Discharge Slope Ecosystem wetlands occur over a mineral soil at the wetland to upland transition.  They are found at the edges of all wetland ecosystems, and at slope breaks on terraces.  They commonly occupy foot- and toe-slope landscape positions at the edge of peatlands or stream  valleys where groundwater discharges or where dense till perches a water table close to the surface.  Discharge slope wetlands are fed either by upslope groundwater storage capacity, or a perched water table atop dense till.  Many are forested.  All wetlands that border uplands contain a Discharge Slope component, though sometimes this component is narrowly restricted, and not map-able at 1:25,000. 

Forested Discharge Slopes are the most difficult wetlands to discern on aerial photography.  Because they occur at the wetland/upland edge, Discharge Slope Ecosystem wetlands encompass plant communities that may also occupy drier sites.  Even after field visits, many discharge slope ecosystem wetlands are difficult to map, because they are wet in some spots, dry at others.  Although 15% of the Lutz spruce discharge slopes we visited were re-classified as upland, Lutz spruce discharge slope wetlands are very common, and many were missed on the National Wetland Inventory mapping done during the late 1970's.

Discharge slopes are important sites of groundwater nutrient supply to peatlands.  Peatlands classified as fens have higher available nutrient concentrations than bogs.  One nutrient source is the cations in groundwater percolating through nearby mineral soil.  Nutrients from other sources, such as tephra, ocean-derived aerosols deposited by precipitation, or deeper groundwater discharge within the peatland may also support fens.

Discharge Slopes are the second most extensive wetland Ecosystem mapped in the project area, even though they are frequently too narrow to map at 1:25,000, especially north of Clam Gulch.  Although they are nearly ubiquitous at upland-wetland fringes by definition, they often cover less than 10%  in any wetland (at 1:25,000), and so are frequently not named as a mapped component.  All wetlands bordering uplands contain a Discharge Slope Ecosystem component whether or not it is named.  Discharge Slopes are more extensive on dense, poorly sorted till, especially south of Clam Gulch.

On the northern Kenai Peninsula, the uphill transition from fen through discharge slope to upland tends to be narrow and abrupt compared to transitions south of Clam Gulch.  This is may be because the landforms in the north are typically on a smaller scale: kames and moraines, which provide much less of an upslope recharge area than the Caribou Hills Uplands in the south.  Till permeability may also play a role.

On the southern Lowlands, Discharge Slopes usually occupy a broader foot- or toe-slope.  Discharge Slope Ecosystem wetlands extend upslope to back-slope positions above some peatlands along the west margin of the Caribou Hills and along the northern shore of Kachemak Bay.

Soils

Peat is typically thin or absent and mineral soils dominate these wetlands.  Common south of Clam Gulch are Chunilna series andisols and Spenard spodosols, north of Clam Gulch, Clam Gulch inceptisols are the most common soils.  When peat extends up the toeslope, Starichkof and Doroshin soils are encountered.  Some dense forests with large trees can be found growing over deep peat. 

Plant Relationships 

The plant communities present depend on discharge, latitude, landscape position, and elevation.  Plant community diversity is high within these wetlands.  At higher elevations with deeper groundwater, Barclay's willow (Salix barclayi) communities dominate; at lower elevations, thinleaf alder (Alnus incana ssp. tenuifolia) communities dominate on flatter toe-slope positions with a water table near the surface.  Intermediate positions typically produce forested communities, usually dominated by Lutz spruce (Picea X lutzii) in the south and black spruce (Picea mariana) in the north.  

South of Clam Gulch, Lutz spruce and field horsetail (Equisetum arvense) occur, from wettest to upland,  with dwarf birch (Betula nana), Barclay's willow, sitka alder (Alnus viridis ssp. sinuata), bluejoint grass (Calamagrostis canadensis), and rusty menziesia (Menziesia ferruginea).  Oakfern (Gymnocarpium dryopteris) replaces field horsetail at the upland transition.  Histosols often transition to mineral soils at fen edges, in the zone where willow becomes dominant (Figure 1.). 

Discharge Slope wetlands underlain by fine-textured alluvial fans support non-forested communities in a few places, such as along the Homer Bluff, or adjacent to steep valley walls.  Bluejoint grass with field horsetail is common, along with thinleaf alder (Alnus incana ssp. tenuifolia).

North of Clam Gulch, black spruce dominates Discharge Slope Ecosystem wetlands.  Black spruce occurs with Labrador tea (Ledum palustre ssp. decumbens) at fen margins,  then with woodland horsetail (Equisetum sylvaticum), as the organic mat thins.  Sometimes dwarf birch and field horsetail quickly grade to a zone with a Barclay's willow and field horsetail.  A rapid transition to upland occurs above the willow and below a lingonberry community, which can occupy wet or dry sites.  Often the black spruce / Labrador tea woodland grades directly to a dry black spruce / lingonberry forest over a distance of a dozen meters.

NWI and HGM

US Fish and Wildlife Service National Wetlands Inventory (NWI) would classify most Kenai Discharge Slope wetlands as forested palustrine wetlands (PFO).  Forested wetlands were frequently overlooked on the NWI, which was mapped at 1:63,360.  Shrub- and herbaceous-dominated Discharge Slopes are classified as PSS and PEM respectively.  

An HydoGeomorphic Model (Tiner, 2003) would classify most as Terrene Slope groundwater-dominated Throughflow wetlands.  If there is no wetland connected upslope, such as along upper terraces or stream valley walls, then they are Terrene Slope Outflow wetlands.  A few have Paludified Slope wetland components, although paludification is uncommon over most of the lowlands.

A bluejoint grass-dominated discharge slope at the base of the bluffs above the Homer bench.

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 Introduction and Key to Plant Communities  

Introduction and Key to Ecosystems

    Kenai Hydric Soils    Map Unit Summary    Methods    Glossary

03 May 2007 18:02