Soil Ecology and Restoration Group

REVEGETATION AND EROSION HILL 760

last update August 2, 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 surface soil particles 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 applied. Restoration activities need to be integrated with training 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. The Hill 760 project consists of two parts. The first part explores and implements methods to slow down the action of erosion in washes.The second part creates a protected area on an alluvial slope at the base of Hill 760 by building berms and planting vegetation around the perimeter to create a physical and visual barrier. These barriers, though not preventing all vehicles from entering the protected region, do discourage vehicle traffic through the central portion of the project site. Inside this area, along with planting vegetation, the topography was slightly modified to encourage volunteer plant establishment.

Site Description

Hill 760 is located at UTM grid coordinates 549000 east by 3911500 north (Figure 1). It is classified as site number 66 in the LRAM Erosion Control Plan. Located in the central corridor of the installation, it is approximately 26 kilometers to the northeast of the cantonment in Training Area “S”. The worksite at Hill 760 is approximately 700 meters in elevation. The site is located in the northern foothills of the Tiefort Mountains and consists of a hill with an alluvial slope at the base that has a north by north-west aspect and a 5-7 degree slope (Figure 2). The soil is a sandy texture with moderately compacted soil structure. The site lies in a creosotebush scrub vegetation zone dominated by Larrea tridentata and Ambrosia dumosa (Table 1).

Table 1. Species found growing at Hill 760.

A)

B)

Figure 2. Hill 760 before installation of project. A) Uphill view of Hill 760 and B) Close up of alluvial slope.

The revegetation site lies in a triangular area of six acres just to the northeast of Hill 760. Small washes occur in this area, but sheet erosion is the predominant erosion process. The site is surrounded by vehicle trails which were marked both visually and physically to prevent any further destruction by vehicular traffic resulting from the desire to take short cuts. The site is a heavily used maneuver zone with many vehicle trails criss-crossing the area. The site also has a trail which runs perpendicular to the various washes coming down off the hill. After heavy rains (more than 0.5 inches at a time), flowing water rushes down these washes. It is feared that gully erosion will begin to occur as the water from the washes moves across the trail.

Materials and Methods

Four catch dams/retention basins were constructed perpendicular to the washes of the hill, four meters in length with a depth of one meter. Each catch dam/basin had a different erosion control treatment: punched broom corn; coir netting; jute netting; and curlex netting (Figures 3 and 4).

Although the contract required that 500 shrubs be planted, the actual number of shrubs planted at the work site for the alluvial slope area in December 1998 was 523 (Table 2). An additional 83 shrubs were planted in the fall of 1999, bringing the total number of shrubs planted to 606. The complete list of shrubs planted is shown in Table 3.

Table 2. Number of nursery grown shrubs originally planted at Hill 760.

A)

B)

Figure 3. Erosion control catchdam/basins at installation. A) Broom corn and B) Curlex.

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B)

Figure 4. Erosion control catchdam/basins at installation. A) Coir and B) Jute.

Table 3. Total number of nursery grown shrubs planted at Hill 760.

Species Ambrosia dumosa Atriplex canescens Atriplex polycarpa Ephedra nevadensis Hymenoclea salsola Isomeris arborea Larrea tridentata TotalNumber 127 90 64 20 84 52 145 606

Using the ITAM tractor’s ripper bar, the surface topography was modified in the open areas between the trails to enhance revegetation from the existing seed bank and reduce erosion. In most places, the soil was heavily compacted and the bars ripped no more than three inches deep. In the sandy areas, the bars ripped to a depth of 10 inches.

