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 (though it can be 20–40 m wide on some pointbars), and up to 100–200 m long. A few examples occur in narrow “flood chutes” between point bars and adjacent uplands. 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, surface sediments range from a thin layer of nearly pure sand to a mixture of sand, gravel, and small cobbles.
The restricted distribution of this community results from a combination of flood-related disturbance and edaphic (soil) conditions along the river profile. Above cobbly low channels, higher river channel 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, adjacent to this community, 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. Forested floodplains often occur above this zone.
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 in the region, the greatest concentration of hairy hudsonia occurs in the sandy river channel barrens in
Characteristic vegetation: Characteristic plant species in river channel portions of this community 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)
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