Soil Ecology and Research Group

last update December 1, 2004

 

Revegetation Methods for the Control of Dust from Desert Soil Disturbances in Antelope Valley
2003 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 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 of farming in Antelope Valley has been seen over the last few years. Most of the land used during the farming season is left fallow during the windy winter season which paves the way for additional dust to be inputted into an already existing air problem. The Dustbusters, comprised of 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, the 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 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 to be experimental plots. It was later discovered that the 50th Street site did not represent recently abandoned farmland, as previously thought, therefore, a third site was selected in June of 2002 as its replacement. The first site is located on 50th Street East, near Palmdale Boulevard. The second site is on 90th Street East, just north of Palmdale Boulevard, and the third site is located on 85th Street and F Avenue. Soil samples were taken in July 1998 from both the 50th, and the 90th Street sites, to determine initial soil nutrient levels. 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 on the 50th and 90th Street sites in January 1999.

The 50th Street site is no longer being monitored for data 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. Survival data was collected for the last time at the 50th Street site in 2003.

The new site located on 85th Street, 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 were grown in the SERG greenhouse at San Diego State University: A. canescens, A. lentiformis, A. polycarpa, L. tridentata and P. glandulosa. A total of 624 seedlings were transplanted on the site in October/ November of 2002. Monitoring of this site will continue until October 2006. 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 2003 through June 2004.

 

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 that had been farmed as recently as 1995, but new information provided by Jim Bort, from the City of Los Angeles Airports, stated that the site had 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 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 second site, located on 90th Street East and Avenue O-8, appeared to have been abandoned at least fifteen to twenty years ago.

The 50th Street site was dominated by scattered groups of non-native annual grasses and Salsola tragus (Russian thistle). By visual and physical inspection of the soil, it was determined to be only slightly compacted, probably due to the recent furrowing activity. The 90th Street site showed no signs of furrowing and was lightly covered with non-native 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. Due to recent agricultural activities, the 85th Street site showed no signs of growth in the late summer / early fall of 2002, and the soil was determined to be slightly compacted.

On November 25, 1998, SERG personnel outlined 15 m (meter) x 30 m shrub plots and 30 m x 30 m 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 of the plots, knock down existing vegetation, and construct windbreaks. Non-native vegetation knocked down on site was incorporated into the 1 m tall by 10 m long windbreaks to provide stabilization and organic matter to the soil. The tractor was again used in December 1998 to alleviate soil compaction. The tractor attachment had 45 cm (centimeter) ripping bars which loosened the soil in all plots to a depth of 30 cm and incorporated dead, non-native, 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 1999 and provided with either TreePees™ 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 TreePees™ as protection from herbivores.

 

EXPERIMENTAL DESIGN

Two types of plot designs (Figures 1) were employed; shrub plots (15 m x 30 m) and wind break plots (30 m x 30 m). 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, and 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. Figure 1 illustrates the designs found on each research site.

 


Figure 1. Plot Design and Treatments

 


Figure 2. Shrub Plot at 85th Street, June 2004

 

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 (Figure 3) plots were established with a 2x5 factorial (Table 1) consisting of two irrigation treatments, perforated pipe irrigation and surface irrigation, and 16 individual plants per windbreak from each of the five native plant species selected. Each site has four windbreak plots and each plot has six windbreaks, three with perforated pipe irrigation and three with surface irrigation. Each windbreak has sixteen seedlings for a total of 1,152 windbreak seedlings.

