Soil Ecology and Research Group last update October 29, 2002

REVEGETATION AND EROSION CONTROL
GRANITE II SITE
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 the surrounding communities.

The purpose of this project is to reduce the amount of sedimentation and gullying occurring across major trails that are used during training activities. Surface erosion occurs when rain channels into drainages, carrying with it the topsoil of the surrounding disturbed areas. Topsoil deposited on trails eventually dries, and particles become airborne when disturbed by vehicles or wind gusts. Dust and large gullies pose driving hazards to those who use the trail by reducing visibility and creating rough trail surfaces. At Granite II Site, erosion control techniques, including straw wattles and catchments, have been used to reduce gullying. Revegetation and direct seeding were then used to stabilize the topsoil, reduce surface flow, and increase infiltration, which will in turn reduce sedimentation.

Site Description

Granite II Site is located twenty-seven kilometers from Cantonment along the western side of the major trail that travels through Granite Pass (Barstow-Granite Main Supply Route). The project site was comprised of two sections: Granite II North and Granite II South. Granite II North is located at UTM Coordinates 544354 E 3922617 N and encompasses about five acres. Granite II South is located at UTM Coordinates 543033 E 3921471 N. This section encompasses about four acres.

Live fire exercises are performed in the vicinity of this site during training activities, leaving much of the area disturbed. Ordnance has left large craters and areas of sparse vegetation surrounding these craters. Tactical vehicle travel is kept primarily on the major trails; however, some less established trails bisect the site. These disturbances are causing surface erosion to occur at an accelerated rate. Large gullies have formed throughout the site and on the trails down-slope from the site (Figure 2).

The project design at both portions of the site involves reducing surface erosion by slowing water down and stabilizing the topsoil. Water flowing down gullies is dispersed across the site before it can reach the main trail. This will reduce the creation of larger, deeper gullies and allow more rain to infiltrate the soil. In addition to helping the existing vegetation, the water will aid in establishing transplanted native shrubs. Revegetation of the site includes the transplanting of greenhouse-grown desert shrubs and direct seeding of native annuals and perennials.

Site-specific information includes soil type and species composition. The soil is of the Venusite series and consists of a coarse, sandy loam that has experienced moderate to high compaction (See Appendix A for a complete soil series description). Native vegetation present in the area includes Larrea tridentata

Figure 2. A large gully threatens the major trail.

(Creosote Bush), Ambrosia dumosa (Burroweed), Hymenoclea salsola (Cheesebush), Eriogonum fasciculatum (California Buckwheat), Psorothamnus arborescens (Mojave Indigo Bush), Salazaria mexicana (Bladder Sage), Lycium andersonii (Desert Thorn), and Achnatherum speciosum (Needlegrass).

Materials and Methods

Granite II North
A triangular area in the southern portion of Granite II North was planted with sixty native desert shrubs (Figure 3). These plants were planted in groups of three with a two-meter diameter basin constructed around each group. Around the periphery of the site, thirty-five more two-meter diameter basins were created and planted with three plants each (See Figure 4 for project design). Holes were dug with a mechanical auger, shovel, and/or a post-hole digger. Each hole was then pre-watered with two gallons. Plants received an additional three gallons during their transplanting. A Treepee or Tubex Shrubshelter was installed on

Figure 3. Basins in a triangular portion of Granite II North.

Figure 4. Project design for Granite II North.

each plant to protect against herbivory and the harsh desert environment. A total of 165 plants were installed in this manner.

Catchments were constructed on the northern, uphill side of two main trails in areas where gullies were forming. A tractor was used to push soil into V-shaped mounds, with each leg approximately two meters long and the mouth three meters wide (Figure 5). Along the trail, nine of these catchments were created and planted with five to six shrubs each. The native shrubs were transplanted in the same manner as those in basins with the exception that rather than one large basin around a group of three plants, a small basin was constructed around each plant.

Figure 5. Catchments are constructed uphill of the main trail using a tractor.

Uphill of the catchments, straw wattles were installed in gullies to disperse and slow water run-off across the site. Eighteen twenty-five-foot wattles were divided into halves to make thirty-six pieces that would be wide enough to span the gullies. Above each catchment, four pieces were spaced nine meters apart. Trenches were dug using pick-axes, shovels, and McCloud digging tools. A wattle was then placed in each trench and secured with three-foot long, three-eighths inch diameter rebar. Two native desert shrubs were then planted just uphill of each wattle (Figure 6). These also received small individual basins around the base of each plant. A total of seventy-two plants were planted next to wattles.

Figure 6. Two plants were planted uphill of each wattle.

A total of 279 plants were planted between 13 March 2001 and 26 June 2001 at Granite II North. Plants were hand-watered on a four to six week basis after the initial planting. Each plant received an average of one and one half-gallon per watering trip. Treepees and Tubex Shrubshelters were removed from the plants that had outgrown their plant protectors beginning in October 2001.

One acre within the interior of the site was seeded with 18.3 pounds of seed provided by ITAM on 18 December 2001. Cactus mix was mixed with the seed in a ratio of 3:1 to add bulk and ensure that the seed was evenly distributed. The seed was sown by hand, and the plot was then sprayed with a light application of Soil-Sement?, a tackifier. The tackifier will keep the seed in place until there is sufficient rain to cause the seeds to germinate. The composition of the seed mix is shown in Appendix B.

