Active and Passive Vernal Pool Restoration Processes in an Urban Park

Winter 2016 Ecesis, Volume 26, Issue 4

Soil blocks collected from the nearby vernal pool complex and placed into the restored pools to introduce fairy shrimp and vernal pool plant species.

Soil blocks collected from the nearby vernal pool complex and placed into the restored pools to introduce fairy shrimp and vernal pool plant species.

Mission Trails Regional Park (MTRP) is one of the largest urban natural parks in the nation, covering over 7,200 acres within the city of San Diego. MTRP contains numerous habitat types — from chaparral and coastal sage scrub to riparian woodlands and vernal pools — as well as over 60 miles of hiking, biking, and equestrian trails; an interactive Visitor and Interpretive Center; boating and fishing opportunities; campgrounds; and historic sites originally used by the Kumeyaay Indians. In addition, several public utilities traverse the park, including the San Diego County Water Authority’s underground water pipelines and associated infrastructure. 

Since its inception in 1944, the Water Authority has aimed to secure a reliable water supply for San Diego County and has incorporated environmental programs and sustainability into its business model. The Water Authority is committed to restoring plant communities that are disturbed or impacted as a result of construction activities, and to conserve natural resources that surround their right-of-way and facilities throughout MTRP. 

In 2006, the Water Authority underwent permitting for the construction of the Mission Trails Flow Regulatory Structure (FRS) II, Pipeline Tunnel, and Vent Demolition Project, the installation of an 18 million gallon buried reservoir, and the replacement of two existing pipelines by a single tunnel pipeline approximately one mile in length. The majority of impacts resulting from construction of the FRS II project will occur in upland habitats. However, some permanent impacts will occur to six basins that ponded water during focused sampling activities conducted during the very wet 2004-05 rainfall season (Black 2005). Four of the six basins had no vernal pool plant or animal indicator species, and, thus, have not been considered vernal pool habitat (USACE 1997). The other two basins had native vernal pool plant indicator species present, and one of these pools had a fairy shrimp (Branchinecta spp.) cyst present in each of two soil samples taken during focused sampling events (Black 2005). The two impacted vernal pools total 1,762 square feet, and are located approximately 30 meters from higher-diversity vernal pools that contain hatched San Diego fairy shrimp (B. sandiegonensis). 

A detailed restoration plan (ERS 2007) was developed for permit approvals that addressed enhancing and creating vernal pools onsite to improve the floral and faunal diversity of the overall vernal pool complex. The plan discussed methods for implementation of a 10-year maintenance and monitoring program and reporting requirements, focusing on the success criteria to be met, maintenance requirements, and monitoring requirements to evaluate success criteria. The plan also included methods of site preparation, seed and soil translocation, pre- and post-grading micro-topographic mapping, plant propagation and installation, upland habitat restoration, and contingency measures should the monitored success criteria fall short of those stated in the plan. This plan was incorporated into the San Diego County Water Authority Master Restoration Plan for Work within Mission Trails Regional Park (MRP; RECON 2009), and further developed by the Mission Trails Regional Park Restoration: Flow Regulatory Structure II (FRS II) Vernal Pool Habitat construction plans (construction plans; RECON 2009).

To address the impacts from construction, the mitigation plan includes restoration of at least 5,413 square feet of vernal pool basin and approximately 1.63 acres of vernal pool watershed. While the project is small in comparison to other vernal pool projects in San Diego, the real challenge lay with the parcel available for restoration. The mitigation site was pre-selected for vernal pool restoration, as it fell within Water Authority ownership; however, the slightly sloping terrain presented a potential problem. Natural vernal pools in San Diego County are typically found on flat mesa tops with basin-and-mound micro-topography that varies by only a few inches to feet. The soil of vernal pool sites also contain a clay or hardpan layer below the soil surface that prevents infiltration of water and results in ponding within the basins. While the Water Authority-owned parcel had the characteristic clay soils of a vernal pool site, the sloping topography created challenges for the successful creation of vernal pools. Since the site did not previously support any vernal pool basins that contained vernal pool species, active restoration would be required to create the appropriate onsite hydrology for vernal pool habitat. In addition, because vernal pool impacts occurred to a highly disturbed vernal pool with a low density of fairy shrimp, the created vernal pools had limited resources from which to collect vernal pool flora and fauna. Despite the challenging site topography and limited inoculum sources, restoration began after approval of the Master Restoration Plan and construction plans.

With a flexible design process, the vernal pools were “field fit” within the existing topography, allowing the site to dictate what the final product would look like. This allowed for the pools to be created with minimal impacts to existing chamise chaparral vegetation and cryptogamic soil crusts while promoting water flow through the site, encouraging the appropriate vernal pool hydrology. In February 2010, the pools were contoured using a small-tracked dozer. The restoration biologist onsite allowed the existing topography to determine the location and shape of each pool, with an emphasis on minimizing soil movement. Existing low points were enhanced, existing high points retained, and the excavated soil was strategically placed in locations that would increase the basin’s ability to capture and retain water, and channel water flow from one pool to another. A laser level was used to determine the appropriate depth of each pool, with pools excavated to a depth of 9 to 12 inches depending on the size of the basin and its location within the watershed. During grading, topsoil containing beneficial soil crust was collected from within the impact areas by field crews. Following pool grading, these crews performed minor grading with hand tools to smooth soil surfaces and create micro-topography too fine to be accomplished by heavy equipment. The salvaged blocks of soil crust were placed within the mounds adjacent to the vernal pools. After the pools ponded the ensuing winter, the maximum ponding area was mapped with a handheld global positioning system (GPS) unit. A total of 6,988 square feet of vernal pool surface area was mapped, exceeding the required square footage by 1,575 square feet (129 percent). 

