Soil Ecology and Research Group last update February 17, 2003

COASTAL SAGE SCRUB RESTORATION
ON 0.6-ACRE MITIGATION SITE ON
MARINE CORPS AIR STATION MIRAMAR,
SAN DIEGO, CALIFORNIA

Final Report

Executive Summary
In November 1998, a portion of Johnson Road was widened to accommodate ordnance vehicles on Marine Corps Air Station Miramar. The road widening extended into 0.3 acres of coastal sage scrub inhabited by the federally threatened coastal California gnatcatcher (Polioptila californica californica). As compensation for the loss of habitat, 0.6 acres of coastal sage scrub restoration was required. On 28 September 1998, Southwest Division awarded a Cooperative Letter of Agreement, at the request of MCAS Miramar, to the Soil Ecology and Restoration Group to locate potential 0.6-acre coastal sage scrub restoration sites, prepare a document outlining these sites, and restore the site selected by the Station. In March 1999, a site adjacent to Austin Avenue was selected and approved.

As part of the standard protocol for evaluating a new restoration site, soil compaction was measured throughout the site. An abandoned dirt road running through the site was found to be as compacted as a nearby actively used dirt road. To study the effects of soil compaction on container seedling survival, an experiment was installed comparing transplants in a rototilled portion of the abandoned road and a control untilled portion. In March 1999, fifty coastal sage scrub seedlings were planted in each portion. Two hundred more seedlings were transplanted in the disturbed area around the abandoned road.

In January 2001, seedling survival was low, so in February 2001, 113 replacement seedlings were planted. Seedling survival comparisons among the heavily compacted road portion of the restoration site, the decompacted rototilled road, and the slightly compacted disturbed non-road revealed that as soil compaction increases, transplant survival decreases. Prior to transplanting the replacement seedlings in February 2001, more sections of the restoration site were rototilled and a new soil compaction experiment was installed. The percent survival of seedlings was estimated again in June 2002 and used to determine the effectiveness of rototilling in improving transplant survival. Results indicate a potential benefit of soil decompaction and reinforce the results of the soil compaction experiment first installed in 1999.

The annual vegetation and soil survey took place again in June 2002 and results were compared to the surrounding undisturbed area. Through all four years of the project, soil characteristics have not indicated a need for soil remediation. Although the Austin Avenue restoration site did not meet all success criteria, remedial action is not necessary. Percent cover of native species and overall survival of transplants were low due to a dry 2001-2002 rainy season and the inability to access the restoration site after the 11 September 2001 terrorist attacks. The situation is temporary as far as percent cover is concerned and species density is high enough to outweigh the deficits. After four years of site preparation, transplanting native coastal sage scrub seedlings, and site maintenance, the restoration site at Austin Avenue is expected to continue to recover on its own. With each additional growing season, the site will begin to more closely resemble the surrounding undisturbed coastal sage scrub habitat.

Introduction
Marine Corps Air Station Miramar, hereinafter referred to as the “Station”, is located in San Diego County, California four miles east of the Pacific Ocean and 13 miles north of downtown San Diego (Figure 1). Road expansion work in November 1998 on Johnson Road just east of Kearny Villa Road and south of Ammo Road extended into a total of 0.3 acres of undisturbed coastal sage scrub habitat. This area is known to support populations of the federally threatened coastal California gnatcatcher (Polioptila californica californica). Mitigation for this project required restoration of a 0.6-acre area. In September 1998, at the request of MCAS Miramar, a Cooperative Letter of Agreement was signed between Southwest Division, Naval Facilities Engineering Command and the Soil Ecology and Restoration Group (SERG) at United States International University (now Alliant International University) to locate potential 0.6 acre restoration sites, prepare a restoration plan for the selected site, and restore the site to coastal sage scrub vegetation. In November 1998, seven sites were selected for potential restoration and described in the document “Proposed Restoration Sites for 0.6 Acre Native Plant Restoration on Marine Corps Air Station Miramar, San Diego, California”, dated 20 November 1998. Site number four was selected from this document and approved by the MCAS Miramar, Public Works Department in March 1999.

