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U.S. Fish and Wildlife Service, Sacramento, CA 95825

June M. De Weese

ABSTRACT.

The Sacramento Field Office of the U.S. Fish and Wildlife Service evaluated 1,543 vernal pools constructed in Placer, Sacramento, and Butte counties between 1988 and 1994, as compensatory mitigation for 25 projects permitted by the U.S. Army Corps of Engineers, Sacramento District, to determine if existing monitor regimes were adequately assessing the target physical and biological properties of federally listed species habitat. Site specific monitoring protocol and performance standards were compared with the draft Vernal Pool Mitigation and Monitoring Guidelines and on-site evaluations were made between March 29, 1994, and April 25, 1996. The most frequently monitored element (24 sites) was the number of vernal pool endemic plant species/pool. Sixteen sites considered federally listed invertebrates and 12 reported their presence in constructed pools. Two sites relocated a federally listed plant community. Each element monitored had a wide range of performance standards with the greatest number of alternatives (16) for vernal pool endemics. Ninety-six percent of the pools met hydrology standards and 69 percent met vegetation standards. Eighty-three percent of the sites met permit compliance. Although 69 percent monitored reference pools, the performance standards between reference and constructed pools did not appear to be site specifically interdependent. In many cases, specific criterion were imposed by impact site conditions that could not be replicated at the mitigation site. Fewer variations in monitoring protocol and performance standards should be accepted for permit compliance.

INTRODUCTION

The Sacramento Field Office (SFO) of the U.S. Fish and Wildlife Service (Service) concluded in their first mitigation follow-up study that constructed wetlands which met performance standards and permit compliance often did not result in optimum habitat replacement values (DeWeese, 1994). Constructed vernal pools received the lowest replacement value ratings in that study. The Service suspected that performance standards for vernal pools were too minimal to assure successful habitat replacement. However, we had not gathered sufficient information to substantiate our concerns.

In January, 1994, an interagency team (Environmental Protection Agency, U.S. Army Corps of Engineers, California Department of Fish and Game, and the Service) was assembled to create the draft Vernal Pool Mitigation and Monitoring Guidelines (Guidelines) (Army, 1994) to assist permit applicants establish a monitoring regime that would adequately assess target physical and biological properties of constructed vernal pools. The Guidelines were peer reviewed by the academic and professional community, presented in public forum prior to distribution (Army, 1994), and released October 4, 1994, with proviso that they would be re-evaluated after one-year of field testing.

In this second mitigation follow-up study, the Service evaluated 25 vernal pool mitigation sites to determine if existing performance standards were too minimal and to assist the interagency team re-evaluate the Guidelines. Conclusions in this document are opinions of the Service and do not represent a consensus of the interagency team.

We analyzed site-specific monitoring protocol, compared existing protocol with the Guidelines, and examined the relationship between monitoring elements, performance standards and habitat replacement evaluation. We found that no field testing of the exact Guidelines protocol has been completed, however, many of the recommended elements are being monitored with a modified protocol. Only 3 sites were using the same performance standards for permit compliance. In spite of numerous variations in the site specific performance standards, a certain percentage of pools had difficulty meeting a performance standard(s) each year. Often performance standards based upon conditions at the impact site could not be duplicated at the off-site mitigation, because baseline surveys were completed in a single season and performance was based upon static criteria. The 5 sites that had on-site avoided pools to monitor as reference pools, and established reference pool-based performance standards, had the highest performance ratings and met permit compliance by year 5. However, the constructed pools at those same sites had harsh geometric shapes with steep, chronically unvegetated side slopes, and were excavated to great depths to reach a water restricting layer.

We concluded that constructed vernal pools did not have minimal performance standards, however, there were too many variations in site-specific performance standards. There should be fewer variations in monitoring protocol and performance standards to facilitate more accurate performance comparisons and assess common problems or improvements in habitat replacement.

METHODS

We selected 25 of 50 records in our mitigation follow-up study database which had impacts to vernal pools with compensatory mitigation requirements and were permitted by the Sacramento Corps District, using the following criteria: 1) the project needed to be an in-kind replacement (not replacing vernal pools with some other type of wetland), 2) at least one annual monitoring report needed to be on file (70% of the 50 records had reports on file), and 3) the total number of sites selected needed to reflect a maximum range in constructed pool age. Twelve of the 25 projects impacted more than one wetland type and required that we separate the information on constructed vernal pools from the total mitigation requirements. We reviewed all of the monitoring reports in the selected project files. To analyze the data, we created a form listing the thirteen Guidelines monitoring elements and recorded the site-specific monitoring protocol and performance standards opposite each element on a separate form for each project. The elements were: Site Selection and Construction Techniques (1), Reference Pools (2), Hydrology (3-5), Vegetation (6-9), Wildlife (10), Invertebrates (including the presence or absence of federally listed species) (11), Water Quality (12) and Site Maintenance (13).

