Soil Ecology and Research Group last update October 29, 2002

REVEGETATION AND EROSION CONTROL
MANIX TANK TRAIL 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 warfighting skills in a realistic environment. Ground maneuvers, live fire exercises, and force on force activities damage the native vegetation of the Mojave creosote/bursage scrub habitat, which in turn causes dust and erosion problems.

The Integrated Training Area Management (ITAM) Program is responsible for land management to identify and repair critical areas on the post. Projects proposed by the ITAM program combine both revegetation and erosion control to mitigate 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 these tiny particles are a health risk to people on the base and to surrounding communities with long-term exposure.

The purpose of this project is to reduce the amount of wind erosion occurring at an intersection of two tank trails. Fieldwork includes revegetation of the fifteen acre site with greenhouse-grown native shrubs as well as direct seeding with locally collected seeds. In addition to re-establishing vegetation on the site, the project is designed to better delineate the surrounding trails and protect existing vegetation.

SITE DESCRIPTION

The Manix Tank Trail intersection is located 7.8 kilometers from Cantonment at UTM Grid Coordinates 533200 easting 3895600 northing. Traffic through this area has created a large, sparsely vegetated area where the trails intersect (Figure 2). Situated between the major trails, the site encompasses approximately 15.1 acres.

The long-term goal of this project is to increase safety at a heavily trafficked intersection. One major focus is to designate trails clearly and prevent vehicles from cutting corners when traveling from one trail to the next. Keeping vehicles on designated trails reduces the rate of erosion and further loss of plant cover. Planting native shrubs not only redefines the trails, but also aids in controlling dust levels by trapping fine particles. Planting around the perimeter of the site offers protection for existing plants to enhance natural recovery. Pitting the soil surface reduces wind speeds across the site and creates microsites to trap blowing silt, seeds, and organic litter thus increasing the long-term productivity of the area.

Site-specific information includes soil type and species composition. The soil is of the Arizo series and consists of a gravelly fine sand and a gravelly fine loam (See Appendix A for a complete soil series description). Native vegetation in the area includes Larrea tridentata (Creosote bush), Ambrosia dumosa (Burro-weed), Atriplex polycarpa (Allscale), and Hymenoclea salsola (Cheesebush). The exotic and invasive Salsola tragus (Tumbleweed) is also prevalent on the site. The slope across the site is minimal, however it is located at the lower end of a large watershed draining toward nearby Langford Dry Lake.

Figure 2. West view of site photographed prior to implementation.

METHODS AND MATERIALS

Planting

Irrigation Line
Along the edges of several major trails, a total of 638 native shrubs were planted along drip irrigation lines. Irrigation tubing was laid out in 20-meter lengths with 15-meter spacing between each line. The lines were situated at an approximate 100° angle from the main road (Figure 3). A total of 2130 meters of drip irrigation line was used to create a total of 64 planting lines.

Once the lines were laid out, greenhouse grown native shrubs were planted every 1.7 meters with about ten plants per 20-meter length. The plants were grown in different types of containers including six inch diameter half-highs (fifteen inch lengths of PVC pipe), and paper plant bands of variable height and diameter (Figure 4). A one-man auger was used to dig the holes in which the plants were installed (Figure 5).

Figure 3. Manix site restoration design.

Figure 4. Two different sizes of plant containers used during planting.

Figure 5. Holes were dug with a one-man auger along irrigation line.

Each hole was pre-watered with 2 gallons to increase soil moisture content in the root zone of the plant. Three additional gallons were used during planting. After each plant was installed, circular basins were built around the base to hold water during irrigation. One 16 liter per hour flag emitter was punched into the line at each plant basin. Plant protectors, including both Treepees and Tubex Shrubshelters, were installed on each plant (Figure 6).

Figure 6. Completed irrigation lines with circular basins and plant protectors.

Basins
Along the western boundary of the site, twenty plant basins were constructed with 2-meter diameters (Figure 3). Three plants were planted in each basin. These basins hold approximately ten gallons and were hand-watered during the maintenance period of the project. Plant protectors, including both Treepees and Tubex Shrubshelters, were installed on each plant.

Perforated Pipe
Five-meter long sections of perforated pipe were laid in three low-lying depressions located throughout the site (Figure 3). These depressions are a result of past engineering activities and pose vehicle hazards when left unfilled. Three sections were laid in the northern depression, with a five-foot spacing between each pipe. The pipes were then covered using the ITAM tractor, leaving each open end of pipe above the soil surface. Along each section of pipe, five plants were planted over the buried pipe for a total of 15 plants. In the central depression, 4 sections of perforated pipe were installed in the same manner, with 5 plants planted along each pipe (Figure 7). Only one five-foot length of pipe was installed in the southern depression and three plants were planted. Plant protectors were also installed on each of the 38 plants.