In March 2010, container plants were installed and native seed was applied within the upland areas. Grasses such as purple needlegrass (Stipa pulchra) and blue-eyed grass (Sisyrinchium bellum) were installed near vernal pool edges, and shrubs such as laurel sumac (Malosma laurina) and spiny redberry (Rhamnus crocea) were installed in upland mounds adjacent to vernal pools. Because grading activities were conducted so carefully, large patches of intact shrubs remained in place after creation of the pools. These areas provided vegetative structure within the uplands where patches of mature vegetation blended with the newly installed container plants. Passive restoration through weed removal was the only restoration practice performed in these areas, which facilitated the entire site in meeting the vegetative goals.  

After sufficient ponding was observed within the vernal pools, soil that had been collected from a vernal pool to be impacted by construction was spread within four of the restored pools. The soil potentially included fairy shrimp cysts and vernal pool plant seed; however, based on observations made regarding the quality of the pool, it was assumed that the salvaged material contained a very low density of fairy shrimp cysts and minimal native seeds. Therefore, the soil was placed in only four pools to assess whether addition of the soil would result in the presence of fairy shrimp and germination of native vernal pool plant species and be beneficial to all pools. Dip net surveys were never performed to avoid unintentional take of any fairy shrimp that did hatch. Instead, biologists visually checked each vernal pool for fairy shrimp approximately every two weeks after the soil was spread within the four pools. Plants observed within the four pools after soil introduction included the widespread wetland species toad rush (Juncus bufonius) and invasive hyssop loosestrife (Lythrum hyssopifolium). After these observations, it was determined that the use of the inoculum within additional pools would not be beneficial for the site, as it could lead to additional unwanted weed infestations and the density of vernal pool plant seed was thought to be minimal. In December 2010, a letter was sent to the USFWS requesting permission to collect additional vernal pool soil from a nearby and more diverse natural vernal pool complex. After working closely with USFWS staff, the USFWS issued an amendment to the Biological Opinion to authorize additional take of San Diego fairy shrimp. This amendment allowed for the collection of soil blocks from nearby established and occupied vernal pools. Fourteen soil blocks measuring 5 by 5 centimeters wide and 2.5 centimeters deep were collected from the donor pool. The soil blocks were distributed amongst the 26 restoration pools. And then the waiting began.

The initial implementation of the project was very active — from vernal pool grading to plant installation to inoculum collection and placement — but in the next phase, the project relied on natural rainfall, plant recruitment, and the spread of fairy shrimp cysts as water flowed from one pool to another. At the end of year 1 (July 2011), after grading and placement of vernal pool soil from the donor pools — although not confirmed by protocol surveys — fairy shrimp were observed in 3 of the 26 pools, toad rush was observed in several pools, and quillwort (Triglochin scilloides, an endemic vernal pool indicator plant species not previously observed onsite) was observed in one pool.

Protocol fairy shrimp surveys began in Year 2 to satisfy USFWS permit conditions, and in July 2012, San Diego fairy shrimp were observed and confirmed in 13 pools with 5 more pools containing immature fairy shrimp. Toad rush was observed in 14 pools, woolly marbles (Psilocarphus brevissimus, another vernal pool indicator species not previously observed onsite) was observed in 12 pools, and quillwort was observed in one pool. By the end of Year 5 (July 2015), the site had satisfied the permit requirements for presence of San Diego fairy shrimp. 

The focus during Year 6 maintenance and monitoring was the establishment of vernal pool plant species. The Water Authority worked with the USFWS and the adjacent Marine Corps Air Station (MCAS) Miramar to gain permission to collect seed from the next nearest vernal pool complex located on the MCAS Miramar military base. In the spring of 2016, vernal pool seed was collected for vernal pool plant species including woolly marbles, Orcutt’s brodiaea (Brodiaea orcutii), annual hairgrass (Deschampsia danthonioides), and the state and federally listed endangered San Diego mesa mint (Pogogyne abrahamsii). The seed will be distributed within vernal pools that contain the appropriate hydrology to support each plant species in winter of 2016, immediately following the onset of winter rains.

While the initial restoration work demanded numerous active tasks to encourage the sloped chaparral habitat to transition into vernal pool habitat, the site required more of a passive approach as the project continued. Vernal pool basins, plants, seeds, and fairy shrimp cysts were actively installed within the site, but the restoration ecologist passively waited for the rains to fill the pools to allow vernal pool vegetation and wildlife to be expressed under their preferred conditions and then spread amongst the pools as water flowed from one pool to another during heavy rain events. Towards the later years of the project, the site relied more and more on natural rainfall and recruitment to move towards final success criteria. For a restoration ecologist, it is important to identify what tasks require a more hands-on approach, when to allow sites the space and time to evolve on their own, and what resources are available around a project that can be utilized to enhance its overall success. The Water Authority recognizes this importance and their confidence and trust in the project restoration ecologist have allowed for this project to move closer and closer towards the final success criteria! — by Meagan Olson, Associate Project Manager/ Restoration Ecologist, RECON Environmental, Inc. molson@reconenvironmental.com

Literature Cited

Black, C. 2005. Focused sampling for the presence of listed fairy shrimp at a site in Mission Trails Regional Park, San Diego, CA. 2004–2005 wet season sampling and 2005 dry season sampling. A report submitted to Brian Mooney Associates.

Ecological Restoration Services (ERS). 2007. Vernal Pool Mitigation Plan for the San Diego County Water Authority Mission Trails Flow Regulatory Structure (FRS) II, Pipeline Tunnel and Vent Demolition Project. September 27.

U.S. Army Corp of Engineers (USACE). 1997. Indicator species for vernal pools in special public notice regional general conditions to the nationwide permits. Los Angeles District. November.