Soil Ecology and Restoration Group
Red Rock Canyon State park is located in the Southwestern Mojave Desert on State Highway approximately 40 miles northeast of the town of Mojave (Figure 1). The average annual rainfall at the park is between 100 to 275 mm and the predominant vegetation is creosote scrub (Figure 2). The first people to reside in the Red Rock Canyon area were Native Americans who collected plants for medicinal and food purposes as well as for basketry and building materials. The area also served as a trade route for other Native Americans. Settlement of non-native peoples began in the late 1800s with ranching and mining being the primary activities. The area was also used over the years as a sheep passage to the north, a stagecoach stop, a railroad route and later, a truck stop. Off Highway Vehicle (OHV) activity began in the 1960's and continues to be the main recreational activity around the park area at this time (Bainbridge, 1995).
Red Rock Canyon officially became part of the State Park system in 1968. In 1989, 4000 acres were added to the park in a land transfer with the Bureau of Land Management. The South Flat area was included in these lands and up until the land transfer had been heavily used by OHVs as a campground with large motor homes, a racetrack and a play area for 4wd vehicles and motorcycles. The State Park staff closed the area in 1989 and fenced the area off preventing further OHV activity. The Soil Ecology and Restoration Group (SERG) began work at the site in 1991 and native species planting began in 1993. The primary research aim of SERG has been to develop cost efficient restoration methods through various site treatments and revegetation procedures (Bainbridge, 1995).
OHV and other human disturbances that occur in desert environments may take hundreds of years to recover without active intervention and restoration efforts. SERG has been successful in the South Flat area through a variety of soil and planting treatments. These restoration efforts, coupled with the restriction of OHV
Figure 1. Red Rock Canyon State Park
Figure 2. View of South Flat dominated by creosote scrub habitat.
activity, has greatly increased the probability of habitat recovery in the South Flat area.
Our efforts for this project were originally focused on the recovery of native annual species, which have been extremely slow to reappear in the more disturbed areas of the South Flat. Desert annuals in the Mojave are an important food source for the endangered desert tortoise (Gopherus agassizii) and other desert wildlife. Our initial objectives were also to evaluate micro-nutrient availability and mycorrhizal activity along a disturbance gradient and to use these findings to create a restoration scheme that would enhance the establishment of desert annuals.
Since Red Rock Canyon State Park had experienced two consecutive dry seasons since 1997 due to the La Niņa effect, we were unable to collect adequate annual seed for a field or greenhouse experiment. Thus the scope of the project was modified to continue revegetating the South Flat area with perennials. Seven hundred seedlings were grown from local seed in SERG greenhouses. Five hundred of the seedlings were planted at South flat in March 2000. Vegetation will be monitored for one year and survival and growth will be analyzed again in March 2001.
Materials and Methods
Soil samples were collected on 12-13 March 1999, 14-15 April 1999 and on 18 March 2000. Samples were collected on three 30 meter transects along a selected disturbance gradient of less disturbed, moderately disturbed and very disturbed soils. Six samples were taken per transect. Soils were analyzed by A&L Western Agricultural Laboratories, in Modesto, CA for available nitrate, phosphorus, total nitrogen, pH, organic matter and micro-nutrients.
Seed Collection and Germination
Seeds from native perennials and several native annuals were collected on 13-14 March 1999 and 15-16 July 1999 from the South Flat and surrounding areas within Red Rock Canyon State Park (Table 1). Five hundred native seedlings were grown from this seed in the SERG greenhouse at San Diego State University. Seeds were germinated in flats that were misted twice daily. Once seeds had germinated, seedlings were transplanted into 2X8 inch plant bands and misted 3 times per week. Once established, plants received water on a weekly basis. One month before planting, plants were watered every 10-14 days. The soil mixture was one part organic matter /one part perlite / one part sand.
