Soil Ecology Restoration Group (SERG)

FINAL PROJECT REPORT

REVEGETATION AND EROSION CONTROL
NELSON LAKE PROJECT AREA
AT NATIONAL TRAINING CENTER (NTC) FORT IRWIN, CA

Project Overview

The National Training Center (NTC) at Fort Irwin is located 35 miles northeast of the city of Barstow, San Bernardino County, California. Fort Irwin encompasses an area of nearly 643,000 acres in the Mojave Desert floristic province. The NTC is dominated by creosotebush scrub, but also contains saltbush scrub, mesquite bosques, alkali playas, dunes, native grasslands, and Mojave desert washes.

As a result of ground maneuvers, live fire exercises, and force-on-force military training activities, the creosotebush, saltbush, and wash communities have sustained significant damage. Furthermore, training activities may also damage the cryptogammic soil crusts, a thin crust of algae, fungi, lichen, and other micro-organisms which bonds the top layer of surface soil together. Once the vegetation and soil crust have been disturbed or damaged, the newly barren surfaces are subject to an accelerated rate of erosion from the normal forces of wind and water. Once the erosion process has begun, the effects will continue to intensify unless corrective action is taken. In severely damaged areas nitrogen and organic contents are decreased and the soils become more alkaline, making it harder for plants to minimize effects of training damage. These effects can be mitigated through restoration activities to insure that the end result is not the loss of realistic environmental conditions for desert warfare training nor excessive dust levels or soil loss.

The Integrated Training Management Program has initiated a five-year program (1997- 2001) to manage areas in a suitable manner. A major portion of this program includes revegetation and erosion control of disturbed lands. This report covers the procedures and results of a project attempting to implement this at a 22 acre site located directly north of Nelson Lake (Figure 1). Field work involved the installation of various erosion control measures to reduce sedimentation and gullying near a major staging area. The larger gullies posed driving hazards and sedimentation on the trail had resulted in increased airborne dust (Figure 2).

Site Description

The Nelson Lake site covers an area of 22.69 acres and is located 22.6 km to the northwest of the cantonment at UTM grid coordinates 519442 east/3922475 north. The slope of the site is less than 5% to the southeast towards Nelson Lake. The topography of the area is characterized by many small gullies and washes which have experienced increased rates of erosion from recent vehicle activity and loss of vegetation. This vehicle activity has also caused high compaction to occur in the coarse sandy-loam Venusite soils (for description, see Appendix B). The heaviest damage was focused in a four acre area in the southeastern corner of the site where almost all mature shrubs had been destroyed (Figure 3). The dominant species present on the site include: Larrea tridentata, Ambrosia dumosa, Sphaeralcea ambigua, Thamnosa montana, Atriplex canescens, and Hymenoclea salsola.

Figure 3: View of Nelson Lake site from a near by hill to the south west.

Materials and Methods

The first task accomplished at the project site was the preparation of the four-acre area south of the main trail, which was to receive the bulk of the planting. This was done with the assistance of ITAM personnel using a mechanical pitter to scarify the soil surface. Then, 1710 ft of 1/2-inch polyethylene tubing was laid out across the area. The tubing was laid in 5 separate non-linear strips in an attempt to maximize surface coverage over the entire four acres. Native shrubs, grown at San Diego State University or Joshua Tree National Park, were then planted along these lines approximately every 3.5 feet (Table 1). Four gallon per hour emitters were installed at each plant to ensure an even flow of water and, like all plants in this project, tubex plant protectors were then placed around each plant to prevent herbivory, and 18 inch diameter basins were constructed to maximize water retention (Figure 4). Lastly, this area was seeded by hand using a native seed mix provided by ITAM (see Appendix C).

