The re-emergence of wetlands vegetation has occurred quite recently, certainly within the past 50 years, allowing many of the low lying areas to once again become red maple swamps. Other on-site wetlands include the two farm ponds, the wildlife pond, a fabulous bog, and several wet meadow areas, each showing the evidence of human activity. The peat bog was presumably used for a period of time as a cranberry production bog. Aerial photographs reveal a series of distinct channels cut in the sphagnum mat. Native cranberries still grow in several locations on the bog.
Types of Wetlands
Four specific types of wetlands can be found in the Arboretum:
- Wet meadow
- Farm and wildlife ponds
- Red maple swamp
Each of the types found were inventoried, and specific points of interest highlighted.
1. Wet Meadow
Wet meadow habitat can be found along the low-lying perimeter of the orchards, frequently in areas adjacent to the drainage ditches. These sunny meadows characteristically exhibit saturated soils, hydric in nature, with an interesting diversity of plants and animals. A classic wet meadow can be found on the east side of the sidewalk leading in from Main Street just behind the formal Butterfly garden.
An informal trail bisects this wet meadow and is a wonderful location for hikers to experience the diversity of late summer native wildflowers inhabiting this full sun environment. Some of the more common native plant species growing in this wet meadow are:
boneset (Eupatorium perfoliatum), blue vervain (Verbenna hastatal), cinnamon fern (Osmunda cinnamomea), grass-leaved goldenrod (Solidago uliginosa), jewelweed (Impatiens capensis), joe-pye-weed (Eupatorium maculatum), New England aster (Aster novae-angliae), New York aster (Aster novi-belgii), reed-canary grass (Phalaris arundinacea), sensitive fern (Onoclea sensibilis), skunk cabbage (Symplocarpos foetidus) and tussock sedge (Carex crinita).
As a result of the profusion of late summer wildflowers, wet meadows are very attractive areas for many species of insects. The abundance of goldenrod, joe-pye weed, and aster species attract the following butterflies:
American copper (Chrysalis), black swallowtail (Papilio Polyxenes), cabbage white (Pieris rapae), eyed brown (Satyrodes eurydice), great spangled fritillary (Speyeria cybele), large wood nymph (Cercyonis pegala), skipper sp. (Hesperiidae), meadow fritillary (Boloria bellona), monarch (Danaus plexippus), mourning cloak (Nymphalus antiopa), painted lady (Vanessa pardoi), question mark (Polygonia interrogationis), red-spotted purple (Limenitis arthemis), tiger swallowtail (Papilio flaucus), viceroy (Limenitis archippos), white admiral (Limenitis arthemis).
In addition to butterflies, many other species of insects including a variety of wasps, bees, and dragonflies are frequent inhabitants of the wet meadow landscape.
Unfortunately, much of the landscape displaying wet meadow characteristics is fast becoming a monoculture of non-native purple loosestrife (Lythrum salicaria). Strategies for controlling the spread of loosestrife in these wetland ecosystems need to be addressed as part of the long-term maintenance of the Arboretum.
2. Farm and Wildlife Ponds
These ponds are located adjacent to the handicapped accessible trail system near the Wildflower Garden. The majority of the larger and upstream pond is almost entirely on private property (433 Main Street.) These ponds are about eight feet deep and have a flat rocked bottom, indicating they were man-made.
Water entering these historic farm ponds arrives via a drainage basin beginning just north of the Arboretum near the town center, with much of the contribution being road runoff from Main Street. Within the drainage basin are many older homes, some still with cesspools, along with an area where horses have, until recently, been boarded.
Each of these factors increases the nutrient supply entering the ponds. This is evident during the summer months when the ponds are completely covered by a thin floating layer of both duckweed (Lemna spp.) and water meal (Wolffia columbiana). While each of the ponds exhibits characteristic features common in nutrient rich water, both still provide excellent wildlife habitat, including vernal pool features.
