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Vernal Pools of Placer County

One of the ‘signature’ landscapes of Placer County are vernal pools. Vernal pools are shallow depressional wetlands, formed when the percolation of rainwater and surface run-off is impeded by the presence of a restrictive, subsurface layer. These pools remain inundated throughout late winter and early spring, evaporating slowly as temperatures rise and precipitation diminishes. Vernal pools can range anywhere in size between 30 square feet to the size of a small lake.

In June of 2000, the Placer County Board of Supervisors requested that the Planning Department initiate preparation of a Natural Community Conservation Plan (NCCP) and a Habitat Conservation Plan (HCP). These plans will serve to: 1) strategize about the conservation of vernal pool communities; and, 2) design and implement resource management objectives.

It is estimated that more than 90% of California’s Vernal Pools have been lost agricultural development and urbanization. There are approximately 22,400 acres of vernal pool habitat in Placer County. Placer County’s remaining vernal pools are primarily located in the western portion of the county at elevations of 30 to 525 feet.

Vernal Pools - Strongholds of Biodiversity
By Joe Medeiros


Wetlands are terrestrial ecological systems where, for various reasons, water persists for a measurable amount of time. Wetlands can be very wet for long periods (mudflats, marshes, bogs, wet meadows and the like) or, in some cases, for very short periods. Regardless of their origins, all wetlands share the characteristic of being wetted by water for time periods long enough to alter what would normally create typical terrestrial ecosystems, dominated by non-wetland vegetation – grasslands, shrublands, forests, etc. In Placer County, a very unique wetland ecosystem type, the vernal pool, exists in valley floor locations and in the county's Sierra Nevada foothills.

Vernal pools derive their name from the Latin vernus which refers to spring. Perhaps this was the season when they were first noticed and described - their showy wildflower displays dotting lush green grasslands. Vernal pools are actually intermittent bodies of water; wet during a portion of the year but bone-dry during the balance of the year. Their sizes vary from only a meter or two in diameter to large vernal lakes of sizeable acreages. Regardless of their size they all hold in common a drastic and characteristic wet and dry, or “boom and bust” ecological economy.

Vernal pools form primarily in regions of Mediterranean climates which have cool, wet winters and hot, dry summers. While five major regions of the world share this climate type, only a few places produce “classic” vernal pools. Such pools are found in southern Oregon, in Baja California, Mexico, in the Cape region of South Africa, and, most spectacularly, in California.

While different sub-types occur, vernal pools share in common the inability to percolate water downward. Impervious layers of clay, hard-pan, lava or bedrock hold the winter’s rainwater within the pool. Instead of trickling downward through the pool’s bottom, water evaporates out of the pool. Depending upon the year's weather and the volume of the pool, this might take only a few weeks or up to a few months. As the pool dries myriad physical and biological changes occur. Beginning cool and clear, the pool gradually becomes warmer and more turbid. Evaporation reduces the pool's total volume and its circumference shrinks accordingly. Earlier levels of oxygen are high, diminishing to low. If you’re a growing vernal pool creature, it gets crowded and dirty in there! Change after change in the pool creates a very dynamic and short-lived ecological system unequaled elsewhere in California.

From what once appeared to be lifeless and desiccated in the summer and fall months emerges a veritable circus train of vernal pool biodiversity. In orderly succession pioneering species are replaced by several stages until climax ecosystems develop. Like their terrestrial counterparts, aquatic ecosystems begin with sunlight and photosynthesis. Without food, none of the pool’s critters could survive, so the all-important algae spring forth from their dormant spores. The algal bloom is grazed by zooplankton (mostly crustaceans) and other invertebrates. Planaria (flatworms) and snails graze the pool’s bottom and even the underside of the water surface. Amphibians make way to the pools to chorus and mate; and leave their fertilized eggs to hatch into thousands of tadpoles of tree frogs and toads. In some pools spadefoots (a unique toad-like amphibian) emerge from their summer slumber in the dried mud below while tiger salamanders (oversummering in squirrel tunnels) join in the frenetic game of ephemeral pool life. Birds don’t miss the opportunity either as they move in for the freshly-stocked warehouses of diverse foods. Stilts, avocets, killdeer, plovers, egrets, herons, ducks, geese, swans and other waterfowl abound.

While no vernal pool species is more important than another, it is perhaps the crustaceans that have been most featured in the news. While other freshwater wetlands are dominated by aquatic insects, the vernal pool is a veritable showroom for their saltwater counterpart – the crustaceans. Related to crabs, shrimp, and lobsters, vernal pool crustaceans are important zooplankton and swimming arthropods that emerge from eggs and cysts dormant in the dried mud from the previous year. Copepods, seed shrimp, clam shrimp, fairy shrimp and "monster" tadpole shrimp occupy various niches during the pool's ecological development and each has its own incredible survival story. Habitat loss and conversion has threatened the future of numerous vernal pool crustaceans – enough to warrant the placement of various fairy shrimp and the tadpole shrimp on state and federal Endangered Species Lists.

Wildflower enthusiasts make regular pilgrimages to vernal pools to bask in artists’ palettes of riotous color. Blooming in concentric rings (as the pool’s circumference shrinks) are meadow foams, popcorns, downingias, goldfields, yellow-carpets, Johnny-tucks, and scores of other botanical beauties. Many vernal pool endemics have evolved in such pools and some of California’s rarest plants are associated only with these unique waterholes.

Unless the public is informed, most would never know of the existence of these unique entities. The landscapes within which they exist are normally dry and mostly perceived as weedy wastelands. But, excepting for the occasional drought year, these short-lived ecosystems return on an annual basis. After the rains, the dry, undulating ground that was once appeared absolutely lifeless will soon teem with thriving life and complex food-webs. Strategies for surviving these ephemeral ecosystems include migration, hibernation, dormancy, seeds, spores, cysts and eggs. For those species that remain, a tolerance of desiccation and extremely high temperatures are required.

No wonder that vernal pools are such biological treasures. They are strongholds of biodiversity as well as precious genetic information. They are discrete, complex, and highly evolved systems. They are essentially our very own “Galapagos Islands” only in reverse. These ecological islands (of water) are surrounded by parched and dry land – intolerable real estate for vernal pool inhabitants. What important secrets lie within these age-old biological libraries? Tolerance to heat and drought? To increasing salinity and pollution? To crowding and stress? Unfortunately vernal pools lie in the pathway of human development. Thousands of vernal pools have already been bulldozed and paved over. Many attempts have been made to mitigate these losses but results of these experiments are still debated. Thousands of acres of functional vernal pools lie within the footprints of various proposed developments. Tenuously protected by state and federal environmental regulations, their future remains at stake.

Can we recreate vernal pool landscapes elsewhere? Can we manage our environmental impacts or otherwise mitigate for the loss of wetlands and habitat? Can we rescue some of the remaining pools and thereby guarantee ourselves a future of beauty and intrigue – and perhaps a future of answers to many critically important questions? Can there be a California without redwoods? Without poppies? Without vernal pools?

Joe Medeiros is Professor Emeritus of Biological Sciences at Sierra College. A trained botanist and ecologist, he has taught at Sierra College since 1990. He recently received the Conservator of the Year award from the Placer Land Trust. Joe can be reached at jmedeiros@sierracollege.edu

Additional information about Placer County’s efforts to preserve vernal pools may be found on the Placer County Conservation Plan website.

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