Protective zones were constructed around the ten largest Larrea tridentata shrubs (Figure 5). This was accomplished using the ITAM tractor and hand tools to build three mini-berms one meter long and one-half meter high. The mini-berms where placed approximately 15 feet away from the Larrea tridentata shrubs in a triangular pattern. One Tubex situated on top of each mound served as a visual aid to inform military personnel that shrubs had been planted around the existing Larrea tridentata plants. Research suggests that Larrea tridentata can act as a nurse plant because its roots bring water from deep down in the soil to the surface. Contrasting this effect, it has also been suggested that Larrea tridentata secretes allelopathic compounds to discourage the growth of other plants nearby. To test the balance of these theories, 10 native seedlings were planted in areas of relatively little disturbance around each Larrea tridentata shrub (total 100). Plants were placed at different distances away from the Larrea shrub to determine any effect on survivorship. Two plants were placed three feet away, two plants five feet away, two plants seven feet away, two plants nine feet away and two plants eleven feet away (Figure 6). A total of 50 Ambrosia dumosa and 50 Atriplex polycarpa were planted. Each shrub was installed with a three inch wide by two foot long pipe and surrounded by a mounded mini-catchment for irrigation. A three inch tube can hold 0.38 gallons of water per linear foot, thus, each deep pipe holds0.76 gallons. As a result of filling the pipe and overspilling into the mini-catchment, every plant received 1-2 gallons of water per watering event. All installed shrubs had been grown in 6 inch diameter by 15 inch long “half-high” containers. Tubex protective devices were not installed due to the large leaf canopy of these shrubs.

Figure 5. Larrea tridentata protection zones at installation. A) Planting shrubs at different distances from host plant and B) Overview of protection zone surrounded by three visual barrier mounds.

Figure 6. Diagram of planting around Larrea tridentata shrub.

In each of the three corners of the site, one large revegetation berm was created with two 20m long arms at a height of approximately 0.75m (Figures 7A, 8 and 9). Each large berm was planted with an average of 105 nursery grown shrubs (317 total). The shrubs were grown in three inch plant bands, four inch citrus containers and six inch “half-highs”, with the majority of the shrubs in the “half-highs”. Once again, all species grown in the large “half-high” shrubs did not receive Tubex after planting, whereas the shrubs grown in the smaller containers did. The shrubs were spaced one meter apart on both sides of the berm about 20 centimeters above the base. A one-half inch flexible polyethylene tubing drip irrigation system was installed on the large berms. To test if irrigation rates effect survivability of planted shrubs, two berms were provided with 2 gallon per hour button emitters and the third with 4 gallon per hour flag emitters, with one emitter per plant. For every watering event, each irrigation line was watered for 30 minutes. Consequently, the shrubs with the 2 gallon per hour emitters received one gallon of water per watering event and the shrubs with 4 gallon per hour emitters receive two gallons of water.

Along the edges of the main trails 20 smaller two armed berms were created with each arm being 1.5m wide x 4m long x 1m tall (Figures 7B and 10). In effect, “v-shaped” catchments were created with the open end facing uphill to capture any sheet flow that might occur as a result of a significant amount of precipitation. Each catchment was planted with five nursery grown shrubs (100 total) on the upslope side of the berm. The shrubs were grown in four inch citrus containers and six inch “half-highs”, with the majority of the shrubs in the “half-highs”. Once again, the large “half-high” shrubs did not receive Tubex after planting. The smaller citrus container grown shrubs, however, did receive Tubex. For irrigation, three inch non-perforated flexible plastic drainage pipes were installed within each berm. A hole was punched in the pipe using an awl wherever a shrub was planted. A three inch tube can hold 0.38 gallons of water per linear foot, thus, each berm receives 9.5 gallons per watering event. As a result, every plant received nearly 2 gallons of water per watering event.

A diagram of the locations of each installation technique at Hill 760 is shown in Figure 11.

Figure 7. Vegetation Mounds Construction Plan.

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B)

C)

Figure 8. Revegetation berms at installation. A) Building the berms, B) Gas powered hand auger and C) Prewatering the holes.

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B)

Figure 9. Revegetation berms at installation. A) Planting the shrubs and B) The finished product

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B)

Figure 10. Catchments at installation. A) Flexible pipe is laid out with rebar marking where future plantings will go and B) Flexible pipe after being covered with soil.