Plants were placed in the numerous bare areas of the South Flat (Table 2). Planting occurred on 11-12 and 17-18 March 2000. Planting holes were dug using a power auger and were saturated with water before and after planting (Figures 3&4). All plants were provided with tree-pees, plastic plant protectors
Seed collected from Red Rock State Park
Eriastrum densifolium spp. Mohavense
Eriogonum fasiculatum var polifolium
Purshia tridentata var. glandulosa
Species used in non-experiment areas
|Number of Plants
Figure 3. Augering planting holes at South Flat.
Figure 4. Completed transect ready for planting.
15-20 cm tall, to reduce herbivory and provide protection from the wind and blowing sand. Watering basins were constructed for each plant.
Preliminary soils data showed a trend toward a micronutrient deficiency in more disturbed soils (Appendix A). Initially we wanted to seed with annuals and apply soil amendments to determine if micronutrients enhanced annual establishment. However, since we were unable to collect enough viable seed, we focused on micronutrient and soil amendment enhancement of shrub establishment.
For the test study, 30 plant (10 Atriplex polycarpa, 10 Isomeris arborea, and 10 Senna armata) were planted along each of the three disturbance transects for a total of 90 plants. Plants were randomly treated with one of the following amendments: fertilizer alone (13-13-13 slow release), fertilizer and microfertilizer (13-13-13 and Azomite), fertilizer and mulch (13-13-13 and cocoa mulch) or no amendments (Figure 5). These plants were also provided with treepees and height was measured for each individual after planting.
Maintenance and Monitoring
Supplemental watering (Figure 6) occurred on a monthly basis, unless enough natural precipitation had occurred, beginning in April 2000. Watering was performed using a SERG four-wheel drive truck with a 180 gallon watering tank. Water was mechanically pumped into containers and plants were hand watered. Each plant received approximately 1/2 gallon of water at each watering. Total amount per plant over the year was about 6 gallons, plus natural precipitation.
On 23 September percent plant survival was measured for total plants and for the experimental plots. Plant basins where reconstructed on an as needed basis and missing plant protectors that had been blown off were located and replaced. Photographs were taken of the site from less disturbed (Figure 7), moderately disturbed (Figure 8) and very disturbed areas (Figures 9).
Plant protectors will remain on the plants until they are interfering in the natural morphology of the plants or until project conclusion, whichever occurs first.
Figure 5. Adding amendments to the experimental plots.
Figure 6. Watering the newly planted seedlings.
Figure 7. View of less disturbed section after planting.
Figure 8. View of moderately disturbed section after planting.
Figure 9. View of very disturbed section after planting.
Percent survival data was collected for the entire site. Species and treatment comparison will be done for the experimental plots once enough data has been collected to evaluate results. Overall survival will be done for the remaining plants.
Overall survival was 56% with Atriplex polycarpa having the highest survival at 87%. The lowest survival rate seen was for Senna armata at 39% (Table 3).
Survival rates for non-experimental plants
Survival at the experimental plots was 55%. Not enough data is available at this time to determine if there is a significant difference between treatments. Of the three species used in the experiments, Atriplex polycarpa did significantly better (p-value = .004) than Senna armata and Isomeris arborea. Percent survival for the three species is shown in Table 4.
Percent survival by species.
Soils collected from South Flat and an undisturbed area nearby were analyzed for available nitrate, phosphorus, pH, organic matter and minor nutrients. This data will be added to existing data (see Appendix A, B) to develop a baseline for further studies of the soils at Red Rock.
Available nitrogen (Figure 1Oa) was slightly higher in the very disturbed than in the undisturbed and disturbed areas. The higher levels in the relatively undisturbed area were higher than might be expected and may reflect deposition of nutrient rich litter and dust. Levels in shrub mounds have reached up to 3 times higher than the N03 concentration observed here.
Phosphorus levels (Figure 1Ob) were high but within previously observed levels. The very disturbed areas also had considerable available phosphorus. This has been observed before and probably results from vehicle oil leaks and detergents used by large camps. The disturbed and relatively undisturbed results were as expected.