A total of 736 plants were planted between 4 January 2001 and 1 February 2001. Table 1 lists the species composition of the site. The site has been watered on a monthly basis since May 2001, with each plant receiving 1-2 gallons per watering trip at all locations. Irrigation lines have been repaired as needed. Appendix C outlines the watering and maintenance schedule for the Manix intersection site.

Figure 7. Perforated pipe being installed in the central hollow.

Table 1.
Species planted at the Manix site.

A seven-acre portion of the site was designated for direct seeding with two different surface treatments (Figure 3). The northern section encompasses about three acres and was to be treated with crimped straw. The southern section encompasses four acres and was to be pitted with a disc pitter. After the surface preparations were completed, the area was seeded.

Several attempts to crimp straw were made in the northern section. On 31 January 2001, a modified planter was used to insert vertical straw bunches into the soil (Figure 8). Difficulties were encountered and site preparation was delayed until 12 December 2001, when the entire site was seeded. On 12 December 2001, straw was laid down and shovels were used to push the straw into the ground. This attempt failed and led to a decision to first broadcast the seed and then cover it with straw. Dirt would then be thrown on top of the straw to hold it in place. The section was seeded on 14 December 2001, and straw was strewn over half of the three acres. Throwing a light layer of soil on top of the straw was ineffective and the idea was abandoned. A fourth attempt to secure the straw was made on 19 December 2001. Planter bars were used to create rectangular depressions in which the straw was placed. The hole was then filled with soil, leaving a single tuft of vertical straw mulch. With approval from Ruth Sparks, ITAM Project Coordinator, the straw treatment was abandoned after this final failed attempt.

The four-acre area directly south of this section was pitted with a cut-out disk pitter towed behind a vehicle in January 2001(Figure 9). The pits and furrows created by the pitter had been smoothed out by wind and rain by the time the section was seeded, so pitting was repeated on 12 December 2001. Pitting
was done by hand, using shovels to create a checkerboard-like pattern of mounds and depressions. The section was then seeded on 14 December 2001.

Approximately 142 bulk pounds of seed mix was applied to the entire seven-acre portion of the site. The seed mix consisted of a variety of native annuals and perennials (see Appendix B for species composition). Before it was sown, the seed was mixed with about 140 pounds of cactus mix to create a 1:1 ratio and provide a more uniform distribution. The mix was hand-broadcast over the seven acres.

RESULTS

Planting

A plant count was conducted on 31 July 2001, and the overall survival rate for transplants after six months was 87%. This figure represents an adjusted total, excluding two plants killed by vehicle damage. A second plant count was conducted on 6 February 2002, and the overall survival rate for transplants after one year is 72%. An adjustment was made for five plants that were killed by vehicles over the year.

Survival rate by individual species is listed in Table 2. All of the species had a decline in survival rate between six months and one year. Atriplex canescens has a survival rate for six months in excess of 100% due to error in identification during the survival count. Ephedra nevadensis had a very sharp decline in survival rate, from 100% to 22.2%. Brickellia incana also had a rapid decline from 92% to 52%.

Table 2.
Percent survival at Manix site by species.

Basins
The use of basins as a watering treatment has been very successful. The survival rate of basin plants after six months was 90%. After one year, the survival rate declined to 78%.

Evidence of traffic through the site appears to occur more frequently in the section containing plants in basins. Grouping plants (three plants within a two-meter diameter circle) does not create as much of a visual deterrence as other treatments.

Perforated Pipe
Transplants planted along perforated pipe have also shown good survival rates. The survival count taken after one year of growth reflects a survival rate of 76%. An error was made in taking the six-month survival count, and survival was not distinguished based upon watering treatment. Results from perforated pipe plants were included in the results for irrigation line plants, therefore no quantitative comparison between the six-month and one-year counts can be made.

A noticeable result of the perforated pipe treatment is the surge of invasive weeds (such as the native Ambrosia psilostachya and the exotic Salsola tragus) in the area surrounding the pipes.

Seeding

All of the techniques attempted at securing straw in the northern section of the seven seeded acres failed. The straw was quickly blown off of the site (Figure 8).

Figure 8. Wind has carried away the majority of the straw in the northern section.