 


Figure 3. Windbreaks at 85th Street, June 2004

 

Table 1.
Shrub Plots and Windbreaks Factorial


Irrigation Method
Deep-pipe
Surface

Soil Amendment
Control
Wood-chip
Mulch

Species
Atriplex canescens
Atriplex lentiformis
Atriplex polycarpa
Larrea tridentata
Prosopis glandulosa

Shrub Plot (2x3x5)

x
x


x
x
x


x
x
x
x
x

Windbreak (2x5)

x
x


N/A
N/A
N/A


x
x
x
x
x

 

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 (5x5x20 cm and 8x8x25 cm). 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 a lathe house at the University of California, Riverside, to aid in acclimation to desert conditions. Plants for the 85th Street site were germinated in April of 2002 and grown following the same techniques used on the 50th and 90th Street plants, with 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 (TreePees™) were used to protect the plants from herbivory. All plants were re-watered after planting. Seven hundred and four plants were put at each site. Of those 704 plants, 320 went to shrub plots and 384 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 a PVC deep-pipe, buried 30 cm underground. 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, 240 plants on the shrub plots and 384 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. Once established, both sites were able to survive without supplemental water. The 85th Street site was provided with supplemental water twice per month through February 2003. Starting at the end of February 2003 until February 2004, the plants have been provided with supplemental irrigation once per month. Supplemental watering ended March 2004. Table 3 illustrates supplemental irrigation intervals and quantities following outplanting.

 

Table 2.
Species Used to Replant 90th Street Site

Species
Ambrosia dumosa (burro-weed)
Atriplex canescens (4-wing saltbush)
Atriplex lentiformis (quailbush)
Atriplex polycarpa (allscale)
Ephedra nevadensis (green ephedra)
Eriogonum fasciculatum (California buckwheat)
Total
1
72
2
146
57
2
Species
Hymenoclea salsola (cheese bush)
Isomeris arborea (bladderpod)
Larrea tridentata (creosote bush)
Lycium andersonii (box-thorn)
Prosopis glandulosa (honey mesquite)
Senna armata (spiny senna)
Total
5
31
57
25
82
9

 

Table 3.
Supplemental Irrigation Quantities and Intervals

Months After Outplanting
1-3
4-6
7-12
Gallons of Water per Plant
1
1/2
1/2
Watering Interval
2 weeks
2-3 weeks
3-4 weeks

 

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 with 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 the majority of the plants on the 85th street site throughout the spring of 2004.

In September 2003 and March 2004, soil samples were taken from the 85th and 90th Street sites and sent to A&L Laboratories, in Modesto CA, to be analyzed for macro and micro-nutrients, organic matter, and pH level. SERG personnel at San Diego State University conducted Europium(III) staining on the samples; a process used to measure fungal and bacterial abundance by using a stain that binds to DNA and fluoresces under UV light. 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 non-native 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 2004, plant survival and biomass data were measured and analyzed for the 85th and 90th Street sites (Figure 4). Both sites were also weeded and cleared of drifting debris in June 2004.

 


Figure 4. Data Collection on 90th Street Shrub Plots, June 2004

 

RESULTS

Survival Data
Survival data was collected for the 85th and 90th Street sites in June 2004. Overall survival at 85th Street was 85%, with shrub plots at 89% and windbreaks at 83%. The highest shrub plot survival was 92%, for the deep-pipe irrigation treatment, while the lowest was 79%, for the deep-pipe irrigation / mulch amendment treatment. Windbreaks with surface irrigation had 78% survival while windbreaks with perforated pipe irrigation had 89% survival. Statistical analysis, using Analysis of Variance (ANOVA), found this to be a significant difference with a P-value of 0.006. Any value less than 0.05 is considered to represent a significant difference. There were no significant differences in irrigation methods or amendment treatments for the shrub plots. Percent survival by treatment is illustrated in Table 4.

The 90th Street site had an overall survival of 60%, with shrub plots having 50%, and windbreaks having 68% survival (Table 4). Shrub plot survival had a low of 42% in the control treatment and a high of 61% in the deep-pipe irrigation with no soil amendment treatment. Statistical analysis (ANOVA) indicated a significant difference between irrigation methods on the shrub plots with deep-pipe irrigation at 56% survival, which outperformed surface irrigation at 44% survival, giving a P-value of 0.02. This was the only significant difference found for the irrigation methods and amendment treatments on the shrub plots. Windbreaks at 90th Street continue to outperform the shrub plots with survival of 68% for perforated pipe irrigation, and 67% for surface irrigation. The difference in survival between irrigation methods for the windbreaks is not statistically significant.

