By Bob Heiser
After a long winter, we all have our favorite signs of spring—the sight of the first bloom, the taste of maple syrup, or simply more hours of sunlight. To many, the sound that harkens the coming of spring is that of the wood frog—usually the first frog to begin calling for mates. The wood frog has the northernmost range of any amphibian or reptile in North America. Relying on sugary “antifreeze” within its cells, much of the remaining liquid in its body can freeze solid in the winter. If you follow the chorus of a group of these “quacking” frogs in the spring, you’re likely to stumble upon a hidden world—a vernal pool.
Just what is a vernal pool? Though definitions vary, vernal pools are generally depressions that fill up with water at spring thaw (vernal meaning spring) and then, in most years, dry out over the course of spring and summer. In Vermont, vernal pools are almost always found in the forest. They are isolated from permanent bodies of water and do not contain fish, a feature important to many vernal pool species. This brings us to the final defining characteristic of vernal pools—they contain species that are “specialists” to these ephemeral habitats, some entirely dependent upon the pools for survival.
Finding vernal pools is easiest in the spring and summer when they are filled with water. You may find egg masses of wood frogs and salamanders, caddisfly larvae, and fairy shrimp, among other species that rely heavily on vernal pools. At other times of the year identification can be harder. Look for depressions, typically without trees, with compressed leaf litter from the weight of standing water. Look also for high-water marks on trees around the edges of the pool. You may be able to dig around and find signs of vernal pool species, such as the shells of fingernail clams or “cases” built by caddisfly larvae from pieces of leaves, needles, or whatever else may be at hand.
The species that rely on vernal pools for part, or all, of their life cycle are in a race against time, and the imminent drying of the pool. The frogs and salamanders that leave egg masses behind need the pool to remain for two or three months for their offspring to hatch, mature, and depart. Fairy shrimp have an even shorter window of opportunity. Once these filter-feeding crustaceans get going they can reproduce several times in the pool, if time allows. But fairy shrimp cannot withstand warm water temperatures, and certainly not the drying of the pool. If all goes well, the fairy shrimp will lay eggs encased in a cyst, which can withstand drying out, freezing temperatures, and abrasion from getting blown about. If you bring a shallow, light-colored tray out to your vernal pool and scoop up some water, you may be able to watch these otherworldly creatures swimming “upside down.”
Not only do vernal pools contain a unique collection of species that add diversity to your woods, they are also inextricably linked to the surrounding woodland. The amount of living material (biomass) contained in all the salamanders in a forest can exceed the total biomass of all the birds, and in some forests, of all small mammals as well. This biomass supports other pool species and those that live in the surrounding woods. A female wood frog will lay up to 1,500 eggs, of which 70 percent—whether in the form of egg, larvae, or frog—will enter the food chain in the first year.
The wood frogs and salamanders that disperse from the pool are highly dependent upon the surrounding woodland habitat. These amphibians are always at risk of drying out; they rely heavily on a damp forest floor and logs lying on the ground that act as sponges and moist oases. If they survive their overland travels, most will return to breed in the pool where they were born.
Woodland management designed to protect vernal pools will address the pools themselves and the surrounding habitat. The first hundred feet or so from the pool are particularly important and can be sensitive to management activities. This area can have high densities of adult and immature amphibians, and its condition can have large impacts on the water quality of the pool. Another 500 feet from the pool is important habitat for adult amphibians, though they will commonly venture up to three times as far.
Here are some things to keep in mind if you are trying to protect a vernal pool on your land:
- By keeping machinery out of the pool and its edges and avoiding felling trees into the pool, you can limit physical alteration of the pool itself.
- Retaining a closed canopy above the pool will prevent premature warming and drying. In surrounding upland habitat, maintaining a significant canopy will keep a cool, moist, forest floor for dispersing amphibians.
- Leaving logs on the woodland floor will provide critical moisture havens.
- Operating heavy machinery only when the ground is completely frozen or completely dry can minimize deep ruts and the creation of “population sinks,” pools that may be attractive to reproducing amphibians but will not lead to the survival of their offspring.
- Consulting a professional forester about management around these habitats is highly recommended.
So the next time you see a broad-winged hawk (an amphibian connoisseur) flying through your woods, you may just find yourself thinking about moist logs on the forest floor, the shade of the canopy, wood frogs, fairy shrimp, and vernal pools.