Swamp white oak floodplain forest (S1)
Swamp white oak floodplain forests are state and regionally rare. In New Hampshire they are restricted to drainageways in the Great Bay watershed and tributaries of the lower Merrimack River in a zone of fertile, silty soils within 30 miles of the coast. Swamp white oaks (Quercus bicolor) are the dominant or co-dominant trees. This community type is similar to low floodplain variant examples of the red maple floodplain forest community.
These floodplain forests are associated with heavy (silty) soils of marine or recent floodplain origin. The lower floodplain is somewhat poorly drained silt loam or fine sandy silt loam with a thin organic horizon. Medium to high floodplain forests are somewhat poorly to moderately well drained with a similar soil profile. Average soil pH is 5.4.
Characteristic vegetation: Both lower and higher floodplains are dominated by a mix of swamp white oak (Quercus bicolor) and red maple (Acer rubrum), with an understory ofmusclewood (Carpinus caroliniana), abundant sensitive fern (Onoclea sensibilis), and variable amounts of northern arrowwood (Viburnum dentatum var. lucidum), nannyberry (Viburnum lentago), winterberry (Ilex verticillata), glaucous carrion-flower (Smilax herbacea), drooping sedge (Carex crinita), and poison ivy (Toxicodendron radicans). White ash (Fraxinus americana) is occasional. River birch (Betula nigra), a rare tree in New Hampshire, sometimes occurs in this community. Loose-stemmed sedge (Carex laxiculmis), an uncommon sedge restricted to silty soils in southern New Hampshire, is also found in this community. There is little or no moss cover.
Variants: Three variants are described. While two are based on floristic differences associated with elevation above the river channel, a continuum of species compositional change is evident across the elevation gradient at most sites. A third variant is based on the abundance of river birch.
1. High variant:
This variant occurs on medium to high elevation floodplains. The herb layer is moderately dense (40–60%) and the shrub layer is moderately to very dense (30–80%). Tree seedling and sapling regeneration in the shrub layer is sparse. There is a greater abundance of upland tree, shrub, and herb species compared to the low floodplain variant. These include shagbark hickory (Carya ovata), white pine (Pinus strobus), red oak (Quercus rubra), black cherry (Prunus serotina), ironwood (Ostrya virginiana), beech (Fagus grandifolia), New York fern (Thelypteris noveboracensis), Canada mayflower (Maianthemum canadense), sessile-leaved bellwort (Uvularia sessilifolia), and lowbush blueberry (Vaccinium angustifolium). Among floodplain forests in New Hampshire, shagbark hickory is most frequent in this variant.
2. Low variant:
The lower floodplain has a moderately dense to dense (40–90%) herbaceous layer, a sparse to moderately dense shrub layer (6–40%), and a light to moderately dense seedling/sapling layer. Green ash (Fraxinus pennsylvanica) is common to abundant, and diagnostic to this community variant among non-silver maple floodplain forest communities. Other species indicative of the low variant include silky dogwood (Cornus amomum), small-tussock sedge (Carex stricta var. strictior), northern blue flag (Iris versicolor), swamp candles (Lysimachia terrestris), and American elm (Ulmus americana).
3. River birch variant:
All of the species indicative of the low variant may occur in this variant. Red maple, swamp white oak, basswood (Tilia americana), white ash, and American elm are all common along with abundant river birch. bulbous bitter-cress (Cardamine bulbosa) and wild garlic (Allium canadense) are rare plants found in this type.
Good examples of the high and low variants of this community occur along the
Swamp white oak floodplain forest along the Exeter River (photo by Ben Kimball)
Swamp white oak floodplain forest at Tuttle Swamp in Newmarket (photo by Ben Kimball)
Edge of a swamp white oak floodplain forest along Beaver Brook in Hollis
(photo by Dan Sperduto)