 

Table 4.
Percent Survival by Treatment

Shrub Plot Irrigation and Amendment
Deep-pipe (DP)/Control
Surface/Control
DP/Mulch
DP/Wood Chips
Surface/Mulch
Surface/Wood Chips
Windbreak Irrigation
Surface
Perforated Pipe
85th Street
92%
92%
79%
80%
90%
88%

78%
89%

90th Street
61%
42%
58%
46%
43%
48%

67%
68%

 

Soils Data
In September 2003 and March 2004, soil samples were collected from the 85th and the 90th Street sites, and from an undisturbed reference site nearby. The samples were then sent to A&L Labs in Modesto, CA, to be analyzed. Results (Table 5) of both sites were then compared to previous samples and to the undisturbed site. Baseline samples at 85th Street were taken in April 2002, and biannual sampling began in March 2003. The site was used for growing onions between these two data collecting occasions. The 85th Street site still shows numbers typical of fertilized cropland. Nitrates and sulfates are decreasing but still remain at high levels. The same pattern can be seen for organic matter which is comparably high to undisturbed land, but decreasing.

The 90th Street site shows a decrease in organic matter while nitrates and sulfates have increased. Zinc and manganese show a decrease, copper remains stable, and iron is increasing.

 

Table 5.
Comparison of Soil Analysis

Site

90th

 

 

 


Undisturbed

 

 


85th

Date

Sep-99
Sep-00
Apr-01
Sep-01
Apr-02
Oct-02
Mar-03
Oct-03
Mar-04
Sep-00
Sep-01
Apr-02
Oct-02
Mar-03
Oct-03
Mar-04
Apr-02
Mar-03
Oct-03
Mar04

% OM

0.5
0.4
1.2
0.93
0.86
0.71
0.57
0.45
0.28
0.5
0.7
0.5
0.55
0.2
0.1
0.15
1.95
1.71
1.48
1.26

Nitrates
(ppm)
19.4
16.2
5.4
6
12.56
10.19
9.69
12.69
19.69
41
7
5.3
40.5
6.5
7
7.5
45.5
55.88
108.5
50.75
Zinc
(ppm)
0.3
0.2
0.8
0.35
0.46
0.44
0.23
0.18
0.19
0.9
1.65
0.9
0.55
0.1
0.1
0.3
0.75
1.51
1.19
1.01
Iron
(ppm)
3.8
3.8
2.6
2.4
1.2
1
4.63
3.44
5.19
9
5.5
5.3
4
5.5
4.5
4
1
2.94
3.75
3.06
Copper
(ppm)
0.4
0.4
0.5
0.3
0.3
0.35
0.6
0.41
0.29
0.6
0.35
0.41
0.15
0.25
0.35
0.2
1.4
1.73
1.83
1.51
Manganese
(ppm)
6.3
5.7
7
6.1
5.1
5.94
2.88
3.69
2.69
12
8
7.6
7
1
2
1
6.5
3.69
12.56
4.56

pH

7.9
8.1
7.9
7.9
7.79
8.21
8.04
8.46
7.5
6.9
6.9
6.75
7.5
7.6
7.75
7.6
7.5
7.53
7.86

Sulfates
(ppm)
8.5
7.9
4.5
10.6
15.8
13.2
5.73
22.6
10
7
7.2
21
6
8
18
926
371.73
538.2
335.33

 

Europium Data
Based on the trend over the past four years data, soil analysis for hyphae length was not conducted fall 2003 or winter 2004. Due to the lack of expected change over time and continued low rainfall it was determined that a final year of data would show no significant change.

Previous fungal hyphal lengths between the various treatments of mulch, wood chips, control and windbreak plots on 85th and 90th Street had demonstrated no significant difference (Table 6). Except for a temporary increase in numbers in October of 2002, they have been demonstrating a slight 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. Shrub plots with wood chip amendment show the highest numbers of hyphae when compared to the other treatments, including irrigation methods, on either site for 2002 / 2003. Though an increase in hyphal lengths was expected to occur as the project developed over time, such activity is highly dependent on precipitation that acts as a catalyst for the chemical process of decomposition. Antelope Valley, as well as all of southern California, has experienced an extended drought since the El Niño year of 1998. Because of this, it appears that the expected increase in hyphal lengths has not occurred. It is expected that once Antelope Valley experiences several years of normal rainfall, hyphal lengths will increase.

