The heart of the forest looks more like a puddle. Small gems of water hidden in the woods — known as vernal pools — teem with life during spring months and play an important role in the ecology of a forest.
It is difficult to walk through the Maine woods in spring and not see a vernal pool. Vernal pools can be as small as a bathtub or larger than many ponds, but all vernal pools lose most of their water by late summer.
In February and March, a vernal pool is nothing more than a shallow frozen depression hidden in the woods under a blanket of snow. As the air warms up, melting snow and spring rains rapidly fill the depression. For the first week or two, the pool is still — with few, if any, signs of life.
The quiet phase of a vernal pool does not last long. By April and May, once-frozen eggs in the mud, now thawed, give rise to fairy shrimp, caddisflies, mayflies, beetles and water fleas (which are far more benign than their name might suggest). Because vernal pools dry down at least once a year, they contain no fish, which would normally devour most of these critters. Some of these species — the fairy shrimp, water fleas and certain beetles — will spend their entire lives in the pool, while others — the mayflies, caddisflies and the rest of the beetles — eventually leave the pools to buzz about the woods.
At roughly the same time, a set of larger and louder species — the amphibians — begin their yearly migration back to the vernal pools they were born in. Wood frogs, which spend their winters mostly frozen solid under fallen leaves or inside of small burrows (and you thought you had a rough time getting up in the morning), are among the first amphibians to enter the pool. The frogs immediately begin looking for mates using their sharp quack-like calls that can often be heard punctuating the more constant chorus of spring peepers.
During this time, other amphibians, including spotted and blue-spotted salamanders, emerge from hibernation and make their way the pools to breed. Each of these amphibian species leaves large, gelatinous egg masses in the pools that develop into tadpoles by early summer.
By August, most vernal pools begin drying up. By this time, the young frogs and salamanders have grown the legs and lungs that allow them to move into upland areas surrounding vernal pools where they will pass the winter. Insects that left the pools return to mate, leaving tiny eggs that will survive both the dry summer and freezing winter. The remaining insects, stimulated by increasing temperatures and dropping water levels, also lay their eggs. Eventually, the pools become a patch of mud or a damp bit of woods, leaving few clues to the life present only weeks before.
The effects of vernal pools on the surrounding forest are seen long after the pools disappear. The insects and amphibians that emerge from the pools in the spring provide plentiful food for birds and mammals well into the fall.
Vernal pools also protect and preserve surrounding lands year-round. Pools fill up like reservoirs after heavy rains, dampening potential floods. Pools also help filter nutrients out of storm runoff water that could otherwise foul lakes and rivers.
While they may be small in stature and in time, vernal pools can nourish the senses in the spring and the forest through the year.
Zachary T. Wood and Jared J. Homola are Ph.D. students in the ecology and environmental sciences program at the University of Maine.