Soil Ecology and Research Group last update October 29, 2002

REVEGETATION AND EROSION CONTROL OF
PIONEER SITE AND PIONEER TRAIL AT
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 war fighting 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.

Projects at two sites, Pioneer Site and Pioneer Trail, were designed to reduce the amount of erosion occurring in the area. The focus at Pioneer Site involves confining military activity to an open, heavily disturbed area and discouraging travel through the surrounding, less disturbed area. Protecting the existing vegetation and planting more native shrubs will help reduce the amount of wind erosion that occurs at this site. At Pioneer Trail, the main objective is to reduce surface erosion on an unused construction trail that runs parallel to the main trail. A variety of erosion control measures were employed at this site.

Site Description

Pioneer Site
Located 25 km from Cantonment at UTM grid coordinates 513800 E 3916000 N, Pioneer site encompasses 19.6 acres (Figure 2). Directly across from a historic N.A.S.A. satellite dish is a 7.1-acre area that has been heavily used during military tactical exercises. South of this area is a 10.6-acre area that has been damaged by moderate military use, adjacent to a 30.4-acre area that has experienced light use.

The heavily used area is to remain open for military use. The construction of a three-foot tall berm along the northern and western perimeters of the moderate and less disturbed areas will discourage vehicles from entering these areas, which in turn will facilitate the recovery of existing vegetation. Along the eastern and southern perimeters, several trails that bisect the site have been closed. In addition to restricting access to allow natural regeneration, greenhouse grown native shrubs have been planted to help the revegetation process. The increased vegetation will reduce wind erosion by slowing wind speed and trapping fine dust particles.

Initial site visits provided site-specific information including soil type and plant species composition. The soil type is of the Yermo series and consists of gravel on the surface, fines with gravel and small rocks in the top six to seven inches, and gravel, sand, and some larger rocks below (A more complete soil series description can be found in Appendix A). Species diversity is low with only two species of native perennial shrubs, Larrea tridentata (Creosote bush) and Ambrosia dumosa (Burro-weed), found at the site.

Pioneer Trail
Just north of Pioneer Site and Pioneer Dry Lake at UTM Coordinates 513125 E 3918300 N is a steep, downhill portion of the Pioneer Trail (Figure 3). At this site, an unused construction trail runs adjacent to and crosses the Pioneer trail. The old construction trail has become a channel for water flowing downhill during

Figure 2. Pioneer Site with the three levels of disturbance.

Figure 3. Restoration design for Pioneer Trail site.

rain events, causing parts of Pioneer trail to erode at an accelerated rate. The five-acre site involved closing off the old trail and installing several different types of erosion control structures to slow surface runoff and reduce gully formation. Native shrubs have been planted to both reduce soil erosion and close off the trails to tactical vehicle traffic.

Initial site visits provided site-specific information including soil type and plant species composition. Soil samples were taken from three pits. The first pit had fines and 3 to 6 inch rocks on the surface, followed by a 4-inch layer of hardpan, and looser gravel and fines below. The second pit had fines and 1 to 2 inch rocks to a depth of 10 to 12 inches. The third pit had an exposed calcic horizon on the slope. These soils are of the Haleburu series (see Appendix A for more specific soil series description). Native vegetation at Pioneer Trail includes Larrea tridentata (Creosote bush), Krameria erecta (Purple heather), Ambrosia dumosa (Burro-weed), Atriplex polycarpa (Allscale), Thamnosma montana (Turpentine Broom), Ephedra nevadensis (Mormon tea), Eriogonum inflatum (Desert trumpet), Yucca brevifolia (Joshua Tree), Opuntia basilaris (Beavertail cactus), and Echinocactus polycephalus (Clustered barrel cactus).

