The Feedbacks between Soil and Biota in Vernal Pool Landscapes near Merced, CA

Transcription

1 Mission Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales Progress Report: , 1/1/ /31/2010 The Feedbacks between Soil and Biota in Vernal Pool Landscapes near Merced, CA Ronald Amundson* and Sarah Reed Project Objectives The undulating Mima mound-vernal pool topography in California s Central Valley has long been of interest, both because of the uncertainty surrounding its origin, as well as the rich biodiversity found in the depressional wetlands. To explore the processes which created the mounded terrain, we are testing a hypothesis which suggests subterranean animals such as pocket gophers have built the small hillocks as a response to perched water tables and saturated soil conditions caused by restrictive underlying soil layers (Dalquest and Scheffer, 1942). This project aims to test the hypothesis, and, ultimately, inform the management of the endangered seasonal wetland habitat by conducting the following analyses: Thorough, quantitative assessment of Mima mound abundance, size, and spatial pattern as a function of soil age and soil slope (level vs. terrace escarpment), using LIDAR data. For five key landforms ranging in age from 0.01 to 3 m.y., quantitatively survey the amount and orientation of soil movement by gophers, and gopher tunnel architecture for a two year period, and monitor soil conditions (soil volumetric water content, temperature and O 2 ) for a one year period. Apply LIDAR, biologic, and environmental measurements to develop a biogeomorphic model which can be used to test proposed processes, better understand (and illustrate) landscape development, and identify biotic and abiotic linkages. Approach and Procedures The study site is located near Merced, CA, and includes a rich distribution of Mima mounds and vernal pools on a series of five landforms which range in age from <0.01 to 3 m.y. (Fig. 1) (Marchand, 1976; Pavich, 1986; Harden, 1987). Each landform in the chronosequence represents a snapshot of the stages of soil formation, and, presumably, the biologic response to changing soil conditions. In order to conduct a spatial analysis of the microtopography, a 35km 2 airborne LIDAR (light detection and ranging) survey was conducted in Eastern Merced County in August Changes in mound morphology (height, diameter, density, slope, and curvature) were analyzed across the age gradient of landforms, using both the current survey and an additional 30 km 2 survey conducted in 2006 (Fig. 1). The spatial patterns observed in the LIDAR data (resolution: 1m) are used to infer mechanisms of mound formation. Gopher soil movement was tracked on five landforms, on three size classes of mounds, and at three mound positions (base, mid, and top). The direction of movement was monitored using small RFID (radio frequency identification) tags (Fig. 2). The tags were mixed with a quantity of soil, placed in active gopher tunnels, and the RFID reader and a differential GPS (global position system) were used to track their subsequent movement, following the methods of Cox and Allen (1987). The quantity of soil moved by gophers was estimated by counting, weighing, and mapping soil surface University of California, Berkeley * Principal Investigator For more information contact Dr. Ronald Amundson (earthy@berkeley.edu)

2 tailings piles on 80 x 80 m plots on each landform (Fig. 3). The data are combined, along with bulk density data in order to estimate rates and vectors of gopher soil movement. Results Table 1 provides a synthesis of LIDAR and environmental data for 0.5 km 2 sections of each of the five terraces. Roughly one-quarter million mounds are estimated to cover the survey areas in an average density of 5000 mounds per square kilometer. Mima mounds generally increase in density and height, and decrease in diameter with geologic age. Movement of RFID markers exhibited patterns roughly similar to the results of Cox and Allen, 1987 (Table 2). Movement was moundward on the mid-aged mounds (Figure 4a and 4b), at the base of mounds (Fig. 4c), and on the shortest mounds (Fig. 4d). The rate of gopher soil movement is highest on the most saturated and the steepest landforms (Fig. 5). Discussion As soils age and become less permeable, we expect an increase in mound size and density as gophers respond to decreasing habitability. In fact, based on the initial LIDAR analysis, Mima mounds generally increase in density and height, and decrease in diameter with soil age. This trend may reflect the different stages in Mima mound formation over time. Results correspond with studies by Smith and Verrill (1998) and Rains et al. (2008) which showed, qualitatively, that mound and pool form and distribution change systematically with landform age. The results in Table 1 are from an analysis of 0.5 km 2 of each landform. Future analyses will improve the mound classification procedure and will include all mounds in the 65 km 2 survey area. In addition, extensive ground truthing and accuracy analysis will further strengthen the results of the LIDAR survey. Soil movement by gophers becomes more moundward with soil age on the youngest three landforms and then reaches an apparent equilibrium on the oldest three where mounds may be tall enough to reduce the environmental pressure on gophers. Consistent with this proposition, movement is also more moundward on shorter mounds. Net zero displacement on the oldest landforms may indicate that the mounds are in dynamic equilibrium. While these results show similar trends as the prototype study (Cox and Allen, 1987), they represent only two repetitions of the experiment. The study will be continued for at least one additional year to ensure consistent, repeatable results and to analyze seasonal differences in biotic sediment transport. We are in the process of applying the LIDAR data (and calculated slope and curvature maps) to a diffusive linear sediment transport model and the mass continuity equation in order to calculate the amount of soil erosion (convex land areas) and deposition (concave areas) on each of the terraces (Heimsath, 1997; Reed and Amundson, 2007). If the landscapes are at steady state, this provides estimates of the mass of soil needed to be moved uphill in order to counter the slope-dependent downward soil creep. The erosion estimates will then be compared with estimates of the rate of gopher soil movement (the upward portion that is moved in counteraction to erosion) across the chronosequence, in order to characterize the soil mass balances on each landscape and to test whether fossorial rodents are contributing to the building and preservation of Mima mounds (Fig. 5). The measurements of soil environmental conditions (moisture, temperature, CO 2 and O 2 ), which will be used to provide the baseline data to demonstrate changes in soil hydrology with soil age, will begin in February The installation of the sensors required to collect this data could not begin until all of the appropriate federal, state, and regional permits which protect vernal pool species were acquired. All necessary permits were granted in late 2010 and the establishment of a soil monitoring network across the chronosequence is in progress. 2