Desert soil is generally alkaline so the pH results (Figure 1Oc) were typical except for the very disturbed area, which was lower than the others. This probably reflects the effects of erosion, with wind blowing away fine calcium particles.
Organic matter levels (Figure 10d) were typical for desert soils, ranging from 0.7-1.1 percent. The highest levels ever measured at RRCSP were under desert Senna at 2.2% (see Appendix B).
For minor nutrients (Figures 11a & b), only potassium, zinc and possibly boron, currently demonstrate a distinct trend, increasing as the disturbance becomes greater. Zinc and boron are probably the result of vehicle operation, with runoff from galvanized materials, gasoline, oils and greases. A fire pit at the south flat measured during an earlier project had 60 ppm zinc, many times higher than the levels observed here. The higher levels of magnesium in the very disturbed site are again probably related to vehicle operation and fires. The practice of burning VW engine blocks has resulted in very high magnesium levels, up to 819 ppm in a fire pit.
No effect of soil amendments was seen from the first year of sampling.
The lack of annual seed due to low rainfall in 1998 and 1999 changed the scope of this project. Revegetating with shrubs became the primary goal, with soil analysis and treatment methods being studied to improve methods used in desert restoration of native perennial species.
The overall survival rate of 56% is slightly better than most previous restoration projects we have conducted at Red Rock Canyon State Park, especially since planting occurred in the middle of a two year drought period. That the two saltbushes, Atriplex polycarpa and A. canescens, had higher survival rates than the other species is to be expected. Saltbush is a very hardy plant that has done well in all our previous Mojave Desert restoration projects, especially on highly impacted soils. The survival rates for the other species are also about average when compared to previous projects in similar habitats.
The experimental plot survival rate at South Flat after six months of 55% is also slightly better than most desert restoration projects. As discussed above, the saltbush, Atriplex polycarpa, survived better than the other two species at 83%. Past experience has shown us that A. polycarpa is an excellent restoration species to use for heavily disturbed areas in the Mojave Desert. It normally has a high initial survival rate and demonstrates quick and heavy growth with minimal water.
Isomeris arborea, though low initial survival rates are common, is also an excellent restoration species. It suffers little from herbivory, being extremely unappetizing to most desert herbivores, and is a prolific producer of seed. As such, it produces many new seedlings through natural recruitment. Once established, it is a fast grower, quickly producing much needed cover and food for wildlife. Hummingbirds have often been found enjoying the flowers of Isomeris on the south flat. The low survival of Senna armata was expected, this species is susceptible to a number of pathogens and is not often found in great numbers.
Several factors from the initial soil sampling data are of interest. First, there are indications that the soil remains adversely affected by the OHV disturbance and camping at the South Flat, even though it has been closed to off-highway vehicle activity for over ten years. Both vehicle operation and camping apparently leave long lasting legacies in changed soil properties. Impacts to desert soils do not quickly heal themselves. The second concerns pH. Desert soils are usually slightly alkaline, having a pH normally above 7.0. Initial results show that soil pH decreases as the amount of disturbance increases. The third item of interest is the apparent increase in zinc and boron as the amount of disturbance increases. All soil samples were within previously observed levels and pose little concern for plant growth, although the elevated levels of boron are approaching the point where they may impact sensitive plants. Proper micro-nutrient availability is important to the growth and survival of all plants and the higher levels of zinc may in fact prove helpful. The lower pH in areas of increased disturbance should also increase micro-nutrient availability and make conditions better for plant establishment. The limiting factor appears to be the changes in soil structure, not nutrients, although the availability of nutrients may be different than the absolute levels. Not enough information is available in such a short time to make any type of judgement concerning the effects of micro-nutrients on plant growth and survival. We will continue monitoring to see what happens in the future.
The effects of amendments on soil are not manifested quickly. When there is little or no rain, the effect of soil amendments is even slower to manifest itself because water is necessary for soil microbial actions to occur. It is expected that soil amendment effects may begin to appear by next year's sampling, but only id there is sufficient precipitation during the winter months to activate the below ground processes.