Soil Ecology Restoration Group

REVEGETATION AND EROSION CONTROL SERVICES FOR SOUTHERN SITES

last update August 3, 2000

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 National Training Center is dominated by creosotebush scrub, but also contains saltbush scrub, 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, many of the vegetation communities present at the National Training Center have been disturbed or damaged. Furthermore, training activities have also damaged the cryptogamic soil crusts, a thin crust of algae, fungi, lichen, and other micro-organisms which bond the surface soil texture together. Once the vegetation and soil crusts have been disturbed, the newly barren surfaces are subject to accelerated rates of erosion from the normal forces of wind and water; the effects of which will continue to intensify unless corrective action is accomplished. Training activities need to be integrated with restoration activities to reduce the loss of realistic environmental conditions for desert warfare training and prevent excessive dust levels or soil loss.

The Integrated Training Area Management Program has initiated a five year program (1997-2001) to manage areas in a sustainable manner. A major portion of this program includes revegetation and erosion control of disturbed lands. This project consists of three tasks: field work at Hill 526; field work at the Bivouac area; and plant propagation at the San Diego State University greenhouse. Field work will involve the building of berms to discourage vehicular traffic through the worksites, and revegetation of the worksites through transplanting nursery grown plants and direct seeding with locally collected seeds.

Site Description

Hill 526

The worksite, originally identified as Hill 526, is actually located at Hill 466. For the sake of continuity with past submittals, the site shall be referred to as Hill 526. The worksite lies 29 kilometers to the east of the garrison at UTM coordinates 550900 east by 3901200 north, located along the border of the central and southern corridors in training area “G” (Figure 1). The worksite has been identified in the LRAM Erosion Control Plan as site number 48. Lying at the northern base of a hill with an elevation of 466 meters, the worksite has an elevation of approximately 450 meters. It is a part of an alluvial slope whose origin is from a distant group of hills to the north. Hence, there is a southern aspect with a 1 degree slope running towards and beyond Hill 466. The area of the worksite is 28 acres and lies within a rectangle of vehicular trails. The area experiences a moderate amount of use for open field combat training exercises, but the impact is not so severe to warrant the construction of a great number of physical and visual barriers. Furthermore, due to the mild slope, erosion control was a low priority. Revegetation work, however, was needed in the more barren areas. The area was divided into three smaller sections based on the amount of remaining vegetation found on the site (Figure 2). The soil is sandy, with surface fines and a moderate compaction of soil structure. The area lies in a creosotebush scrub vegetation zone, with the dominant plants being Larrea tridentata, Ambrosia dumosa and Ambrosia acanthicarpa.

Bivouac

The Bivouac worksite lies 30 kilometers east of the garrison at UTM grid coordinates 549500 east by 3903400 north, along the border of the central and southern corridors in training area “G” (Figure 1). This site is not listed in the LRAM Erosion Control Plan because the disturbance occurred rather recently, in June 1998, when the site was used as a bivouac area for a training exercise. There was a general loss of vegetation throughout the area due to encampment activities, leaving the area denuded in the middle and partially denuded at the edges of the encampment (Figure 3). The area encompasses 11 acres and is surrounded by vehicle trails. There are several trails running through the site that were closed off to discourage further damage to the area. The soil texture is sandy with surface fines and has low to moderate compaction of soil structure. The worksite is part of an alluvial slope whose origin is from a distant group of hills to the north, leading to a southern aspect with a mild slope of 1 degree. Although unlikely, with the mild slope the site may be prone to sheet erosion. Heavy replanting of vegetation would slow this process down. The area lies in a creosotebush scrub vegetation zone with the dominant plant species being Larrea tridentata and Ambrosia dumosa.

Figure 2. Hill 526 before installation. A) Seeding section, B) Less dense vegetation section and C) More dense vegetation section

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Figure 3. Bivouac site before installation. A) Drip irrigation area and B) Trail to be closed down.