The third and most recently excavated of the three ponds, is located just off the right branch of Orchard Loop accessible trail, not far from the Taylor Road parking lot. This small, full-sun pond was constructed in 1990 by the town of Acton. The intent was to create a wildlife pond near the maintained orchards and meadows, within viewing distance of the accessible trail. Many interesting developmental wetlands changes have occurred during the pond's short existence. Cattails (Typha latifolia) have colonized the pond, and green frogs and American toads use the pond as an annual breeding area. Because of the extreme seasonal fluctuation in groundwater in this area of the Arboretum, the pond periodically dries up and; as a result, has no fish population. Both elements make this pool an ideal wetlands for amphibian breeding.
Water quality issues related to the health of the two old farm ponds need monitoring. The recently excavated pond should be considered because much of the mowed (lawn) section of the Arboretum is up-gradient of this pond.
3. Red Maple Swamp
By far the most common wetland type throughout Acton is red maple swamp. This is also true in the Arboretum where more than 4 acres are forested wetlands with an overstory consisting primarily of red maple (Acer rubrum). Red maple swamps represent a very late successional stage of forest/wetlands evolution and will be the dominant wetlands feature in the Arboretum for decades to come.i
The Arboretum's red maple swamps are most commonly found in high groundwater areas bordering the three major drainage ditches. The map of the area illustrates the landscape pattern created by the north-south orientation of the ditches and associated wetlands.
Originally, the drainage ditches were excavated to channel seasonal runoff and the red maple swamps were cleared to develop lowland hayfields. In the late 1960s, active farming in the area ended, and red maples once again became the dominant species colonizing the wetlands. Along with the red maples came an interesting and diverse collection of understory shrubs, ferns, and herbaceous plants.
Encroachment into the Bog
Red maple has established a firm foothold along the boundaries of the Arboretum's bog, where the water is richer in minerals and dissolved oxygen and has a slightly higher pH than in the bog's center. The maples continue to encroach out onto the bog. This red maple/bog interface needs to be addressed in this plan.
One of the most fascinating wetlands features in the Arboretum is the bog. The origins of this surficial, geologic feature has already been described in detail. This section further reviews the origin of the bog and describes the limiting factors for life in and on the bog and surrounding red maple swamp.
By definition, the 2.5 acre bog is considered a flat or level peatland, decomposing sphagnum moss (Sphagnum spp.) and leaf litter (peat) concealed beneath a living sphagnum mat. The level bog is classified as oligotrophic, acidic, poorly fed, receiving water from a spring north of the bog near the base of the esker. Rain is the only other source of water entering the bog.ii
Evolution of the Bog
About fifteen thousand years ago, our bog would have appeared as a small, 15 to 20 foot deep pond, residing in the barren, treeless landscape, just beyond the boundary of the retreating glacier.iii The landscape on all sides of the bog featured other newly-forged glacial remnants such as an esker and acres of outwash sands and gravel strewn with large boulders left behind as the climate warmed and the glaciers receded.
The melt waters contained an abundance of seeds and spores stowed away for centuries in the ice finally to be deposited in the bog sediment. The stage was set for the evolution of the bog. iv
Nutrient Supply and Oxygen
As with most peat lands, no stream flows into the Arboretum's bog. This lack of a direct inlet significantly limits the nutrients entering the bog. There is no constant supply of oxygen, which is necessary to fuel the bacterial decomposition process. Both of these factors slow the rate of biologic decomposition of the organic matter. The excessively slow rate, along with other factors causes the bog water to become acidic, often with a pH of less than 4.
This sterile, isolated environment, limited in available nitrogen for plant growth provides the necessary habitat for a most interesting host of plant characters. In the following section covering wildlife habitat, life on the bog will be inventoried in greater detail.
i Ecology of Red Maple Swamps in the Glaciated Northeast: A Community Profile, by Francis C. Golet, Aram J.K. Calcoun, William R. DeRagon, Dennis J. Lowry and Arthur J. Gold. (Fish and Wildlife Service, U. S. Department of the Interior, 1993.)
ii Bogs of the Northeast (Charles W. Johnson, University Press of New England, 1985.)
iii Depth probing in March, 1998, by Richard Hatfield and Tom Tidman revealed a maximum bog depth of 16 feet.
iv New England Natives (Sheila Connor, Harvard University Press, 1994)