The arid deserts of the southwestern United States experience frequent dust problems that can often lead to air quality problems for populations in and around these areas. Antelope Valley, the westernmost arm of the Mojave Desert, is an area experiencing this problem. Located approximately fifty miles to the north of Los Angeles, Antelope Valley is bordered to the south by the San Gabriel Mountains, to the west by the Coastal Mountain Ranges and to the north by the Sierra Nevada Mountains. This area of the Mojave Desert was once intensely farmed, though farming operations have greatly decreased over the past 10-20 years due to increased water costs. Abandoned farmland, dirt roads and other human related disturbances, combined with high winds have led to a severe air pollution problem, including high levels of PM₁₀, in Lancaster and Palmdale. The Soil Ecology and Restoration Group, located at San Diego State University, has joined the Dustbuster Task Force, consisting of local farmers, the California Air Resources Board, South Coast Air Quality Management District, The Natural Resource Conservation District, City of Los Angeles Department of Airports and Southern California Edison, to address the issues of dust mitigation in the Antelope Valley.

The Soil Ecology and Restoration Group (SERG) was tasked with researching cost efficient methods by which revegetation with native plant species could be used for the suppression of airborne particulate matter. The reestablishment of native plant species in arid and semi-arid areas, on land that has been disturbed by human activities, including farming, off-highway vehicle activity and commercial or residential construction are often hampered by physical impacts caused by these activities. These impacts include soil compaction, biological and chemical impacts such as the loss of soil microbes and nutrients, and by the lack of available water inherent in such areas. Our role is to delineate exactly what the specific impacts are, what the best methods are to mitigate for these impacts, what native species are best suited for revegetation of such disturbed lands and how best to provide the necessary water to transplanted and establish seedlings in an overall environment that normally receives very little precipitation.

In June of 1998 two sites were selected near Palmdale Blvd. The sites were examined for soil nutrient availability and the following five native plant species were selected; Prosopis glandulosa (mesquite), Larrea tridentata (cresosote bush), Atriplex canescens, Atriplex polycarpa (shadscale), and Atriplex lentiformis (quailbush). Fourteen hundred seedlings were grown from local seed in SERG greenhouses and planted in January 1999. Vegetation was surveyed in June 1999, January 2000 and July 2000. Soil samples were taken in August 1999 and April 2000. Maintenance in the form of irrigation, weeding and removal of plant protective devices has continued through the summer of 2000. Vegetation monitoring and soil analyses will be conducted biannually for 4 years beginning in January 2000. This report details the construction of the plots and planting procedures as well as monitoring efforts up until August 2000.

Two experimental sites were chosen on land provided by the Los Angeles City Airport; an abandoned agricultural field that had been left fallow between three to five years, (50th street east, N 34' 37.425" W 118' 02.450"), and an agricultural field that appears to have been abandoned a minimum of fifteen to twenty years (90th street east, N 34' 36.559" W117' 59.017"). The sites are approximately 7 miles east of downtown Palmdale.

The 50^th Street site, the site most recently abandoned, still showed signs of furrows and was dominated by scattered groups of exotic annual grasses and Salsola tragus (Russian thistle). The soil was slightly compacted, though the furrowing prevented extreme compaction from occurring. The second site, at 90^th Street, showed no signs of furrowing and was lightly covered with exotic annual grasses, Salsola tragus (Russian thistle) and scattered native species such as Chrysothamnus nauseosus (rabbitbrush), an early seral invader. The soil at this site was also slightly compacted, much like the 50^th Street site. Native shrubs found in undisturbed areas in the Antelope Valley near the two sites include Larrea tridentata (creosote), Atriplex spp (saltbush), Opuntia spp (Cholla), Yucca brevifolia (Joshua tree), and Ambrosia dumosa (Burro weed).

On November 25, 1998 SERG personnel outlined shrub (15m x 30m) and windbreak plots (30m x 30m) at both 50th and 90th street sites. Palmdale Regional Airport supplied a rubber tired John Deer tractor with a skip loader and a tractor operator. The tractor was used to back blade all plots, knocking down existing exotic vegetation. In December 1998 the tractor, containing 18" ripping bars, was used to rip all plots to a depth of 12 inches. Plots were ripped in order to loosen soil and to incorporate the dead exotic material into the soil providing additional organic matter. During the same period the tractor and skip loader were used to construct windbreak plots. Again, the dead exotic vegetation was incorporated into the berms, which measured 1m high, and 15m in length.

In January of 1999, SERG personnel installed perforated pipe irrigation into windbreak mounds at both 50th and 90th street sites. Plants were also installed at both sites in January and provided with amendment treatments and irrigation systems in accordance with the experimental design, described below.

Two types of plot designs were employed; shrub plots (15m x 30m) (Figure 1) and wind break plots (30m x 30m) (Figure 2). Shrub plots were established using a 2x3x4 factorial with two types of irrigation methods (surface watering or deep pipe), three types of surface applied soil amendments (wood chips, compost, control) and twenty individuals from the five native plant species selected. All treatments were randomly assigned within a plot, with each of the sites having two shrub plots for a

total of 640 shrub plot seedlings. Windbreak plots were established with a 2x4 factorial consisting of two irrigation treatments, perforated pipe and surface irrigation, and 16 individual plants per windbreak. Each site has two wind break plots and each plot has six windbreaks, three with perforated pipe and three with surface irrigation. Each windbreak has sixteen seedlings for a total of 760 windbreak seedlings (Figure 3).