During the first year of this project (1998-1999), soil strength measurements on the restoration site showed a high degree of compaction on the dirt road bisecting the site. One-half of the road was rototilled to test the effects of soil compaction on seedling survival. In April 1999, 300 coastal sage scrub seedlings grown in the SERG greenhouse were planted on the site. Vegetation surveys and soil analyses were conducted on the site in May 1999 and compared to surrounding undisturbed areas.

In March 2000, during the second year of the project, 26 additional seedlings were planted to replace those lost during the dry summer months of 1999. The May 2000 vegetation survey results met required success criteria for percent survival of transplanted seedlings and species density, but fell short of the required species percent cover. Visual observations indicated that rototilling a site to decompact the soil may improve transplant survival and aid in weed suppression.

In January 2001 of the third year of the project, percent survival data indicated a need to transplant additional seedlings in order to meet success criteria. In February 2001, 113 additional seedlings were transplanted to the restoration site. Survival data was also used to confirm visual observations from the previous year that soil decompaction improves transplant survival. Because of these results, more areas of the restoration site were rototilled before the February replacement planting began in order to increase transplant survival. Results of the annual vegetation and soil surveys met success criteria.

The draft final report covers the life of this project from 1 October 1998 to 30 September 2002. It includes a summary of work carried out during the first three years and details of work carried out during the final year of the project (1 October 2001 to 30 September 2002) (see Appendices 1 and 2). During this period, maintenance, monitoring, and weed control activities continued similar to previous years.

(Figure 1)
Figure 1. Austin Avenue restoration site located on Marine Corps Air Station Miramar.

Site Description
The restoration site is located approximately 30 meters (m) west of Austin Avenue and 100 m northwest of the South Gate off Harris Plant Road, just east of San Clemente Canyon (Figure 1). This site was selected primarily because no future Public Works projects were scheduled in that area. San Clemente Canyon is composed of a coastal sage scrub community dominated by Laurel sumac (Malosma laurina), Toyon (Heteromeles arbutifolia), California sagebrush (Artemisia californica), Flat-top buckwheat (Eriogonum fasciculatum), and Yerba santa (Eriodictyon crassifolium).

The area requiring restoration is approximately 2.5 acres in size; however, restoration is only being conducted on 0.6 acres, an area sufficient to fulfill the mitigation requirement. Native vegetation at the restoration site before work began consisted of Laurel sumac, Flat-top buckwheat, Foothill needle grass (Nassella pulchra) and Tarweed (Hemizonia fasciculata). Exotic vegetation on the site included Avena and Bromus exotic grass species and Red filaree (Erodium cicutarium). Fauna (visual sightings and presence of tracks) consists of coyotes, rabbits, and rattlesnakes. Initial disturbance appears to have been caused by off road vehicle activity.

Summary of Previous Years’ Work
October 1998-September 1999
In March 1999, a penetrometer was used to measure soil compaction in and around the restoration site. Results revealed that an unused road that bisects the restoration site was as compacted as a nearby actively used dirt road. Compaction on the remainder of the site was similar to that in the undisturbed reference habitat. To study the effects of soil compaction on container seedling survival, an experiment was installed comparing a rototilled plot and a control untreated plot situated within the path of the abandoned road. The compaction of the rototillled plot was measured and found to be similar to the undisturbed soil. In April 1999, fifty coastal sage scrub seedlings were planted in each plot plus 200 more in the non-experimental area of the restoration site. Seedlings were protected from herbivory through the installation of TreePee and Tubex shelters around each transplant and all exotic vegetation was removed to a radius of 18 cm around each plant.

Installation of a temporary irrigation system in the form of ½” irrigation tubing and 2 gallon per hour drip emitters was completed in May 1999. Seedlings were watered twice each month, May through September 1999. Due to continuous damage to the irrigation system from coyotes, the system was removed in August 1999 and seedlings were hand-watered for the remainder of the summer.

In June and September 1999, percent survival by species of container planted seedlings was determined. Seedling survival in the rototilled plot was greater than in the control plot both times (96 and 90 percent versus 80 and 50 percent). Overall plant survival in June 1999 was 88 percent, but dropped to 76 percent by September 1999.

Also in June 1999, soil and vegetation surveys were conducted to determine soil health and the species, percent cover, and density of vegetation within both the restoration site and the surrounding undisturbed area. Soils were collected and sent to A&L Western Agricultural Laboratories (Appendix 3) for analysis of pH, percent organic matter, nitrate, total nitrogen, and phosphorus. The initial vegetation survey was used to establish success criteria for the restoration site. Results of the soil and vegetation surveys are shown in Tables 3 and 4 and Figure 3.