Methods and Materials

Hill 526

In the northeast section of the site, the area with the most dense vegetation, 60 one meter diameter basins were constructed with the ITAM tractor operated by SERG personnel. Two nursery grown container shrubs were planted in each basin (Figures 4 and 5) and treated with root stimulator. In the northwest section of the site, the area with less existing vegetation, 142 one meter diameter basins were constructed with two plants placed in each basin. Root stimulator was not applied to these plants. To test if deep pipes in basins have a beneficial effect on survivorship compared to surface watering, 87 of the basins were randomly assigned two deep pipe irrigation systems (Figure 6C). The deep pipes are 3 inches wide by two feet long and can hold 0.76 gallons of water when full. A total of 404 nursery grown plants were outplanted, with the species listed in Table 1.

Table 1. Species planted at Hill 526.

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Figure 4. Installation of basins at Hill 526. A) Planting in a basin and B) The finished product.

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Figure 5. Installation of basins at Hill 526. A) Watering a basin and B) Overview of basins with ripped and seeded area in between.

Direct seeding occurred in the unplanted southern section and the planted northwest section. The areas were pitted using the ITAM pitter, primary vehicle and personnel. Seeding was accomplished by SERG personnel. A native seed mix was supplied by the ITAM Program. The seeding application rate was 250 live seeds per square meter, roughly equivalent to a 3:1 mixture of carrying agent to seed mix. The seed mix carrying agent consisted of Cactus mix and planting mix in a 1:1 mixture. The seed mix contained forbs (Amsinkia tesellata, Camisonia sp., Cryptantha sp., and Geraea canescens ); shrubs (Ambrosia dumosa, Brickellia incana, and Encelia sp.); and grasses (Achnatherum hymenoides and Nassella speciosa). A water sprayed plot was set up in the southern section to investigate techniques for increased germination rates. A diagram of the locations of installation techniques is shown in Figure 7.

Bivouac

Four tank trails that lead into the area from the perimeter of the worksite were closed. There were no access routes to the interior of the worksite once this was accomplished. The trails were closed by creating mounds (1.5m wide x 10m long x 0.75m tall) using the ITAM tractor and SERG personnel. Two mounds were placed 5 meters apart at the entrance to each trail (Figure 8) for a total of eight mounds. Three inch by two foot deep pipes were then installed for the irrigation of 10 nursery grown shrubs planted on each mound (Figure 6A). To test plant survivorship based on plant location, 40 plants were placed at the base of the mound while 43 plants were placed near the top of the mound.

The closed trails behind the mounds were hand pitted. Seed was then broadcast by hand to the hand pitted areas using a native seed mix provided by ITAM personnel (Figure 9). The seed mix carrying agent consisted of Cactus mix and planting mix in a 1:1 mixture. Seed application was 100 live seeds per square meter and consisted of forbs (Amsinkia tesellata, Camissonia sp., and Geraea canescens ) and shrubs (Ambrosia dumosa, Brickellia incana, and Encelia sp.).

In the center of the site, the most damaged area of the worksite, 720 plants were placed every five feet along a polyethelyne one-half inch drip line system laid down in a serpentine pattern to mimic natural spatial dynamics (Figures 6B, 10 and 11). Every planted shrub received a four gallon per hour emitter. A total of six lines were installed, four 500 foot lengths and two 600 foot lengths. The watering points occur at the start of each line, which were placed six feet from the perimeter road so watering could easily be accomplished with a 4,000 gallon water truck. The water truck has an engine driven water pump which can control the rate of water flow. This ensures that the plants at the end of the line receive as much water as those at the beginning of the line. The nursery grown transplants totaled 803, with species listed in Table 2. A diagram of the locations of installation techniques is shown in Figure 12.

Figure 7. Diagram of Hill 526 - Purple Line- 142 Basin Area. -Blue Line- 60 Basin Area. Red Line- Seeding Area . Black Arrow - To Langford Lake Road.

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Figure 8. Installation of mounds at the Bivouac site. A) Mounds before planting with hand pitted area beyond and B) Mounds during planting.