 

Table 6.
Meters of Fungal Hyphae per Gram of Soil

Site

90th
90th
90th
90th

85th
85th
85th
85th

Treatment

Shrub Plot Control
Shrub Plot Mulch
Shrub Plot Wood Chip
Windbreak
Undisturbed
Shrub Plot Control
Shrub Plot Mulch
Shrub Plot Wood Chip
Windbreak

9-99
m/g
0.07
0.08
0.13
0.05
-
-
-
-
-
4-00
m/g
0.05
0.05
0.02
0.03
0.03
-
-
-
-
9-00
m/g
0.05
0.03
0.03
0.05
0.07
-
-
-
-
4-01
m/g
0.03
0.06
0.08
0.08
0.07
-
-
-
-
9-01
m/g
0.01
0.02
0.01
0.04
0.04
-
-
-
-
4-02
m/g
0.01
0.02
0.01
0.02
0.03
0.08
-
-
-
10-02
m/g
0.04
0.03
0.02
0.01
0.01
-
-
-
-
3-03
m/g
0.01
0.02
0.01
0.01
0.01
0.01
0.01
0.08
0.01
9-03

-
-
-
-
-
-
-
-
-

3-04

-
-
-
-
-
-
-
-
-

 

Bacteria numbers are all showing a decrease over time with a high of 372 million bacteria per gram of soil seen in the control shrub plots on 85th Street and a low of 54 million seen on the shrub plots with wood chip amendments on 90th Street (Table 7). Numbers at 85th Street are higher than those on 90th Street, as would be expected. 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 over time. Undisturbed samples have decreased from their original numbers. There were no significant differences (ANOVA) seen between amendment treatments or irrigation methods. The decrease in bacteria numbers, though expected as the site progresses over time, is probably caused more from the drought mentioned above than by competition from an increased fungal population that would normally occur on a desert restoration site that experienced average precipitation. Again, as stated above, once Antelope Valley has experienced several years of average rainfall, bacteria and fungal measurements will more accurately reflect the actual soil processes occurring on the restoration sites.

 

Table 7.
Millions of Bacteria per Gram of Soil

Site
90th
90th
90th
90th

85th
85th
85th
85th

Treatment
Shrub Plot Control
Shrub Plot Mulch
Shrub Plot Wood Chip
Windbreak
Undisturbed
Shrub Plot Control
Shrub Plot Mulch
Shrub Plot Wood Chip
Windbreak
9-99
201
278
392
397

-
-
-
-

4-00
110
121
20
80
561
-
-
-
-
9-00
858
956
20
80
590
-
-
-
-
4-01
420
823
661
286
584
-
-
-
-
9-01
437
1116
562
297
437
-
-
-
-
4-02
771
448
459
589
584
3861
-
-
-
10-02
273
457
421
252
119
-
-
-
-
3-03
263
151
136
88
162
364
537
303
157
9-03
587
346
406
189
65
403
520
452
550
3-04
78
142
54
136
31
372
363
162
162

 

Biomass Data
Biomass data was collected at the 85th and 90th Street sites in June 2004. Total biomass for the shrub plots and windbreaks combined was 775.39 m3 for 90th street (Table 8) and 311.44 m3 for 85th Street (Table 9). These numbers reflect a total of 53% growth during the last year for 90th street. For the same site, the highest percent change was seen at the shrub plots with deep-pipe irrigation / mulch surface amendment, which had 80% growth. The least amount of growth occurred at the shrub plots with surface irrigation and no amendment treatment, which had a growth of 13%. Windbreaks on 90th Street grew 67%.