Further details for the work conducted during the first year of this project can be found in “Coastal sage scrub restoration on 0.6-acre mitigation site on Marine Corps Air Station Miramar: First year annual report 1999” (Heffernan, 1999).

October 1999-September 2000
In March 2000, 26 additional seedlings were planted to replace those lost during the dry summer months of 1999. Seedlings were irrigated monthly from May through September 2000. A vegetation and soil survey took place again in May 2000 and results were compared to the success criteria established by surveying the surrounding undisturbed area. At 9.5 percent, native species cover (Table 3) fell below the established success criteria of 20 percent and at 2, 040 shrubs per hectare, native species density (Table 4) at the restoration site was higher than the required 700 plants per hectare. Percent survival of transplanted seedlings was 75 percent which met the success criterion of 70%. The low percent cover measured in May 2000 was likely due to the unusual La Niña event during the 1999-2000 rainy season. Since native species density and transplant survival met the criteria, no remedial action was taken to compensate for the low percent cover.

Visual observations were made regarding the health and size of plants within the rototilled portion of the restoration site as compared to that of planted seedlings elsewhere. The seedlings in the rototilled section of the site stayed greener longer into the dry season and outgrew their protective shelters much faster than seedlings on the rest of the site, indicating that rototilling compacted areas before planting may increase survival and health of container seedlings. This may be due to the fact that decompacting the soil eases growth of plant roots, enabling them to reach groundwater much more efficiently. In addition, fewer weeds grew in the rototilled section of the site, so rototilling may also aid in the suppression of weeds.

Results of the chemical analyses of soils collected from the restoration site and from undisturbed areas in May 2000 are shown in Figure 3.

Further details for the work conducted during the second year of this project can be found in “Coastal sage scrub restoration on 0.6-acre mitigation site on Marine Corps Air Station Miramar: Second annual report” (Johnson, 2000).

October 2000-September 2001
In January 2001, overall percent survival was 40 percent, indicating a need to transplant additional seedlings in order to meet the 2001 success criterion of 70 percent. In February 2001, 113 replacement seedlings were planted to ensure that by May 2002, percent survival would meet the required success criterion of 65%. Seedlings planted in February 2001 were watered once every two weeks until the Station was closed to non-military personnel after the 11 September 2001 terrorist attacks. Seedlings planted in March 1999 were watered once in June 2001.

The January survival data was also used to test visual observations from the 1999-2000 season that soil decompaction improves transplant survival. Seedling survival comparisons among the heavily compacted road portion of the restoration site, the decompacted rototilled road, and the slightly compacted disturbed non-road revealed that as soil compaction increases, transplant survival decreases. Therefore, prior to transplanting the replacement seedlings in February 2001, more sections of the restoration site were rototilled and a new experiment was installed. For the new experiment, the entirety of the abandoned road within the restoration site was rototilled. In addition, two new areas were rototilled within the non-road portion of the restoration site. The survival of 115 transplants within these rototilled areas was compared to 118 transplants within the slightly compacted remainder of the site. In order to quantify the degree of compaction within the two areas, soil bulk density, the ratio of the mass of soil to its total volume and a measure of soil compaction, was evaluated for the soil in the tilled area of the site, in the slightly compacted area, and in the surrounding undisturbed habitat. The soil compaction data showed that the rototilled and undisturbed portions of the restoration site were nearly identical and were less compacted than the untilled/slightly compacted portion of the site.

In May 2001, a licensed herbicide applicator applied three gallons of 2% Round Up Pro herbicide solution to the site on one occasion and determined that spraying would not be necessary until after the next rainy season. Weeds, primarily Erodium cicutarium, were also removed by hand from transplant basins.

The annual vegetation and soil survey took place again in May 2001 and results were compared to the surrounding undisturbed area. Percent native species cover was 59 percent (Table 3) and native species density (Table 4) was 2,240 plants per hectare. Both results exceeded the required success criteria for 2001 of 20 percent cover and 700 plants per hectare.

