What is a Vernal Pool?
Vernal pools are seasonal 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. Vernal pools are a type of wetland. They can be surrounded by many communities/species including deciduous forest, grassland, lodgepole pine forest, blue oak woodland, sagebrush steppe, succulent coastal scrub, and prairie. These pools are characteristic of Mediterranean climates but occur in many other ecosystems. Most pools are dry for at least part of the year, and fill with the winter rains, spring snowmelts and rising water tables. 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. Typically, though, a vernal pool has three phases each year: it is inundated in the Winter (Inundated Phase), it dries slowly in the Spring (Flowering Phase), and in the summer it completely dries (Dry phase).
Image 1: Phases of Vernal Pools
Soils in Vernal Pools:
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 hardpan layer which causes the retention of water in the pools. The hardpan clay basin accumulates water due to the small particle size and therefore reduced porosity. This permits flooding and development of vernal pools.
Types of Natural Vernal Pools:
Floodplain Vernal Pools: Vernal pools are often formed in the floodplains of streams and rivers. During floods, a stream will overflow its banks and enter the floodplain where the rushing high waters scour pockets in the floodplain. Some of these pockets retain water well and become vernal pools. The natural meandering of a stream channel across a floodplain over the years can also create vernal pool habitats in the form of oxbow wetlands.
Image 2: Floodplain Vernal Pool
Glacial Vernal Pools: Glaciers covered much of New Jersey during the last ice age. Glaciers created many isolated depressions, through the actions of ice melt and erosion that fill with precipitation and snowmelt.
Sag Pond Vernal Pools: These types of vernal pools are found near active fault lines. Strike-slip faults may create depression. When rivers, streams, rainfall or snowfall fills the low-lying area, a sag pond vernal pool forms.
Image 3: Formation of a Sag Pond
Pingo Vernal Pools: In the mountainous regions where permafrost is found, a common feature is a pingo. Pingos are formed by freezing and thawing processes on moist slopes. The upper layers of soil thaw first and slowly slide over underlying frozen soils. The resulting landscape appears to be scarred with crescent-shaped wetlands. Open canopied pingos are often full of marshy vegetation. Pingo vernal pools frequently have groundwater influences that extend their hydroperiod.
Logging Requirements:
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How much water was at the vernal pool at the GZ?
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Using your observations at the GZ, what phase is this vernal pool in during your visit?
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What type of vernal pool is seen at the GZ? Why?
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Sources:
https://en.wikipedia.org/wiki/Vernal_pool
http://www.naturalheritage.state.pa.us/VernalPool_Geology.aspx
https://en.wikipedia.org/wiki/Sag_pond
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