 

Table 8.
Biomass in meter3 at Shrub Plots and Windbreaks on 90th Street

TREATMENTS
Shrub Plot
Windbreak w/Perforated Pipe
Windbreak w/Surface Irrigation
6-2000
18.00
6.55
5.96
6-2001
49.43
38.93
26.52
6-2002
105.63
137.60
81.00
6-2003
202.74
176.88
126.95
6-2004
268.66
296.75
209.98

 

Table 9.
Biomass in meter3 at Shrub Plots and Windbreaks on 85th Street

TREATMENTS
Shrub Plot
Windbreak w/Perforated Pipe
Windbreak w/Surface Irrigation
6-2003
35.09
23.99
23.95
6-2004
133.34
92.83
85.26

 

Combined increase in growth on 85th Street was 275%, with shrub plots and windbreaks growing by 280% and 271% respectively. The biggest increase in growth was seen on the shrub plot treatment with deep pipe irrigation/mulch amendment at 476%. Shrubs on windbreaks with perforated pipe irrigation grew 287% compared to 256% for surface irrigated shrubs. Figure 5 shows the 85th Street site in November 2002 compared to June 2004.

 


Figure 5. Comparison of the 85th Street Site, November 2002 (left) and June 2004 (right)

 

DISCUSSION

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 ability of the soil 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 60% in June 2004. With an extremely dry winter in 2002, followed by a normal winter in 2003, and a very dry winter in 2004, these plants are at 60% survival, which is the expected survival rate in desert restoration projects. The windbreaks, with 68% survival, have a higher survival rate than the shrub plots, which have an overall survival of 50%. As expected, shrub plot survival numbers for irrigation methods show deep-pipe irrigation significantly higher than surface irrigation with (P-value 0.02) 56% and 44%, respectively. There is no significant difference (ANOVA) seen in survival numbers between the amendments used. An additional factor in the low shrub plot survival is believed to be herbivory. This is based on grazing marks seen on the shrubs and from observing jackrabbits and various burrows on site. The site has two shrub plots, one eastern and one western plot. The plots are equal in design and identical in the proportions of irrigation methods, surface amendments, and species. From the start of the project almost all signs of herbivory on the shrub plots have been seen on the eastern plot, as well as the majority of the burrows. By separating and comparing the numbers between the two plots, a difference in survival is obvious in all treatment methods applied (Table 10).

 

Table 10.
Shrub Plot Survival at 90th Street

Western Plot
DP / MUL.
SUR / WC
DP ONLY
SUR / MUL.
CONTROL
DP / WC
Overall:
80%
60%
79%
65%
63%
68%
70%
Eastern Plot
DP / MUL.
SUR / WC
DP ONLY
SUR / MUL.
CONTROL
DP / WC
Overall:
30%
35%
35%
20%
21%
25%
30%

 

Ignoring the results from the eastern plot at 90th Street, and using the data collected only from the western plot at 90th Street, the survival rate is 70% for the shrub plots. This is higher than the windbreak survival of 68%. As expected, deep-pipe irrigation numbers are significantly higher than all surface irrigation numbers, 77% and 63%, respectively (P-value 0.05). Deep-pipe irrigation with a mulch treatment is still at 80% survival, and deep-pipe irrigation with no surface amendment is at 79%. Total survival on 90th street would be 68% rather than 60% if the eastern plot data was ignored. Why the eastern area is so heavily grazed is unclear, but the western area is approximately 40 meters from 90th Street (the actual road) and traffic may keep the grazers away from this end of the site.
With almost normal precipitation during the first season, and below normal during the second season, overall survival at 85th Street was 85%. Shrub plots had a higher survival rate at 89%, compared to 83% for the windbreak plots. The only significant difference seen in survival rates is between the perforated pipe irrigation and surface irrigation methods used on the windbreaks. No significant difference can be seen between irrigation methods on the shrub plots, however, more significant trends are expected to appear on 85th Street as the project progresses. Last year's data showed a significant difference in irrigation methods on the shrub plots, favoring surface irrigation over deep-pipe irrigation. This contradicted the initial expectations of the experiment. This year the difference is no longer significant and deep-pipe irrigation is expected to yield significantly higher survival in the long run.