Results of the chemical analyses of soils collected from the restoration site and from undisturbed areas in May 2001 are shown in Figure 3.

Further details for the work conducted during the third year of this project can be found in “Coastal sage scrub restoration on 0.6-acre mitigation site on Marine Corps Air Station Miramar, San Diego, California: Third annual report” (Johnson, 2002).

Final Year’s Work (October 2001-September 2002)
Methods
Access to the Austin Avenue site is obtained by driving within the flightline gates. Due to the terrorist attacks of 11 September 2001, this was not permitted until November and not without a Marine escort. Because of this access problem, two scheduled watering trips were missed in October 2001. The replacement transplants planted in February 2001 were watered after the 2001-2002 winter rains ended in June and July 2002.
Plants received 1/2 gallon per watering. Water from a source located just inside the closed Station gate next to Harris Plant Road was used to fill a 180-gallon water tank in the back of a pickup truck. Hoses were run from the tank to the site and, with the help of a small gas-powered pump, transplants were hand-watered.

In June 2002, the annual vegetation and soil surveys were conducted. The vegetation survey consisted of estimates of percent survival of transplanted seedlings, percent cover of native species, and density of native species. The percent survival of transplants was estimated for the site overall and also compared between the two soil compaction treatments (decompacted/rototilled and slightly compacted). Percent cover of native species was measured in two ways. First, by the transect method which estimates species cover using two 50-meter transects with data points every half meter. This method is most accurate for shrubs and larger herbs. In addition to percent cover of plants, the transect method was also used to estimate ground cover. The second sampling method estimates percent cover using three one-meter square quadrats and more accurately quantifies small species of low occurrence. Native species density was measured by counting the number of individuals located within 2.5 meters of either side of the 50-meter transect line.

The soil survey consisted of collecting three soil samples each from the restoration site and from the undisturbed reference site. These samples were sent to A&L Western Agricultural Laboratories (Appendix 3) and analyzed for phosphorus, total nitrogen, pH, and percent organic matter. In previous years, nitrate levels were also determined. This year, however, the soil analysis laboratory did not run this test even though they were instructed to do so.

During site visits throughout the year, the site was assessed for the need for weed control.

Results
Overall transplant survival in June 2002 was 54% (Table 1). This fell below the 65% required by the success criteria for summer 2002. The results of percent survival of transplants between the two soil compaction treatments on the restoration site (decompacted/rototilled and slightly compacted) were nearly the same at 69.5 and 70.3 percent, respectively. Table 2 lists the number of plants found alive in the two sections in January 2001, the number of new seedlings added to these sections in February 2001, and the number still alive over one year later in June 2002. Figure 2 is a graphical representation of the percentages of survival.

Table 1
Overall percent survival of transplants in June 2002.

Table 2
Survival of transplanted seedlings by soil treatment.

Figure 2. Percent survival in rototilled and slightly compacted site sections.

Results from the June 2002 vegetation survey showed 8.5 percent native shrub cover (Table 3). Native species density at the restoration site was 1,580 perennials per hectare (Table 4). The success criteria for 2002 state that percent cover of native species should be 30 percent and species density should be 650 plants per hectare. Species density meets the requirement, but percent cover falls short.

There are no success criteria for percent cover of small herbs, but the vegetation survey results indicate that cover was lower in the restoration plot than in the undisturbed site (Table 3). Ground cover at the restoration site was 82 percent litter, 2 percent rock, and 16 percent bare compared to 79 percent, 3 percent, and 18 percent at the undisturbed site (Table 3).

Table 3
Percent cover by species on the restoration and undisturbed sites.

Table 4
Density of native species on the restoration and undisturbed sites.

Results of the chemical analyses of soils collected from the restoration site and from undisturbed areas in June 1999, May 2000, June 2001, and June 2002 are shown in Figure 3. Phosphorus and pH are nearly the same at both sites and have followed the same trends throughout the years. Organic matter and total nitrogen levels have been erratic over the years at the undisturbed site while they have remained steady at the restoration site. Nitrate levels were not evaluated in 2002, but in 2001, the restoration site had higher levels of nitrates than the undisturbed site.

Figure 3. Soil analysis results of samples collected on Austin Ave. restoration site and the surrounding undisturbed area in 1999, 2000, 2001, and 2002.

