The Vernal Pools of Lynn Woods

by Mark Maguire

(originally published in July 2003 newsletter)

On a brisk day in late March in the Lynn Woods, signs of spring are few in number. But while the rest of the forest is barely beginning to awake from the New England winter, vernal pools are already brimming with life. Near-deafening choruses of wood frogs and spring peepers transform these recently-filled wetlands into forest hatcheries.

Venal pools are small, temporary wetlands that are abundant throughout the New England landscape. Most of these "spring ponds" hold water for only a few months out of the year. Certain amphibian species have evolved to take advantage of the temporary nature of vernal pools. These organisms, referred to as obligate species, require these fish-free breeding grounds to complete their life cycles. A variety of other terrestrial and aquatic species, ranging from insects to large mammals, also utilize the many resources vernal pools provide.

vernal pool comparison

My vernal pool project in the Lynn Woods capped two years of research at Boston University in the Department of Geography. Though the study of vernal pool organisms is a classic exercise in biology, research in vernal pool ecology fits well into the spatial discipline of geography. The goal of my research was to develop a geographic framework for identifying vernal pools and associated vernal pool habitat using Lynn Woods as an example.

spotted salamanderThe first step was to locate and map all vernal pool sites in the Lynn Woods study region. After months of fieldwork and much map scrutiny, I identified 47 "true" vernal pool sites in the Lynn Woods, which were confirmed if they sustained breeding populations of obligate vernal pools species or otherwise fit the classic definition of a vernal pool. Three obligates were found to inhabit pools of the Lynn Woods: Spotted Salamanders (Ambystoma maculatum), Wood Frogs (Rana sylvatica), and Fairy Shrimp (Eubrahnipus sp.). Most interesting was the distribution of breeding and non-breeding sites over the study region for each species. Only 17 out of 47 pools were utilized by breeding spotted salamanders. Using Walden Pond to divide the Woods into north and south sections (of relatively equal area), I did not uncover any salamander activity in the north section. Wood frogs were found throughout both sections of the Woods, though they were more sparse north of Walden Pond.

fairy shrimpMy research attempted to uncover variables that could be causing this interesting spatial gradient in the Lynn Woods. One factor could be the quality of the vernal pool itself, which may influence the type and intensity of breeding. The statistical model employed, which incorporated environmental data collected at each site and larger scale landscape data, did uncover some interesting relationships in terms of amphibian site selection. For example, spotted salamanders were more often found in sites in close proximity to permanent water sources, while wood frogs preferred sites with relatively open tree canopies.

Other factors specific to the Lynn Woods may be equally important, but could not be explicitly tested in this research. Although you may not be able to notice it, the park has undergone many changes over the last decade. The landscape has been impacted by to some degree by wildfires, forest succession, and human development at the periphery of the park. The conifer tree population was decimated due to gypsy moth infestation and white pine blight at the turn of the 20th century. Interestingly, this loss was most severe north of Walden Pond. It is possible that these factors may have shaped the vernal pool landscape of the Woods.

forest vernal pool

Statistical models and serious science aside, the Lynn Woods is truly a great spot for the vernal pool enthusiast. No two sites are alike in physical appearance and in the types of animals found there. Sites in the Woods ranged from high-elevation, shallow, and sunlit ponds to low-lying, deep, and heavily-vegetated depressions. In addition to spotted salamanders and wood frogs, these ponds are utilized by a host of forest creatures. In early spring, pools are saturated with American Toad tadpoles, fighting for space with the much larger wood frog tadpoles. Spring peepers, though rarely seen, are certainly heard throughout. Bullfrogs and green frogs bask in the sunlight, feeding on healthy supplies of insects, including dragonfly larvae, whirligig beetles, and predacious diving beetle larvae.

The various "ecozones" that make up the Lynn Woods create a unique blend of wetland life. A series of pools located around Mount Moriah at the southwest portion of the reservation are heavily used by wood frogs. The high elevation and minimal overhead tree canopy provided an excellent sunning spot for young tadpoles. Down the slope to Birch Pond lies a trio of pools which harbor a strong local population of spotted salamanders. Heading north, a few hundred feet from the westernmost arm of Walden Pond, is a small, extremely temporary depression, which attracts dozens of spotted salamanders from the high upland around surrounding it.

One active wood frog site located in the woods just south of Burrill Hill is hidden amongst a dense expanse of young birch trees, aptly named the "Birch Pool." Similarly, at a site on Bow Ridge near the water tower, a thick, near-impenetrable wall of brambles and high-bush blueberry encircles the shoreline (a sort-of KEEP OUT sign for unsuspecting researchers). An attractive site just off Oxbow Road north of Walden Pond is a haven reserved for fairy shrimp, an obligate vernal pool crustacean. The pool contains a handful of granite boulders, great for standing on and peering into the water for these miniature fish-like creatures.

See Also

July 2003

Kids' Day 2003

Plant Sale a Success

Concert Series Starting Soon

Old Names in Lynn Woods

Earthfest 2003

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