Priority Natural Communities
The Massachusetts Natural Heritage & Endangered Species Program (NHESP) defines Priority Natural Communities as "natural communities that have limited statewide or global distribution."
NHESP's Natural Communities Datalayer (2006) includes two Priority Natural Communities associated with the Mill River at Arcadia Wildlife Sanctuary:
- a small river floodplain forest
- a high terrace floodplain forest
NHESP ranks both of these communities as S2 imperiled, indicating they are very vulnerable to extirpation from the state. S2 designees have 6 to 20 occurrences, very restricted range, and few remaining acres or miles of stream.
Additionally, we have surveyed and mapped another imperiled community—Black Gum-Pin Oak-Swamp White Oak Perched Wetland.
Small River Floodplain Forest
"Small-River Floodplain Forests are silver maple/green ash forests occurring on alluvial soils of small rivers and streams. They occur on small tributaries of the Connecticut and Nashua Rivers and along some small rivers of eastern Massachusetts. Alteration of the natural hydrologic regime that created floodplain forests and their natural closed-canopy forest structure threatens these communities, particularly by allowing the establishment of invasive species.
"This moderate-sized example [covering approximately 26 acres ] of Small-River Floodplain Forest is one of the three best examples of this type in the state. It has good structure and diversity, and is well buffered by natural vegetation." — NHESP
High Terrace Floodplain Forest
"High-Terrace Floodplain Forests are deciduous hardwood forests that occur along riverbanks, above the zone of annual flooding. Although they do not flood annually, they flood often enough for the soil to be moderately enriched. Changes in natural flooding patterns that create floodplain forests threaten these communities. Most high terraces have been converted to agriculture or developed, leaving small, fragmented, and disturbed examples. This example of High-Terrace Floodplain Forest is quite small [less than 2 acres] and in somewhat degraded condition." — NHESP
Field work by Arcadia staff indicates that the High Terrace Floodplain Forest is of greater extent and quality than NHESP recognizes. This survey and mapping includes over 12 acres of High Terrace Floodplain Forest, ranging from a narrow band near the mouth of the Mill River, to approximately 200 feet wide along parts of the Fern Trail and River Trail, and tapering north toward the old dump site.
This community is not degraded except for a concentration of oriental bittersweet in the ground layer near part of the Fern Trail. These vines are cut periodically when they begin to climb the trees.
Black Gum-Pin Oak-Swamp White Oak Perched Swamp
Arcadia staff has also surveyed and mapped three forested wetland tracts that fit this S2 Natural Community description. In Massachusetts, this community type is limited to the Connecticut River Valley where low-lying areas create a perched water table over the impervious clay sediments of Glacial Lake Hitchcock.
Red Maple is usually the dominant tree species, but Black Gum (Tupelo), Pin Oak, and Swamp White Oak are important components of the canopy. Hemlock, American Beech, and Yellow Birch may also occur. Shrubs include Winterberry, Highbush Blueberry, arrowwood, Serviceberry, and sometimes Mountain Laurel. Cinnamon Fern characteristically grows on the higher parts of the hummock and hollow terrain. Sedges may grow in the hollows.
The three sites—Red Maple Swamp, Tupelo Swamp, and Hitchcock Swamp—differ somewhat in the dominance of the canopy species, but all are well delineated by Cinnamon Fern, and the clay layer is between 5-24 inches below the surface. Pin Oak and Swamp White Oak are absent from Red Maple Swamp, while large specimens occur in Hitchcock Swamp.
Tupelo Swamp transitions from mainly hardwoods (red maple, black gum, pin oak, swamp white oak, white oak) to a mixed canopy (hemlock, red maple, black gum, swamp white oak, white oak, and American beech). Virtually the entire western forest is perched on Lake Hitchcock clays, with some areas better drained than others. Hummock and hollow topography is common. Drainage ditching is evident around Tupelo Swamp.
Transitional Floodplain Forest
Additionally, NHESP maps a third area of floodplain forest—a Transitional Floodplain Forest—along the inside bank of the Oxbow, opposite from Arcadia. NHESP notes that this area is moderate in size [approximately 21 acres] but is in good condition, and is one of only five areas of its type known to occur in the state.
Transitional Floodplain Forests are "Riverside Silver Maple-Green Ash-American Elm forests that experience annual floods. Of the three floodplain forest community types, these communities are intermediate in vegetation and soils." As with other floodplain forests, alteration of the natural hydrologic regime can threaten the distribution and long term viability of these woodlands.
A total of 31 vernal pools occur on the sanctuary. Amphibian egg mass tallies have been conducted annually for more than a decade. These pools support varying populations of Wood Frogs as the main vernal-pool breeding amphibian; low numbers of Spotted Salamanders also use some pools. There are 21 pools that are formed by Connecticut and Mill River flooding that support Fairy Shrimp.