3 In a recent decision by the 9 th Circuit Court of Appeals (Home Builders Association of Northern California vs. U.S. Fish and Wildlife Service, 2010), the designation of 850,000 acres of critical vernal pool habitat in California and Oregon was upheld. In their finding, the Court identified the mound-swale topography as one of three key factors necessary for the formation of vernal pools. Without a clear understanding of how the topography was originally formed or is currently maintained, the management and conservation of vernal pool wetlands is in jeopardy. By continuing observations of biologic activity across the chronosequence and by advancing the LIDAR analysis across the 65 km 2 survey region, we will be able to shed further light on the role of biologic sediment transport in both the genesis and the continued functioning of the wetland ecosystems. Table 1. Compilation of LIDAR and environmental data for five terraces. Roughly one-quarter million mounds are estimated to cover the survey areas in an average density of 5000 mounds per square kilometer. Environmental data are from Arkley, 1960; Marchand, 1976; and NRCS,

4 Table 2. Comparison of to-date results of current study and previous study by Cox and Allen (1987). Trends are roughly similar, but to verify robust results, additional iterations of the experiment are planned. This study Cox and Allen (1987) Number of markers emplaced to date Average horizontal displacement (cm) + = moundward Net horizontal displacement (cm) Average vertical displacement (cm) - = upward 1 4 Net vertical displacement (cm) % of markers recovered % of markers moved from implant site % of markers redeposited at surface

5 Figure 1. Eastern Merced County regional map showing geologic surfaces as mapped by Marchand (1976) and the outline of the LIDAR survey areas (totaling 65 km 2 ). Gopher monitoring locations are marked with blue circles. 5

6 Figure 2. Photograph of the RFID (radio frequency identification) tags in comparison with the sediment on the ~250 ky Riverbank soils. The tags were mixed with approximately 2 pints of soil and placed in active gopher tunnels. The RFID reader was used to track their movement over time, following the methods of Cox and Allen (1987). Figure 3. Example of above-ground gopher tailings mapping. All recent gopher tailings piles in a ~80 x 80 m region (red box) were marked with a differential GPS and a representative set were weighed with a spring scale. 6

7 Figures 4a-d. Gopher soil movement patterns considered in terms of soil age, mound position, and mound height. 7

8 8

9 Figure 5. The rate of moundward gopher soil movement (cm/1000 yr) is highest on the most saturated (Riverbank) and steepest (Laguna sloping) landforms. Average mound curvature values were estimated and used to calculate mound erosion rates on all landforms. Gopher soil transport is generally equal to or greater than estimated local erosion rates, except on the oldest Laguna formation. 9

10 References Arkley, R.J Soil Survey of the Merced Area, California. California Agricultural Experiment Station. United States Department of Agriculture Soil Conservation Service, 1950, Cox GW and DW Allen. (1987) Soil translocation by pocket gophers in a Mima moundfield. Oecologia 72, Dalquest W. W. and Scheffer V. B The origin of the Mima mounds of western Washington. J. Geol. 50, Harden, J. 1987a. Soils developed in granitic alluvium near Merced, California: USGS Bulletin, 1590-A. 65. Heimsath A. M., Dietrich W. E., Nishiizumi K. and Finkel R. C The soil production function and landscape equilibrium. Nature. 388, Home Builders Association of Northern California vs. U.S. Fish and Wildlife Service, No , U.S. App. LEXIS (Aug. 9, 2010). Marchand, D.M Preliminary geologic maps showing Quaternary deposits of the Merced area, eastern San Joaquin Valley: 76 sheet(s). Pavich, M.J., Brown, L., Harden, J., Klein, J., and Middleton, R Be distribution in soils from Merced River terraces, California: Geochim. Cosmochim. Acta, Rains, M.C., Dahlgren, R.A., Fogg, G.E., Harter, T. and Williamson, R.J Geological control of physical and chemical hydrology in California vernal pools. Wetlands. 28, Reed S. E. and Amundson R. G Sediment, Gophers, and Time: A Model for the Origin and Persistence of Mima Mound Vernal Pool Topography in the Great Central Valley. In Vernal Pool Landscapes. (eds. R. A. Schlising and D. G. Alexander). California State University, Chico, CA Smith D. W. and Verrill W. L Vernal pool-soil-landform relationships in the Central Valley, California. Ecology, conservation, and management of vernal pool ecosystems. California Native Plant Society, Sacramento, California, USA Soil Survey Staff, Natural Resources Conservation Service (NRCS), United States Department of Agriculture. Soil Survey Geographic (SSURGO) Database for [Merced, CA]. Available online at accessed [7/2010]. This research was funded by the Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales, Mission ( The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California. 10