To the west of this area where washouts were developing, ten points were chosen, in conjunction with ITAM personnel, for erosion control measures. This was accomplished through the use of straw wattles: 25-foot lengths of 12-inch diameter, flexible straw-flake tubes. Rice grass straw encased in photo-degradable plastic netting was used to minimize any possible harmful effects to local wildlife. These wattles were laid perpendicularly across the washouts at an obtuse angle, and held in place using five 2-foot lengths of 1/4-inch steel re-bar. The angle was put in the wattle to break up the flow of water and force it in two directions, thereby reducing the water’s capability to initiate erosion gullies. Small, shallow holes were dug at both ends of the wattle to collect any displaced water and silt (Figure 5).

At the opposite end of the project area, on the north side of the road, twenty more wattles were installed in a similar manner. This time, however, three native shrubs were planted along the uphill side of each wattle (Table 1). Irrigation line was not installed as these plants were to be watered individually by hand. In developing washes that were too deep to use wattles, rock check dams were constructed. A total of 28 check dams were built using rocks ranging in size from 8-18 inches in diameter that were collected from areas surrounding the project site. In one area adjacent to

TABLE 1
Species planted by area

Figure 5. Typical wattle installation with re-bar.

the pitting site, twenty-five 4 foot long dams were spaced every 10 feet perpendicular to the flow of water. In the eastern-most portion of the project site, in one exceptionally large wash, three 50 foot50-foot long dams were constructed with twenty foottwenty-foot spacing between each dam. Thirty shrubs were then planted along the uphill side of each dam, for a total of 90 plants (Table 1). Plants were again laid out along irrigation lines in a non linear-linear manner (Figure 6).

Figure 6. Plants and irrigation lines along rock check dams.

Lastly, fifty "V" shaped catchment basins were constructed along the north side of the main road for further erosion control and to hold and supplement water for both existing and planted shrubs. Catchments were 6 feet wide at the mouth, with each arm of the "V" being 3 feet long and 18 inches high. These were constructed using a small tractor equipped with a front-end loading bucket. Each catchment was then planted with two plants (Table 1). To prevent water flow from eroding and damaging the basins, two barrier fences were placed in a staggered fashion at the mouth of each catchment (Figure 7). The fences were 18 inches high x 42 inches long and made of coir netting, a durable material made of woven coconut fibers. The coir was then stapled to wooden stakes, which were pounded into the ground. Unfortunately, the ground was extremely compacted in some areas, causing the wooden stakes to splinter when pounded. In these cases steel re-bar was used, to which the coir was then attached using bailing wire.

The preparation of the site and installation of the plants and erosion control measures took place over a three week period during the end of December and the beginning of January. Follow up visits to the site for watering and maintenance were planned for every six weeks, however, due to scheduling conflicts, only three trips were made during the six month study. During these trips, each plant received at least 2.5 gallons of water. Also, any damage to the irrigation lines was repaired by splicing in new sections of line.

Figure 7. Catchments with silt fences used to control run-off.

Results

Six months after completion of planting, the overall survival rate at Nelson Lake was 92%, (Table 2) exceeding the required survival rate by 22%. Plants responded well to all treatments with the highest survival seen along the drip lines in the seeded area at 97% (Table 3). The next highest survival rate was found in the catchments at 94% (Table 4). Survival was next highest for plants along the wattles at 83.5% (Table 5) and then for the plants along the drip lines next to the rock check dams at 79.5% (Table 6).

The ANOVA computer program was used to determine the effect of the four different planting treatments, and a significant difference was found between treatments in overall survivorship (p-value = 0.0001) (Table 7). Using Fisher's Protected Least Significant Difference (LSD) test to compare the different treatments, it was found that significantly greater survivorship occurred on the large drip line plantings compared to both the wattle and rock dam plantings and in the catchment plantings compared to the rock dam plantings.

After six months, the coir netting, rock check dams, and straw wattles remained in good shape with indications that all had been successful at retaining aeolian dust deposits (Figures 8 and 9). Due to the small amount of precipitation, however, these erosion control measures were not fully tested for their effectiveness in controlling water flow.

The four acre pitted area seemed to have a fairly good amount of regeneration from seed, but it consisted mainly of Sphaeralcea ambigua and a few species of annuals.

TABLE 2
Total species survival rates