H. Freshwater Wetland Ecosystems

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n the North Atlantic Coast

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Distribution of freshwater wetlands in NAC

The larger wetland complexes in the ecoregion are forested, with red maple swamp, Atlantic white cedar swamp and pitch pine wet woods (pitch pine savanna) being the dominant types. Pitch pine wet woods is the most restricted in distribution, with the southern end of NAC as the core of its range (see Forested Wetlands description for more detail). Of the open wetland types, coastal plain pondshores (outwash plain pondshores) and related coastal plain bogs are the most restricted to this ecoregion.

Detailed wetland mapping has been done by many state programs as well as nationally by the National Wetlands Inventory (NWI; Cowardin, et al. 1979). We excluded wetlands under one acre; to filter out small mapping errors and create a data set of manageable size. For many wetland types, the larger examples will capture the full range and better expression of the type than these tiny occurrences. However, several important wetland types will be under-represented in this analysis including seasonally flooded pools (vernal pools and others), headwater seeps, and springs. The assumption is that these communities will be captured as nested targets within the unfragmented coastal blocks, the matrix forest blocks and target species sites. Ideally, a more detailed analysis would be run in sample locations to test this assumption.

Conifer and mixed swamps

Hardwood swamps

Floodplain Forests (some overlap with types above; see floodplain section)

Shrub Swamps and Marshes (includes some riparian wetlands)

Shrub Swamps

Marshes

Bogs and Fens

Dwarf Shrub Bogs

Acidic Fens

Calcareous Fens

Lake or Pond – palustrine component (primarily Coastal Plain Pondshores)

Although the freshwater wetland model encompasses some examples of floodplain and coastal plain pondshore communities, they were not consistently mapped using the NWI based methodology. Both of these wetland types have highly fluctuating hydrologic regimes and are typically dry for large parts of the year. To address this issue, we developed a separate floodplain/riparian model for river related ecosystems and for the well inventoried pondshores we limited our analysis to confirmed ground survey points provided by the heritage programs.

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n the North Atlantic Coast Ecoregion

A total of 24,183 forested freshwater wetlands ranging in size from 1 to 26,404 acres were identified using the NWI-based ecosystem models. Mean elevation for the wetlands was 23 meters, with a range from sea level to 165 meters (Table 13). Since this ecoregion lacks the extensive conifer bogs of the north, most of the forested wetlands in NAC can be broadly described as forested swamps including floodplain forest, forested seeps, and broadleaf and conifer swamps. The forested bogs tend to be quite small. Differences in dominant species, flooding regime (seasonally to semi-permanently flooded) and substrate (peat or mineral soils) are used to describe the different types. The most common forested wetlands in this ecoregion are red maple swamp, Atlantic white cedar swamp and pitch pine wet woods.

photo © J. Lundgren, TNC

Table 13. Properties of forested wetland occurrences (MO’s) in North Atlantic Coast Ecoregion

Attribute	Range	Mean	Std deviation
Size (acres)	1 to 26,404	25.6	257.0
Elevation (meters)	0 to 165	23	19.0
LCI value (land cover index)	0 (best) to 312 (worst)	87	66.0
GAP12	0 to 100%	4.7%	18.8
GAPtotal	0 to 100%	14.0%	31.8
Wetland Complex size (acres)	1 to 800,139	37,810	166,112

Over 73% of modeled occurrences were less than 10 acres and only 7% were greater than 50 acres (Table 14). However, the larger wetlands (greater than 50 acres) accounted for more than 70% of the total forest wetland acreage across the ecoregion. Based on literature of breeding requirements supplemented by data from nearby ecoregions, wetlands greater than 50 acres typically support a larger percent of forest wetland species. Thus we focused our selection on these larger occurrences to serve as a coarse filter for all species, and selected small examples where other evidence pointed to biodiversity elements that might otherwise be missed (e.g., forest seeps, viable element occurrences for other species, etc.).

Size class	Size range (acres)	Total # of occurrences	Percent of occurrences		Total area (acres)	Percent of area
1	1 – 10	1,766	73%		62,978	10%
2	10.1 – 50	4,877	20%		105,763	17%
3	50.1 – 500	1,472	6%		193,594	31%
4	> 500	160	<1%		256,443	41%
Total		24,183			618,777

Forested swamps with their tangled structure, abundant water and often impenetrable understories are rich in species. Typical animals that breed in this ecosystem include beaver and water shrew, birds such as wood duck, Canada warbler, northern parula, prothonotary warbler, northern waterthrush, and a variety of reptiles and amphibians like the four-toed salamander and wood turtle. Rare species include the Hessel’s hairstreak and plants such as swamp pink, pine barren gentian, and pine barrens smokegrass.

Small patch forested communities that are likely to be missed in this model include forested seeps and vernal pools. These will be captured to some degree within upland forest targets.

The baseline criteria for an occurrence to qualify as a critical site was set as follows:

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  Size: minimum 100 acres. In some instances forested wetlands 50-100 acres with exemplary condition, strong corroborating evidence and good landscape context were considered qualifying especially when advocated for by a trusted source.
  
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  Land Cover Index Class: LCI <60. (LCI value 1-30 was considered best and LCI value 31-60 was considered adequate).
  

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  Condition: verification by an element occurrence or expert.
  
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  Corroboration: not necessary for larger wetlands, but those in the 50-100 acre range were accepted if there was substantial evidence of the quality and importance of the occurrence.
  

The assumption was that larger forested wetlands contained a higher diversity of vegetation types and wetland species. With the exception of a few small patch communities, most forested wetlands depend on natural processes at a larger scale and are better captured by larger occurrences or complexes. Criteria for selecting small patch types was better met by species or natural community occurrences that were identified as targets in the ecoregion. Manual review was necessary for borderline examples to assess whether biodiversity components are present and modifications to the selection were made accordingly. Some small wetland occurrences fell within protected areas or portfolio sites and were categorized as defacto supporting wetlands. Expert review was allowed to override criteria in some cases. Additional candidate occurrences can be confirmed and added in the future – if needed to meet goals – assuming they are corroborated with element occurrences and other evidence.

To insure that the portfolio would have adequate replication and redundancy of various forest wetland types, we set a minimum goal of an average of 20 occurrences per subsection, weighted by percent of forested wetlands in each subsection. There are 10 land-based subsections in NAC, but we also included one adjacent Lower New England ecoregion subsection that contains a large number of wetlands on the NAC border. Thus 11 subsections times 20 wetlands per subsection provides a minimum goal of 220 forested wetlands in NAC. Because many forest wetlands contain more than one ecoystem type (e.g., atlantic white cedar swamps often contain or co-occur with red maple swamps), additional replication for ecosystem types was not used. However we stratified the selected occurrences across all bedrock types to insure the capture of the full spectrum of forested wetland types (Table 16).

The goal per substrate type was roughly met in all but the “granite” category. It may be that the wetlands on this substrate tend to be smaller and under the cut-off of our size criteria or that landscape context is poorer in these areas. Additional wetlands in this category may have been captured by the matrix blocks, the coastal unfragmented blocks or certain species occurrences.