Soil Ecology and Research Group
last update July 23, 2001
Revegetation Methods for the Control of Dust from Arid/Desert
Soil Disturbances in the Antelope Valley
2000 Annual Report
The Antelope Valley, located fifty miles north of Los Angeles in the Mojave Desert, is 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 Sierra Nevada 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 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. The Dustbusters, a coalition of local farmers, the California Air Resources Board, South Coast Air Quality Management, The Natural Resource Conservation District, The City of Los Angeles Department of 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 delineate exactly what the specific impacts are, the most cost effective methods to mitigate for these impacts, what native species are best suited for restoration on such disturbed lands and 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 near Palmdale Boulevard while the second is on 90th Street just north of Palmdale Boulevard. Soil samples were in July 1998 from both sites to determine initial soil nutrient levels and it was determined that five native plant species would be used for the study. Fourteen hundred seedlings comprised of Atriplex canescens (saltbush), Atriplex lentiformis (quailbush), Atriplex polycarpa (shadscale), 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 1999 Annual Report, including site preparation, planting and a review of data collected for the 1999 report and then detail all activities and data collected at the plots from July 2000 through June 2001.
The Los Angeles City Airport provided two sites for SERG to conduct revegetation experiments. The first site, located on East 50th Street (34' 37.4" N, 118' 02.4"W), is abandoned farmland. We had originally been informed the site had been farmed as recently as three to five years ago, but new information provided by Jim Bort, from the City of Los Angeles Airports, states the site has lain fallow for the past twenty-five years. The site has, however, been used by the Los Angeles County Sanitation District for the disposal of secondary effluent as recently as five years ago. 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 East 90th Street, (34' 36.5"N, 117' 59.0"W), appears to have been abandoned a minimum of fifteen to twenty years ago.
The 50th Street site was dominated by scattered groups of exotic annual grasses and Salsola tragus (Russian thistle). The soil was 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, and a few native species, including Chrysothamnus nauseosus (rabbitbrush), an early seral invader. 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 which was used to back blade all the plots, knocking down existing vegetation and to construct windbreaks (Figure 1). Exotic vegetation knocked down on site was incorporated into the 1m tall by 15m 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 pipe irrigation 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 (Figure 2). In addition, each shrub plot was provided with amendments and irrigation treatments as required by the experimental design described in the following section.
Two types of plot designs were employed, shrub plots (15m x 30m) and wind break plots (30m x 30m). Shrub plots were established using a 2x3x5 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 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 640 shrub plot seedlings. Windbreak plots were established with a 2x5 factorial consisting of two irrigation treatments, perforated pipe and surface irrigation, and 16 individual plants per windbreak. Each site has four windbreak 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 768 windbreak seedlings.
Figure 1. Windbreak construction at 90th Street December 1998.
Figure 2. Srub plots at 50th Street June 1999.
Seed Collecting and Planting
The fourteen hundred seedlings used for this project were grown in the SERG greenhouse at San Diego State University. The seed was obtained from S and S Seeds and had been collected from the Mojave Desert. Plants 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 also fertilized for the first three months. In November 1998 the plants were transferred to the University of California, Riverside lathe house to acclimate them to desert conditions.
In January 1999 the 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 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 (Figure 3). Designated sub-plots within the shrub plots were covered in compost or wood chips.
Maintenance and Monitoring
In March 1999 numerous plant protectors that had been blown off in high winds were replaced at the 90th Street site. At this time it was noticed that many of the plants had suffered heavy herbivory and would have to be replaced. 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.
Figure 3. Larrea with deep pipe irrigation treatment.
All replacement plants were grown from Mojave Desert seed at the San Diego State University greenhouse (Table 1).
Species Used to Replant 90th Street site
The 50th Street site was provided with supplemental water through July 2000. The 90th Street site, because it was replanted, continued to be watered monthly until January 2001. Both sites are now established and can survive without supplemental water (Figure 4 and 5).
Figure 4. 50th Street shrub plots May 2001.
Figure 5. 90th Street windbreak May 2001.
In July 2000 SERG personnel began removing plant protectors from both sites. Plant protectors were only removed from plants that were large enough to withstand herbivory. The protectors were removed on a regular basis through out the year with all removed by June 2001.