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Figure 9. Seeding at the Bivouac site. A) Preparing seed and B) Distributing seed by hand.

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Figure 10. Installation of drip lines at the Bivouac site. A) Four thousand gallon water truck at source point on perimeter road and B) Prewatering a hole.

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a) b) Figure 11. Installation of drip lines at the Bivouac site. A) Irrigation after planting and B) Overview of drip line serpentine pattern.

Table 2. Species planted at the Bivouac site.

Project Monitoring and Maintenance

Project schedules for Hill 526 and Bivouac work sites are shown in Table 3. All installation work was completed by 15 January 1999. Watering was conducted using a contracted water truck and commenced one week post-planting and continued at approximately three to four week intervals. Due to the scant amount of rainfall, the watering schedule remained continuous through the one year monitoring with the exception of July and November. At Hill 526, each basin received approximately six gallons of water. Thus, each plant received approximately three gallons of water per watering event. Root stimulator was applied to the treated plants during every watering event. In many cases, sand had filled up the deep pipes rendering them ineffective (Figure 13A). At the Bivouac site mounds the deep pipes did not fill up with sand and could be watered as planned. Individual plants with deep pipes received approximately 0.75 gallons of water per watering event. Each drip line system at the Bivouac site was watered for 30 minutes. As a result, every plant received 2 gallons of water per watering event.

Figure 12. Diagram of Bivouac site. Blue Line- Boundary Green Line- Irrigation Lines Yellow Line- Seeded Areas Red Line- Berms

Table 3. Project Timeline.

Plant survival was monitored twice, on 30 June 1999 for six month survivorship and 4 January for one year survivorship. At the six month monitoring, approximately 100 of the Tubex were removed from those plants where the Tubex had filled up with sand (Figure 13B) or loosened itself from the soil. All remaining Tubex were removed at the one year monitoring. Irrigation devices were left at the site and will be removed at the request of ITAM personnel. After one year, 48 planted shrubs had been run over by military vehicles; 27 at the Bivouac site and 21 at Hill 526.

Results

Hill 526

Overall survival of transplanted shrubs after six months was 84 percent. This is the adjusted total which accounts for 14 plants damaged by vehicles. Overall survival of transplanted shrubs after one year was 77 percent (Figure 14). This is the adjusted total which accounts for 21 plants damaged by vehicles. A breakdown of survival by individual species is shown in Tables 4 and 5.

ANOVA was used to statistically analyze the root stimulator and deep pipe treatments. At the six month monitoring, there was no significant difference in survivorship (p = 0.94) between those plants which received root stimulator (79 percent) versus those that did not (80 percent). There was also no significant difference in survivorship (p = 0.82) between those plants that received the deep pipes (81 percent) and those that did not (80 percent). At the one year monitoring, there was still no significant difference in survivorship (p = 0.09) between those plants which received root stimulator (69 percent) versus those that did not (75 percent). There was also no significant difference in survivorship (p = 0.41)between those plants that received the deep pipes (70 percent) and those that did not (74 percent).

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Figure 13. Problems at Hill 526. A) Sand in deep pipe and B) Sand in Tubex.

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Figure 14. Hill 526 one year after installation. A) Basins in area of less dense vegetation and B) Basins in area of more dense vegetation.

Table 4. Six month survival at Hill 526.

No seedlings were observed in the seeded areas at either the six month or one year monitorings (Figure 15).

Table 5. One year survival at Hill 526.

Figure 15. Hill 526 one year after installation. Seeded area.

Bivouac site

At the six month monitoring, overall survival at the Bivouac site was 86 percent. This is the adjusted total which accounts for 26 plants damaged by vehicles. At the one year monitoring, overall survival at the Bivouac site was 77 percent. This is the adjusted total which accounts for 27 plants damaged by vehicles. A breakdown of survival by individual species is shown in Tables 6 and 7.

Table 6. Six month survival at the Bivouac site.

Table 7. One year survival at the Bivouac site.