We conducted on-site evaluations between March 29, 1994, and April 25, 1996, to observe monitoring protocol and compare performance standards with permit compliance and replacement values. We recorded the number of pools meeting site-specific performance standards and, if problems were identified, which standard was not met. Ten sites were evaluated twice to observe periods of inundation and dessication (Zedler, 1987). During our evaluations, we discovered that one project constructed pools at two different sites, had two different consultants, used two distinct monitoring regimes, therefore the results reflect 26 sites, rather than 25 projects. Each site visit was conducted with the project consultant present, to answer questions and provide us with additional specific information that may not have been discussed in the monitoring reports. The author of this study also observed construction activities and monitored pools at some sites to further evaluate specific protocol.

Additional items we investigated were: number of wetted acres constructed; number of pools impacted versus number of pools constructed; if the mitigation was on- or off-site; gross acres of preserve site; wetland density at the preserve site after implementation; and whether the preserve constructed only vernal pools or multiple wetland types.

RESULTS

Only five percent of the projects we evaluated had constructed pools the same year as the impacts. Forty-four percent of the projects had two years or more lapse between project impacts and implementing the mitigation. Seventeen sites (144.7 constructed vernal pool acres) were in Sacramento County, 8 sites (49.3 constructed vernal pool acres) were in Placer County, and 1 site (6 constructed vernal pool acres) was in Butte County. Sixty-four percent (16) of the projects implemented mitigation off-site; 36 percent (9) of the projects implemented the mitigation on-site. The combined projects constructed 1,543 vernal pools, or approximately 200 wetted acres. The greatest number of pools were constructed in 1994 (472).

Pool construction and subsequent monitoring reports were the product of 11 different consulting firms, however, one of the consulting firms monitored 11 of the 26 sites. The element most frequently monitored (24 sites) was the number of Vernal Pool Endemic (VPE) plant species/pool (Ikeda, 1990). The element least frequently monitored (0 sites) was site maintenance. Two sites had additional monitoring requirements for a special status plant population, Butte County Meadowfoam (Limnanthes floccosa ssp. californica) and Boggs Lake Hedgehyssop (Gratiola heterosepala). To facilitate comparison, the following results for specific monitoring elements include paraphrased recommendations from the Guidelines in italics.

Site Selection

Give priority to sites that historically supported vernal pools or have appropriate soil type (preferably same series as impact site) and will be adequately buffered (Castelle, 1994). Preserve sizes ranged between a minimum of 4.5 gross acres and a maximum of 520 gross acres, with the mode (6 sites) between 25 and 50 gross acres and 8 sites greater than 100 gross acres. Nineteen had constructed more than one type of wetland and many included restoration measures for existing wetlands at the mitigation site. Five sites included an intermittent stream traversing the mitigation site. Seven sites had exclusively constructed vernal pools. Thirteen projects (52%) constructed their compensatory wetlands at an off-site mitigation area which was shared with mitigation requirements for other permits, but was not an official mitigation bank. One site was an established, interagency approved, mitigation bank. Eight sites constructed vernal pools within existing vernal pool complexes, converting a low density complex into a high density complex. Three sites were constructed on former rice farms. Twelve of the mitigation sites were created in locations undesirable for wildlife habitat. For example, two sites were within utility easements with pools constructed underneath high voltage power lines, five sites were adjacent to freeways, and five sites were created on parcels that are less than 13 gross acres, surrounded by development, and inadequately buffered.

