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Policy and Guidelines on Vernal Pool Mitigation

Adopted March 5, 1994 (PDF Version)

Vernal pools constitute a unique natural resource by virtue of their flora and fauna, and contain many plant species endemic to the vernal pool habitat. The continuing expansion of California's population has resulted in the loss (at a rapid rate) of lands that surround and include natural vernal pools; a large percentage of vernal pool habitat has already vanished, and

Much of the natural world of vernal pools remains relatively unknown to science, as for example the nature, requirements, and types of microorganisms, algae, and crustaceans populating the pools, the importance of various vectors such as birds, insects, and wind in maintaining the genetic diversity of the pool ecosystems, and the interactions involving upland- and soil-based species in making vernal pools viable ecosystems. Adequate methods for comparing the distribution of plants within pools are lacking, and

Creation of vernal pools is increasingly practiced as a mitigation for loss of natural vernal pools, although creation is not an inexpensive or easy alternative when performed properly. This practice of creating vernal pools to mitigate for loss of natural pools relies on incomplete scientific methodology, in that criteria for success are seldom based on detailed biological data and monitoring for success is therefore usually incomplete; created pools frequently are built in existing vernal pool fields and thus alter, and may mar or damage, the existing functional natural ecosystems; and created pools built in areas away from natural pools generally have greatly lowered success rates, and

Mitigation by establishing small "postage stamp" preserves for vernal pools can lead to non-viable preservation on a long term basis because of increased "edge effects", and small preserves often lack adequate uplands to maintain hydrologic requirements necessary for vernal pools and to support species important in maintaining the genetic health of pool species, and

It is perhaps impractical to think that the continued expansion of the population of the state of California will occur without some loss of vernal pools, since this habitat type is particularly suitable for development activities, and there needs to be some mechanism to preserve other natural vernal pools to compensate for the loss of natural pools, so that this relatively rare habitat type does not disappear from the California landscape, and

Wetlands, a category that includes vernal pools, are an extremely valuable natural resource, and there exists a national direction and policy that there should be no net loss of wetlands. For these reasons, CNPS adopts the following:

Policy

The best practice in developing lands that contain vernal pools is to configure the development to avoid impacts to existing vernal pools, and to set aside the pool ecosystem as a preserve with adequate upland area to ensure that hydrologic requirements are met and that sufficient buffer area exists to ensure its long-term sustainability.

If avoidance is not practicable and it is determined that the development has an overriding benefit to society that necessitates loss of certain vernal pools, mitigation shall focus on establishing preserves that contain other natural vernal pools.

To accomplish this, the preferred mitigation, if part or all of the vernal pool ecosystem on a project site is to be destroyed or damaged, shall consist of both off-site or on-site preservation of vernal pools systems not presently preserved and additional off- or on-site creation of wetlands. Alternative mitigation mechanisms, which should be considered if important rare species values are involved, include restoration and preservation of damaged or obliterated natural vernal pool systems and creation of vernal pools using controlled scientific studies. If vernal pools are created as part of mitigation for loss of natural pools, the created pools shall not negatively impact natural ecosystems involving vernal pools.

Guidelines

The preferred mitigation shall consist of both of the following:

a) Off-site or on-site preservation of vernal pool systems not presently preserved, using a preservation/loss area ratio of not less than 2:1, via mechanisms such as direct purchase or establishment of conservation easements. If rare species are displaced, a 3:1 or 4:1 pool area mitigation ratio should be required. Preserves should be sufficiently large as to ensure viability and manageability, and should include adequate adjacent natural uplands and buffer areas to prevent encroachment by adverse human activities. Preserves should be located as near as possible, have a similar hydrological regime, and be on the same soil type as the system being lost in order to conserve local genetic interactions and provide habitat for species important in these interactions. Exceptions may be made if an accepted regional plan dictates otherwise. Adequate provisions for maintenance of the preserve in perpetuity should be developed.

b) In addition, off- or on-site creation of wetlands (not necessarily in-kind) of some type to provide habitat for wetlands species with an area commensurate (greater than 1:1, to allow for possible failure) with the area of vernal pools being lost. The created wetlands need not, but may, be vernal pools. It may be scientifically and ecologically preferable to compensate vernal pool loss with wetland types that are easier to establish and maintain.

An alternative mitigation mechanism, which should be considered if important rare species values are involved, may be one of the following:

a) Restoration and preservation of damaged or obliterated natural vernal pool systems is strongly supported by CNPS, provided that a greater than 3:1 mitigation ratio is used, and that there is a reasonable expectation of ultimate return to near-natural conditions. This mechanism of mitigation is an option when the vernal pools to be lost contain important populations of rare species which might be preserved by translocation to the mitigation site. However, translocation is typically not successful. The CNPS "Mitigation Guidelines Regarding Impacts to Rare, Threatened, and Endangered Plants" should be consulted for detailed discussion of considerations involved in mitigation for impacts to rare plants. Background data on biota of the damaged or degraded vernal pools or nearby functional pools should be collected prior to restoration to provide a baseline to evaluate success of restoration.

b) Creation of vernal pools using controlled scientific studies (with a mitigation ratio much greater than 1:1) to establish a factual basis for understanding the macro- and micro-ecology of the vernal pool system may be used to attempt conservation of rare species. This mitigation by creation of vernal pools shall be accompanied by preservation of pools not presently preserved on at least a 2:1 basis. The use of created pools as mitigation for loss of natural pools should be undertaken to obtain knowledge of the effects on all important species involved (including algae, insects, crustaceans, and birds, as well as plants) and with comprehensive monitoring (which may in part use evaluation of suitable indicator species), including collection of adequate background data, as a basis for evaluating the success of establishment. Created vernal pools shall have a similar hydrological regime and occupy the same soil types as those being replaced, unless dictated otherwise by an accepted regional plan. It is appropriate to evaluate the success of pool construction efforts before destruction of the natural system. The ratio of wetlands to uplands should not exceed that of natural vernal pool fields in the area. If vernal pools are created as part of mitigation for loss of natural pools, the created pools should not negatively impact natural ecosystems involving vernal pools; e.g., created vernal pools are inappropriate in pristine or high density natural vernal pool fields.

When restoration or creation of wetlands is accomplished, evaluation of biota should include baseline data for the vernal pools being lost (preferably at least two years) and must include a minimum of five years monitoring (and preferably more) to indicate the success of restoration or creation. In every case, monitoring should be of sufficient length that the success of the criteria selected is ensured.