In April 2001,soil samples were taken from both sites and sent to A&L Laboratories,
in Modesto CA, to be analyzed for macro and micronutrients, organic matter and
pH. Fungal and bacterial measurements were taken and analyzed at San Diego State
University by SERG personnel. At the request of Rob Farber, samples were also
taken from the suppressant plots located at 50th Street. These were also sent
to A&L Laboratories for analysis. In April 2001 it was observed that 50th
Street had weeds to such an extent as to interfere in the growth of the plants,
while 90th Street, although weedy, was not excessively so. In April 2001 50th
Street was weeded and in June 2001 plant survival/biomass was measured and analyzed.
Percent survival data was collected for both the 50th and the 90th Street sites in June 2001. Overall survival at 50th Street was 71%, with shrub plots at 65% survival and windbreaks at 75%. The highest shrub plot survival was 78% for the deep pipe/mulch treatment while the lowest was 55% for the surface water/wood chip treatment. The control subplot was only one percent higher at 56%. Windbreaks with surface irrigation had 78% survival while windbreaks with perforated pipe had 71% survival. No significant difference was found for any of the windbreak treatments. The only significant difference between treatments for the 50th Street shrub plots was deep pipe over surface watering (p-value = 0.01). Percent survival by treatment is illustrated in Table 2.
The 90th Street site had an overall survival of 86%, with shrub plots having 81% and windbreaks having 91% survival. Shrub plot survival had a low of 63% in the control subplot and a high of 98% in the deep pipe/mulch treatment. A significant difference in survival between shrub plot amendments was found with mulch performing significantly better (p-value = .04) than the control subplot but not better than the wood chip subplot. Windbreak treatments were separated by only 2% survival with perforated pipe having 92% and surface irrigation having 90% survival. No significant difference between irrigation methods on the windbreaks is seen at this time. Although not statistically significant, shrub plots with deep pipe and windbreaks with perforated pipe had higher survival than those that received surface water. Percent survival by treatment is illustrated in Table 2.
Percent Survival by Treatment
Deep pipe (DP)
In April 2001, soil samples were collected from both 50th and 90th Street sites, the suppressant plots and from a site currently being exposed to secondary effluent and sent to A&L Labs in Modesto, CA to be analyzed. Results of the 50th and 90th Street were then compared to previous samples from both sites and from an undisturbed area.
The 50th Street remains higher than both 90th Street and the undisturbed site for all parameters. Organic matter at 50th Street has remained stable, available nitrogen has risen and all minor nutrients at 50th Street have fallen with a significant fall seen in both iron and manganese.
The 90th Street site demonstrates an increase in organic matter and a decrease in available nitrogen compared to 1999 samples. Both zinc and copper are stable and there is an increase in iron and manganese. The suppressant and secondary effluent plots demonstrate levels similar to those found on the 50th Street restoration site. Table 3 shows the overall averages for all sites.
Comparison of Soil Analysis
Fungal hyphal lengths between the various treatments of mulch, wood chips, windbreak and shrub plot control have not yet demonstrated any significant difference at either the 50th Street or 90th Street site. Fungal lengths range from 0.03 to 0.08 meters of hyphae per gram of soil with both 50th and 90th Street averaging about 0.05 meters of hyphae per gram of soil (Table 4).
Meters of Hyphae per Gram of Soil
Shrub plot control
Bacterial numbers also show no significant difference at this time between either treatments or the two sites. However, mulch treatments on both 50th and 90th Street have fairly high bacteria numbers, relative to other treatments, and they appear to be increasing over time. The 50th Street site also appears to have larger numbers of bacteria than the 90th Street site overall. Bacteria numbers range from a high of 28.1E+08 in the 50th Street treatments to a low of 0.2E+08 in the 90th Street wood chip treatment (Table 5).
Bacteria per Gram of Soil
Shrub plot control
Biomass data was collected at both sites in June 2001. The total biomass at the 50th Street site was 451,017,825 cm3. Total biomass for the 90th Street site was 114,879,556 cm3 (Table 6). These numbers reflect an average of 273% growth in the last year with 50th Street having a 269% increase and 90th Street a 276% increase. The highest percent change is seen at the 90th Street windbreaks where the perforated pipe site had a 494% increase in growth. The least amount of growth occurred at the 90th Street shrub plots, which had an increase of 175%.