Materials and Methods

Pioneer Site
Native shrubs were planted along the northern and western perimeters of the site along drip irrigation lines. Thirteen lines of half-inch polyethylene tubing were laid in a non-linear pattern, with hose connections at the perimeter of the site and closed ends in the interior of the site. The majority of these lines lie in the moderately disturbed area. A total of 6000 feet of drip line was used. Along the irrigation lines, plants were planted at six-foot intervals. Each hole was dug to the depth of the planting container using a one-man auger, shovel, and/or post-hole digger. The hole was pre-watered with two gallons before planting and an additional three gallons of water afterwards (Figure 4). Water infiltration throughout the site varied based upon levels of compaction. A circular basin was built around each plant to hold water during irrigation. A total of 1000 plants were planted on irrigation lines.

Figure 4. Holes pre-watered with 2 gallons of water before planting and an additional 3 gallons afterwards.

Along the eastern and southern perimeters of the site, sixteen trails that entered the site were closed off by the installation of cyber stakes and ten to twenty plants per closure. Large rocks were placed around and in between the plants to offer additional protection from vehicles. The native shrubs were installed in a similar manner as the plants along irrigation line, with each shrub receiving five gallons of water. A circular basin with an average diameter of twenty inches was built around the base of each plant to hold water during irrigation. These basins were built slightly larger than those on irrigation lines to ensure that two gallons could be delivered to these hand-watered plants. A total of 254 plants were planted at road closures.

All planting was completed between 26 March and 21 June 2001. Table 1 lists the number of each species planted at the site. During planting, each plant was protected with either a Treepee or a Tubex Shrubshelter secured with rebar. Watering trips were made on a monthly basis, delivering approximately two gallons per plant. Plants were pruned on 18 October 2001 and 28 November 2001. Appendix C gives a complete schedule of work completed at the site.

The final step involved constructing 2500 feet of intermittent berms along the western and northern perimeters. This task was completed using a D-4 Bulldozer to push soil from the highly disturbed area (north of site) up to a height of three feet. The berms filled in gaps between well-established Creosote bush shrubs that lay on the site perimeter. This created a continuous barrier between the area left open for military use and the rehabilitation site.

Table 1.
Numbers of species planted at Pioneer site.

Pioneer Trail
Much of the project could not be implemented as planned because of the difficulty in working with the soil. The project coordinator, Ruth Sparks, successfully modified the project to overcome the challenges. As described in the Project Overview section, just below the rocky surface lies a four-inch layer of hardpan. This layer was very difficult to penetrate, even with rebar, making it impossible to install some of the planned erosion control structures, as well as greatly reducing the probability of transplant survival. Portions of the task description that were accomplished are described below. An equivalent amount of labor/materials was added to the Red Pass MSR projects for those portions of the Pioneer Trail task description that were not completed.

Planting
Planting was completed on 20 and 21 June 2001, with installation of a total of 106 native desert shrubs (Table 2). These shrubs were planted at the intersections of an old construction trail and the Pioneer Trail to close off access to the old construction trail. Holes were dug using pick axes, shovels, and dig bars to the depth of planting containers (mechanical augers could not penetrate the hardpan layer). Each hole was pre-watered with approximately two gallons, and an additional three gallons was used during planting. Due to the heavy compaction of the hardpan layer, water infiltration was extremely slow. A twenty to twenty-four inch circular basin was constructed around the base of each plant. Most of the plants were covered with a Treepee plant protector. Because of a lack of supplies, some plants did not receive plant protectors on the day of planting. Treepees were taken to the site three weeks later on the next watering trip but installation of these was not possible due to the hardpan layer. Those plants have remained unprotected.

Plants were hand-watered on a four to six week basis after the final planting date for a total of 5 maintenance visits between June and December 2001. An irrigation trip planned for 28 November 2001 was cancelled due to sufficient natural precipitation that had occurred the prior weekend. Approximately two gallons were delivered on each trip. Appendix C outlines the schedule of all work completed at Pioneer Trail.

Table 2.
Numbers of species planted at Pioneer Trail

Erosion Control
Several different erosion control techniques were implemented at Pioneer Trail. In a drainage running along the west side of the trail, six straw bale dams were installed on 19 and 20 June 2001 to slow water that is channeled into the drainage (Figure 5). Pick axes, shovels, and dig bars were used to create a rectangular hole the size of the straw bale. The bales were then buried halfway and the downhill side of the bale was reinforced with large rocks.

