Natural Community Systems -- Photo Guide

Montane/near-boreal floodplain system

montane/near-boreal floodplain system (photo by Dan Sperduto for the NH Natural Heritage Bureau)
montane/near-boreal floodplain system flanking a gravel barren along the Saco River
(photo by Dan Sperduto)


Description:  This floodplain system is associated with montane and northern rivers in New Hampshire that often have flashy flood regimes and relatively high-gradients compared to other river systems.  Flashy flood regimes are those affected by high intensity, short-duration floods from mountain runoff events.  The most diagnostic natural communities are sugar maple and balsam fir floodplain forests, and occasionally red maple floodplain forest.  When silver maple floodplain forests are present they typically form a narrow border and are not the dominant forest type.  Moderate gradient sandy-cobbly riverbanks are typically adjacent to these floodplains, although some examples occur on higher- or lower-gradient sections of river.  Some smaller, northern river floodplains contain balsam fir floodplain/silt plains and alder thickets that lack sugar maple floodplain forest communities.

Examples along larger rivers with sugar maple - ironwood - short husk floodplain forest contain mixes of sugar maple (Acer saccharum), ironwood (Ostrya virginiana), and other common upland trees.  Shrubs are generally not dominant, except at forest edges.  Compared to average northern hardwood forests, the herb layer is often more lush, commonly with a high total coverage and a species composition indicative of semi-rich conditions.  Sugar maple - silver maple - white ash floodplain forest can occur on lower adjacent floodplains, marked by the appearance of silver maple (Acer saccharinum) and more mesic site plants.  Balsam fir floodplain/silt plains have a somewhat less floristically rich flora that lacks ironwood and contains more softwoods and common wet-site herbs of northern NH.

Diagnostic natural communities:

   Floodplain forests

      • Sugar maple - ironwood - short husk floodplain forest (S1)

      • Sugar maple - silver maple - white ash floodplain forest (S1S2)

      • Balsam fir floodplain/silt plain (S2)

   Herbaceous and shrubby floodplains

      • Herbaceous riverbank/floodplain (S2S4)

      • Bluejoint - goldenrod - virgin’s bower riverbank/floodplain (S3S4)

      • Alder alluvial shrubland (S3)

      • Alluvial mixed shrub thicket (S4)

      • Emergent marsh (S5)

Peripheral or occasional natural communities:

      • Silver maple - false nettle - sensitive fern floodplain forest (S2)

Landscape settings: floodplains (above bankful) along moderate-gradient sections of rivers and large streams

Soils: sandy alluvium (loamy sand, sandy loam, silt loams, and occasionally sand over gravel or cobble); moderately minerotrophic

Spatial pattern: large patch, extensive broad-linear (<150+ acres); meandering linear and semi-circular zones parallel to riverbanks or corresponding to floodplain terracing and oxbow, slough, or over-flow channel formations

Physiognomy: forest, woodland, sparse woodland, tall to medium-height shrub, herbaceous, and aquatic

Distribution: primarily found on flashy northern rivers in the White Mountains or north country, and occasional in north-central New Hampshire

Characteristic species:
Sugar maple - ironwood - short husk floodplain forest:
      Acer saccharum (sugar maple)
      Ostrya virginiana (ironwood)
      Quercus rubra (red oak)
      Fraxinus americana (white ash)
      Prunus serotina (black cherry)
      Pinus strobus (white pine)
      Solidago caesia (blue-stemmed goldenrod)
      Uvularia sessilifolia (sessile-leaved bellwort)
      Toxicodendron radicans (poison ivy)
      Aralia nudicaulis (wild sarsaparilla)
      Carex pedunculata (long-stalked sedge)
      Brachyelytrum erectum (northern short husk grass)

Balsam fir floodplain/silt plain
   Trees and shrubs
      Abies balsamea (balsam fir)
      Acer rubrum (red maple)
      Pinus strobus (white pine)
      Alnus incana ssp. rugosa (speckled alder)

      Calamagrostis canadensis (bluejoint)
      Carex stricta var. strictior (small-tussock sedge; non-tussock forming)
      Spiraea alba (eastern meadowsweet)
      Coptis trifolia (goldthread)
      Cornus canadensis (bunchberry)
      Aster acuminatus (whorled aster)

Associated natural community systems:  Moderate gradient sandy-cobbly riverbank systems are typically adjacent to these floodplains, although some examples occur on higher- or lower-gradient sections of river.

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