that bioturbation plays a dominant role in maintaining Mima mound terrain. Finally, I use LIDAR measurements and results from published gopher physiology research to develop a model that approximates the energy required for the formation of Mima mounds (shearing, pushing, and uplifting soil) and their maintenance (counteractions to erosion). This energy estimate was compared to estimates of energy available to gopher populations in the region. The results indicate that gophers have ample energy to build typical Mima mounds in as little as 100 years, thus strongly supporting a biotic mechanism of Mima mound development and maintenance.

Advisor: Amundson, Ronald