On the western side of Pioneer Trail a steep downhill portion of the old construction trail remains. Waterbars were installed on 21 June 2001 on this portion of the construction trail to deflect water to an adjacent vegetated area before it could reach and wash away Pioneer Trail at the base of the slope. A D-4 Bulldozer was used to push soil into ten, thirty-foot long berms that were spaced ten to twenty meters apart. The waterbars were angled so that the water would be diverted west of the old construction trail.

Figure 5. Straw bale dams installed in the drainage between Pioneer Trail (left) and the old construction trail (right).

The final erosion control technique used at Pioneer Trail was the installation of forty-five straw wattles between 23 and 25 October 2001. Twenty-five foot long wattles were installed along the sparsely vegetated slopes of a major drainage on the western side of Pioneer Trail. Pick-axes, McCloud digging tools, and shovels were used to dig the trenches, and three-foot long, three-eighths inch diameter rebar were used to secure the wattles in the trenches. Twenty-four wattles were installed on the southern slope of the drainage (Figure 6), and thirteen on the northern slope.

In the wash at the base of the slopes, five wattles were installed. Two wattles were cut in half, with the two halves stacked on top of each other before securing them with rebar. Another was divided into one-third and two-thirds pieces to fit the width of the wash. The remaining two wattles were installed as twenty-five foot long sections.

Figure 6. Straw wattles on the southern slope west of Pioneer Trail.

On the eastern side of Pioneer Trail, three wattles were installed. Original plans involved spacing several wattles ten to fifteen meters apart on a steep downhill grade of the old construction trail where gullies are forming. Because of the high level of compaction of this soil, the wattles could not be properly secured with rebar. One wattle was installed; however, a small section that could not be secured with rebar was braced with several large rocks on the downhill side. The other two wattles were placed along a sparsely vegetated area between Pioneer Trail and the old construction trail.

Results

Pioneer Site
Planting
The survival for Pioneer site was a low 53%. This represents an adjusted survival rate, excluding the results of Achnatherum hymenoides (Indian rice grass) and plants destroyed by vehicles from the survival count. This agreement was reached during the implementation of the project, because many of the Indian rice grass plants provided by ITAM were unhealthy and success or failure of their establishment could not be based solely on care during and after the transplanting. Of the 1,254 plants planted at Pioneer site, 32 were Achnatherum hymenoides and 14 were destroyed by vehicles, therefore, the adjusted total number of plants for the site is 1,208. A survival rate for each species was calculated with results provided in Table 3.

Prosopis glandulosa (Mesquite) had the highest survival rate of any of the species at an amazing 100% survival. The second highest survival rate was with Larrea tridentata (Creosote bush), at 86.6%. Ephedra nevadensis had the third highest survival rate at 78%, and Lycium andersonii was fourth with 75% surviving after one year from the time of planting. All other species planted had survival rates less than 70%.

Table 3.
Survival rate for each species at Pioneer Site.

Pioneer Trail
Planting
The overall survival rate for Pioneer Trail was a low 52.4%. An error in plant numbers planted at the site appears to have been made at the time of planting which resulted in erroneously low survival rates for a few species. Table 4 shows the survival rate for each species planted at Pioneer Trail. Species are listed in descending order of success rate.

Table 4.
Survival rate per species at Pioneer Trail.

Erosion Control
The straw wattles and straw bale dams installed remain intact and appear to be functioning well. However, there has not been enough rainfall since implementation (Figure 7) to be able to come to any conclusions as to the effectiveness of these forms of erosion control. The total rainfall from May 2001 through June 2002 was only 1.07 inches and the most it rained in one month was less than half an inch.

Also, the waterbars created along the old construction trail have been damaged by tank training exercises (note tank tracks in Figure 8.), not damaged by water run-off as previously reported.

Figure 8. Waterbars damaged by tank training exercises.

