Vernal pool

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Vernal pool with clay hardpan bottom, Vina Plains Nature Conservancy Preserve, Calif.

Vernal pools, also called vernal ponds or ephemeral pools, are temporary pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe development of natal amphibian and insect species unable to withstand competition or predation by fish. Certain tropical fish lineages (such as killifishes) have however adapted to this habitat specifically.

During most years, a vernal pool basin will experience inundation from local surface runoff, followed by desiccation from evapotranspiration. These conditions are commonly associated with Mediterranean climate. Most pools are dry for at least part of the year, and fill with the winter rains or snow melt. Some pools may remain at least partially filled with water over the course of a year or more, but all vernal pools dry up periodically. Some authorities restrict the definition of vernal pools to exclude seasonal wetlands with defined inlet and outlet channels. Such seasonal wetlands have larger drainage basins contributing higher concentrations of dissolved minerals, and increased probability of periodic scouring flows through the wetland. Low dissolved mineral concentrations of smaller vernal pool basins may be characterized as oligotrophic, and poorly buffered with rapid pH shifts due to carbon dioxide uptake during photosynthesis.[1]

They are called vernal pools because they are often, but not necessarily, at their maximum depth in the spring ("vernal" meaning of, relating to, or occurring in the spring). There are many local names for such pools, depending upon the part of the world in which they are found. Vernal pools may form in forests, but they are more typically associated with grasslands. While many vernal pools are only a few meters in width, playas and prairie potholes are usually much larger, but are still otherwise similar in many respects, with high water in wet periods, followed by dry conditions.[2] Some exclude desert playas from the definition of vernal pools because their larger closed drainage basins in areas with high evaporation rates produce higher concentrations of dissolved minerals, with salinity and alkalinity favoring different species. Playas may be inundated less frequently than vernal pools, and inundation typically coincides with colder weather unfavorable for plant growth.[3]

Despite being dry at times, once filled, vernal pools teem with life. The most obvious inhabitants are various species of frogs and toads. Some salamanders also utilize vernal pools for reproduction, but the adults may visit the pool only briefly. Other notable inhabitants are Daphnia and fairy shrimp, the latter often used as an indicator species to decisively define a vernal pool. Other indicator species, at least in New England, are the wood frog, the spadefoot toad, and some species of mole salamanders. Certain plant species are also associated with vernal pools, although the particular species depend upon the ecological region. South African vernal pools, for example, have a different flora from Californian vernal pools. In some northern areas, tadpole shrimp are also common.


Vernal pools can form anywhere that a depression fills with water. They can be found on bedrock of many kinds, or in grasslands that form over a variety of soil types containing silts and clays. They can develop hydric soils which are typical of flooded areas, including accumulations of organic matter, but this may not happen in drier areas. In some cases there is a hard pan layer which causes the retention of water in the pools.[4]


Vernal pool flowers, with different species occurring in zones related to soil moisture and temperature gradients formed as the pool dries out. Sacramento National Wildlife Refuge, Calif.

In vernal pools, flowering occurs simultaneously because of the seasonality of favorable conditions. Vernal pool ecosystems may include both cosmopolitan species and endemic species adapted to unique environmental conditions. These include moisture gradients, salinity gradients, and reduced levels of competition.[5] Many vernal pool plants have buried seeds which accumulate in the soil. Different species are suited to different moisture levels, and as water evaporates from the edges of a pool, distinctive zonation of species can be seen.

Many upland perennial plants are unable to withstand the duration of vernal pool inundation; while many wetland plants are unable to withstand desiccation. Wetland plants may also be nutrient limited within small drainage basins. When dissolved carbon dioxide is depleted by daytime photosynthesis, vernal pool species like Howell's quillwort (Isoetes howellii) and pygmyweed (Crassula aquatica) collect carbon dioxide nocturnally using Crassulacean acid metabolism. Vernal pool basin habitats favor annual plants with some uniquely adapted perennials which suffer extensive mortality resembling annual reproduction. Annuals comprise approximately 80 percent of vernal pool flora. Listed below are some genera of the approximately one hundred vascular plant species associated with California vernal pool habitats; although a typical pool will include only 15 to 25 species.[6]

Cosmopolitan aquatic flora
Vernal pool specialists

Upland plants commonly found at vernal pools in California include yellow pansies, several sweet-scented clovers, yellow and bright lavender monkeyflowers, star lilies, and yarrow.

Vernal pools are often threatened by development in the same way that other wetlands are. As a result, most pools have been converted into residential zones, roads, and industrial parks. That is why most extant pools occur on protected or private land such as national parks, and ranches.

A large number of rare, endangered species, and endemic 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. A third example is the herb Limnanthes vinculans endemic to Sonoma County, California.


Vernal pool in northern California

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. In New York state the endangered tiger salamander (Ambystoma tigrinum) is dependent on vernal pools to breed as described above. A few other obligate vernal pool species are the marbled salamander (Ambystoma opacum), Jefferson's salamander (Ambystoma jeffersonianum), the blue-spotted salamander (Ambystoma laterale) and the spotted salamander (Ambystoma maculatum).

Some other species, notably the fairy shrimp, lay eggs capable of entering a state of cryptobiosis. They hatch when rains replenish the water of the pool.

See also[edit]


  1. ^ Witham, Carol W. Ecology, Conservation, and Management of Vernal Pool Ecosystems California Native Plant Society (1998) ISBN 0-943460-37-9 p.1
  2. ^ Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. p. 48-52,65.
  3. ^ Witham, Carol W. Ecology, Conservation, and Management of Vernal Pool Ecosystems California Native Plant Society (1998) ISBN 0-943460-37-9 pp.10&11

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