Construction Techniques

Excavate side slopes and pool bottoms that mimic impact site pools, to duplicate hydrologic depth, surface area, and inundation period. The earlier constructed pools used slope ratios of 3:1 and 4:1 and excavated to a maximum depth of 13 to 18 inches. Recently constructed pools, have slope ratios between 7:1 and 10:1, with maximum depths as shallow as 4 to 6 inches. Constructed pools at several sites were inundated for longer periods than natural pools, especially during the first two years after construction when soils may remain densely compacted. Final site density should not exceed 30 percent. The sites we evaluated had densities which ranged between 3 percent and 26 percent, pool acres to gross site acres, without consideration of other wetland types on-site. When all on-site wetland types were considered, the highest density was 44 percent after construction. Inoculum should not be stored for more than one year, to avoid adverse effects to the establishment of vegetation (Leck, 1989). Only five percent of the projects we evaluated had installed inoculum within the same year collected. Often the inoculum for both vernal pools and seasonal swales are collected and stored together. One site collected inoculum from pools on a volcanic substrate and installed the rocky-strewn soils on claypan. One of the projects constructed half of the pools one year, and stored the remaining inoculum for an additional year. The differences in plant vigor and absolute cover between the two halves were readily apparent. The pools inoculated with the longer stored soil performed poorly during the first three years of monitoring (Sugnet, 1993). Excavation spoils should be hauled off-site. Eight sites had not hauled off spoils.

Reference Pools

The establishment of biological viability can only be verified by comparing constructed pools with natural vernal pools from the same immediate area. Eighteen sites were monitoring reference pools. Five projects were monitoring avoided reference pools at the impact site. Nine projects were monitoring reference pools at the mitigation site. Four projects were monitoring reference pools somewhere within the immediate area. Seven projects did not monitor any reference pools.

Hydrology

Install two staff gauges (one deep, one shallow, where 70% pool bottom is lower) in all created and reference pools, monitor weekly at wet season. Document depth, area, and duration of inundation results with hydrographs, photographs, and aerial photography. One site had installed two gauges per pool. Seven sites had placed single staff gauges in the deepest part of the reference and constructed pools. Three sites included aerial photographs in the monitoring reports and two sites included photo-documentation of hydrology in sample pools. Five sites included hydrographs in their monitoring reports. Three sites set up a condition that hydrology be monitored for 1 year prior to installing the inoculum.

Vegetation

Measure absolute cover and relative cover (Barbour, 1987) using transects with point intercept, square meter quadrats, photo documentation and graphing; identify species with 20 percent relative cover or greater; indicate status and relative cover of hydrophytics (Reed, 1988); and determine vernal pool endemic species/pool (VPEs). Twenty-one sites monitored absolute cover, 12 sites monitored relative cover, 24 sites monitored VPEs, and 13 monitored dominance of wetland species. Most sites used visual estimates to measure absolute and relative covers. Four sites used permanent transects and 2 sites a square meter quadrat. Eleven sites included graphs depicting relative and absolute cover for each pool in their monitoring reports. Four sites included photodocumentation of vegetative cover in sampled pools. The VPE measurement was completed by identifying maximum species per pool, with the aid of a checklist of species most frequently encountered in vernal pools, indicating whether native, non-native, wetland, upland, and tallying VPEs. Fifteen sites documented a specific number of VPEs to meet permit compliance. Five sites stated VPEs had to be a specific percentage of the reference pool species. Five sites measured VPEs with a Vernal Pool Floristic Index (Sugnet, 1991).

Wildlife and Listed Invertebrates

Monitor on a case by case basis. Three sites noted all wildlife and 10 sites monitored birds. Twelve of 16 sites monitoring for invertebrates found federally listed species, either vernal pool tadpole shrimp (Lepidurus packardi) or vernal pool fairy shrimp (Branchinecta lynchi) present in constructed pools. One site found listed species in 23 of 25 constructed pools in year three. One five-year old site has found tadpole shrimp in some of their constructed pools every year.

Water Quality

Monitor on a case by case basis. Eight sites monitored temperature, turbidity, and conductivity.

Site Maintenance

Monitor for uncontrolled human disturbance, i.e., all terrain vehicles (ATVs), trash, and other unexpected conditions, i.e., soil piping, erosion, water run-off pollutants, wildlife mortality. No monitoring reports reviewed included a discussion of routine site maintenance. Three reports mentioned specific impacts (one site suffered arson, two sites photodocumented ATV damaged pools).

Performance Standards

The Guidelines performance standards are based upon establishing a reference site and primarily consider hydrology and vegetation. The performance standards for hydrology are: maximum depth of inundation within range of reference pools and longest period of inundation not greater than 125% of reference pools. The performance standards for vegetation are: absolute cover and relative cover by VPEs in each constructed pool shall be no less than the minimum recorded in the reference pools; each constructed pool must support no fewer than the lowest number of VPEs recorded in reference pools; VPEs shared by both the impact and reference pools shall be as vigorous and reproductively active in the constructed pools as the reference pools; and, by the last year of monitoring, any VPEs that are dominant (relative cover of a least 20%) in at least 30% of the reference pools shall be present as a dominant species in the constructed pools. Only 3 sites were using the same performance standards for permit compliance. Each element monitored had a wide range of performance standards with the greatest number of alternatives (16) relating to species diversity (VPEs). The most frequently shared performance standard was for avian surveys (11 sites). Table 1 lists the most frequently used site-specific performance standard for each element monitored and number of alternative standards being used at other sites. All three hydrology elements had high performance ratings: depth of inundation (96%), period of inundation (91%), and area of inundation (94%). For vegetation, 66% of the sites met the absolute cover criterion, 78% met the relative cover criterion, 89% met the species diversity criterion, and 77% met the hydrophytic criterion. Four (50%) of the 8 sites which required an invertebrate performance criterion met permit compliance.

