Hudsonia - silverling river channel (S1)
The globally-rare hudsonia - silverling river channel community is currently known only from river channel barrens along the Saco River in New Hampshire and Maine. It forms narrow strips of vegetation on channel shelves, just above the river and below the forest edge. Grasses, forbs, mosses, and lichens are the dominant life forms, although vegetation is sparse overall. Widely scattered shrubs and saplings may be present. The association of the two rare perennial species hairy hudsonia (Hudsonia tomentosa) and silverling (Paronychia argyrocoma) is otherwise known only from ridge-top barrens on Panther Knob in West Virginia.
This community occupies the uppermost zone of the river channel, corresponding to the two-year floodplain at approximately 1.5–2.5 m above the river at low water. It is generally only 5–15 m wide, and up to 100–200 m long. It can also occupy pointbar cutoff channels or “flood chutes” where the barren is separated from the lower, intensely scoured pointbar by a wooded vegetation strip. The community is wider (often 20–40 m) in pointbar cutoff positions. Most examples are roughly 0.2 ha, and larger examples range from 0.5 ha to approximately 1 ha.
Substrate consists of water-sorted sediments, with variation occurring both within and between sites. At a given site, the substrate on the surface ranges from a matrix of nearly pure sand to a mixture of gravel, small cobble, and sand. A thin surface layer of sand often covers coarser material. Sand also sometimes forms low, tear-shaped mounds whose long axes are aligned with stream flow direction; these are the result of eddy deposition behind vegetation clumps. Coarser sediments, especially gravels, are more common upstream; sites with nearly pure sand occur downstream in
The restricted distribution of this community likely corresponds to a combination of flood-related disturbance and edaphic (soil) conditions along the river profile. Above open cobble shores, river channel barren zones are scoured enough to prevent most woody plant growth, but stable enough for perennial species such as hairy hudsonia and silverling to survive and sometimes thrive. Still higher is a sandy, depositional environment, often heavily vegetated by shrubs, saplings, and a variety of grasses and forbs. Presumably at this height in the channel, floodwater velocities have slackened to the point where depositional processes outpace erosion. The forested floodplain occurs above this zone.
The distribution of plants reflects substrate variability within and between sites. Perhaps the most telling plant distributions are those of the community’s signature species. Within sites, hairy hudsonia is most frequently associated with the sandier zones found near the highest areas of the channel, which are often farthest from the river itself. Among known examples of this community, the greatest concentration of hairy hudsonia occurs in the sandy river channel barrens in
Characteristic vegetation: Characteristic plant species include hairy hudsonia (Hudsonia tomentosa), silverling (Paronychia argyrocoma), little bluestem (Schizachyrium scoparium),
A good example occurs at the Dahl Sanctuary along the Saco River (Conway).
Hudsonia - silverling river channel usually occurs as part of a larger moderate-gradient sandy-cobbly riverbank system.
hudsonia - silverling river channel community along the Saco River (photo by Ben Kimball)
hudsonia patch at the upper edge of a hudsonia - silverling river channel
community along the Saco River (photo by Ben Kimball)
silverling in bloom (foreground) along the Saco River in Conway (photo by Ben Kimball)
silverling in bloom (foreground) in a patch of
hudsonia - silverling river channel community
along the Saco River (photo by Ben Kimball)
back to Natural Communities of NH Photo Guide