Soil Ecology and Research Group last update March 9, 2004

Revegetation Methods for the Control of Dust from Arid/Desert Soil Disturbances in the Antelope Valley

2002 Annual Report

INTRODUCTION

Antelope Valley, located fifty miles north of Los Angeles in the Mojave Desert, has been experiencing air quality problems caused by frequent dust storms. Bordered to the south by the San Gabriel Mountains, to the west by the Coastal Mountain Ranges and to the north by the Tehachapi Mountains, Antelope Valley was intensely farmed up until 10-20 years ago. Increased water costs caused many farmers to abandon farming, leaving vast tracts of bare disturbed land. These abandoned farms, miles of dirt roads, increased construction, summer brushfires and other human disturbances, combined with high winds, has led to an air pollution problem with high levels of PM₁₀ in and around Lancaster and Palmdale. A resurgence in farming has been seen over the last few years but most of the land used during the farming season is left fallow during the windy winter season providing additional dust to an already existing air problem. The Dustbusters, a coalition of local farmers, the Antelope Valley Resources Conservation District, the California Air Resources Board, South Coast Air Quality Management, The United States Department of Agriculture, Natural Resource Conservation Service, The City of Los Angeles Department of World Airports and Southern California Edison, invited the Soil Ecology and Restoration Group (SERG) of San Diego State University to join the group in addressing issues of dust mitigation in the Antelope Valley.

SERG was given the task of researching cost efficient methods of revegetating abandoned farmland with native plant species. The goal of this project is to suppress airborne particulate matter and restore the soil to the point that native plant communities can be reestablished. The process of reestablishing such native plant communities in arid and semi-arid areas is hampered by the physical impacts caused by human activities. These activities, including farming, off-road vehicle use and construction impact the soil through compaction, loss of soil microbes and changes in soil nutrient levels. The role of SERG is to determine the specific impacts, develop the most cost-effective methods to mitigate these impacts, determine which native species are best suited for restoration on such disturbed lands and establish how best to reestablish a self-sufficient plant community. In June of 1998 two sites were selected for experimental plots. The first site is located on 50th Street East near Palmdale Boulevard and the second site is on 90th Street East just north of Palmdale Boulevard. Soil samples were taken in July 1998 from both sites to determine initial soil nutrient levels and five native plant species were selected to be used in the study. Fourteen hundred seedlings, comprised of Atriplex canescens (four-wing saltbush), Atriplex lentiformis (quailbush), Atriplex polycarpa (allscale), Larrea tridentata (creosote bush) and Prosopis glandulosa (mesquite) were grown in the SERG greenhouse at San Diego State University and outplanted in January 1999. This report will give an overview of the project, including site preparation, planting, maintenance and monitoring methods followed by a detailed account of all activities and data collected at the sites from July 2002 through June 2003.

In addition to the review of past work and detailed information on collected data, this report will include data and discussion from a new site, located on 85th Street East and Avenue F in Lancaster, that was selected to replace the 50th Street site. The 50th Street site (Figure1) is no longer being monitored for data, except survival, as it has been determined to be non-representative of recently abandoned farmland. This conclusion was reached after reviewing new information stating the site had been regularly flooded with secondary effluent for several years after farming operations had ceased. The secondary effluent dramatically increased the soil nutrient levels over those usually found on farmland and therefore the site is not representative of recently abandoned farmland. This is the last year data will be collected from the 50th street site.

The new site, donated by Phillip Giba of Giba farms, was farmed as recently as the 2002 season. The experimental design is the same as that used on the 50th and 90th Street sites. Five species of plants A. canescens, A. lentiformis, A.polycarpa, L.tridentata and P. glandulosa were grown in the SERG
Greenhouse at San Diego State University and transplanted on the site in October/ November of 2002. Monitoring of this site will continue until March 2005.

