It is spring and time to head out to some of my favorite vernal pools. Vernal pools are small pools of water that appear and dry up as the seasons change. Vernal pools are isolated wetlands because they are not permanently connected to other bodies of water. They are fascinating but rare ecosystems.
Vernal pools happen all over the globe. The temporary pools are called by different terms depending on the local language and history. In the western part of the United States, temporary pools are called prairie potholes or in grasslands, playas.
A staggering percentage of vernal pools across North America have been destroyed. In California, 90% have been destroyed. The pools were paved, built on or drained to be replaced with human-built structures. Some vernal pools were deepened and used to water cattle.
Creating new vernal pools is difficult as humans often can’t get the slopes right or the edges the right width.
Vernal pools appear in swallow depressions:
- in clayey mineral soils
- in an upland (higher elevation) area
- in a wetland area in a low lying area
- in the floodplain of a stream or river
- in a forested landscape or historically forested area in northeastern North America
- in an agricultural field or residential area
Vernal pools fill with winter rains and snow melt. As the weather warms, the water in vernal pools evaporate. The temporary pools disappear and may return with the next winter’s snow and rain. Some vernal pools dry out every year, other every few years. But the drying out period is key to having the ecosystem of a temporary or vernal pond. If a temporary pond is filled with water in the autumn, it is often called an autumnal pond.
Animals which breed and spend part of their life cycle in vernal pools thrive without a top predator such as fish. These temporary wetlands are a safer breeding ground for frogs, toads, and salamanders than wetlands where fish live. Fish would eat the eggs, larva, and juveniles of these animals if they were present. This dry cycle prevents almost all fish from surviving in a vernal pool.
Pennsylvania’s vernal pool indicator animals include:
- Marbled Salamander (Ambystoma opacum)
- Spotted Salamander (Ambystoma maculatum)
- Jefferson Salamander (Ambystoma jeffersonianum)
- Blue-spotted Salamander (Ambystoma laterale)
- Wood Frog (Lithobates sylvaticus)
- Eastern Spadefoot (Scaphiopus holbrooki)
- Fairy Shrimp (Eubranchipus spp.)
- Clam Shrimp: Euroamerican clam shrimp (Limnadia lenticularis), diversity clam shrimp (Eulimnadia diversa)
The Pennsylvania vernal pools are often in a closed canopy forest dominated by Pin Oaks (Quercus palustris). There may also be mosses present. Some vernal pools don’t have any growing plants. The other plants present tend to be wetland plants. So, if you see a stand of wetland plants (like arums and grasses) but little or no standing water the spot may be a temporary pool.
In spring, I seek out vernal pools to watch and listen to the frogs and try to spot baby salamanders. While walking through the woods or grassy areas you can learn to recognize an eastern vernal pool even in the dry stage. http://www.naturalheritage.state.pa.us/VernalPool_DryPhase.aspx
More information on Vernal Pools
Information on the Internet
Sparsely Vegetated Vernal Pool Community
What are Playa Lakes?
Panhandle Playa Lakes
Prairie Pothole Joint Venture http://ppjv.org/
What are Praire Potholes?