Fourteen Hundred seedlings, Atriplex canescens, A. polycarpa, A. lentiformis, Larrea tridentata and Prosopis glandulosa, were grown in the SERG Greenhouse at San Diego State University beginning in June 1998. Seeds were obtained from S and S Seeds and had been collected from the local Mojave Desert area (Table 1). Plants were grown in plant bands (2x2x8 and 3x3x10) and were watered twice each week for the first three months and then on a monthly basis. All seedlings were fertilized monthly for the first three months. In November 1998 all seedlings were transferred to a University of California, Riverside lathe house to allow the plants to become acclimated to desert conditions prior to scheduled planting in January 1999.

In January 1999 planting holes were dug using a power auger at both sites and then saturated with water prior to planting. All plants were provided with plant protectors in the form of either tubex or tree-pees, plastic coverings 15-20cm tall, to reduce herbivory. All plants were re-watered after planting was completed. Of the fourteen hundred seedlings, seven hundred per site were planted with three hundred and twenty in the shrub plots and three hundred eighty in the wind break plots (Figure 4).

Figure 3. Shrub plot and windbreak map.

Supplemental watering occurred on a monthly basis beginning in February 1999. Irrigation was performed using mechanically pumped water onto the site from a 800-gallon watering truck supplied by the Palmdale Regional Airport. Students from Antelope Valley Community College were trained to water and monitor plant growth at each site. In March of 1999, plant protectors had to be replaced at 90th Street due to the loss of many of the original protectors from high winds. It was noticed at this time that a large number of the newly planted seedlings had suffered heavy herbivory after the protective devices had been lost. Replacement plant protectors were installed and secured with rebar that was staked into the ground 4-6 inches deep. In June 1999, weeding was done at the 50^th Street site because numerous exotic annuals had appeared in and around the newly planted seedlings.

On 25 June 1999, 25 January 2000 and 7 July 2000, percent plant survival was measured at both the 50th and 90th street sites. Documentary photos were taken of the plot layouts at both sites in June 1999. Pesticide (diazanon) was also applied by Los Angeles Airport personnel in June 1999 to help control herbivory. On 7 September 1999 soil samples were collected for nutrient analyses and fungal/bacterial measurements. Samples were analyzed for macro and micronutrients as well as organic matter, pH/EC and texture. Soil analyses were done at the Davis Analytical Lab at the University of California, Davis. On 7 June 2000, biomass, through plant volume by using the three axis method, was taken at both sites.

Percent survival data was collected for both the 50^th and the 90^th Street sites, but for this report only the data collected for 50^th street will be analyzed. The loss of plant protectors due to high winds at the 90^th Street site resulted in an unusually high number of plant losses leading to the re-planting on that site. Data from the 90^th Street site will be used for all future reports.

Percent survival at the 50^th Street site was 85% in June 1999 and 79% in July 2000. These numbers reflect all plants from both the shrub plots and the windbreaks. Percent survival by treatment is illustrated in Table 2.

Windbreak plot survival was high with no demonstrated difference in percent survival between irrigation methods. Overall percent survival for the shrub plots is also fairly high, with both control plots and mulch plots having the lowest survival rates at 68% and 65% respectively. Though not statistically analyzed, survival rates between the two types of plant protectors were compared at the 90^th Street site where it was observed that seedlings protected by Tree Pees demonstrated a higher percent survival than those protected by tubex.

Table 2. Percent survival at 50^th Street.

Soils collected from the 50^th Street site, the 90^th Street site and an undisturbed area was analyzed for organic matter (OM), nitrates, ammonium, and minor nutrients. To develop a baseline for further studies we looked at three groups of below ground parameters; organic matter, nitrogen and a selection of minors, which included zinc, iron, copper, and manganese.

Results of the soil analysis show 50^th Street to be higher than 90th Street and the undisturbered area in all areas tested, with the single exception of available nitrogen (NO₃). Results at 90^th Street were much closer to the undisturbed area than 50^th Street. The overall averages for all three sites are shown in Table 3.

Table 3. Comparison of Soil Analysis

Hyphal lengths showed no significant difference between the pipe and no pipe treatments (Figure 7a). Hyphal lengths of shrub treatment plots varied (Figure 7b). At the 50^th street site, hyphal lengths were highest on plots with wood chips / surface irrigation (.190 m/gm) and control deep pipe plots (.145 gm/m). At the 90th street site the hyphal lengths were highest on the control deep pipe (.398gm/m) and mulch / deep pipe plots (.195 gm/m). Bacterial counts show no significant difference between pipe and no pipe treatments for windbreak plots (Figure 7c). Bacterial counts on shrub treatment plots appeared to be higher at the 90^th street site with high counts (above 3.21E+08) for all plots with the exception of wood chips / deep pipe plots (1.67E+08). At the 50th street site the highest counts of 5.87E+08 and 5.01E+08 were found on the control surface area plots and wood chips / deep pipe plots. The lowest numbers occurred on the wood chips / surface area (1.88E+08) and control deep pipe plots (1.67E+08) (Figure 7d).