Figure 1. 50th Street Windbreaks 2003

SITE PREPARATION

The Palmdale Regional Airport provided the original two sites for SERG to conduct revegetation experiments. The first site, located on 50th Street East and Avenue N-8, was thought to be abandoned farmland. We had originally been informed the site had been farmed as recently as 1995, but new information provided by Jim Bort from the City of Los Angeles Airports stated the site has lain fallow for twenty-five years. The site had, however, been used by the Los Angeles County Sanitation District for the disposal of secondary effluent as recently as 1997. The furrows, originally believed to be evidence of recent crop activity, were actually used to divert the secondary effluent evenly over the fields. The second site, located on 90th Street East and Avenue O-8, appeared to have been abandoned a minimum of fifteen to twenty years ago. The replacement site, located on 85th Street East and Avenue F, was used for growing onions as recent as 2002 according to the owner of the land, Mr. Phil Giba.

The 50th Street site was dominated by scattered groups of exotic annual grasses and Salsola tragus (Russian thistle). By visual and physical inspection of the soil it was determined only slightly compacted, probably due to the recent furrowing activity. The 90th Street site showed no signs of furrowing and was lightly covered with exotic annual grasses, Salsola tragus (Russian thistle), and a few native species, including an early seral invader Chrysothamnus nauseosus (rabbitbrush). The soil at 90th Street was also only slightly compacted.

On November 25, 1998 SERG personnel outlined 15m x 30m shrub plots and 30m x 30m windbreaks at both the 50th and 90th Street sites. The Palmdale Regional Airport supplied a John Deer tractor that was used to back blade all the plots, knock down existing vegetation and construct windbreaks. Exotic vegetation knocked down on site was incorporated into the 1m tall by 10m long windbreaks to provide stabilization and organic matter to the soil. The tractor was again used in December 1998, this time using 18" ripping bars to loosen the soil in all plots to a depth of 12" and to incorporate dead exotic plant material into the soil to provide additional organic matter.

In January 1999, SERG personnel installed perforated irrigation pipe into the windbreak mounds at both 50th and 90th Street sites. All plants were outplanted in January and provided with either Tree-pees or Tubex as plant protectors. In addition, each shrub plot was provided with amendments and irrigation treatments as required by the experimental design described in the following section.

Once the decision was made to abandon the 50th Street site and replace it with a new site located on 85th Street, site preparation began. In September 2002 Giba Farms supplied a John Deer 4450 tractor equipped with a back blade that was used to construct 24 windbreak mounds. Perforated irrigation pipe was installed for the designated windbreaks and shrub plots were outlined. Preparations, planting, and set up of the experimental design was completed by SERG personnel in October and November of 2002. Shrubs were provided with Tree-pees as protection from herbivores.

EXPERIMENTAL DESIGN

Two types of plot designs were employed; shrub plots (15m x 30m) and wind break plots (30m x 30m). Shrub plots (Figure 2) were established using a 2x3x5 factorial (Table 1) with two types of irrigation methods (surface watering or deep pipe), three types of surface applied soil amendments (wood chips, mulch, control) and twenty individuals from each of 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 880 shrub plot seedlings. The initial design for the 50th and 90th Street sites was to have four surface applied soil amendments. During site implementation only three surface amendments were available thus leaving both sites with two control amendments rather than one. The 85th Street site was implemented according to design with only with one control amendment. This causes the shrub plots on the 50th and 90th Street sites to have 80 more shrubs per site compared to the shrub plots on the 85th Street site. Windbreak plots were established with a 2x5 factorial (Table 1) consisting of two irrigation treatments, perforated pipe and surface irrigation, and 16 individual plants per windbreak from each of the five native plant species selected. Each site has four windbreak plots (Figure 3) and each plot has six windbreaks, three with perforated pipe and three with surface irrigation. Each windbreak has sixteen seedlings for a total of 1152 windbreak seedlings.

