Alder alluvial shrubland (S3)
This variant is based on two samples in northern
Alder alluvial shrubland is a patchy shrubland dominated by either speckled alder (Alnus incana ssp. rugosa) or smooth alder (Alnus serrulata). Other woody species and herbs are usually sparse in moderate-energy settings, but moderately dense to dense in lower-energy floodplains and settings with silt soils. A variety of herbaceous plants comprise the understory, which can be highly variable in composition and density.
This community often occurs as a narrow band (<1–10 m wide) along rivers, sometimes stretching for hundreds of meters, or forms wider expanses on silt plains in broader valley bottoms. It occurs in moderate- and low-energy settings along the banks of small to large rivers, and on low floodplains of large streams adjacent to meadow marshes, though it can also occur in higher energy settings such as alluvial floodplains and riverbanks. It forms on a variety of substrates, including shallow muck soils. It often flanks herbaceous riverbank and upland forest communities. While widely distributed, it is common in the northern part of the state, and more sporadic south of the mountains.
Alders may out-compete other shrubs and trees along riverbanks because they bend in strong currents, rather than uprooting or breaking, and tolerate long periods of flooding. Shrub stems lean in the direction of river flow and exhibit signs of flood scour on the upstream sides of their stems. Alders have an additional advantage over competitors with their nitrogen-fixing root nodules that provide nitrogen to the shrubs in nutrient-poor, coarse substrate.
Mineral soils range from cobble, gravel, and sand-silt mixes (in the typic and bryophyte variants) to silt (in the silt variant). This community is seasonally to infrequently flooded, with water levels near the surface for substantial portions of the year. When it occurs at a beaver pond, it often succeeds to meadow marsh communities following the drop in water level caused by beaver dam abandonment.
Variants: Three variants are described. All are dominated by moderately dense thickets of alder (15 to >60% cover).
1. Typic variant:
This moderate-energy variant has 1% or less bryophyte cover and 25–40% alder cover. Substrate is variable, from sand and silt to mixtures of sand, gravel, cobbles, and boulders. The pH of the sandy patches averages 5.1. This variant is often flanked by herbaceous riverbank communities and upland forests. Examples of this variant are likely to be found on many moderate-energy stretches of rivers and streams in
Speckled alder (or, less frequently, smooth alder) dominates this variant. Bryophytes are almost entirely absent. Though not abundant, common associates are red osier dogwood (Cornus sericea), silky willow (Salix sericea), bluejoint (Calamagrostis canadensis), goldenrods (Solidago spp.), flat-topped white aster (Doellingeria umbellata), flat-topped goldenrod (Euthamia graminifolia), swamp candles (Lysimachia terrestris), meadowsweet (Spiraea alba), and panic grasses (Panicum spp.). In some examples, herbs and shrubs contribute greater cover (5–15%) including winterberry (Ilex verticillata), western black raspberry (Rubus occidentalis), virgin's bower (Clematis virginiana), sensitive fern (Onoclea sensibilis), and red maple (Acer rubrum). The state-threatened
2. Bryophyte variant:
3. Silt variant:
This variant occurs on silty soils found in generally lower-energy settings compared to the other variants. These settings can occur along streams, minor rivers, and silt plains formed from glacial lakebed deposits or stream deltas adjacent to lakes. It is characterized by a moderate to dense herbaceous layer in contrast to sparser herbaceous cover found in the other two variants. There is also an increase in the number and abundance of species indicative of richer and/or wetter soil conditions. Balsam fir (Abies balsamea) and/or white spruce (Picea glauca) form a sparse woodland canopy (<25% cover) in many examples. This variant can transition into balsam fir floodplain/silt plain forest on relatively higher terrace positions. Also, cut over examples of that community can approximate the composition of this variant. This variant is described from northern
Frequent species include red osier dogwood (Cornus sericea), bluejoint (Calamagrostis canadensis), tall meadow-rue (Thalictrum pubescens), sensitive fern (Onoclea sensibilis), dwarf raspberry (Rubus pubescens), northeastern mannagrass (Glyceria melicaria), flat-topped white aster (Doellingeria umbellata), spotted touch-me-not (Impatiens capensis), intermediate wood fern (Dryopteris intermedia), rough goldenrod (Solidago rugosa), spotted Joe-pye-weed (Eupatorium maculatum), and purple-stemmed aster (Symphyotrichum puniceum). Occasional species include paper birch (Betula papyrifera), squashberry (Viburnum edule), foamflower (Tiarella cordifolia), northern lady fern (Athyrium filix-femina var. angustum), red elderberry (Sambucus racemosa), royal fern (Osmunda regalis var. spectabilis), northern short husk grass (Brachyelytrum septentrionale), purple avens (Geum rivale), hooked buttercup (Ranunculus recurvatus), Jack-in-the-pulpit (Arisaema triphyllum), crested wood fern (Dryopteris cristata), white turtlehead (Chelone glabra), inflated sedge (Carex intumescens), and zigzag goldenrod (Solidago flexicaulis). Good examples of the silt variant occur along the
A good example of this community occurs along the South River (Effingham).
Alder alluvial shrublands often occur as a component community of emergent marsh - shrub swamp systems, low-gradient silty-sandy riverbank systems, moderate-gradient sandy-cobbly riverbank systems, high-gradient rocky riverbank systems, montane/near-boreal floodplain system, major river silver maple floodplain system, and temperate minor river floodplain system.
This variant is based on two samples in northern
alder alluvial shrubland along a stream at Coleman State Park in Stewartstown (photo by Ben Kimball)