Statistical analysis shows no significant difference in biomass for windbreak treatment, however, the shrub plots at 90th Street showed a significant difference in growth with wood chips being superior to both control (p-value = .01) and mulch (p-value = .03).
Biomass at Shrub Plots and Windbreaks
In 1999 the 90th Street site was subjected to severe herbivory due to the loss of many of the plant protectors from high winds. The site was replanted but data from 90th Street was not used in the 1999 Annual report. This year's data was collected and analyzed from both the 50th and the 90th Street sites.
Overall survival at 50th Street was 71%, compared to last year's 79%. At 71% survival is still at an acceptable level for a desert restoration project. There was still no significant difference seen between amendment treatments, though survival in the deep pipe/mulch treatment was highest at 78%. This lack of any difference is to be expected since the normally low amount of rainfall occurring in the Antelope Valley leads to an extremely slow decomposition of the recalcitrant amendments of wood chips and mulch. Treatment differences, if they are to occur, may not appear until 3-4 years into the project.
A significant difference was seen between irrigation methods used on the shrub plots. The significantly higher survival rates seen with deep pipe treatment compared to surface irrigation was expected due to high evaporation rates normally seen when surface irrigation systems are used. The difference between windbreak/irrigation treatments, though not statistically significant, with surface irrigation having higher survival rates at 78% than perforated pipe at 71%, was unexpected. It is generally believed that deep pipe and perforated pipe systems will perform better than surface irrigation due to lower evaporation rates. It is possible that as the experiment goes on, the windbreak plants with perforated pipe will out-survive the plants with surface irrigation. However, it is also possible that because the windbreaks are man made, the soil is loose enough to allow the surface irrigation water to percolate fast enough into the mound to minimize water loss from evaporation, therefore allowing more water to reach the plant roots. This could explain the lack of any significant difference in survival rates between the perforated pipe and surface irrigation systems.
Percent survival at 90th Street was extremely high at 86%. Because 90th Street was replanted in March of 2000, this is not unexpected. Survival the first year of an experiment can often be high if the newly planted seedlings are consistently watered to help them become established. Windbreaks had the highest overall survival and this may be because many of the replants went onto the windbreaks. Windbreaks with perforated pipe and shrub plots with deep pipe had higher survival than those with surface irrigation, which was expected for the same reasons explained above.
Results of the soil analysis are similar to those reported in the 1999 Annual report. The unusually high levels of nutrients at the 50th Street site are most probably the result of the site being flooded with secondary effluent as recently as five years ago. The high levels of nitrates, sulfates, zinc, iron, and copper are comparable to those seen at the site that is currently being flooded with secondary effluent. The acidity of the soil at 50th Street is also comparable to the currently flooded site, both having extremely low pH for desert soils. The addition of lime to the soil would correct the pH, but in order to determine cost effective restoration methods, we recommend the soil be allowed to correct itself with the help of the native plant species now growing there. The macro and micronutrient levels should decrease over time, as they currently seem to be doing with the possible exception of available nitrogen. As this happens, it is expected the growth pattern of the plants will continue to slow down, reflecting a more natural growth pattern.
The 90th Street soil continues to have lower levels of nutrients than the 50th Street site, and are therefore more typical of a desert environment. Nitrates at this site have fallen significantly in the last year. This drop was most likely caused from the seedlings using much of the available nitrogen that was in the soil for their initial growth. To replace the nitrates used, the organic matter provided by the plants must be mineralized, a process that is extremely slow in desert communities. After the initial growth spurt, plant growth will typically slow down and upon reaching maturity, these native plants will require a minimum of nitrates and other nutrients. As this happens, nutrient levels in the soil should stabilize at levels comparable to the undisturbed site. Organic matter (OM) at the 90th Street site has risen and it is from this that the nitrates will be replaced.
Measurements of fungal hyphal lengths and bacteria numbers have, not unexpectedly,
shown no significant differences between treatments or sites even after two
years. The mineralization process, by which organic matter is broken down into
its chemical elements by both fungi and bacteria, is an extremely slow process
in semi-arid and arid habitats such as the Mojave Desert. Since water availability
is a key factor in mineralization, the limited amounts of precipitation experienced
by 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 which serve as the primary decomposer for the more complex recalcitrant organic matter. 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 time for both 50th and 90th Street sites. Fungi have demonstrated no such increase. Since desert plant communities are shrub dominated, and therefore provide mostly recalcitrant matter to the soil, desert soils normally have fungi dominated mineralization cycles. It is expected that as the project develops over time, fungal activity will increase while bacteria numbers will level out as a self-sufficient plant/soil desert community develops.
