Soil Ecology and Research Group last update October 29, 2002

REVEGETATION AND EROSION CONTROL OF
AMMO ROUTE
NATIONAL TRAINING CENTER
FORT IRWIN, CA 92310

Project Overview

The National Training Center at Fort Irwin, CA is a training facility for the United States Army. Located 35 miles northeast of the city of Barstow, CA, Fort Irwin occupies an area of nearly 643,000 acres within the Mojave Desert. Training activities conducted at the installation provide units and commanders the opportunity to practice large-scale warfighting skills in a realistic environment. Ground maneuvers, live-fire exercises, and force on force activities damage the native vegetation of the Mojave Desert creosote/bursage scrub habitat, which in turn causes dust and erosion problems.

The Integrated Training Area Management (ITAM) Program is responsible for land management and rehabilitation of disturbed areas on the post. Projects proposed by the ITAM program combine both revegetation and erosion control to minimize the environmental impacts of training activities. Dust contributes to poor air quality as well as reduced visibility. Controlling levels of particulate matter less than 10 microns (PM 10) is of particular importance, as long-term exposure to these tiny particles is a health risk to people both on the base and in surrounding communities.

This project involves reducing the width of a portion of Ammo Route, a tactical vehicle trail, to both protect the remaining vegetation along the sides of the trail and reduce wind erosion. The dust created by traffic on this trail greatly reduces visibility in the area. Because of the close proximity to the cantonment area, the effect of the fugitive dust on air quality is also a primary concern. Setting defined trail boundaries, combined with revegetation, will help minimize further damage to the area. Increasing the amount of vegetation by transplanting greenhouse-grown native shrubs and direct seeding of annuals and perennials will also reduce dust levels by trapping fine particles.

Site Description

Ammo Route is located within the boundaries of the Cantonment area at UTM coordinates 531050 E 3900850 N. The site encompasses about three acres along the sides of a 1,232 meter length portion of a major trail. This trail is used by military personnel to transport ammunition to the field and is also a road test area for tactical vehicles. The heavy use has destroyed the vegetation along the sides of the trail, increasing the width. The island of vegetation west of the main trail is also damaged by vehicles cutting through the area.

The project design (Figure 2) involves narrowing the east trail and closing off trails that run through the vegetated area west of Ammo Route. Narrowing the trail was accomplished by building catchments along each side of the east trail to encourage vehicles to stay in the center of the trail. Planting native shrubs and direct seeding within these catchments will help to establish a more defined trail boundary. Trails bisecting the vegetated area to the west were closed off by planting shrubs at the intersections with the Ammo Route training loop. Closing off the trails to vehicles will allow natural regeneration. Pitting the trails will help to accelerate the recovery by creating micro-sites for wind-blown seeds and by increasing water infiltration.

Site-specific information includes soil type and species composition. The soil is of the Goldivide series and consists of heavily compacted horizons under a layer of loose aeolian deposits on the surface (See Appendix A for a more complete soil series description). Native vegetation includes Atriplex polycarpa (Allscale), Larrea tridentata (Creosote bush), Pleuraphis rigida (Big Galleta), Ambrosia dumosa (Burroweed), Senna armata (Spiny senna), Hymenoclea salsola (Cheesebush), and Ephedra nevadensis (Mormon tea). A population of the invasive, exotic Salsola tragus (Tumbleweed) also inhabits the site.

Figure 2. Project design for Ammo Route.

Materials and Methods

Site preparation
Site preparation for the main trail involved building catchments every twenty feet along both sides of the trail. ITAM personnel indicated the placement of each with a pin flag prior to implementation (Figure 3). Catchments were constructed using a D-4 bulldozer (Figure 4) to push soil into a V-shape. The blade of the bulldozer was angled to create a slope into the bottom of the catchment, with the berm facing the direction of oncoming vehicles. Each catchment was made with one push of an 8.5-foot blade to a depth of approximately 1.5 feet. The top of each berm was leveled off by hand to a height of about two feet. A total of 184 catchments were created on the east side of the trail, and a total of 170 catchments were created on the west side of the trail. This work was completed on 28 February 2001.

Figure 3. Pink pin flags indicate catchment placement prior to implementation.

Figure 4. A D-4 bulldozer was used to build catchments.

Site preparation for the ten trail closures involved pitting the trails with a mechanical disc pitter that was towed behind a tractor. This work was completed on 13 June 2001.

Planting
Two plants were planted in every other catchment for a total of 354 plants. Holes were dug using a mechanical auger and shovels. Each hole was pre-watered with two gallons before the plants were planted. An additional three gallons were used during planting, for a total of five gallons. Compacted clay layers made digging difficult and water infiltration slow. A circular basin was created around the base of each plant. Each of the 354 plants was protected with a Treepee.

Ten to twenty plants were planted on both ends of each trail closure. Each was installed in the same manner as those in catchments. The majority of plants were protected with either Treepees or Tubex Shrubshelters, but some remained unprotected due to lack of supplies during planting. On the next watering trip, an attempt to cover all of the plants with plant protectors failed because the ground was impenetrable. A total of 250 plants were planted at trail closures.

A total of 599 plants were planted at the site between the dates of 28 February and 12 July 2001. Table 1 lists the numbers of each species planted at Ammo Route. All plants have been watered on a monthly basis since the final planting date.

Table 1.
Species planted at Ammo Route.

Figure 5. Catchment contains a handful of seed and two shovels full of gravel.