Figure 2. 50th Street Shrub Plot 2003

Table 1
Shrub Plots and Windbreaks Factorial

SEED COLLECTING AND PLANTING

The native plant seedlings used for this project were grown in the SERG greenhouse at San Diego State University. The seed, obtained from S&S Seeds, had been collected from the Mojave Desert in and around Antelope Valley. Plants for the 50th and 90th Street sites were germinated in June 1998 and then transplanted to plant bands (2"x2"x8" and 3"x3"x10"). Seedlings were watered twice each week for the first three months and then on a monthly basis. Seedlings were fertilized for the first three months using half strength commercial fertilizer. In November 1998 the plants were transferred to the University of California, Riverside lathe house to acclimate them to desert conditions. Plants for the 85th Street site were germinated in April of 2002 and grown following the same treatment used on the 50th and 90th Street plants, the one exception being that plants were hardened off in San Diego rather than Riverside.

In January 1999 the 50thand 90th Street sites were prepared for planting. A power auger was used to dig holes and each hole was saturated with water prior to planting. Plant protectors were used to protect the plants from herbivory. All plants were re-watered after planting. Seven hundred plants were put at each site. Of the seven hundred, three hundred and twenty went to shrub plots and three hundred and eighty four went to the windbreaks. Surface catchments were built for plants on the windbreaks that did not have perforated pipe installed. The shrub plots were divided equally so that half of the plants had catchments and half were provided with PVC pipe for deep watering. Designated sub-plots within the shrub plots were covered in mulch or wood chips.

In October and November of 2002 the 85th Street site was prepared for planting. Planting and setup was accomplished utilizing the same methods as for the previous two sites with the exception that holes were dug by hand instead of using a power auger. Six hundred and twenty four seedlings were put on the site. Two hundred and forty plants on the shrub plots and three hundred and eighty four on the windbreak plots.

MAINTENANCE AND MONITORING

In March 1999 numerous plant protectors were blown off by high winds at the 90th Street site. Many of the unprotected seedlings suffered heavy herbivory damage and had to be replaced. Plant protectors were replaced on the remaining unprotected plants and secured with rebar. In March of 2000, 489 plants were replanted at the 90th Street site. Since there were only a limited number of the original species available, other species were used. All replacement plants were grown from Mojave Desert seed at the San Diego State University greenhouse (Table 2).

The 50th Street site was provided with supplemental water through July 2000. The 90th Street site, since it was replanted, continued to be watered monthly until January 2001. Both sites are now established and can survive without supplemental water. The 85th Street site has been provided with supplemental water twice per month through February 2003. Starting at the end of February

Table 2
Species Used to Replant 90th Street Site

2003, the plants have been provided with supplemental irrigation once per month. Due to consistent rainfall in the early months of 2003, supplemental
watering on a biweekly cycle was not necessary for the months of February and March. Plants will continue to be provided with supplemental irrigation through the summer of 2003.

In July 2000 SERG personnel began removing plant protectors from the 50th and 90th street sites. Plant protectors were removed from plants when it was determined they were interfering in the natural morphology of the plant. The protectors were removed on a regular basis throughout the year with all removed by June 2001. Based on the same criteria as with the two earlier sites, protectors have been removed from a majority of the plants on the 85th street site throughout the spring of 2003.

In October 2002 and March 2003, soil samples were taken from the 90th Street site, and in March 2003 from the 85th Street site, and sent to A&L Laboratories, in Modesto CA, to be analyzed for macro and micro-nutrients, organic matter and pH. SERG personnel conducted europium staining on the samples, a process used to measure fungal and bacterial growth, at San Diego State University. This process is used to determine the ratio of bacteria to fungal hyphae per gram of soil. A shrub dominated habitat, such as those naturally found in the Mojave Desert, typically has a fungal dominated mineralization cycle. Disturbed habitats, dominated by exotic annual species, will have a bacteria dominated mineralization cycle. By comparing the fungal to bacteria ratio it can be determined if the disturbed mineralization cycle is actually being rehabilitated.

