Natural Communities of NH -- Photo Guide

Northern hardwood - black ash - conifer swamp  (S2)

Northern hardwood - black ash - conifer swamps
are seepage swamps occurring on shallow peat or mineral soils. The most common tree species in the diverse overstory are black ash (Fraxinus nigra), yellow birch (Betula alleghaniensis), balsam fir (Abies balsamea), and red spruce (Picea rubens). The herb layer is lush and indicative of mineral-rich conditions. Common herbaceous species include purple avens (Geum rivale), Robbins’ ragwort (Senecio robbinsii), water pennywort (Hydrocotyle americana), and small enchanter’s nightshade (Circaea alpina). Several rare plant species may be found in this community, including green adder's mouth (Malaxis unifolia), lily-leaved twayblade (Listera convallariodes) and heart-leaved twayblade (Listera cordata). In New Hampshire they are more common in the north, though examples are known from central parts of the state.

A hardwood - conifer mix is typical, though some examples have a dominance of either hardwoods or conifers. It shares numerous species with the red spruce - hardwood - violet variant of the red spruce swamp community, but several notable species differentiate this more nutrient-rich type. It occupies sites influenced by seepage or upland runoff, including gently sloping hillsides, headwater areas of drainages, minerotrophic upland margins of streamside or basin swamps, and local groundwater discharge areas within other swamp types. It is concentrated in northern regions of the state where surficial deposits of intermediate to base-rich composition are more prevalent. Most are fairly small (1-10 acres), though some examples in peatland basins may exceed 40-50 acres.

Black ash (Fraxinus nigra) is characteristic and may occur in nearly pure stands, though it is not always abundant or dominant. Although black ash does not grow as fast as red maple or American elm, it is fairly long-lived (>250 years). Its ability to stump sprout may be an important reproductive strategy in response to disturbance.

This is a moderately rich swamp type influenced to some degree by groundwater seepage or near-surface upland runoff. When present, hummocks tend to be modest in size (ca. 0.3 m tall), though they are occasionally absent, particularly in sloped examples. There is often a gentle but discernable slope to these wetlands. Soil water pHs are subacid to circumneutral (ranging from 5.2-6.3, average=5.8) and conductivity measures range from 20-70 uS (average=36). Perennially saturated soils are typical and surface rivulets and springs are occasional. Peat or muck horizons generally consist of well decomposed organic matter and are very shallow to absent in some examples, particularly on slopes with gray (gleyed) silty or sandy mineral soil often mottled near the surface (mineral histic, histic epipedons, and shallow Histosols). Examples in flat peatland basins have moderately deep peats, averaging 127 cm (range=70-265 cm).

Tree cover ranges from 25-80%. The most abundant and constant species in the tree canopy are various combinations of black ash, yellow birch (Betula alleghaniensis), balsam fir (Abies balsamea), and red spruce (Picea rubens). Other frequent species include northern white cedar (Thuja occidentalis), red maple (Acer rubrum), white spruce (Picea glauca), balsam poplar (Populus balsamifera), white ash (Fraxinus americana), eastern larch (Larix laricina), black spruce (Picea mariana), and, mostly south of the White Mountains, hemlock (Tsuga canadensis).

The shrub layer is typically sparse to moderately well-developed and may include speckled alder (Alnus incana ssp. rugosa), swamp winterberry (Ilex verticillata), mountain holly (Nemopanthus mucronatus), poison ivy (Toxicodendron radicans), witherod (Viburnum nudum var. cassinoides), Canadian honeysuckle (Lonicera canadensis), meadowsweet (Spiraea alba), and red osier dogwood (Cornus sericea). Highbush blueberry (Vaccinium corymbosum) is notably absent.