DISCUSSION

Performance Standards Two important trends were discovered when the number of constructed pools meeting site-specific performance standards were compared with the total number of pools constructed per year: 1) pools constructed between 1988 and 1990 had the highest performance ratings and 2) in spite of numerous variations, a certain percentage of pools each year had difficulty meeting their performance standards. Six projects were constructed between 1988 and 1990. All of the projects were five or more years old and expected to fully meet performance standards on the fifth monitoring year. Five of the 6 sites monitored reference pools and stated reference pool-based performance standards between reference and constructed pools. Four sites stated the hydrology performance standard as "within range of the reference pools". The vegetation performance standards did not appear to be as site-specifically interdependent with reference pools. For example, two sites stated 7 species/pool and one site stated 12 species/pool, but did not state whether this number was an average, the minimum, or based upon a single year at the reference site. In addition, there should be no absolute or static numbers if monitoring at the reference site for comparison is every year. One project stated constructed pools should have 75% VPE relative cover, however, average reference pool absolute cover was not stated and no absolute cover was required for constructed pools. Therefore, if a pool with 80% bare ground had 75% VPE relative cover, it would pass the performance standard. Four sites required a specific percentage of absolute cover presumably based upon reference pool data, but variations over time were not documented.

In some cases, specific criterion were imposed by impact site conditions that could not be replicated at the mitigation site. Four of the 6 projects between 1988 and 1990 monitored reference pools at on-site preserves to compare with off-site constructed pools. The worst performance rating (50% pools not meeting performance standards) among this group of pools was at a site with standards based upon information gathered at the impact site over a single season. The project's vegetation standards were based upon each pool achieving 80% VPEs found in the inoculum-source pools. Surveys completed at the project site found numbers as high as 46 species per pool, because source pools had been grazed solely by horses, resulting in much richer flora than pools grazed by cattle (Balance, 1994). The constructed pools easily met the standards of the Guidelines and had a higher species diversity than existing pools at the mitigation site, yet, were not meeting permit compliance.

The earliest constructed pools in our study (1987) had poor performance in hydrology. The crucial element for vernal pool construction is the presence of a vernal pool forming soil, which includes the presence of a water restrictive layer (Stromberg, 1994). The Guidelines recommend that site selection include historic vernal pool soils to ensure success. Vernal pools constructed on historic vernal pool soils could equally be considered restoration. Table 2 lists the total number of pools constructed and how many failed (either or both) the hydrology or vegetation standards each year. The figures indicate that no pools have failed hydrology since 1991. This clearly indicates that more pools are being constructed on the proper soils. As of November 13, 1992, it has been the policy of the Sacramento Corps District not to authorize the use of bentonite linings to create wetlands, including vernal pools, because subsequent impacts, i.e., cattle, ATVs, could cause the bentonite to suspend in the water column (Norton, K., pers. comm.) and damage to the liner would drain the pool and prevent further ponding.

Pools constructed in 1994 had the lowest performance ratings (60% pools failing a performance standard). Further examination revealed that 125 pools at one site had required hydrology monitoring for one year prior to inoculation, and hence, were not yet meeting the vegetation standard. At another site, 155 pools were not meeting the 80% absolute cover standard because the site was only one year old. Neither site needed remediation, because both were on track to meet performance standards by year five. The average percent of pools constructed between 1987 and 1994 that needed remediation was 35%.

Habitat Replacement Evaluation

Vernal pool mitigation sites are selected and constructed based on economy of scale and density to get the greatest wetland acreage within the smallest land area. The impact site is typically comprised of uplands, seasonal wetlands, vernal pools, interconnecting swales, and perhaps an ephemeral stream. The average vernal pool compensation is a fragmented replacement, resulting in a potential loss of functions. Compensation vernal pools are often separated from other mitigation components and homogeneously combined with additional projects. Because swales are not easily re-created, additional isolated depressions are excavated at the vernal pool site to create seasonal wetlands as compensation for the swale acreage.