The southern, pitted section has been very successful. After mechanically pitting the four acres in January 2001, many volunteer seedlings have sprouted and new vegetation is establishing. Although the pits did not hold their shape for long, they did serve their purpose. Pitting the area again in December 2001 has helped keep the sown seeds on the site (Figure 9). Hand pitting on this occasion preserved the seedlings that had established over the prior eleven months.

Figure 9. Pits have been effective in keeping some seed on the site.

Almost two months after the seeding was completed, observations indicate that the majority of seed scattered on the site has been carried away by wind gusts. In the northern section, nearly all of the straw and seed was carried away. Seeds from a few species whose seeds are small and dense have remained within the pits in the southern portion. Those seeds that are larger and wind-dispersed were carried away very quickly. Ravens were also observed eating seed.

CONCLUSIONS AND RECOMMENDATIONS

Planting

The overall survival rate for plants on irrigation line after one year (70%) suggests that this watering technique is efficient and effective for large-scale projects. Numerous problems were encountered in delivering equal amounts of water to each transplant. Had this not been the case, survival rates may have been higher.
Typically, watering each plant with two gallons every month is sufficient to establish the plant without creating an unrealistic and unnatural situation for the plant. The objective is to supply enough water for establishment while encouraging root development so that the plant begins to take advantage of natural soil moisture. Providing container plants with too much water could be detrimental to long-term survival when they are no longer maintained. The plants at Manix intersection did not get watered for nearly four months after they were planted. Although the 1.35 inches of rainfall received in February 2001 was sufficient to postpone irrigation until the following month, there was not sufficient rainfall in March and April (0.81 inches) to postpone irrigation until 23 May 2001, the first time the site was watered following planting. This did not appear to drastically effect survival rates after six months; survival rates on irrigation lines at this time ranged from 80% to 96%. Although lack of irrigation was not catastrophic in this case, it is not recommended to delay initial irrigation beyond one month after the final planting date.

The decline in survival rate that occurred after the six month survival count was taken suggests that the plants did not receive enough water from 31 July 2001 to 6 February 2002. This is more than likely due to having improper emitters and improper irrigation line length. Many of the flag emitters installed during planting were too slow, allowing only 2 liter per half-hour of watering, while others allowed 8-liters. This mistake was not noticed until 2 July 2001. Lack of supplies delayed replacing the 4 liter/hr emitters with 16 liter/hr emitters until 5 September 2001. Sixteen-liter per hour emitters deliver just over 2 gallons of water per plant in one-half hour. These should be used whenever possible to ensure enough water reaches the plant in a timely manner. The length of the line also affects water distribution. Lines one and three were very long lines, containing over 100 plants each. On October 17, 2001 these two lines were divided
into four shorter lines, increasing the water pressure and allowing water to reach all of the plants equally. When spacing plants six feet apart, no more than 100 plants should be planted along any length of line to ensure equal distribution to all plants. Perhaps if these mistakes had not been made or were corrected immediately after planting, each plant would have received two gallons on a monthly basis and survival rates would be higher.

Basin plants had a higher survival rate (78%) than plants along irrigation lines (70%) after one year. This may be due to more consistent watering. Hand-watering forces personnel to give individual attention to each basin, as they must be watered one at a time. Plants along irrigation line, on the other hand, are watered all at once. Repairing leaks in drip line, replacing emitters, and adjusting emitters to get proper flow can take a great deal of time. By the time personnel have checked the entire line for proper flow and made any necessary repairs, it is generally time to disconnect the line from the water source. Plants closer to the connector that were checked first may have received the two gallons intended, but the plants at the end of the line may have only received half the amount. Irrigating plants within basins eliminates this chance for unequal water distribution.

The benefits of hand-watering would indicate the use of basins in future
projects. Basins do, however, have limitations on scale and purpose of the restoration site. On several occasions, tire marks were found running through the section of the site containing basins. Spacing plants in groups of three with twenty or more feet between each group does not create as much of a visual deterrence as if the plants had been spaced out one every six or seven feet along irrigation line (Figure 10). The use of basins alone as a watering treatment is not recommended for defining boundaries. Perhaps using basins in conjunction with irrigation line or an earthen berm would be appropriate for establishing boundaries.

Figure 10. Basin spacing (left) does not create as much of a visual deterrence as irrigation line (right) in defining a boundary.