There is a broad diversity of herbaceous species indicative of minerotrophic conditions, including purple avens (Geum rivale), Robbins’ ragwort (Senecio robbinsii), sensitive fern (Onoclea sensibilis), foamflower (Tiarella cordifolia), water pennywort (Hydrocotyle americana), golden saxifrage (Chrysosplenium americanum), small enchanter's nightshade (Circaea alpina), spotted touch-me-not (Impatiens capensis), tussock sedge (Carex stricta; particularly var. strictior), drooping woodreed (Cinna latifolia), perfect-awned sedge (Carex gynandra), fowl mannagrass (Glyceria striata), northeastern mannagrass (Glyceria melicaria), zigzag goldenrod (Solidago flexicaulis), marsh fern (Thelypteris palustris var. pubescens), white turtlehead (Chelone glabra), violets (Viola spp.), rein-orchids (Platanthera spp.), and Sphagnum squarrosum (peat moss). Other occasional species include bunchberry (Cornus canadensis), goldthread (Coptis trifolia), and northern wild licorice (Galium kamtschaticum).

Mosses are usually abundant and may form a continuous carpet. The so-called “Brown mosses” (species primarily of the Amblystegiaceae family) are typically present along with Mnium spp., and various liverworts. Potential rare species that may occur in this community include green adder's mouth (Malaxis unifolia), Pursh's goldenrod (Solidago purshii), heart-leaved twayblade (Listera cordata), and lily-leaved twayblade (Listera convallarioides).

Species either absent or in lower abundance and constancy in this type compared to red spruce swamps include three-seeded sedge (Carex trisperma), cinnamon fern (Osmunda cinnamomea), and peat mosses (Sphagnum spp.).

This swamp type is essentially a northern version of the red maple - black ash swamps and red maple - sensitive fern swamps found in central and southern NH. It differs from these more southern swamps by having less red maple and a low abundance (or absence) of southern species such as swamp saxifrage (Saxifraga pensylvanica), skunk cabbage (Symplocarpus foetidus), and northern spicebush (Lindera benzoin). Examples transitional to red maple - black ash swamps appear to occur at somewhat lower elevations (ca. 500 ft.).

VARIANTS: Three variants are described.

1. Typic variant:

   Moderately to weakly acidic (mediacid to subacid) conditions characterize this variant; species restricted to basic conditions are absent. It generally occurs in small to large, flat or very slightly sloped open-basins that are more seasonally flooded than sloped variants described below. This is the northern analogue to the red maple - sensitive fern swamp community.

2. Circumneutral variant

This type is similar to the typic variant but contains a more significant presence of calcicoles. Conditions appropriate for these species may only occur as localized areas where base-rich groundwater discharges within an otherwise more extensive and acidic swamp. particularly in small canopy opening areas. These species include American alder buckthorn (Rhamnus alnifolia), showy lady's slipper (Cypripedium reginae), chestnut sedge (Carex castanea), Bailey's sedge (Carex baileyi), Bebb's sedge (Carex bebbii), large yellow lady's slipper (Cypripedium parviflorum var. pubescens), and Loesel's twayblade (Liparis loeselii). Many of these species are normally found in association with rich fens or northern white cedar swamps.

3. Sloped black ash variant

This black ash dominated swamp forms on gently sloped terrain, often with visible evidence of surface seepage and non-entrenched drainage channels. Soils are grayish or gleyed silty loams, sometimes with a shallow muck layer. The sloped terrain, shallow organic layer, and dominance of black ash may justify designating this variant as a distinct natural community type. This variant has a distinctly northern distribution, occurring only north of (and including) Lancaster.

Good examples of this community occur at Coleman State Park (Stewartstown) and Umbagog State Park (Errol).

Northern hardwood - black ash - conifer swamps are a diagnostic community of near-boreal minerotrophic peat swamp systems and forest seep/seepage forest systems.

Northern hardwood - black ash - conifer swamp at Coleman State Park (photo by Ben Kimball for the NH Natural Heritage Bureau)
Northern hardwood - black ash - conifer swamp at Coleman State Park (photo by Ben Kimball)

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