Although nutrient levels remain high, plant growth at 50th Street has slowed down over the past year and appears to be moving towards a more natural growth pattern. Total biomass at 50th Street has increased 269% compared to 276% at 90th Street. The 90th Street data reflects first year growth for approximately 70% of the plants because 489 of them had to be replaced, therefore the growth rate is probably due to the initial growth spurt commonly seen the first season after plants have been outplanted. When comparing 90th Street's growth from this past year at 114,879,556 cm3, to 50th Street's growth its first year at 122,100,220 cm3, it still appears that the 90th Street plants are growing at an overall slower rate than 50th Street as previously reported. The 90th Street growth for the past year includes 30% second year growth and yet the total biomass is still smaller than the biomass measured at 50th Street in 2000.
No significant differences, when comparing biomass, were seen between amendment treatments or irrigation treatments at the 50th Street site. Although a significant difference was seen between amendment treatments at the 90th Street site, it is possible that the primary reason for increased growth seen in the wood chip sub-plots is due to less water evaporation rather than the amendment contributing to increased soil quality. In an arid environment, it generally takes years before amendments like wood chips and mulch have broken down enough to change the nutrient levels in the soil.
Since the loss of plant protectors at 90th Street in March 1999, it has been observed that not only is survivorship higher for plants in Tree-pees but the quality of the plants are also better. Tubex are often small in diameter, usually 4-6 inches and cylindrical in shape. Tree-pees are wider at the bottom, 8-10 inches, and narrow to 4-5 inches at the top. The tubex produce tall narrow plants while the Tree-pees appear to provide enough room for the plants to branch out. Both styles of plant protectors should be removed before the plant starts growing out the top to keep the plants from being misshapen. Tubex are less expensive and lighter, but they are difficult to install firmly in the ground. Tree-pees have wire stakes on the bottom which make them easier to install in many desert soils and the weight and cone shape keep them from blowing away as easily as the tubex. Tree-pees can be more difficult to install in real rocky ground and they are more difficult to store because of their size. Although tree-pees are more expensive and harder to store, they appear to provide better protection to the plants while allowing them to grow a more natural shape.
It seems that the 50th Street site was not removed from active farming approximately 5-8 years ago as was originally thought. The extremely high nutrient levels compare quite closely to the neighboring suppressant plots and to an adjacent area that is currently being flooded with secondary effluent. It appears that the site, as verified by Jim Bort of the Los Angeles Airport, was for many years subjected to flooding with secondary effluent and is the direct cause for the high nutrient and low pH levels found throughout the site. These levels are most probably the cause of the extremely high initial growth rates seen at the 50th Street site. Over time, these levels should decrease, as it appears they are beginning to do. As this happens, growth rates should decrease proportionately until they fall more in line with what is seen in undisturbed areas.
The 90th Street site, with its low nutrient levels and higher pH, appears to
have been abandoned from active farming for at least 20 years as was originally
reported. With nutrient and pH levels more in line with those found in undisturbed
areas, it appears this site has had time to slowly return to more "normal"
soil conditions. This explains the lower initial growth rates, and especially
the lower biomass amounts, experienced by the 90th Street plants when compared
to those at the 50th Street site. This site has a shorter road to travel to
achieve a natural, self-sustaining plant/soil ecosystem than 50th Street.
Data collected this year indicates the possibility that treatments, both irrigation and soil amendment, may be starting to have effect on some of the parameters being analyzed. Though currently too early to tell if actual trends are being formed, upcoming third year results may be significant enough to develop some assessments on treatment effects. All parameters will continue to be monitored during the third year of the project with the goal that some conclusions as to what restoration methods are the most cost effective and efficient for controlling dust from soil disturbances in the Antelope Valley will be developed.
First Annual Report (November 1, 2000)
Third Annual Rreport (October 29, 2002)
Fourth Annual Report (March 9, 2004)
Fifth Annual Report (December 1, 2004)