Discussion
In 2002, the final year of the Austin Avenue coastal sage scrub restoration project, both overall percent survival of transplants and percent cover of native species fell below the required success criteria. Regardless, remedial action will not be taken to improve these numbers for several reasons. First, the terrorist attacks of 11 September 2001 prevented access to the Station and the restoration site for the months of September, October, and part of November 2001. As a result, several scheduled trips for the purpose of irrigating new transplants were missed, leading to the death of many of the seedlings. In addition, after the abnormally dry 2001-2002 rainy season, many well-established transplants that had been in the ground for more than one year suddenly died. The dry rainy season also undoubtedly caused the low 2002 estimate of the percent survival of native species. Very few annuals germinated at the restoration site in 2002 in sharp contrast to the numerous individuals present in 2001. The rains in 2001 yielded a percent survival that was nearly seven times higher than that in 2002. Percent cover of native species is likely to recover after a normal rain year. These explanations for low survival and percent cover coupled with a native species density that is well above the 2002 requirement (1580 compared to the required 650 plants per hectare) makes additional planting at the restoration site unnecessary.

The experiment that was installed to test the difference in transplant survival between a decompacted/rototilled area and a slightly compacted area revealed little difference between the two treatments. This was unexpected as results from the comparison in 2001 which tested three different levels of compaction revealed that as soil compaction increases, transplant survival decreases. However, considering the fact that the rototilled plots contained nearly twice as many new replacement seedlings as the slightly compacted area and that newly transplanted seedlings are more susceptible to death during the first year than those that have been in the field for more than one year, it is still possible to conclude that rototilling the soil increases overall transplant survival. The more compacted a soil is, the fewer air spaces exist between the soil particles. This can cause slower drainage of water through the soil (soil is less aerated), lower overall water-holding capacity (soils dry out more quickly during dry periods), and poor conditions for root penetration (nutrient absorption by plants is impeded). All of these factors reduce plant health and survival.

Results of the chemical analyses of soils collected from the restoration site and from the undisturbed reference site revealed phosphorus and pH levels that are of no concern. Organic matter, total nitrogen, and nitrate levels, however, have been different between the restoration site and the undisturbed reference site throughout the four sampling years. Organic matter and total nitrogen have fluctuated erratically over the years at the undisturbed site while remaining relatively steady at the restoration site. The undisturbed reference site is considered a stable habitat, so soil parameters are expected to remain much the same from year to year. The fact that the results of soil analyses for the undisturbed site changed greatly between 1999 and 2002 lead to the conclusion that spatial variability is heavily influencing the results. Soil samples are collected each year during the soil survey from the same general area, but not from the exact same location. Due to variability in vegetation and animal activity, a sample that is collected only a few feet away from another may differ greatly in terms of nutrient content. After four years, the primary conclusion that can be drawn from these soil analyses is that in future restoration projects, a larger number of samples must be collected from both the undisturbed and restoration sites each year in order to minimize spatial variability at both sites. Soil nitrate levels were not analyzed in 2002, but 2001 levels were high. Once again, this difference is likely due to spatial variability of the soil. Since nitrate levels in 1999 and 2000 were similar at the restoration and undisturbed sites, the difference in 2001 is not enough to necessitate the addition of soil amendments.

Conclusions and Management Implications
Alhough the Austin Avenue restoration site did not meet all success criteria as of September 2002, remedial action is not necessary. The low percent survival and percent cover are a product of drought and the temporary inability to properly irrigate new transplants. The situation is temporary as far as percent cover is concerned and species density is high enough to outweigh the deficits. After four years of site preparation, transplanting native coastal sage scrub seedlings, and site maintenance, the restoration site at Austin Avenue is expected to continue to recover on its own. With each additional growing season, the site will begin to more closely resemble the surrounding undisturbed coastal sage scrub habitat (Figures 4 and 5).

Figure 4. The Austin Avenue restoration site in 1998.

Figure 5. The Austin Avenue restoration site in 2002.

The results of the soil compaction experiment conducted during the restoration of the Austin Avenue site yielded interesting preliminary results indicating that soil decompaction before transplanting seedlings may improve survival. For future restoration projects, it may be beneficial to examine soil compaction as part of site preparation before planting begins.

First Annual Report (January 13, 2001)

Second Annual Report