Eighty-eight percent of the plants survived that were irrigated by the drip lines at the six month monitoring and 80 percent at the one year monitoring (Figure 16).

After six months, sixty-nine percent of the plants that were planted on the mounds survived (Figure 17A). A chi-square contingency test was used to statistically analyze the location of planting treatment. There was no significant difference (p "e 0.05) between the shrubs planted high on the mounds (58 percent) versus those planted at the base of the mounds (72 percent). After one year, 60 percent of the plants that were planted on the mounds survived. There was a significant difference (p "d 0.05) between the shrubs planted high on the mounds (42 percent) versus those planted at the base of the mounds (78 percent).

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Figure 16. Bivouac site one year after installation. A) Irrigation line and B) Vehicle damage.

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a) b) Figure 17. Bivouac one year after installation. A) Mound planting and B) Hand pitted seeded area.

No seedlings were observed in the seeded areas at either the six month or one year monitorings (Figure 17B).

Discussion and Recommendations

Hill 526

Overall survival of transplanted shrubs at the site was 84 percent after six months and 77 percent after one year. Both survivorship rates exceed the contract required 75 percent survivorship after six months and 60 percent after one year. The basins proved to be an effective method of irrigation. By using a pickup truck, a 180 gallon water tank, a gas powered water pump and a garden hose, six gallons of water could be applied to each basin in about 30 seconds. A caveat for using basins on a restoration worksite is placement. The basins placed in the interior of the worksite proved to be problematic. One person can easily manage 100 feet of garden hose, but beyond that it is a two-person job. We could have parked at the perimeter of the site and with two people dragged an extended garden hose from basin to basin, but it was a quicker process to drive the pickup truck from basin to basin. Compared to military vehicles, the pickup truck is light and it’s tracks are lost in the shifting sands; however, repeated passings undoubtedly would eventually lead to soil compaction, further disturbing the area to be restored. In the future, it is recommended that basins be placed within 100 feet of the perimeter of a worksite. An alternative form of irrigation, such as drip lines, should be used to water the interior of the site.

Many of the basins filled up with sand and other basins will more than likely continue to be so affected. When transplanted shrubs are irrigated, crown rot can also be a problem if wet soil contacts the base of the plant, and was noted on approximately ten shrubs. However, the advantages of the water holding capabilities of basins outweigh such disadvantages as blowing sand and soil-borne plant pathogens, so it is recommended that the basins continue to be irrigated by ITAM personnel for a minimum of one more year. The high survivorship after one year at Hill 526 suggests that the use of basins may be advantageous in areas that have sandy surface soil and a loamy sand subsoil. At another Fort Irwin SERG project called the Bunker site, installed in the spring of 1998, basins were also used. The soil at the Bunker site was noticeably sandier than at Hill 526. The basins at the Bunker site, although irrigated in the same manner, were never seen to hold standing water. At the Bunker site there was a 42 percent survival after six months, compared to the 84 percent survival at Hill 526. It appears that the use of basins in locations where the soil is very sandy, with no loamy sand subsoil, is less effective.

There was no significant difference in survivorship between basin plants that received deep pipes and those that did not. Hill 526 is an area that is sandier than most at Fort Irwin and the deep pipe openings were not covered, leaving them exposed to the elements. When the wind blows hard, as it often does, sand moves around the site at vertical heights several feet above the surface. As a result, many of the deep pipes were quickly filled up with sand, rendering them ineffective. For this reason it is difficult to compare the basins with and without deep pipes. However, seeing as an adequate amount of water could be applied to the basins and a high rate of plant survivorship was achieved without the deep pipes, it is fair to say that the deep pipes were not needed. Furthermore, at the end of the contract period, the deep pipes will have to be removed from those plants that did not survive, thus the deep pipes entail a greater amount of work, not to mention the amount of non-degradable plastic left at the site. The deep pipes will be left for the plants that did survive so as not to disturb the roots. Based on these results, the use of deep pipes is not recommended as a supplemental irrigation device for basin plantings.

