Mather Vernal Pools
When rain falls on a vernal pool grassland, some water sinks into the ground and the rest flows into streams or into depressions in the landscape. The water cannot move deeper into the ground in a vernal pool grassland because a hardpan blocks its path.
Hardplan is a layer of clay or minerals that water cannot pass through easily. Once the soils are saturated, rainwater perches on top of the hardpan. Only where there are depressions in the landscape can you see the perched water - as ponds of all shapes and sizes called vernal pools. The only way for the water to leave the vernal pools is by slowly moving through the ground or by evaporation.
Vernal pools are a special kind of wetland. Wetlands are a transition between water and dry land. Throughout most of the world wetlands serve many important functions: They hold rainwater that might otherwise cause flooding. They purify storm runoff by removing sediment and nutrients. They are home to hundreds of species of plants and animals. Many of these plants and animals are rare and endangered due to the destruction of our nation's wetlands.
During the winter, vernal pools are home to many dozens of species of aquatic organisms, all linked together in a complex food web. With the warmth of spring, water evaporates from the vernal pools, revealing concentric rings and patches of showy wildflowers. While winter ponds are common throughout the world, the special animals and plants in California's vernal pools make them unique.
Nearly 90% of California's original wetlands have already been destroyed by the conversion of land to agriculture and other types of development. Despite laws intended to protect them, wetlands continue to be lost every day. Preserving vernal pools and other wetlands is vital to preserving rare and endangered species, and the quality of the water we drink. So, we help ourselves when we watch out for wetlands where we live.
The Dry PhaseDuring the summer and fall, the vernal pools are completely dry and the vegetation is brown. Although the critters and plants are gone, they have left behind microscopic eggs, cysts, spores and seeds that will survive one or more long hot summers to produce the next generation.
Even when they appear dry and barren, the grasslands and vernal pools support many animals throughout the summer and fall. Seeds left behind by the flowering plants provide food for summer residents such as insects, birds, voles and gophers. Other dry-season residents include hawks, coyotes, snakes, and toads.
The Wet PhaseDuring the winter, vernal pools appear as ponds of water dotting the landscape. Rainwater collects in these low depressions and cannot immediately soak into the ground because of a hardpan beneath the soil surface.
During the wet phase, vernal pools are teeming with life. At the bottom of the food chain are last year's decaying matter and microscopic bacteria, algae and protozoa. Higher on the food chain are fairy shrimp, tadpole shrimp and tadpoles. Many of these animals live no where else on the Earth and are important food sources for migratory waterfowl.
The Flowering PhaseAs the weather warms, the pools begin to dry down and the flowering phase begins. Because each species of plant has unique requirements for water, warmth and light, you can often see concentrated rings, or patches of different colored flowers in vernal pools.
Flowering vernal pools are like snowflakes in that no two are alike. While each pool usually has 15 to 20 different species of flowers in it, the mix can be different in every pool. Goldfields (Lasthenia spp.) and Sky Blues (Downingia spp.) are just two of the more than 60 plant species in California that live only in vernal pools. The peak bloom of each plant species coincides with the emergence of the pollinator insect that will visit the plant.
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