The drawbacks of hand-watering also limit the use of basins. Watering by hand generally takes more time than watering along irrigation line. On a small-scale project, this would not be a very big problem. On a large-scale project with a thousand or more plants, this would be a much greater concern. Also, proximity of the basins to a trail is very important. Hand-watering requires that the water source be constantly moving. Running a hose from the water source to a closely placed basin is quick and efficient. If the basins are placed a distance back from the trail, buckets must be filled and carried back and forth from water source to basins, making irrigation much more inefficient. At Manix Intersection, basins were situated close to a trail, making hand-watering very quick and easy.

The sections of perforated pipe did well with an annual survival rate of 76%. Although this irrigation treatment has a higher percent survival than plants along irrigation line, a direct comparison cannot be made due to different species composition.

Table 3 lists the species composition of plants planted along perforated pipe. Four plants were unidentifiable at the time the data was taken and are listed as Unknown in the table. The majority of shrubs planted were Atriplex polycarpa and Larrea tridentata. These species have proven to be more hardy species at Manix intersection, with overall survival rates of 85.5% and 84.3% respectively. This undoubtedly increased the probability for the species to do well with the perforated pipe treatment. Success can not be solely attributed to type of irrigation.

Table 3.
Species composition along perforated pipe.

One undesirable effect of this supplemental watering method is an increase in the population of weed species. Compared to the rest of the site, the areas surrounding the perforated pipe treatment have become heavily populated with native and exotic annuals, such as Ambrosia psilostachya and Salsola tragus (Figure 11). The surge of annuals utilizes water and nutrients which may adversely affect availability for transplanted perennial shrubs.

Figure 11. Ambrosia psilostachya and Salsola tragus have inhabited the area surrounding the perforated pipes.

The increase in the population of Ambrosia psilostachya and Salsola tragus may be attributed to the soil disturbance that occurred during installation of the perforated pipe. The soil pushed back over the perforated pipe by the ITAM tractor was loose, allowing for easy rooting of annual seedlings. Water infiltrates this soil more readily than the surrounding heavily compacted soil. The micro-topography was also altered, leaving mounds over the buried pipes. This acted as a trap for windblown seeds.

One other concern with perforated pipe treatments is the removal of the plastic pipe once plants have established themselves. Because the pipes are located directly in the root zone of the plant, pulling the pipe out could potentially damage the root structure of the plants. In the past, the exposed part of the pipe has been cut off at the soil surface rather than removing the entire pipe. Perhaps biodegradable tubing, such as soaker tubes made of recycled rubber, would be a better alternative and should be tried in the future.

Seeding

All of the methods used to secure straw did not work well with the compacted soil at this site. The modified planter could not rip a trench deep enough to secure a vertical bunch of straw. Crimping straw by hand also failed. The shovel would not push the straw into the soil; instead, it cut it in half. A third plan was made to scatter straw on top of the seed and then secure it with a thin layer of soil. This also failed when the soil fell through the spaces between individual straw pieces. The final attempt made was the most promising. A planter bar was used to create a depression. A bunch of straw was placed vertically in the depression, and the remainder of the hole was filled in with soil. The vertical mulch appeared fairly secure, but the idea was abandoned. Three acres of implementing this would have been heavily labor-intensive. A mechanical crimper should have been tried to complete the task. However, based upon the difficulty in penetrating the soil surface, it is doubtful that this method would have been successful either.

Wind became a major limiting factor in the success of the seeding portion of this project. During two consecutive weeks after seeding, there were several days of winds in excess of fifty miles per hour. All of the straw that had been broadcast over the site was carried away, taking with it the majority of the seed. It is highly unlikely that straw, regardless of how it is secured, could withstand such winds. Alternatives should be sought in areas where wind is a major factor. One alternative that may have been successful at Manix is spraying the seeded site with a light covering of tackifier.

The four-acre area that was mechanically pitted in January 2001 needed additional treatment before seeding. Mounds and depressions created by pitting were flattened and filled by months of wind and rain. Hand-pitting was necessary in December 2001 to avoid uprooting the vegetation that had established over the prior eleven months. This was very labor intensive and repeated work could have been avoided had the site preparation been done just before seeding.

A positive outcome of the improper timing of site preparation is the establishment of seedlings within the one-year maintenance period of the project. The four-acre pitted section has much more growth than the three-acre section that received no treatment (Figure 12). Observations made during the hand-pitting also indicate that the soil in this section of the site is less compacted than the soil of the crimped straw section.

Figure 12. The pitted area (left) had much more natural growth than the untreated area (right) after one year.

Pitting has proven to be effective in increasing establishment of native plants and is highly recommended for use in revegetation projects, whether or not direct seeding is intended. For direct seeding projects, pitting should be completed just prior to broadcasting the seed.