Alder - dogwood - arrowwood alluvial thicket (S4)
Alder - dogwood - arrowwood alluvial thickets are one of several riverbank communities that are heavily dominated by shrub thickets. They frequently contain a dense mix of several shrub species, though trees, woody vines, and herbs may be present as well. This community is more common in the southern part of the state.
This is a broadly defined community consisting of shrub thickets on loam and fine- to medium-grained sands. It is subjected to flooding early in the year. As river water levels fall during the growing season, the sandy soils become exposed and less wet. Shrub cover is typically moderate to high and often consists of a mix of several shrub species. Variation in species composition and cover likely relates to the variable environmental conditions and disturbance dynamics of the stream channels, as well as propagule availability. This community may occur as broad floodplain thickets along major rivers or as narrow zones along large streams and rivers.
Characteristic Vegetation: Common shrubs that may be locally dominant include silky dogwood (Cornus amomum), speckled alder (Alnus incana ssp. rugosa), smooth alder (Alnus serrulata), red osier dogwood (Cornus sericea), willows (Salix spp.), northern arrowwood (Viburnum dentatum var. lucidum), winterberry (Ilex verticillata), and meadowsweet (Spiraea alba var. latifolia). Trees, woody vines, and other shrubs include grapes (Vitis spp.), brambles (Rubus spp.), sweet pepperbush (Clethra alnifolia; near the coast), nannyberry (Viburnum lentago), witherod (Viburnum nudum var. cassinoides), swamp rose (Rosa palustris), alder-buckthorn (Frangula alnus), red maple (Acer rubrum), silver maple (Acer saccharinum), steeplebush (Spiraea tomentosa), common elderberry (Sambucus canadensis), American elm (Ulmus americana), and poison ivy (Toxicodendron radicans).
A moderate diversity of herbs may also be present, including swamp candles (Lysimachia terrestris), bluejoint (Calamagrostis canadensis), blue-grasses (Poa spp.), grass-leaved goldenrod (Euthamia graminifolia), goldenrods (Solidago spp.), asters (Aster spp.), bedstraws (Galium spp.), Joe-pye-weeds (Eupatorium spp.), sensitive fern (Onoclea sensibilis), virgin's bower (Clematis virginiana), sedges (Carex spp.), bent grasses (Agrostis spp.), mannagrasses (Glyceria spp.), fringed loosestrife (Lysimachia ciliata), marsh fern (Thelypteris palustris var. pubescens), ferns (Osmunda spp.), false nettle (Boehmeria cylindrica), common water horehound (Lycopus uniflorus), common woodreed (Cinna arundinacea), spotted touch-me-not (Impatiens capensis), common water hemlock (Cicuta maculata), violets (Viola spp.), and tall meadow-rue (Thalictrum pubescens).
This community is distinct from alder alluvial shrubland, which occurs on coarser sand in more moderate-energy settings. Some examples are dominated by one species [specifically, red osier dogwood (Cornus sericea)] and warrant consideration as their own community.
A good example of this community occurs along the South Branch of the Piscataquog River (New Boston).
Alder - dogwood - arrowwood alluvial thickets usually occur as part of a larger low-gradient silty-sandy riverbank systems.
Alder thicket along Blackwater Brook (photo by Bill Nichols)
Alder - dogwood - arrowwwod alluvial thicket at Saint-Gaudens National Historic Site
(photo by Ben Kimball)
Inside an alder thicket at Tuttle Swamp (photo by Ben Kimball)