Vernal pool
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A vernal pool is usually a shallow, natural depression in level ground, with no permanent above-ground outlet, that holds water seasonally. In the northeast United States vernal pools fill with the rising water table or with the meltwater and rain of spring. Many vernal pools in the northeast are covered with ice in the winter. By definition, vernal pools are fishless, and thus allowing the safe development of natal amphibian and insect species.
Vernal pools are best known in California, Oregon, Maine, and Washington. In southern California they form on the coastal plateaus where the clays below the surface soil prevent water from percolating into the soil. Further north they form on ancient lava flows where lavas perform the same function as the clays further south. They also proliferate in the California Central Valley. They can form anywhere where winters are wet and summers are dry and conditions hold water in the pools.
The pools are dry most of the year and fill with the winter rains. They are called vernal pools because they are at their peak in the spring, ("vernal" meaning of, relating to, or occurring in the spring). Despite being dry most of the year, once filled they are teeming with life. The most obvious inhabitants are various species of frogs and toads. Living in the water the most notable inhabitants are daphnia and fairy shrimp. Further north tadpole shrimp are also common.
One of the best vernal pool habitats in the world is Frog Pond in Orono, Maine.
Flowers make vernal pools colorful. Different species are suited to different moisture levels, and as water evaporates from the edges of a pool, rings of flowers blossom around it. The color patterns change as the wet season wears on. The rings may form swirls and layers, with the green of new grass surrounding the whole pattern. Flora commonly found at vernal pools include Downingia and lupine species, yellow pansies, several sweet-scented clovers, a variety of goldfields, button parsleys, yellow and bright lavender monkeyflowers, star lilies, yarrow, and endangered grasses such as Solano grass.
Vernal pools tend to be on land that is prized for development. As a result most pools have all but disappeared except on protected land. A large number of rare and endangered species occur in vernal pool areas. For example, the San Diego mesa mint, a highly endangered plant, is found exclusively in vernal pools in the San Diego area. Another example is the wildflower Lasthenia conjugens, which is found in limited parts of the San Francisco Bay Area.
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Many of the amphibians that breed only in vernal pools spend most of their lives in the uplands within hundreds of feet of the vernal pool. Eggs are laid in the vernal pool, then the juveniles leave the pool two or three months later, not to return until the following spring to breed. Therefore, the upland areas surrounding a vernal pool are critical for the survival of these species.