Figure 11. Diagram of the locations of installation techniques.

Project Monitoring and Maintenance

The project schedule for work completed at Hill 760 is shown in Table 4. All installation work was completed by 31 December 1998. Watering was conducted using a contracted water truck commencing one week post-planting and continued at three to four week intervals. With the exception of a precipitation event in July, the watering schedule remained uninterrupted through the one year maintenance period. Shortly after planting, a vehicle damaged 13 planted shrubs on one of the large berms (Figure 12 A) and three more on one of the catchments. SERG personnel replanted 15 Ambrosia dumosa shrubs at the site to make up this difference. A total of 52 plants were damaged by vehicles after year one.

The unprotected large shrubs soon proved to be a target for herbivory by rabbits (Figure 12 B). Two months after planting, approximately 50 plants had been reduced to stubs. To prevent further damage to those severely herbivorized plants, Tubex plant protectors were installed. The unprotected large shrubs were also susceptible to erosion caused by settling of the berms. On two separate occasions, eight man-hours were dedicated to removing soil from around the base of these plants. The basins around plants with Tubex required no more resculpting until the Tubex was removed. In May and June 1999, the exotic plant Salsola tragus began to appear (Figure 12 C). Those found growing near planted shrubs were removed by hand.

A)

B)

C)

Figure 12. Problems occurring at Hill 760. A) Thirteen plants damaged by vehicle, B) Herbivory of Ambrosia dumosa by rabbits and C) Salsola tragus growing on revegetation berm.

The catch dam/basins and plant survival were monitored as required at six months after planting on 30 June 1999. Due to the low plant survivorship at six months, an additional 83 shrubs were planted in the fall of 1999 from 14 September through 9 November. One year monitoring of plant survivorship occurred on 22 December 1999. The irrigation devices were left intact and Tubex were left on the smaller plants. The Tubex will be removed in the spring of 2000 and the irrigation devices will be removed at the request of ITAM personnel.

Table 4. Project Timeline

Results

The catch dam/basins are in good shape. No degradation of materials has occurred after one year (Figures 13 and 14). Due to the small amount of precipitation, the catch dam/basins were not fully tested for their erosion control capabilities. Consequently, analysis cannot be adequately completed in the one year report. Based on the installation of 500 plants, as required by the contract, overall survival of planted shrubs at the site after six months was 65% (Table 5). Sixteen plants were damaged by vehicles, with 13 occurring on the large revegetation berms and three occurring in the catchments. Due to the low six month survivorship, 83 additional shrubs were planted at the site in the fall of 1999. Based on the contract required 500 plants, overall survival of planted shrubs at the site after one year was 71% (Figures 15 and 16, Table 6). Throughout the site, a total of fifty-two plants were damaged by vehicles after one year.

Table 5. Overall plant survival six months after installation.

Figure 13. Erosion control catchdam/basins one year after installation. Curlex.

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B)

Figure 14. Erosion control catchdam/basins one year after installation. A) Coir and B) Jute.

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B)

Figure 15. Revegetation berms one year after installation. A) Close up of berm plantings and B) Overview of berm plantings.

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B)

Figure 16. Hill 760 one year after installation. A) Catchment and B) Larrea tridentata protection zone.

Table 6. Overall plant survival one year after installation.

Adjusted total 448 316 71 (% contract success)

Survival of plants in the Larrea tridentata protection zones after six months was 53 percent. ANOVA statistical analysis was used to investigate the effect of plant placement. Although the highest survival was at the seven foot distance, overall there was no significant effect of distance of planting from the host Larrea tridentata plant (p = 0.44). There was a trend (p = 0.11) suggesting that Atriplex canescens (62% survival) grew more successfully than Ambrosia dumosa (46% survival). Not counting the seven surviving replants, survival of plants in the Larrea tridentata protection zones after one year was 39 percent. Although the highest survival was still at the seven foot distance, overall there was no significant effect of distance of planting from the host Larrea tridentata plant (p = 0.25). At the end of one year, the trend suggesting that Atriplex canescens grew more successfully than Ambrosia dumosa was no longer apparent. Though not significantly different (p = 0.27), Ambrosia dumosa (44% survival) had a higher survival rate than Atriplex canescens(33% survival). Results are shown in Figure 17.