In June 2003 plant survival was taken for the 50th Street site and survival/biomass was measured and analyzed for the 90th and 85th Street sites. The 85th Street site was also weeded in June 2003 (Figure 4).

Figure 4. Weeded Windbreaks on 85th Street 2003

RESULTS

Survival Data
Survival data was collected for the 50th, 90th, and the 85th Street sites in June 2003. Overall survival at 50th Street was 48%, with shrub plots at 52% and windbreaks at 44%. The highest shrub plot survival was 60% for the deep pipe/mulch treatment while the lowest was 25% for the wood-chips/surface treatment. Windbreaks with surface irrigation had 45% survival while windbreaks with perforated pipe irrigation had 44% survival. Statistical analysis, using Analysis of Variance (ANOVA), found a significant difference in irrigation methods and amendment treatments for shrub plots at 50th Street. For irrigation methods, deep pipe watering outperformed surface watering with a p-value of .0336. In the scientific community, any value less than .05 is considered to represent a significant difference. For amendment treatments, mulch outperformed wood-chips with a p-value of .0114. Percent survival by treatment is illustrated in Table 3.

The 90th Street site had an overall survival of 64%, with shrub plots having 55% and windbreaks having 71% survival (Table 3). Shrub plot survival had a low of 45% in the surface/mulch treatment and a high of 65% in the deep pipe/mulch treatment. Statistical analysis (ANOVA) indicated no significant difference between amendment treatments or irrigation methods. Windbreaks at 90th Street continue to outperform the shrub plots with survival of 68% for surface irrigation and 74% for perforated pipe irrigation.

Seven months after initial planting, the 85th Street site had an overall survival of 92%, with shrub plots having 95% and windbreaks having 90% survival (Table 3). Shrub plot survival had a high of 100% in the control treatment and a low of 90% in the deep pipe/mulch treatment. Statistical analysis (ANOVA) found a significant difference in irrigation methods but no difference in amendment treatments for the shrub plots. Surface irrigation outperforms deep pipe irrigation with a p-value of .0308. Statistical analysis (ANOVA) also found a difference between irrigation methods for the windbreaks. The perforated pipe method outperformed the surface method with a p-value of .0281.

Table 3
Percent Survival by Treatment

Soils Data
In October 2002 and March 2003 soil samples were collected from 90th Street site and from an undisturbed reference site nearby. Soil samples were collected
from the new site on 85th Street in March 2003. The samples were then sent to A&L Labs in Modesto, CA to be analyzed. Results (Table 4) of the 90th and 85th Street sites were then compared to previous samples and to the undisturbed site. Data from the 85th Street site has only been collected twice. Baseline samples were taken in April 2002, and biannual sampling began March 2003. The site was used for growing onions in between these two data collecting occasions. Changes in soil nutrient levels caused by the planted shrubs can therefore not be determined until more data is collected. The 85th Street site shows numbers typical of fertilized cropland.

The 90th Street site shows a decrease in organic matter while nitrates and sulfates remain stable. Zinc and manganese show a decrease, copper remains stable, and following a winter with above normal rainfall iron has ended a long trend of decreasing numbers.

Table 4
Comparison of Soil Analysis

Europium Data
Fungal hyphal lengths between the various treatments of mulch, wood chips, control and windbreak plots on 85th and 90th Street have demonstrated no significant difference (ANOVA). Except for a temporary increase in numbers in October of 2002, they have remained extremely stable over the past two years and are demonstrating a downward trend over the course of the experiment. Though the undisturbed samples began to rise in the winter of 2000-01, they have decreased to numbers below their original levels (Table 5). Meters of hyphae per gram of soil are slightly lower in numbers for 85th Street compared to those found on 90th Street except for the shrub plots with wood-chip amendment. These show the highest numbers of hyphae when compared to the other treatments, including irrigation methods, on either site for 2002/ 2003. The numbers found are similar to the initial soil samples taken before the site was implemented.