The liquid based root stimulator was used to reduce plant shock during transplanting and then used in nearly every watering event thereafter. There was no significant difference in survivorship between those basin plants that received root stimulator and those that did not. Though statistically insignificant, the use of root stimulator actually reduced plant survivorship by eight percent after one year. According to this experiment, the use of root stimulator on desert shrubs proved to be ineffective, and is not recommended as a practical means of increasing transplant survivorship.

Due to the scant amount of rainfall, no seedlings where observed growing in the seeded areas. This includes the seeded area treated with supplemental irrigation applied from the side sprayer of a 4,000 gallon water truck. Results for this treated area are tentative at best because, due to logistic problems, the area was only treated three times throughout the course of the year.

In summary, basins in the proper soil type appear to be a valuable restoration tool for the following reasons: they are easy to build, they require no maintenance, they can accept a large amount of water in a short time and they provide high survivorship rates.

Bivouac

Overall survival of transplanted shrubs at the site was 86 percent after six months and 77 percent after one year. Both survivorship rates exceed the contract required 75 percent survivorship after six months and 60 percent after one year. Overall survival of the shrubs planted along irrigation lines was 88 percent after six months and 80 percent after one year. Although the sites were nearby, plant survivorship was higher at the Bivouac site than at Hill 526. This was probably due to two compounding factors. One, the shrubs at the Hill 526 site were subjected to the adverse effects of sand deposition in the basins. Two, even though the Bivouac plants received only two gallons of water per watering event, compared to the three gallons of water per watering event at Hill 526, the drip irrigation water is applied directly to where it is needed. Also, the slower application rate allows for less evaporation and deeper penetration of water. An added benefit of the drip lines is they can be easily removed from the project upon completion.

Drip emitter systems, however, do not come without problems. Some troubles occurred with emitters becoming clogged with sand or insects, but all clogged emitters on one line could be cleaned out within ten minutes of the start of irrigation. Also, coyotes and rabbits have been known to gnaw on the one-half inch polyethelyne tubing, though this did not occur at this site.

Overall survival of transplanted shrubs on the mounds was 69 percent after six months and 60 percent after one year. Although not significantly different at the six month monitoring, survivorship at the one year monitoring for plants placed on top of the mound was significantly lower (42 percent) than for plants placed at the bottom of the mound (78 percent). Results from the one year monitoring suggest that planting at the bottom of the mound is preferred. Most likely, a greater amount of irrigation water is retained at the bottom of the mound due to mound sand acting as an insulator. Furthermore, the plants at the bottom of the mound are, in part, growing in the native undisturbed soil. Overall, mound plant survivorship was substantially lower than for plants irrigated with drip lines. Drip line plants received more water and were also planted at the native undisturbed soil level. If mounds are to be built as visual and physical barriers, it may be beneficial to plant shrubs either at the base of the mound or, possibly, offset approximately one foot from the mound.

Due to the scant amount of rainfall, no seedlings where observed growing in the seeded areas. This raises the question as to the appropriateness of seeding during years where a La Niña weather pattern is predicted. Although the viability of many desert seeds is often greater than one year, if immediate results are required, it is recommended that direct seeding only be conducted during years where above average rainfall is predicted, such as during an El Niño event.

Tubex that were loosened from the soil by wind were removed from approximately 40 of the drip line plants. Unlike the Tubex at Hill 526, none of the Tubex at the Bivouac site filled with sand. This most probably occurred because the existing vegetation around the Bivouac site slowed down the wind erosion process. Some of the plants at the Bivouac site, as well as Hill 526, have begun to emerge from the top of the Tubex. The decision was made to leave the Tubex on because the effects of herbivory are more devastating than the effects of an altered plant morphology. Past evidence has shown that plants cannot always recover from herbivory, but they can usually reform to their natural morphology once the Tubex is removed. There is a plastic mesh-type photodegradable plant protection device available that, ideally, would limit herbivory to the core of the plant, yet allow the plant to grow through the mesh in a more natural state. This protection device is recommended for testing in future experiments.