Figure 17. Summary of Larrea tridentata protection zone plantings. Light stippled columns are results from six months after installation and dark stippled columns are results from one year after installation.

A chi-square contingency analysis was used to compare plant survivorship based on the different irrigation techniques used at the site and a significant difference (p £ 0.05) was found. Overall survival at the six month monitoring (Table 7) was 68 percent for the drip irrigation on the large revegetation berms, significantly higher than the 62 percent on the flex pipe irrigated catchments and the 53 percent on the deep pipe irrigated Larrea protection zone plants. After one year, there was still a significant difference (p "d 0.05) in survivorship of plants based on irrigation technique (Table 8). This time, however, the flex pipe irrigated catchments had a significantly higher plant survival rate of 69 percent, followed by 57 percent on the irrigation lines and 47 percent on the deep pipes.

For the drip line irrigation experiment, chi-square contingency analysis was used to compare the two gallon per hour emitter to the four gallon per hour emitter survival results six months after installation. There was no significant effect (p ³ 0.05) on survivorship with 71 percent for the two gallon per hour emitters and 60 percent for the four gallon per hour emitters. One year after installation, however, the two gallon per hour emitters had a significantly (p "d 0.05) higher survivorship of 61 percent versus 46 percent for the four gallon per hour emitters.

Table 7. Comparison of irrigation techniques at Hill 760 six months after installation.

Table 8. Comparison of irrigation techniques at Hill 760 one year after installation.

Discussion and Recommendations

The catch dam/basins are in good shape with no degradation of materials occurring after one year. Due to the small amount of precipitation, the catch dam/basins were not fully tested for their erosion control capabilities. The catch dam/basins did have a tendency to trap dried Salsola tragus plant species. If not removed, the catch dam/basins could act as a nursery for new S. tragus seedlings. Not only would this growth of S. tragus compete with the growth of native plant seedlings, it may impede the effectiveness of the erosion control materials installed on the catch dam/basins. Based on this experiment, it is difficult to give a strong recommendation, either positive or negative, for the use of catch dam/basins as an effective erosion control device.

Overall contract success survivorship of planted shrubs at the site after six months was 65 percent, with actual surviorship being 61 percent. The contract required a 75 percent survival rate after six months. Replanting of shrubs was required, so 83 additional shrubs were planted in the fall of 1999. The cooler temperatures at that time of year provided for a more appropriate planting environment than in the hot summer months. Overall contract success of planted shrubs at the site after one year was 71 percent, with actual survivorship being 52 percent. The contract required a 60 percent survival rate after one year.

The reasons for the lower than expected survival rate at both the six month and one year monitoring events derive from the compounded effects of several different situations. First, the weather patterns of La Niña created a drier than normal winter. No winter rainfall occurred before planting to moisten the soil and only in late winter/early spring, after planting was completed, did occasional rainfalls occur. However, these rainfalls were light and scattered and were not enough to warrant a break in the manual watering schedule. Not until July 1999 did a significant amount of precipitation occur, allowing for a break in manual watering schedule. Unfortunately, no rainfall occurred for the remainder of the contract. Natural precipitation is more beneficial than manual watering because it contains atmospheric nutrients, which are beneficial for the plants, and wets the entire soil surface, lessening the effects of wicking by dry soil that commonly occurs with manual spot watering.

