A vernal pool is a pool formed after a period of rainfall
which dries up later in the year during the dry
season. Vernal pools may be just a few feet across, but can be hundereds of yards wide and several feet deep. Although these pools can be found in any area with a wet season and a dry season, they are especially common in the central valleys of California
, where conditions including a fairly wet rainy season, a prolonged dry season, and 'hardpan
' clay soils are found.
Vernal pools are a unique ecosystem, and many specialized animals and plants have evolved to take advantage of this habitat type. Plants growing in these pools must be able to survive a wide spectrum of conditions ranging from submergence to parching, and hardpacked soils. Vernal pool plants are generally annuals, which germinate when the pool is submerged, or shortly after. They then must go through their life cycle and produce seed before the soil completely dries out. Since these pools remain wet longer than surrounding areas, they often appear as 'pools' of vibrant yellow, white, or blue wildflowers when the upland areas have already gone to seed. When a pool is deep and dries up slowly, cocentric rings of different types and colors of flowers may form colorful 'bullseyes' on the ground. Vernal pool plants include goldfields, popcorn plant, and various adapted grasses, as well as many endemic species which are found nowhere else.
One of the most fascinating vernal pool animals are the fairy shrimp, tiny shrimp which thrive in the pools while they are full of water. These shrimp fill the pools with life for a few months, quickly completing their life cycle before the pool dries up. By the time the pool is dry, they have died and produced eggs which can survive the summer (or a dry stretch of several years) before hatching when the pool fills again. Other animals, such as various amphibians, employ the same strategy. When the pool dries, however, animal life does not leave the area. A wide range of native pollenators, such as endemic native bees, thrive amongst the flowers. Many species feed on only one type of flower, which in turn is pollenated by only that species of bee. Without one, the other could not exist.
Vernal pools are among the most threatened ecosystems in the world. According to the California Department of Fish and Game, only 6% of the vernal pool complexes in California's Central Valley are on public land. The rest are on private land which may be overgrazed, or even worse, plowed or developed. Fish and Game also recognizes over 70 potentially threatened species found only in vernal pools. Once the hardpan is plowed and broken up, the vernal pools, and their associated biota, can never return. Restoring or creating new vernal pools has never been successful. It just is not possible to create perfect soil conditions, bring in every type of plant, AND find every pollenator for every plant. In some cases, flowers have been coaxed into germination in artificial vernal pools, but without their pollenators, they can not reproduce and die out.
Ironically, the US Government has proposed building the new University of California, Merced, an 'environmental' campus, on top of one of the finest vernal pool areas in the valley. Despite much protest, the plans are still going forward, and although some mitigation efforts have been suggested, they can never make up for the pools which will be lost. This is especially ridiculous in an area with so much open land.
If you are in the Sacramento area, and would like to visit some vernal pools, check out Jepson Prairie Preserve or the Cosumnes River preserve. If you are in Southern California, check out the Santa Rosa Plateau Preserve.