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Vernal Pools
Vernal pools are
usually quite small and ponded only during the wetter part of the year. |
Description
Vernal pools are seasonal depressional wetlands that occur under the
Mediterranean climate conditions of the West Coast. They are covered by shallow
water for variable periods from winter to spring, but may be completely dry for
most of the summer and fall. These wetlands range in size from small puddles to
shallow lakes and are usually found in a gently sloping plain of grassland.
Although generally isolated, they are sometimes connected to each other by
small drainages known as vernal swales. Beneath vernal pools lies either
bedrock or a hard clay layer in the soil that helps keep water in the
pool.
Climatic changes associated with each season cause dramatic changes in the
appearance of vernal pools. The pools collect water during winter and spring
rains, changing in volume in response to varying weather patterns. During a
single season, pools may fill and dry several times. In years of drought, some
pools may not fill at all.
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The term vernal pool originally referred only to small, intermittently filled wetlands
found in the Mediterranean-type climate of the western United States. Today it
is used more broadly to include other small ephemeral wetlands found
country-wide. |
In the spring, wildflowers often bloom in brilliant circles of
color that follow the receding shoreline of the pools. By early summer, the
water has evaporated, and the clay pools appear brown, barren, and
cracked.
Functions & Values
However, appearances may be deceiving. The unique environment of vernal pools
provides habitat for numerous rare plants and animals that are able to survive
and thrive in these harsh conditions. Many of these plants and animals spend
the dry season as seeds, eggs, or cysts, and then grow and reproduce when the
ponds are again filled with water. In addition, birds such as egrets, ducks,
and hawks use vernal pools as a seasonal source of food and water.
Status
Vernal pools are a valuable and increasingly threatened ecosystem, often
smaller than the bulldozer that threatens to destroy them. More than 90% of
California's vernal pools have already been lost. Great efforts are being made
to protect the remaining vernal pools, as their disappearance marks the loss of
rare and important habitat and some of the associated plant and animal species
as well.
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Eastern vernal pools are
critical spawning areas for the Marbled Salamander (Ambystoma opacum,
inset) and other amphibians. |
Eastern Vernal Pools
These systems are very different from the numerous small, intermittent,
frequently unvegetated depressions that occur in the mid-western and eastern
United States, which are also refered to as vernal pools. The mid-western and
eastern vernal pools are particularly important for amphibians. |
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