Second, due to continual heavy use as a training area, the soil was heavily compacted in many of the areas which were planted. This most probably hindered the roots from spreading out of the loose soil in the planting holes into the compacted native soil, thereby decreasing the chances for plant survival.

Third, herbivory by rabbits was a serious problem on plants not protected by Tubex. It was thought at the time of planting that the larger, six inch half high container plants, would not be susceptible to heavy herbivory by rabbits. The trunks on many of these plants were one-half inch in diameter and the leaf canopy was larger than the size of the Tubex. The effects of La Niña, however, led to a decrease in the nutritional value of the natural shrubs from the lack of rainfall, causing the rabbits to concentrate on the more nutritious, irrigated shrubs of the project. Within two months of planting, approximately 50 of the shrubs, particularly Atriplex sp. and Ambrosia dumosa, were chewed down to a stub. At this point, Tubex was applied to the plants. Since nearly all of the stubs still had an active cambium layer, it was believed that because of the large size of the half-high container rootball, the plants would have enough stored energy to generate new growth. This new growth would then be protected by the Tubex. Unfortunately, this was not the case, and many of the stubs developed no new growth. In the future, it is recommended that larger shrubs planted in areas of known herbivory, or during La Niña conditions, be pruned to allow the Tubex to fit over the leaf canopy, or a larger, alternative form of herbivory protection, such as wire cages or Treepees, be used.

Fourth, the validity of planting on the mounds remains suspect the shrubs planted at the lower portion of the berms were covered up by soil as the berms settled down. For the smaller plants protected by Tubex, the settling soil was prevented from covering up the base of the plants; however, for the larger plants not protected by Tubex, the soil buried the base of the plants to a depth ranging from one to five inches. This soil contact with the plant, combined with irrigated water, caused crown rot on approximately 20 of the shrubs, resulting in death.

At the Bivouac site, where installation occurred at the same time as the Hill 760 site, a mound planting experiment was conducted. Results showed that overall survivorship for those mound plants after one year was 60 percent. There was a significant difference (p "d 0.05) between shrubs planted high on the mounds (42 percent) versus those planted at the base of the mounds (78 percent). Those shrubs which were planted on flat ground with irrigation tubing had an 80 percent survivorship. All plants received Tubex. At the Hill 760 site, for one of the two gallon per hour drip emitter irrigated berms, all the shrubs received Tubex and had an 80 percent survival. This parallels those results for the base of the mound and the flat ground plantings found at the Bivouac site. The two gallon per hour drip emitter irrigated berm at Hill 760, in which approximately 70 percent of the plants received Tubex, had a survivorship of 50 percent. Thus, it appears that the use of Tubex for shrubs planted at the base of the mounds raised the survivorship to 80 percent. The Tubex apparently provides temporary protection against the adverse affects of mound plantings, such as settling soil and crown rot, which can lead to lower survivorship.

Once the Tubex are removed from these mound plants, the mortality rate will most likely increase slightly. Such was the case with the Bunker site mound shrubs which were planted in spring 1998. Plant survivorship was 70 percent at six months. All Tubex were removed at the six month monitoring and, within one month, survivorship plummeted to approximately 40 percent. Although Tubex secures adequate results on mound plants in terms of contract success, if results are substandard after their removal then other planting techniques need to be explored. For instance, planting on or at the base of mounds may be more effective after the mounds have settled. Another technique, as practiced in phase two of the Hill 760 project, was to place the shrubs two feet away from the base of the berms. Results from that experiment show an 85% survival after six months. Continued monitoring of mound plantings versus flat land plantings after Tubex removal will show results of the long term effectiveness of mound plantings.

Only 53 percent of the Atriplex canescens and Ambrosia dumosa species planted in the Larrea tridentata protection zones remained alive after six months. None of these plants were protected with Tubex, so they were all subject to herbivory and dessication. Results of the distance planting from the Larrea tridentata nurse plant experiment proved to be inconclusive at the six month monitoring. Two trends, however, were beginning to show. Atriplex canescens had a higher rate of survival at 62 percent than the Ambrosia dumosa species at 46 percent; and, plants placed at seven feet from the leaf canopy of the Larrea tridentata nurse plant had the highest survival rate.