Granite II South
Uphill of the main trail, eleven 'V'-shaped catchments were constructed in low -lying areas (See Figure 7 for project design). A tractor was used to push dirt into a 'V' shape, with each leg two meters in length and the mouth about three meters across. Eight or nine plants were planted in or around each catchment, for a total of ninety plants. Holes were dug with a mechanical auger, shovels, and/or a post-hole digger. Each hole was then pre-watered with two gallons and planted with an additional three gallons. A Treepee or Tubex Shrubshelter was installed on each plant. Small, circular basins were created around the base of each shrub to facilitate maintenance watering.

Figure 7. Project design for Granite II South.

Straw wattles were installed on the uphill slope above each catchment. Twenty-two twenty-five-foot long wattles were cut in half and tied off to form forty-four smaller pieces. Above each catchment, four wattle pieces were placed in gullies spaced at nine meters apart. Installing the wattles involved digging a trench with a pick-axe, shovel, and/or McCloud digging tool, placing the wattle in the trench, and securing the wattle with rebar. Two plants were planted on the uphill side of each wattle in the same manner as those planted uphill of the catchments. Eighty-eight plants were planted in association with the wattles.

Twenty basins were created in the southern portion of the site. Each basin contains three plants that were planted in the manner previously described. However, rather than individual basins around each plant, one large (two-meter diameter) basin was constructed around each group of three. A total of sixty plants were planted this way.

All 241 plants at Granite II South were planted between 13 March 2001 and 26 June, 2001. The species composition for Granite II North and Granite II South is listed in Table 1. An average of one and a half gallons of water was delivered to each plant on a four to six week basis. Treepees and a Tubex Shrubshelters have been removed from the plants that have outgrown their plant protectors.

Table 1.
Species planted at Granite II North and Granite II South.

Granite II North and South
Overall survival rate for the entire site after one year was 85.7%. This figure represents results from both sections of the site and an adjustment for the five transplants that were killed during training exercises. For the entire site, survival rate per species was calculated, and the results are illustrated in Table 2. All species planted showed excellent survival rates. Prosopis glandulosa (Honey mesquite), Lycium species (Desert thorn), and Eriogonum fasciculatum (California buckwheat) are the strongest of all of the transplants, with survival rates greater than 85%. The remaining species planted did quite well with survival rates all above 78% one year after implementation.

Table 2.
Survival rates based upon species for entire Granite II Site.

Figure 8. Damage to wattle and shrubs caused by training activities.

Conclusions and Recommendations

Granite II North and South
The plants have done extremely well surviving at this site. Success can probably be attributed to the proper placement of transplanted shrubs and the higher precipitation at this site in relation to the rest of the Base. Planting native shrubs within close proximity of the erosion control structures allows them to receive additional water during rainfalls. Catchments stop the water flow, saturating the soil around them. Similarly, the shrubs planted uphill of the wattle section also received additional water from surface run-off. Passive capture of water from natural rain events for plant establishment is favored over irrigation at this site.

Granite II Site is located in an area that receives considerably more rain in relation to other areas of the Fort Irwin National Training Center. Data collected from weather stations throughout the post from 1 January 2001 to 30 September 2001 indicate that Granite Pass received the highest amount of precipitation (4.35 inches) over the nine-month period. The transplants benefited from the high soil moisture content at the time of planting. Since the final planting date Granite Pass has received a total of two and eleven-tenths inches of rain. The slight downhill slope (2-8%) of the site in conjunction with sufficient precipitation favors the use of wattles and catchments in establishing native plants at this site. Although ponding of water could be a potential problem for establishing plants in catchments, it has not been a concern with the well-drained soil at Granite II Site. Soil type should always be considered when planting within catchments.

Each plant received an average of one and a half gallons every four to six weeks. When maintaining a hand-watered site, the size of the basin surrounding each plant is very important. Many of the basins constructed were too small, with diameters of 12 to 15 inches. These basins could hold only about one gallon. This became a limiting factor in how much water could be delivered on each trip and was evident as the summer began. Final planting, consisting of about twenty shrubs, occurred on 26 June 2001 in the Granite II South section of the site. Because it was beyond the planting season, basins on these plants were constructed with 24-inch diameter basins to ensure they received enough water. Each basin could hold two or more gallons. Despite the stress caused by planting in mid-summer, these plants did quite well, having received almost twice as much water as the surrounding plants. We recommend that basins be built with a diameter of 20 to 24 inches around plants that are to be hand-watered.

Due to a very dry 2001/2002 wet season it is difficult to determine the extent of success or failure of the seeding project. The seeding was completed on 18 December 2001 and the total rainfall from then through June 2002 was less than three-tenths of an inch. Also, it never rained over a quarter of an inch in any one month during this time (Figure 9). It will take a normal wet season to be able to determine whether or not the seeding project was a success or a failure.

All of the methods of erosion control seem to be working extremely well. The wattles have been filling in the areas in front of and behind them with sand, effectively filling the gullies from previous rains with sand. The catchments have held up quite well and they have prevented any erosion from endangering the main trail nearby. In addition to preventing erosion, both the wattles and the catchments, have helped capture surface water run-off during rain events and made this water available to the surrounding transplants on the sites.

The rehabilitation project at the Granite sites had very good transplant survival rates a year after implementation in comparison to other projects at Fort Irwin. This can be attributed to several factors. Granite Pass typically receives more rain than any of the other places on the Base that there are currently projects at. The majority of the planting was completed in the spring before temperatures soared and the soil moisture content declined. And lastly the soil was loose and not heavily compacted from vehicles and training exercises.