Conclusions and Recommendations

Pioneer Site
The overall survival rate of Pioneer Site (53%) is below the required minimum of 60% survival. Several factors, including insufficient irrigation, late season planting time, high levels of soil compaction, and competition with the invasive weed, Salsola tragus, have contributed to the higher rates of plant mortality at this site.

Irrigation line 2.1 and 2 are adjacent lines in the northeastern corner of the site. In order to water the irrigation lines efficiently, the connectors were installed along the perimeter of the site for easy access. This made the connectors for Line 2.1 and Line 2 slightly downhill from the rest of the line. On several of the irrigation trips, a 500-gallon water trailer was used to water the lines. The pump on the trailer was not strong enough to distribute water equally to all the plants. Some of the shrubs at the far end of the drip-line have received approximately half the amount of water as those near the connector. On 26 September 2001 this problem became evident and the plants on these lines were hand-watered to ensure equal distribution. Irrigation after this date was completed using a 4,000-gallon water truck. The water truck has a more powerful pump and water flow can be more controlled. The lack of sufficient water to these lines during the hot and dry summer months had a detrimental effect on the success of these plants. When laying irrigation line on a slope (no matter how slight), the connector should always be installed on the uphill end of the drip-line.

Beginning in May, temperatures were often in excess of 100º F. This is not an optimal time for planting because the transplanted shrubs are put under even greater stress. The shrubs on Line 2.1, Line 12, and on the Road Closures were planted between 16 May and 1 June 2001. The rest of the site was planted earlier during the spring when temperatures were less extreme and soil moisture was higher. Higher mortality rates for Line 2.1, Line 12, and the Road Closures can partially be attributed to planting occurring during the hot summer months.

Planting in heavily compacted soils has also affected plant mortality. Water does not infiltrate as easily, and root growth is impeded. The soil did not receive any preparatory treatment to relieve compaction. The below average survival rate of 53% may also be due to the high levels of soil compaction. Perhaps if these trails had been ripped prior to planting, the transplants would have been more successful. It is strongly recommended to relieve soil compaction during site preparation for rehabilitation projects on or near roads and trails.

Another major factor in high mortality rates at the site was competition with the invasive weed, Salsola tragus (tumbleweed). Tumbleweed was pulled out of the ground during the maintenance period of the project; however, not enough was done to keep up with its quick growth habits. Irrigation necessary for the survival of transplanted shrubs also accelerated the establishment of invasive species. By July, the entire northeast corner of the site was covered with this weedy species. Salsola tragus is a difficult weed to control. Once it sets seed, little can be done until the next growing season.

Pioneer Trail
Planting
It was not surprising that the overall survival rate for plants at Pioneer Trail was so low (52.4%). Late-season planting in conjunction with poor soil quality, which prohibited deep watering, contributed to the lack of success at this site.

Digging the holes in this highly compacted soil was an extremely difficult task. Mechanical augers could not penetrate the hardpan layer just beneath the surface. The only tool that could break up the compacted soil was a pick axe. Planting this site was extremely labor-intensive, and the probability of transplants surviving in such poor quality soil was low. A combination of ripping and amending the soil would likely have favored transplant success; however, this would have been both costly and difficult considering the slope at the site.

Another factor affecting transplant survival was planting time. The 106 plants planted at Pioneer Trail were planted on 20 and 21 June 2001. Temperatures were above 100º F during planting, making the soil extremely hot and dry. Also, some of the plants did not receive plant protectors, which left them exposed to the elements. Plant protectors should be installed immediately after planting to avoid leaving such exposure. Often, after the water used to plant the shrub has dried, the ground becomes rock hard and installing plant protectors is no longer an option.

Erosion Control
The 2001/2002 wet season amounted to very little rainfall, thus it is difficult to determine the effectiveness of the different erosion control measures. The straw wattles and straw bale dams are intact and appear to be functioning to slow water and topsoil running down the slopes. The waterbars were damaged by tank training exercises. A more durable method, such as the use of rock weirs, is recommended for use in areas where fast flowing water is expected to occur.