Results

Planting
The one-year survival rate for the transplants at Ammo Route was 72.6%. This represents an adjusted total, excluding six plants that were killed by vehicles from the total number. Table 2 shows the survival rate for each species at the site. Species with the highest success rates are listed first.

Table 2.
Survival rate based on species.

The two species of plants that had the highest survival rates of 84% at the end of one year were Lepidium fremontii and Atriplex polycarpa. Chilopsis linearis, Prosopis glandulosa, Larrea tridentata, Ambrosia dumosa, and Atriplex canescens all had survival rates between 81% and 82.5%. The plant that had the lowest survival rate was Isomeris arborea, at 30% survival.

Seeding
Approximately seven months after the seeding was completed, the site was revisited to evaluate seeding success. At this time, no seedlings had sprouted. The gravel was effective in keeping the seed trapped within the catchment. Due to a very dry 2001/2002 wet season the seeds have not sprouted. It will take a close to normal wet season for the seeds to germinate. However, as time passes the seeds are likely to lose their vitality and less will likely germinate. El Niño is currently predicted for the 2002/2003 wet season. If it brings heavier precipitation, as it is expected to do, the majority of the seeds can be expected to germinate.

Conclusions and Recommendations

The use of catchments to narrow a trail has been very successful at Ammo Route. Building catchments along both sides of the trail has created a clear boundary between the trail and adjacent vegetated areas, helping to protect the existing vegetation on both sides of the trail (Figure 6). Only a few incidents have occurred where a vehicle has drifted off of the trail and into the catchments.

Figure 6. Catchments create a clear boundary between the road and vegetated areas.

Pitting the unnecessary trails bisecting the vegetated area between the major trails has not shown any significant results. No new vegetation appears to have established in the areas that were pitted. This can be attributed to several factors, including soil type and time of year the pitting was completed. The soil along these trails is very sandy in the first few inches but hard pan underneath. The pits and furrows created by the mechanical pitter were composed of this very loose sand and were quickly smoothed out by gusts of wind. The degraded pits could not serve their purpose as traps for windblown seeds and rain. Additionally, pitting was completed on 13 June 2001 when precipitation levels were low. During the months of June, July, and August a weather station in the vicinity of the site recorded 0.16 inches of rain. The minimal precipitation was probably instrumental in the lack of establishment of new plants. Seed germination might have been more successful if the pitting had been completed between January and March when precipitation amounts totaled 3.6 inches.

The survival rate of the transplants (72.6%) is just above the contract requirement of 70%. The success can partially be attributed to both proper planting time and the effectiveness of catchments at maintaining the soil moisture. Two hundred fifty-five of the 354 plants installed within catchments were planted from 28 February 2001 to 2 March 2001. Sufficient rainfall during these two months (2.71 inches) was beneficial to the establishment of the transplants. Catchments worked very well to collect precipitation flowing across the slightly sloped site, especially on the east side of the trail. The catchments along this side of the trail appear to collect more rainfall because of their more uphill position on the gradual slope of the site.

The plants at trail closures, in general, did poorer than those planted in the catchments. This can be attributed to improper planting time and high levels of compaction along the trails. All of the plants planted at trail closures were planted between the dates of 11 June and 12 July 2001. Lack of rainfall and extremely high temperatures created extra stress for these transplants. Precipitation during these two months totaled 0.16 inches, and temperatures were often in excess of 100ºF. High levels of compaction along these trails were discovered during the implementation phase. Mechanical augers had a very difficult time penetrating the soil surface, wearing down a carbide screw bit within three days of work. Although each hole was successfully dug to the proper depth, the soil surrounding the hole remained highly compacted. This limits establishment of roots for the transplants by creating a physical barrier that new roots must penetrate. Water infiltration is also greatly reduced in compacted soils. Ripping the trails prior to planting would have helped to relieve some of the compaction and should be considered in future projects.

Planting ten to twenty plants at each trail closure was effective in closing off the trail to tactical vehicles (Figure 7). One shrub planted close to the main trail was killed by a vehicle drifting into the site, but there has been no evidence of vehicles continuing to use the trails. Several new trails have been created through the vegetated area, however, indicating an apparent need for tactical vehicles to move from one main trail to the next. Perhaps leaving one or two strategically located trails open within this vegetated area would have eliminated the formation of new trails.

Figure 7. Vehicle travel along a closed off trail has ceased by planting 10-20 plants at each intersection with the main trails.

Direct seeding within catchments is highly advisable. Catchments create a wind barrier for the seed, helping to keep them in place until enough rain falls to induce germination. They do not create enough of a barrier to obstruct all wind gusts, so additional treatments must be made to ensure that the seed remains in place. Rock mulch was an easy and inexpensive treatment that worked well in combination with the catchments. This would not be advisable for a large-scale seeding project or at a site where vehicles cannot access the area to be seeded. Gravel becomes very heavy when transported by hand. Although there has not been sufficient precipitation since the seeding dates (11 December and 18 December 2001) to germinate any seeds, they have remained in the catchments and should germinate with sufficient rainfall.

The success of the plants surviving over 70% one year after the completion of the implementation is largely due to the early planting time, while the soil moisture content was still high and outside temperatures relatively low, and the soil moisture holding capacities of the catchments. The survival rate of the transplants may have been further enhanced had the trails bisecting the project site been ripped prior to planting the transplants. It is suggested that all future project sites that have high levels of soil compaction be ripped before planting.