Tuesday, July 6, 2010

A rare plant community grows along the shore of Hawksnest Pond

The Plymouth gentian found at Hawksnest and Black ponds, is a species of Special Concern.

Hawksnest Pond was formed when the retreating glacier left behind a huge block of ice, half-buried in the sand.  When the ice melted, it left a large depression called a kettle pond, or a coastal plain pond.  A coastal plain pond is one of the rarest wetland types in North America.

The plants growing along the sandy shore of Hawksnest Pond together make up the Coastal Plain Pondshore Community.  We'll call it the Pondshore Community for short.  It's composed of a mixture of herbaceous and grass-like plants, growing between the shallow water and the shrubs that surround the pond.

They grow in soil ranging from patches of sand, sandy or muddy peat, to cobbles.

The pondshores in the Six Ponds District of Harwich are considered an "imperiled plant community" by the State.  Not only is this plant community home to several rare species, but it also helps safeguard the pristine water quality of Hawksnest.

A constantly changing environment

The Pondshore Community occurs in those ponds with no surface inlet or outlet, and with a gradual slope to the shore. The community develops best in small ponds or bays of larger ponds--places that avoid the wave and ice damage typical of large ponds.

The ponds are windows into the groundwater, which moves easily through the sand surrounding the ponds. As a result, the water level rises and falls with the water table through the seasons, which in most years leaves exposed shores expanding throughout the summer.

Many of the plant species are able to start growth from seed, perennial basal leaves, or roots while inundated with water in the spring.  They grow in the increasingly dry, nutrient poor soils as the season progresses.  Other plants may germinate only when exposed.

Hawksnest during high water, usually early in the year. 
Flooding keeps shrubs back, enabling Plymouth gentian to compete.

In wet years, when the water level does not recede as far as in dry years, the plants may grow vegetatively while submerged, with little flowering, or may not grow or germinate at all.

Not only do the water levels change through the year, but between years as well.  Only one year in about 5 may be dry enough for the community to develop fully. The lowering of water levels during the growing season is probably the single most important factor in providing suitable habitat for the plants of the Pondshore Community.

Hawksnest during drought, 1993. Because the plants require low water to reproduce, they are most vulnerable to foot traffic at this time.

The waters of coastal plain ponds tend to be nutrient poor and acidic. The plants of the Pondshore Community are particularly adapted to the nutrient poor conditions--so they are able to compete with plants from other communities that require more nutrients.
 
The periodic flooding of the shore also helps to keep out shrubs and upland plants, and the periodic drying keeps out aquatic plants.
 
Characteristics of the community
 
The Pondshore Community contains a number of plants which seldom occur elsewhere. Some may be locally abundant, mixed in with more common marsh emergents such as rushes, sedges, Boneset and Purple Gerardia.
 
The plants of the community appear to form zones between the water and the shrubs around the pond. The driest zone, inundated only in the highest water, may have New England boneset* or Maryland meadow-beauty, both considered rare in Massachusetts. The higher shoreline is home to Thread-leaved sundew (common on these ponds but uncommon elsewhere), and Spatulate-leaved sundew.  The mid to upper level is home to redroot (a species of special concern) found in the Six Ponds District.
 
Threats and Management

Pondshore Communities have several threats caused by human disturbance. The community requires natural fluctuation of the water levels along the shore. Artificially maintained high water levels reduce the area of shore available for the Pondshore Community. Most of the plants of the community can withstand high water for a few years, which happens naturally, but most need to be out of water to reproduce.

Human use of the pondshores, including walking, offroad vehicles, and beach building, restricts plant growth. Experiments have shown that a few walking trips can create a trail where no plants grow. In areas of heavy use, the plants of the Pondshore Community can easily be eliminated.

Nutrient enrichment from septic systems poses a serious long-term threat to the natural balance** of ponds.  This can change the character of the ponds, allowing algae and pondweeds not native to the ponds to grow and reduce the habitat available to the plants of the Pondshore Community.

Excessive drawdown from pumping at town wells reduces natural fluctuations and allows woody species to advance down the shores.

Gallery of members of the community

The carnivorous spatulate-leaved sundew, by Debbie Barnegat. Not rare, but uncommon.


Maryland meadow beauty, by Ken Clark. Shown on maps in Brewster. Last seen in Harwich in 1918--I don't know if it has been seen at Hawksnest.

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*    New England boneset (Eupatorim novae-angliae) appears to be a hybrid of two other plants, but can persist in an area without both parent species present, so is though to be a distinct species.  Not known to have occurred at Hawksnest.

**  The upset in natural balance of ponds from too many nutrients, causing fast "aging" of the pond and blooms of toxic algae, is called "eutrophication."

This article is quoted and condensed from a fact sheet.
You can find a list of rare species for Harwich here.