Biomass data was collected at both sites in July 2000. The data collected at 90^th street has limited use because the site had to be replanted. One of the shrub plots on 90^th did not lose plant protectors in the high winds so the plants were not subjected to herbivory.

The biomass from this shrub plot was compared to the data from the shrub plots at 50^th Street. The total biomass of the shrub plots at 50^th was 22,336,307 cm³ and 17,038,598 cm³. The biomass collected from the 90^th Street shrub plot was significantly smaller, measuring 8,206,785 cm³. The biomass data for all shrub plots as well as the biomass data from the windbreaks is included in Table 4.

Table 4. Total Biomass shown in cm³

The 90th Street site was subjected to severe herbivory early on due to the loss of plant protective devices from high winds. This caused both individual species and overall survival rates to be extremely low, with the overall survival rate for 90^th Street after one year at 30%. The 50^th Street site experienced only a minimal loss of protective devices which resulted in an overall first year survival rate of 79%. Because of this unexpected event, analyses of treatment effects were conducted only at the 50^th Street site. Though treatment effects for both soil amendments and irrigation methods demonstrated no significant differences after one year, this came as no surprise. Reclacitrant soil amendments, such as wood chips and compost, are slow to decompose and require water to activate the mineralization process. With Antelope Valley experiencing a La Niña effect during 1999, precipitation was well below average, leading to little or no mineralization activity occurring.

Additionally, lack of rainfall due to the La Niña event was most probably also the cause of no significant difference between the irrigation methods being tested. Results of soil analysis done on the sites show above ground activity to be a reflection of below ground activity for both sites. The 50^th Street site has shown active growth, as seen in the biomass data, despite harsh dry conditions. This unexpected growth, especially noteworthy for the several saltbush species, is most probably explained by the soil analysis results. All parameters, when compared to results from the undisturbed reference site, were extremely high. This increased amount of nutrients has likely led to the explosive growth experienced at the 50^th Street site. Since this site was only recently abandoned, and the past several years have been extremely dry with below average precipitation, thus minimizing any leaching activity, the nutrients could possibly be left over from when the site was actively farmed. A second possible source of nutrients could be from overspray from the surrounding active croplands. As time passes, nutrients left over from previous farming should be reduced and the plants at 50^th Street will eventually demonstrate the more natural growth patterns typical of an arid environment.

Plant growth measured at 90^th Street, though limited in numbers because of the problem of lost protective devices and subsequent herbivory has been at a steady, but much lower rate, as reflected in the biomass data, without the explosiveness experienced at 50^th Street. Soil analysis results at 90^th Street are overall much lower than at 50^th Street, and more closely reflect the undisturbed site results. Since 90^th Street has been abandoned for more than 20 years, and does not have active cropland adjacent to it, this site has been allowed to return to the more natural condition of low nutrient availability that is normally found in arid and semi-arid habitats. Thus, growth at 90^th Street reflects a more "natural’ path.

Supplemental irrigation is used sparingly when watering native desert seedlings; mainly to develop a hardy and tough plant that will be able to survive the rigors of extreme heat and lack of rainfall prevalent in arid habitats such as Antelope Valley. Our watering schedule was aimed at only providing enough water to keep the seedlings alive, not to necessarily experience a growth spurt. Competition for water between the seedlings and, most notably, with exotic annual grasses, is the process by which survival rates are most effected. With little to no competition from annual grasses due to the lack of precipitation, and little to no competition among the seedlings due to minimal root and shoot growth due to a combination of lack of rainfall and minimal irrigation, no significant difference between irrigation systems is what would be expected. In our experience, one year is normally too early to determine the effects of any treatments on plant growth. The La Niña experienced by Southern California in 1999 further reduced our ability to determine a difference in plant growth between treatments because the lack of natural precipitation caused additional stress.

The low survival rates experienced by the plants at the 90^th Street site provided the opportunity to conduct an unplanned comparison experiment on the difference between Treepees and tubex as plant protective devices. We were able to see a dramatic difference in the survival rates of those species that retained their protective devices. Seedling species with Treepees proved to have between 2-3 times higher survival rates than those with tubex protection. No such difference was seen at the 50^th Street site with its extremely high overall survival. From this information, it would appear that under stressful conditions, such as were experienced at 90^th Street, Treepees provide much better protection than do tubex. The stress most probably involved the wind that apparently affected the site; quite possibly from sandblasting, the drying out effect of the hot wind or a temperature difference provided by the Treepee in comparison to the tubex. At any rate, we will continue to monitor the effects of the two types of protective devices being used.

The first year resulted in little data to be analyzed, mostly due to the La Niña effect experienced by all of southern California during the winter of 1999. The ability to measure such variable parameters as seedling growth, soil amendment effects and soil microbial activity over several years, covering both dry and wet years, should provide more than sufficient data to determine the most cost efficient native revegetation methods to be used in the Antelope Valley.