Mixed alluvial shrubland
Mixed alluvial shrubland (S4)
(formerly alluvial mixed shrub thicket)
Mixed alluvial shrublands occur on sandy to cobbly channel shelves. This broadly defined community forms a woody transition zone between more open riparian communities such as low riverbanks closer to the river channel and floodplain forests at higher elevations. It is distinguished by thickets of tree saplings and tall shrubs, often interspersed with sizable patches of bare, coarse substrate. Tree saplings and shrubs total more than 25% cover, though low shrub, grass, and herb species can also be common. It has a variable mixture of medium to coarse sand, gravel, cobbles, and boulders, and is typically very well drained at low water. Most examples are less than 0.5 ha in size. They occur both as narrow bands and as broader patches. They are highly dynamic and likely shift with point bar migration. Invasive plant species are common and occasionally abundant in this natural community.
The community occupies unstable, depositional areas characterized by sand and leaf-litter accumulation. The sand frequently forms low, linear mounds (with long axis parallel to stream flow) around the bases of the shrub thickets. Leaf-litter accumulates as pockets of mulch in hollows adjacent to these mounds, along with woody debris. High-energy water leaves its mark on the shrubs; they may lean in the direction of river flow, and their bark is often scraped by ice on the upstream side.
Similar riparian shrub communities include two alder-dominated types: 1) alder alluvial shrubland; and 2) alder - dogwood - arrowwood alluvial thicket.
Characteristic Vegetation: The most characteristic species include eastern meadowsweet (Spiraea alba var. latifolia), willows (Salix spp.; esp. silky willow (Salix sericea) and stiff willow (Salix eriocephala)), birches (Betula spp.), and other saplings. Other species that may be present include southeastern silky dogwood (Cornus amomum), red oak (Quercus rubra), silver maple (Acer saccharinum), white ash (Fraxinus americana), smooth alder (Alnus serrulata), white pine (Pinus strobus), sand cherry (Prunus pumila), Virginia creeper (Parthenocissus quinquefolia), staghorn sumac (Rhus typhina), red maple (Acer rubrum), and aspens (Populus spp.). Herbs and low shrubs can be common and include goldenrods (Solidago spp.), little bluestem (Schizachyrium scoparium), deertongue (Dichanthelium clandestinum), woodgrass (Sorghastrum nutans), reed canary grass (Phalaris arundinacea), bluejoint (Calamagrostis canadensis), and sweet fern (Comptonia peregrina). The state-rare Virginian mountain mint (Pycnanthemum virginianum) occurs in some examples.
Invasive species are common and occasionally abundant in this natural community. They may include alder-buckthorn (Frangula alnus), Japanese knotweed (Fallopia japonica), Morrow's honeysuckle (Lonicera morrowii), black swallowwort (Cynanchum louiseae), purple loosestrife (Lythrum salicaria), black locust (Robinia pseudoacacia), and ninebark (Physocarpus opulifolius).
Good examples of this community occur along the Pemigewasset River (Woodstock) and at Livermore Falls (Holderness).
Mixed alluvial shrublands often occur as part of moderate-gradient sandy-cobbly riverbank systems, emergent marsh - shrub swamp systems, and montane/near-boreal floodplain systems.
Mixed alluvial shrubland at Livermore Falls
(photo by Dan Sperduto)