Table 5
Meters of Fungal Hyphae per Gram of Soil

Bacteria numbers are all showing an increase over time with a high of 537 million bacteria per gram of soil seen in the mulch shrub plots on 85th street, and a low of 88 million seen on the windbreaks on 90th Street (Table 6). 85th Street numbers are higher than those on 90th Street. In 2003 a decrease was seen in all of the treatments on 90th Street, and a downward trend can also be seen for all the treatments when viewed overtime. Undisturbed samples have decreased

Table 6
Millions of Bacteria per Gram of Soil

from their original numbers. There were no significant differences (ANOVA) seen between amendment treatments or irrigation methods.

Biomass Data
Biomass data was collected at the 85th and 90th Street sites in June 2003. Total biomass for the shrub plots and windbreaks combined was 506.58m3 for 90th street (Table 7) and 83.04m3 for 85th. These numbers reflect an average of 56% growth during the last year for 90th street. The highest percent change was seen at the shrub plots which had a 92% increase in growth. The least amount of growth occurred at the windbreaks with perforated pipe, which had a growth increase of 29%.

Table 7
Biomass in m³ at Shrub Plots and Windbreaks on 90th Street

DISCUSSION

When reviewing the results of soil analyses and biomass at the 50th Street site, it came to our attention that the nutrient levels were high even when compared to actively farmed soils. Examples of these high levels include nitrogen levels above 60ppm, iron and sulfate levels above 20ppm, and manganese as high as 39ppm. In addition, the tremendous growth of the plants was beyond our expectations. When considering the source of the growth we suggested it could be attributed to over-spray from the neighboring sod farm, but this did not explain the nutrient levels in the soil. We eventually learned that the land had actually been fallow for over twenty-five years and during that time it had been regularly flooded with secondary effluent by the Los Angeles County Sanitation District. In April 2001 we took samples from a site recently flooded with secondary effluent and found nutrient levels were comparable to the 50th Street site soil samples.

At the September 2001 Dustbuster meeting this information was discussed and all members agreed that the 50th Street site was not representative of recently abandoned cropland and would no longer be monitored, except for survival. Discussions with Phil Giba of Giba Farms led to the donation of a new site. Located on 85th Street in Lancaster, California, it had been used for onion crops as recently as 2002. This new site was prepared and planted in October and November of 2002. The experimental design was the same as that used on the 50th and 90th Street sites. Due to the implementation of this new site, the 50th street site is considered obsolete and no further data will be collected.

Survival at 50th Street is currently 48%, a drop from last years 60%. This drop in survival can be attributed to extremely low rainfall in the second half of 2002 and to plant loss due to competition. One of the distinguishing factors seen at 50th Street is the explosive growth of the Atriplex species. When taking survival data this year it was observed that P. glandulosa and L. tridentata plants planted between Atriplex spp. had been choked out over the past year. The majority of survivals for these two species are located on the edges of the shrub plots and windbreaks with at least one side left clear for growth.

A significant difference is seen between irrigation methods on the shrub plots. As expected, survival is higher for shrub plots with deep pipe irrigation versus surface irrigation. A significant difference is also seen in surface amendments for the shrub plots. Survival for shrubs treated with mulch is higher than those treated with wood-chips. Mulch normally decomposes quicker than wood-chips and will therefore provide a quicker increase in organic matter. This increases the soils ability to retain water. Increased water retention helps plants establish and thus are more likely to survive a period of drought. There is no significant difference between irrigation methods on the windbreaks.

Overall survival at 90th Street was 64% in June 2003. With an extremely dry winter in 2002, followed by a normal winter in 2003, these plants are doing better than the 60% survival rate expected in desert restoration projects. The windbreaks, with 71% survival, have a higher survival rate than the shrub plots which have an overall survival of 55%. This may be attributed to the windbreak design, which slopes towards the plant on two sides, allowing these plants to receive more water from rain runoff than plants in the shrub plots. Although shrub plot survival numbers for deep pipe irrigation are higher than those of surface irrigation, statistical analysis (ANOVA) showed no significant difference between watering methods used at 90th Street. There is no significant difference seen between the amendments used. Still to be seen are the effects of the 2003 rains. Based on results and observations from 50th and 90th Street, 85th Street will probably experience increased losses in survival, especially for P. glandulosa and L. tridentata, due to overcrowding as the Atriplex spp. increase in size.

