Vernal Pools in Oak Woodlands: Puddles or Unique Habitats?

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Vernal pools are seasonal bodies of water that form in shallow depressionsfollowing winter rains. Because of an underlying hardpan layer, water maypersist for several months providing a wetland oasis for many uniquely adaptedspecies of plants and animals. More prevalent taxa such as waterfowl, frogs,salamanders, dragonflies, and numerous aquatic insects readily use the poolsfor feeding, breeding, and juvenile development.

Over tens of thousands of years, a number of highly specialized plantsand macro-invertebrates such as fairy shrimp, have evolved the ability toexploit the flood/drought environment associated with vernal pools. Often,species abundance and composition in one pool is quite different from thatof a neighboring pool. The environmental and biological factors responsiblefor these differences have helped to produce a highly specialized groupof obligate vernal pool species found nowhere else in the world.

For over three decades scientists have been intensively studying vernalpools and have gained insights into the structure, function, and compositionof these unique ecosystems. Spatially well-defined, vernal pools have contributedgreatly to our understanding of the effects of habitat fragmentation onsmall, isolated populations. Long thought of as mud puddles, mosquito breedingsites, or simply ignored all together, vernal pools are now recognized asunique natural resources worthy of preservation.

Although most common on terrace soils bordering the east side of theCentral Valley at the base of the Sierran foothills, vernal pools are scatteredthroughout the state on a variety of soils and landforms. Historically,vast stands of valley oak dominated the better developed vernal pool landscapesof the Central Valley, but most of these oaks and the associated pools werelost early on due to agricultural development and then later by urban expansion.Today, many intact pools are located throughout the oak woodlands of theNorth Coast Ranges and the lower elevation foothills of the Sierra Nevada,in sites traditionally grazed by livestock.

Located in the rugged terrain of the Mayacamas Mountains, the U.C. HoplandResearch and Extension Center (HREC) has many vernal pools within its 5,300acres of oak woodland, grassland, and chaparral. Twelve of the larger vernalpools at HREC were studied and found to vary considerably in landform, soiltype, mean area and volume, and species composition (table 1). Researchershere are currently collecting baseline data on plant and aquatic invertebrates,as well as on soil characteristics and water chemistry. This informationis essential for future experiments involving treatments and manipulations.For example, one current research project is evaluating the consequencesof sheep grazing on plant and macro-invertebrate composition, and the potentialbenefits of grazing for controlling exotic annual grass encroachment. HoplandResearch and Extension Center provides a unique research opportunity tostudy vernal pool ecology and implement science-based management necessaryto preserve the biological diversity of North Coast vernal pools. This typeof applied ecological research will hopefully provide direction and guidanceto landowners, managers and policy makers who are trying to balance humanpopulation growth with conservation goals and objectives.

prepared and edited by Richard B. Standiford and Pamela Tinnin