Constructing fewer and larger pools is more cost effective than a direct replication of the impact site (Francisco, R., 1994), hence, most of the projects impacted a greater number of pools than were constructed at the mitigation site. In addition, we did not observe any concerted effort to create microtopographic pool bottoms for enhancing plant distribution and invertebrate habitat in the constructed pools we evaluated. The resulting change in hydrologic regime on the transplanted vegetation is readily apparent when compared over a five-year period. The first two years, there is comparible diversity and most of the plant species captured in the inoculum appear. Starting in the third year and sometimes sooner, it appears that a shift in species cover class (Braun-Blanquet In Barbour, 1987) occurs, with the floristics which prefer longer inundation i.e., Eleocharis macrostachya, beginning to dominate. These conclusions are based upon our observations of the same mitigation sites over time and comparing five years of cover class detailed in the monitoring reports. However, to fully determine what plant species may be significantly reducing in number, due to lack of adaptation to the re-created habitat, would require additional extensive research.

One consultant stated that the 2:1 mitigation ratio reduced available inoculum by one-half and resulted in sparce cover during the first three years (Whitney, K., 1994). However, the sites we evaluated had a variable replacement ratio, usually lower than 2:1. For projects implemented after September 1, 1995, the Service added a preservation component which reduces the creation ratio to 1:1, if the impacted vernal pools also are habitat for listed species. We hope to see a more rapid establishment of vegetation with this reduced ratio, and will be tracking the results.

The steep slopes of early constructed pools were the subject of vigorous criticism, because of their unnatural appearance, and the resulting "bathtub ring" due to vegetation not establishing on the slopes. The steep slopes also did not provide optimum habitat for shorebirds and migratory waterfowl (Recher, 1966). More recent constructed pools have gentle slopes that are not only more aesthetically pleasing, but also are less likely to have unvegetated slopes. To further prevent bare slopes, many of the consultants "double seed" slopes by raking some upland topsoils downward towards pool bottoms and some inoculum upwards, overlapping the soils on pool slopes. Pools shapes are randomly designed and, over time, have evolved from harsh geometrics to shapes which more accurately mimic nature.

Routine site maintenance needs to have a higher priority and discussed in monitoring reports. The adverse impacts most frequently observed were trash dumping, ATV ruts, and uncontrolled weeds. Often invasive non-native plant species readily adapt to the recently disturbed sites and contribute to increased fire hazards. Some of the consultants have attempted weed control by hydroseeding pool perimeters immediately after construction. Routine maintenance, such as mowing or hand weeding, is labor intensive and often ignored. Most mitigation sites are too close to urban development for controlled burns and generally, we do not recommend the use of pesticides or herbacides within vernal pool habitats. One of the sites requested managed grazing to keep weeds under control and recently received approval from the Corps and the Service.

Numerous passerines, shorebirds, waterfowl, and jackrabbits are attracted to the compensatory wetlands we evaluated, regardless of whether they exactly replicate vernal pools. We observed 15 different wildlife species at one of the mitigation sites, including members of various levels in the food chain, culminating with a coyote. One of the potential problems we have noticed with constructed pools is that they often do not exhibit small mammal burrowing or deep hydric cracks in pool bottoms the first and second years, presumably because the soil remains densely compacted and soil ped formation has not occurred. These conditions could potentially delay establishment of species that utilize burrows and cracks for estivation habitat, such as tiger salamanders and spadefoot toads. Small mammal burrowing also creates additional microtopography which enhances the pool habitat for plants and invertebrates.

The art and science of constructing vernal pools has greatly improved over the past eight years. The technology for constructing wetlands that will provide viable habitat for rare plant populations, federally listed invertebrates, migratory waterfowl, and other wildlife will continue to improve if we can specifically document what has been successful and what has failed. Our study concluded that, if we are to enable valid performance comparisons over time, fewer variations in monitoring protocol and performance standards should be accepted for permit compliance.

ACKNOWLEDGEMENTS

We gratefully acknowledge the Sacramento Corps District personnel and the private consultants who granted access to the mitigation sites and fully cooperated with our investigations; the original interagency team members who wrote the Guidelines: Tom Cavanaugh (Corps), June DeWeese (Service), Julie Horenstein (CDFG), Paul Jones (EPA), Mark Littlefield (Service); and Brian Cordone (Service), who completed seven of the site evaluations. This study was partially funded by EPA, Region 9 Water Management Division, San Francisco, CA.

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