With a normal precipitation first season, overall survival at 85th Street was 92%. Shrub plots had a higher survival rate at 95% compared to the 90% for the windbreak plots. Presently, surface irrigation is showing a significant difference in survival compared to the deep pipe irrigation for the shrub plots, however, the benefit of deep pipe irrigation over surface irrigation is expected to be evident following the summer, when significant amounts of surface water will be lost to evaporation. Although no significant difference can be seen be seen for the amendment treatments the 100% survival seen on the control plots reflects a good rain year and the high levels of nutrients found in the soil. Windbreak survival numbers were, as predicted, significantly higher for perforated pipe irrigation than for surface irrigation.

Figure 5. Atriplex Seedlings on 90th Street 2003

When taking survival data, it was noted that both shrub plots at 90th Street had new Atriplex seedlings appearing (Figure 5), indicating a successful seed bank is being developed on this site. No seedlings were observed at the 50th Street site, although many of the Atriplex species were producing large quantities of seed. As expected, no seedlings were observed at the recently implemented 85th Street site.

When reviewing soil analysis data it is important to remember the extreme variability inherent in soil/plant restoration projects. There are many outside factors that can affect the data, including extreme weather conditions, animal and insect damage or byproducts and human disturbances. For this reason multiple samples are collected and data sets are analyzed for variability and any number that falls outside the range is excluded from analysis. For example, when analyzing the sulfates on 90th Street, all of the data fell between one and thirty six part per million except for one that was sixty parts per million. A number this high indicates an outside influence and is not considered representative of the site.

The results of the March 2003 soil samples taken at 90th Street show a decrease in organic matter. A decrease in organic matter indicates that it is being broken down and made available to the plants in the form of nitrogen. Nitrogen levels often go up following organic matter break down, but according to soil analysis there is a slight decrease. Nitrogen plays an important part in the growth cycle of plants, and the biomass data indicates that recent plant growth has occurred, thus the plants have potentially used the available nitrogen leading to the nitrogen decrease in the soil. Sulfates at 90th Street have fallen from 15.8 in the fall of 2002 to 13.2 in the spring of 2003. It is very common, particularly in arid climates, for there to be a wide fluctuation in sulfate concentrations as environmental conditions play a large role in their mineralization process. During the winter season sulfates usually drop as they are leached out by rainfall. In extremely dry years they begin to build up, but seldom reach levels that are harmful to plants. Sulfate levels in desert soils can exceed 20ppm without any adverse effect on the plants.

Soil nutrient levels on 85th street are typical for recently farmed land. Soil samples have only been analyzed twice for the site, once before planting, and once after planting. The site was used for growing onions in between data collecting occasions, and effects on soil nutrient levels caused by the planted shrubs and surface amendments can not be determined until further data has been collected.

Measurements of fungal hyphal lengths and bacteria numbers have shown no significant difference between treatments at this time. The mineralization process by which organic matter is broken down into its chemical elements by fungi and bacteria is an extremely slow process in arid habitats such as Antelope Valley. Since water availability is a key factor in mineralization, the limited amount of precipitation in the Antelope Valley retards the mineralization process, thus preventing a rapid increase in soil microorganisms such as fungi and bacteria.

