Natural Communities of NH -- Photo Guide

Riverwash plain and dunes  (S1)

Inland dunes are a rare geologic phenomenon in New Hampshire, and only occur in the globally rare riverwash plain and dunes community. The dominant vegetation in this primarily barren habitat is a sparse to moderate cover of drought-tolerant grasses, sedges, and forbs, along with a fragile cryptogamic crust.

In New Hampshire, this globally rare community is known only from high terraces associated with the broad meanders of the upper Merrimack River in Canterbury. These terraces have been periodically scoured during major flood events, burying the point bars with fresh sand and gravel deposits. Subsequent re-working of sand deposits by wind has formed 0.5–1.5 m dunes on portions of these plains; dunes are a very rare geologic phenomenon away from the coast in New England. The two largest examples of this community are 6-12 ha. in size and 1.5–3 m above the current average river level. Flood control dams may preclude the large flood events that sustained this community.

The bottomlands along this part of the upper Merrimack River consist of glacio-lacustrine deposits (lake-bed and river delta sediments) associated with glacial Lake Merrimack, which existed between 14,300 to 14,750 years ago. Thinly-bedded, alternating layers of silt and clay lake-bed deposits (called “varves”) found near the current river level were covered by river delta deposits, which were in turn capped by deep fluvial deposits of more recent origin that eventually filled Lake Merrimack. Since then, the river has cut its way through about 25 m of these deposits to reach its present day level, maintaining a series of broad, sandy peninsulas that support this and other communities.

Flood and fire dynamics have been operating on these terraces for thousands of years. Soils consist of fine sand near the surface, with a fine gravel “pavement”. Topographic maps, air photos, and recent surveys indicate that these areas have been sparsely vegetated since at least 1927, though open areas have diminished as trees and other vegetation have re-colonized portions of the plain and dunes. A relatively large 5 ha.+ area on one of the plains has remained open with little vegetation for at least 60-75 years, supporting a very sparse cover of vascular plants and a variable cover of cryptogamic organisms.

Cryptogamic crusts are ecologically important biotic layers at the soil surface found in arid and semi-arid landscapes worldwide, and consist of some combination of mosses, lichens, fungi, bacteria, and algae. In many cases their development takes decades. Cryptogamic organisms have not been examined closely in this community, but deserve detailed study given their potential ecological importance and rarity in the region.

Characteristic vegetation: This community ranges from unvegetated to forested cover, depending on degree of natural and human disturbance. The more open areas include active dune zones and flat riverwash plains areas, with a moderate to sparse cover of drought-tolerant grasses, sedges, forbs, and a cryptogamic crust. Shrubs and tree saplings are widely scattered overall, but are more abundant at the margins of the open plains and dunes.

The open riverwash plains contain less than 1% cover of vascular plants and zero to 100% local cover of cryptogamic organisms. Vascular plants here include little bluestem (Schizachyrium scoparium), shaved sedge (Carex tonsa var. tonsa), seabeach pinweed (Lechea maritima), and jointweed (Polygonella articulata). These plants are also found in active dune areas along with stiff-leaved aster (Ionactis linariifolius), panic grasses (Panicum spp.), red sorrel (Rumex acetosella), big bluestem (Andropogon gerardii), and the rare sandbur (Cenchrus longispinus).

Other plants in open to semi-wooded dune areas include Canada bluegrass (Poa compressa), linear-leaved panic grass (Dichanthelium linearifolium), deertongue (Dichanthelium clandestinum), tumble grass (Eragrostis spectabilis), rough bentgrass (Agrostis scabra), branching needle grass (Aristida basiramea), fall witchgrass (Digitaria cognata), Greene's rush (Juncus greenei), bulblet umbrella sedge (Cyperus dentatus), beach umbrella sedge (Cyperus filicinus), bluecurls (Trichostema dichotomum), northern evening primrose (Oenothera parviflora), silverrod (Solidago bicolor), northern gray goldenrod (Solidago nemoralis), blue ground-cedar (Diphasiastrum tristachyum), red raspberry (Rubus idaeus), ground juniper (Juniperus communis var. depressa), and sweet fern (Comptonia peregrina).

Trees form a woodland cover in some areas and include scarlet oak (Quercus coccinea), black oak (Quercus velutina), red oak (Quercus rubra), white pine (Pinus strobus), pitch pine (Pinus rigida), and black cherry (Prunus serotina).

A good example of this community occurs along the Merrimack River (Canterbury).

Riverwash plain and dunes at Muchyedo Meander (photo by Dan Sperduto for the NH Natural Heritage Bureau)
Riverwash plain and dunes at Muchyedo Meander (photo by Dan Sperduto)

Riverwash plain and dunes at Muchyedo Meander (photo by Dan Sperduto for the NH Natural Heritage Bureau)
Riverwash plain and dunes at Muchyedo Meander (photo by Dan Sperduto)

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