Sunderland Bridge Islands

Connecticut River at Sunderland, MA

For inquiries contact Jennifer Kearsley, Wetland Plant Ecologist,Massachusetts Natural Heritage and Endangered Species Program

UNIQUE HABITATS OF THE SUNDERLAND BRIDGE ISLANDS

From where the Sunderland Bridge crosses the Connecticut River in the towns of Sunderland and Deerfield to the confluence of the Deerfield and Connecticut Rivers in Greenfield, MA, there is a nine-mile stretch of river with four islands appropriately called Sunderland First, Second, Third and Fourth Islands. These islands are biologically interesting because of their unique physical environments, habitats and vegetation. The alluvial deposition of cobbles, sand and silt during high spring flood events created the islands, and annual flooding across the islands have created a gradient of substrate types and therefore unique habitats and vegetation. Scouring and flooding are most intense on the upstream end which receives the full force of ice sheets and debris-laden flood waters. Large cobbles are the only particles heavy enough to drop out of suspension in fast-moving water, so the exposed, upstream ends are mostly cobbles mixed with some pebbles and sand. As the water crosses the island it slows down and smaller particles drop out creating a continuum from cobbles and pebbles to sand and silt on the more protected downstream end of each island. That continuum correlates to a change in vegetation communities.

VEGETATION COMMUNITIES OF THE SUNDERLAND ISLANDS

Each island typically has two vegetation communities: a High-Energy Riverbank community on the upstream end and Floodplain Forest on the downstream end. The extent of each depends on the size and shape of the island and the physical processes of flooding and ice scour.

THE BROAD HIGH-ENERGY COBBLE COMMUNITY OF SECOND ISLAND ON UPSTREAM END; FLOODPLAIN FOREST ON SILTIER, DOWNSTREAM END

THE MUCH SMALLER COBBLE AREA AT THE UPSTREAM TIP OF THIRD ISLAND

The Massachusetts Natural Heritage and Endangered Species Program (MNHESP, part of the Division of Fisheries and Wildlife) designates both community types Priority Natural Communities due to their rarity in the state. High-energy riverbank communities are rare because they can only form in steep-gradient, high flood areas such as those that occur on the Connecticut, Deerfield and Westfield Rivers. Floodplain forests were at one time quite common in the state, particularly on the extensive alluvial silt deposits of the Connecticut River Valley, but they have been largely converted to agricultural land due to their high fertility. Only the lowest, most frequently flooded areas remain.

The cobble and sandy areas on the upstream ends of the islands have open vegetation composed of low-growing herbs and grasses.

HIGH-ENERGY RIVERBANK COMMUNITY ON SECOND ISLAND

These riverine cobble bars are unique because they are the only areas in the state where grasses typical of the Midwestern prairies, such as big and little bluestem (Andropogon gerardii and A. scoparius), are found growing. There are also several state-listed herbaceous species that are specific to this community type. Intense flooding and ice scour prevents establishment and growth of trees or tall shrubs. However, short shrubs such as shadbush (Amelanchier sanguinea), silky dogwood (Cornus amomum) and two state-listed species form a vegetation zone at the downstream edge of the grassy cobbles, bordering the floodplain forest. The openness of this riverine community provides suitable habitat for migrating shorebirds, such as Dunlins and Spotted Sandpipers, and for other birds that prefer open habitats, such as Killdeer. Several of our state-listed dragonfly species inhabit boulders and washed-in tree trunks in the cobble bar community.

Moving downstream from the high-energy cobble zone, the substrate becomes sandier and trees are able to establish and grow. It is on the downstream end of the Sunderland Islands that floodplain forests have developed. Floodplain forests of the islands are sandier than floodplain forests found along the river banks. The floodplain forest canopy is a mixture of silver maple (Acer saccharinum), cottonwood (Populus deltoides), and sycamore (Platanus occidentalis). Box elder (Acer negundo) and slippery elm (Ulmus rubra) occur in the subcanopy. There are few shrubs within the undisturbed forest, but vines, such as riverbank grape (Vitis riparia) and Virginia creeper (Parthenocissus quinquefolia) are abundant. Pale jewelweed (Impatiens pallida), ostrich fern (Matteucia struthiopteris), cut-grass (Leersia virginica) and stinging nettle (Laportea canadensis) are the most common herbaceous species.

FLOODPLAIN FOREST OF FIRST ISLAND SHOWING LARGE SILVER MAPLE GROWING WITH PALE JEWELWEED AND OSTRICH FERN

CONSERVATION ISSUES

Due to the rarity of the vegetation communities and the presence of state-listed plant and animal species on the Sunderland Islands, it is important that we protect these unique systems. The biggest threat to the islands is disturbance from people camping on them. Many campers have cleared large openings within the forest to place their tents, dug toilet pits in the sand and cut down trees to use as firewood. These activities alter the natural community structure and also allow exotic plant species to invade. Japanese knotweed (Polygonum cuspidatum) now occurs on all of the islands in cleared areas. It is a highly invasive, fast-growing exotic plant that grows three to six feet tall, and it appears to establish only in disturbed, open area in full sun. Once established, it spreads rapidly and shades out lower-growing native species. Japanese knotweed is very difficult to eradicate, and the best way to deal with it is to prevent its establishment. This can in part be accomplished by eliminating suitable habitat for the plants by preventing disturbance and clearing within floodplain forest areas. Other exotic plants that are becoming a problem on the islands are ground ivy (Glechoma hederacea) in the floodplain forest and lady's thumb (Polygonum persicaria) and purple loosestrife (Lythrum salicaria) on the cobble bars. To prevent these exotics from spreading farther, we must take care not to disturb the natural structure of these unique Massachusetts communities.

References:

Motzkin, G. 1993. Uncommon plant communities of the Connecticut Valley, Massachusetts.

Unpublished report to MNHESP.

Veneman, P.L.M. and R.W. Tiner. 1990. Soil-vegetation correlations in the Connecticut River

floodplain of western Massachusetts. Biol Report 90 (6). U.S. Fish and Wildlife Service.

Rowinski, T.S. 1988. Notes on riverside vegetation. The Nature Conservancy Magazine 38:24-25.

Armour, C.L. Some aspects of bottom types and benthos of a 32-mile segment of the

Connecticut River. M.A. Thesis.