Natural Community Systems -- Photo Guide

Major river silver maple floodplain system

Major river silver maple floodplain system along the Merrimack River (photo by Dan Sperduto for the NH Natural Heritage Bureau)
Major river silver maple floodplain system along the Merrimack River (photo by Dan Sperduto)

Silver maple floodplain forest along the Merricack River (photo by Ben Kimball for the NH Natural Heritage Bureau)
Silver maple floodplain forest along the Merrimack River (photo by Ben Kimball)

 

Description:  Floodplains corresponding to this system occur along major rivers in New Hampshire (e.g., the Connecticut and Merrimack) and are indicated by the dominance of one or both types of silver maple floodplain forest, and sometimes oxbow marshes and various meadow and thicket communities.  Flooding on these rivers is affected by snowmelt from the White Mountains that peaks a bit later in the spring than melting snowpacks along more minor tributaries, and by the much larger water volume found on these main-stem rivers.  Forest canopies are dominated by mature Acer saccharinum (silver maple), which forms a tall, arching, cathedral-like ceiling above the level floodplain adjacent to the river channel.  Whereas shrubs are poorly represented, vines (including grapes) tend to be abundant, especially in canopy gaps and along forest edge transitions to other communities.  The silver maple - wood nettle - ostrich fern floodplain forest type is most common along the Connecticut River, while the silver maple - false nettle - sensitive fern floodplain forest type is most common on the Merrimack and its larger tributaries (e.g., Ashuelot, Contoocook, Suncook).  The latter type exhibits different dominant herbs and generally more grasses and sedges.  On more minor rivers, silver maple floodplain forest communities are reduced to a narrow band or relatively small portion of the floodplain system compared to other forest types or disappear altogether.  Red maple and other floodplain forest types predominate when silver maple disappears.

Floodplain forests of this system often form a mosaic with more open floodplain communities.  Shrub thickets and herbaceous meadows may occur on low floodplains and adjacent riverbanks.  Aquatic beds, emergent marshes and shrub thickets in oxbows may also occur and are typically flooded annually.  Riverwash plain and dunes occur on a few sandy pointbar floodplains of the Merrimack River and are kept open by some combination of infrequent scouring by major floods and subsequent shifting windblown sands.  Flood regimes have been altered considerably by dam control of major flood events, lengthening the return interval of medium and high floodplains along some sections of river.  Invasive plants are problematic in many examples of this system, particularly Celastrus orbiculatus (Asian bittersweet) and Berberis thunbergii (Japanese barberry).


Diagnostic natural communities:

   • Silver maple - wood nettle - ostrich fern floodplain forest (S2)

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

   • Herbaceous riverbank/floodplain (S2S4)

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

   • Alder alluvial shrubland (S3)

   • Alder - dogwood - arrowwood alluvial thicket (S4)

   • Buttonbush shrubland (S3) 

   • Aquatic bed (S4S5)


Peripheral or occasional natural communities:

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

   • Red maple floodplain forest (S3)

   • Riverwash plain and dunes (S1)


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

Soils: sandy to silty alluvium (loamy sand, sandy loam, silt loams); moderately to strongly minerotrophic

Spatial pattern: large patch, extensive broad-linear (< 1–50+ 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: found primarily along the main-stems of the Connecticut and Merrimack Rivers, and occasionally on lower reaches of major tributaries


Characteristic species:

Silver maple - wood nettle - ostrich fern floodplain forest
   
Trees
      Abundant species:
      Acer saccharinum (silver maple)

      Occasional to locally abundant:
      Fraxinus americana (white ash)
      Ulmus americana (American elm)
      Populus deltoides (eastern cottonwood)
      Celtis occidentalis (hackberry)
      Juglans cinerea (butternut)

   
Herbs and vines
      Dominant to abundant species:
      Matteuccia struthiopteris (ostrich fern)
      Laportea canadensis (wood nettle)

      Occasional, never dominant:
      Onoclea sensibilis (sensitive fern)
      Athyrium filix-femina (northern lady fern)
      Cinna arundinacea (common woodreed)
      Boehmeria cylindrica (false nettle)

      Impatiens capensis (spotted touch-me-not)
      Thalictrum pubescens (tall meadow-rue)
      Arisaema triphyllum (Jack-in-the-pulpit)
      Vitis riparia (river grape)

      Rare species:
      Arisaema dracontium (green dragon)

Silver maple - false nettle - sensitive fern floodplain forest
   
Trees
      Abundant species:
      Acer saccharinum (silver maple)
      Occasional to locally abundant:
      Ulmus americana (American elm)

   
Herbs and vines
      Dominant species:
      Onoclea sensibilis (sensitive fern)
      Boehmeria cylindrica (false nettle)

      
Occasional to locally abundant species:
      Toxicodendron radicans (poison ivy)
      Cinna latifolia (drooping woodreed) 
      Cinna arundinacea (common woodreed)
      Carex crinita (drooping sedge)
      Carex intumescens (inflated sedge)
      Athyrium filix-femina (northern lady fern)
      Impatiens capensis (spotted touch-me-not)
      Thalictrum pubescens (tall meadow-rue)
      Arisaema triphyllum (Jack-in-the-pulpit)
      Matteuccia struthiopteris (ostrich fern)


Associated natural community systems:  Most often this system is adjacent to the low-gradient silty-sandy riverbank system and sometimes the moderate-gradient sandy-cobbly riverbank system.  It often transitions to river terraces dominated by upland forest types, and in some locales to rich sugar maple - oak - hickory terrace forest.



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