For the past two seasons it has been noted that both shrub plots at 90th Street had new Atriplex seedlings appearing, indicating a successful seed bank is being developed on this site. No seedlings have been observed as of yet at the 85th Street site, although, many of the Atriplex species were producing large quantities of seed and natural recruitment should begin to appear over the next several years.

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 their 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 8 and 39 ppm, except for one, which was 80 ppm. A number this high indicates an outside influence and is not considered representative of the site.

The results of the March 2004 soil samples taken at 90th Street show a decrease in organic matter. A decrease in organic matter indicates that it is being broken down (by
fungi and bacteria) and made available to the plants in the form of nitrates. Nitrogen levels often go up following organic matter break down, something which is indicated in the data. Average sulfate levels at 90th Street have increased from 13.2 ppm in the spring of 2003, to 22.6 ppm in the spring of 2004. 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 20 ppm without any adverse effect on the plants. The below average rainfall seen during the 2003-2004 rainy season is likely to have contributed to the high levels of sulfate.

Soil nutrient levels on 85th street are typical for recently farmed land. Soil samples have only been analyzed four times for the site; once before planting, and three times after planting. The site was used for growing onions between the first and second data collecting occasion. Some expected changes seen in the data collected following outplanting were decreases of organic matter, nitrates, and sulfates. Further decreases are expected as the data still show that nutrient levels are comparable to the high levels characteristically found in agricultural soil.

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 micro-organisms such as fungi and bacteria. Over the past several years rainfall has been extremely low. Analysis of hyphae length for the final year of the 90th Street site was unnecessary as four years of data collection had shown no significant trend. Analysis of the 85th Street site will continue starting fall 2004 and if normal rainfall occurs will provide valuable information about the fungal/bacteria cycle.

Bacteria play a major role in the mineralization of non-recalcitrant matter, as opposed to fungi, which serve as the primary decomposers for the more complex, recalcitrant, organic matter. Non-recalcitrant matter is made up of simple carbon compounds found in both non-native and native annuals. Recalcitrant matter is made up of more complex carbon compounds such as those found in bark, wood chips and perennial, woody 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(III) 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 mixed 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 and normal precipitation occurs. 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 an analysis of variance (ANOVA) 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 (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, decomposition of the wood chips by fungal networks within the soil may have started sooner than would be expected for an arid climate like that found in Antelope Valley. Supplemental watering during the early stages of the restoration project was also a possible factor in the early increase in fungal hyphae.

The relationship between microorganisms and soil nutrients displays itself as a trend when looking at the levels of organic matter, nitrogen, and bacteria, over time. As bacteria break down non-recalcitrant material such as grasses, herbs and forbs, the percent of organic matter in the soil drops and levels of nitrogen increase in the form of nitrates. The high level of nitrates in the soil facilitates more growth of non-native annuals, which are nitrophilic organisms. When a desert community is dominated by perennial shrubs, bacteria cannot begin the decomposition process first because the organic matter is too complex; fungi need to break down this recalcitrant, woody, material to more simple compounds. With this condition in the soil environment, nitrates are made available at a much slower rate, curbing the rampant growth of non-native annuals. Observing a restoration project in the desert through time, it can be expected to see decreases in the number of bacteria. Once the restoration sites experience several years of normal rainfall, the expected rise in fungal hyphae and decrease in bacteria numbers should occur.

Biomass at 90th Street has increased 53% over the past year, with the highest increase seen at the windbreaks at 67%. Shrub plots with surface irrigation and no surface amendments, with a 13% 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, may have occurred because of an underground water leak. Between the two northernmost windbreaks there is a water shut-off valve that is turned off, but 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. 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 five years of data collected on the 90th Street site, 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 than 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 at the site. During the sixth year of data collection for this project, monitoring will cease at the 90th Street site, whereas all parameters will continue to be monitored at the 85th Street sites. 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 Antelope Valley.


First Annual Report (November 1, 2000)

Second Annual Report (July 23, 2001)

Third Annual Report (October 29, 2002)

Fourth Annual Report (March 9, 2004)