Bacteria are a primary mineralizer of non-recalcitrant matter, as opposed to fungi that serve as the primary decomposer for the more complex recalcitrant organic matter. Non-recalcitrant matter is made up of simple carbon compounds found in both exotic and native annuals. Recalcitrant matter is made up of more complex carbon compounds such as those found in bark, wood-chips and perennial shrubs. With very few exceptions, recalcitrant organic matter must first be broken down from complex to simple carbon compounds by fungi. Once this occurs, bacteria will finish the decomposition process. Because of this, changes in bacterial numbers should develop much faster than changes in fungal activity. Changes in fungal activity would, due to their role in the slower decomposition of more complex organic matter, be much slower to appear. Results from the europium analysis seem to demonstrate this fact with an increase in bacteria numbers appearing over the first few years at the 90th Street site. This increase was most probably the result of the decomposition of the non-recalcitrant annual grasses that were worked into the plots during site preparation. The latest results show a decrease in bacteria numbers, which indicates a decrease in non-recalcitrant matter.

Since desert plant communities are shrub dominated, and therefore provide mostly recalcitrant matter to the soil, desert soils normally have fungi dominated mineralization cycles. Fungi have, so far, demonstrated no increase. Based on the recent decrease in bacterial numbers, an increase in fungal numbers is expected to follow in the near future as a self-sufficient plant/soil desert community develops. As expected, soil analysis from 85th Street shows bacterial counts higher, and fungal numbers lower, compared to the respective data from 90th Street. Even though analysis of variance shows no significant difference between amendment treatments, it is worth mentioning that fungal numbers are much higher for shrub plots treated with wood-chips than any other surface amendments. Wood-chips, or recalcitrant matter, provide microorganisms, such as soil fungi, with additional carbon that will help them increase in numbers. An increase in fungal numbers due to the wood chip amendments was not expected at such an early stage of the experiment. Because of the normal rainfall in early 2003 the decomposition of the wood-chips, which is how the carbon is made available, is thought to have started sooner than typical for the arid climate found in the Antelope Valley, thus the additional fungal growth.

The relationship between microorganisms and soil nutrients can be seen when looking at the levels of organic matter, nitrogen, and bacteria. When examined over time, a trend can be seen among these three parameters. As nitrogen levels go up, so do the bacteria numbers in the soil. As bacteria break down the non-recalcitrant organic matter, the percent of organic matter in the soil drops. Over time, this non-recalcitrant organic matter will be slowly released with the more recalcitrant organic matter provided by the native permanent shrubs.

Biomass at 90th Street has increased 56% over the past year with the highest increase seen at the shrub plots at 57%. The windbreaks with deep pipe irrigation, with a 29% increase, had the least amount of growth. Many of the P. glandulosa and L. tridentata shrubs had actually lost biomass due to herbivory, but are still extremely healthy. On the second set of windbreaks a large growth spurt, occurring on the northernmost edge, is contributed to a suspected underground water leak. Between the two northernmost windbreaks there is a water shut off valve that is turned off but is most likely leaking small amounts of water near the root level of the plants. In the fall of 2002 several soil samples were taken on the 90th street site using an Oakfield Soil Sampler, and increased moisture levels were found close to the suspected leaking valve. Thus it is apparent that the additional moisture in the ground close to the windbreak in question, possibly due to a leaking water valve, is the contributing factor to the increased growth.

When reviewing the four years of data collected on the 50th and 90th Street sites, a trend can be seen indicating that deep pipe and perforated pipe irrigation methods are superior to surface irrigation. It also appears that over the course of the experiment, shrub plots treated with amendments have higher overall survival then those without. This probably has more to do with water retention than an improvement in soil quality as the lack of significant rainfall has limited the amount of microbial activity seen on the site. During the fifth year of data collection of this project all parameters will continue to be monitored at the 90th and 85th Street sites, whereas all monitoring will cease at the 50th Street site. Our continued goal is to use collected data to determine what restoration methods will be the most cost-effective and efficient for reducing dust in the Antelope Valley.

First Annual Report (November 1, 2000)

Second Annual Report (July 23, 2001)

Third Annual Report (October 29, 2002)

Fifth Annual Report (December 1, 2004)