After one year, only 39 percent of the Atriplex canescens and Ambrosia dumosa species planted in the Larrea tridentata protection zones remained alive. Atriplex canescens had a lower rate of survival at 33 percent than the Ambrosia dumosa species at 44 percent, thus reversing the trend found at the six month monitoring. Results of the distance planting from the Larrea tridentata nurse plant experiment proved to be inconclusive at the one year monitoring, however, as with the six month monitoring, those shrubs planted seven feet away from the host plant had the highest survivorship at 56 percent. Perhaps the plants placed closer to the nurse plant suffered from the secreted allelopathic compounds and the plants placed farther from the host plant did not benefit from any extra moisture brought up to the soil surface by the host plant. Based on this experiment, it is recommended, though tenuously at best, that shrubs be planted seven feet away from the Larrea tridentata host plant. Although only two species were used in this experiment, Ambrosia dumosa, a species most often found in conjunction with Larrea tridentata, responded more favorably than Atriplex canescens when planted near the host plant. It is recommended that Ambrosia dumosa be used in future similar planting situations.

Total survival on the large revegetation berms at six months was 68 percent. Although not statistically significant, the survivorship for the shrubs watered with two gallon per hour emitters was 71 percent, while 60 percent for plants with four gallon per hour emitters. Total survival on the large revegetation berms one year after planting was 57 percent. Although not statistically significant at the six month monitoring, survivorship for the shrubs watered with two gallon per hour emitters was significantly higher at the one year monitoring, with 65 percent survivorship for the two gallon per hour emitters and 46 percent for plants with four gallon per hour emitters. The difference might possibly be attributed to the fact that on one of the two gallon per hour emitter revegetation berms, many of the plants were smaller and protected with Tubex, thus raising survivorship to 80 percent. The other two gallon per hour emitter revegetation berm had the same proportion of large plants, unprotected by Tubex, as the four gallon per hour emitter revegetation berm, and produced similar results in survivorship, 50 percent for the two gallon per hour emitters and 46 percent for the four gallon per hour emitters. Based on these results, the four gallon per hour emitters produced slightly lower survivorship numbers. However, logistically, the four gallon per hour emitters are less expensive and easier to clean when clogged, thus making them be better suited for field work. It should be kept in mind, however, that the increased moisture may contribute to increased herbivory of unprotected transplanted shrubs as well as increased exotic plant growth. Although these occurrences were not be documented in this study, it is recommended that the four gallon per hour emitters be used for the drip emitter system.

Overall, the irrigation systems produced significantly different survivorships. At the six month monitoring (p "d 0.05), the drip lines had significantly higher survivorship (68 percent) than both the flex pipes (62 percent) and the deep pipes (53 percent). At the one year monitoring (p "d 0.05), the flex pipes had significantly higher survivorship (69 percent) than both the drip lines (57 percent) and the deep pipes (47 percent). The one year results, however, were skewed in favor of the flex pipes due to the higher percentage of fall replants that they received. Thus, the six month results are a more accurate indicator of the effectiveness of irrigation techniques. The drip lines could water more plants in a shorter period of time than the flex pipes and deep pipes. The contracted water truck, with one operator, could water 300 plants in the time it took two personnel, a pickup truck, a 180 gallon water tank and a gas powered water pump to water both the 100 catchment plants and the 100 Larrea tridentata protection zone plants. The drip lines also have another advantage over the flex pipes and the deep pipes in that once the transplanted shrubs are established, the lines can be easily removed. The flex pipes and the deep pipes will probably have to remain in the ground for fear of damaging the plant’s roots upon removal. At this point, drip line irrigation appears to be the most cost efficient and successful irrigation procedure for use at Fort Irwin.