Natural Resources
Conservation Service
Ecological site F220XY427AK
Maritime Forest Gravelly High Floodplain
Last updated: 3/10/2025
Accessed: 03/31/2025
General information
Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.
MLRA notes
Major Land Resource Area (MLRA): 220X–Alexander Archipelago-Gulf of Alaska Coast
The Alexander Archipelago-Gulf of Alaska Coast area consists of a narrow arc of islands and lower elevation coastal mountains in the Southern Alaska Region. This area spans from the Alexander Archipelago in southeastern Alaska, north and west along the coast of the Gulf of Alaska and Prince William Sound, and further west to the southern tip of the Kenai Peninsula and the northeastern islands of the Kodiak Archipelago. The area makes up about 27,435 square miles (USDA 2006). The terrain primarily consists of low to moderate relief mountains that are deeply incised. Throughout the area glaciers, rivers, and streams have cut deep, narrow to broad valleys. The broader valleys have nearly level to strongly sloping flood plains and stream terraces. Alluvial and colluvial fans and short footslopes are common in the valleys along the base of the mountains. Rocky headlands, sea cliffs, estuaries, and beaches are common along the coast.
This area includes the Municipality of Juneau, Alaska's capital, and a number of smaller coastal towns and villages. Federally administered lands within this MLRA include Admiralty Island National Monument and part of Misty Fjords National Monument, Tongass National Forest, Chugach National Forest, and Glacier Bay, Wrangell-St. Elias, and Kenai Fjords National Parks and Preserves. The southern terminus of the Trans-Alaska Pipeline is in Valdez.
During the late Pleistocene epoch, the entire area was covered with glacial ice. The numerous fjords of the Alexander Archipelago and Prince William Sound were formed chiefly as a result of glacial scouring and deepening of preglacial river valleys. Most glacial deposits have been eroded away or buried by mountain colluvium and alluvium, which cover about 90 percent of the present landscape. The remaining glacial and glaciofluvial deposits are generally restricted to coastal areas. During the Holocene epoch, volcanic activity within and adjacent to this area deposited a layer of volcanic ash of varying thickness on much of the landscape in the southeastern and northwestern parts of the area. Paleozoic, Mesozoic, and Lower Tertiary stratified sedimentary rocks and Cretaceous and Tertiary intrusive rocks underlie much of the area and are exposed on steep mountain
slopes and ridges (USDA 2006).
The dominant soil orders in this MLRA are Spodosols, Histosols, and Entisols. Soils in the area typically have a cryic soil temperature regime, an udic moisture regime, and have mixed minerology. Spodosols are common on mountains and hills having been formed in gravelly or cobbly colluvium, glacial till, and varying amounts of silty volcanic ash. These Spodosols commonly range from shallow to deep, are well to somewhat poorly drained, and typically classify as Humicryods or Haplocryods. Histosols that are poorly to very poorly drained occur on footslopes, discharge slopes, and valley floors. These wet histosols commonly classify as Cryosaprists, Cryohemists, and Cryofibrists. Histosols that are well drained occur on steep mountainsides. These dry Histosols commonly classify as Cryofolists. Entisols are common on flood plains, stream terraces, and outwash plains having been formed in silty, sandy, and gravelly to cobbly alluvium. These Entisols are generally deep, range from well to somewhat poorly drained, and commonly classify as Cryaquents and Cryofluvents. Miscellaneous (non-soil) areas make up about 23 percent of the MLRA. The most common miscellaneous areas are avalanche chutes, rock outcrop, rubble land, beaches, river wash, and water.
This area represents the northern extent of the Pacific temperature rainforest and is characterized by productive stands of conifers. Western hemlock and Sitka spruce are the dominant trees on mountains and hills at the lower elevations. Due to warmer temperatures, western red cedar and Alaska cedar are more prevalent in the southern part of the area. Black cottonwood and mixed forest types occur on flood plains. Areas of peat and other sites that are too wet for forest growth support sedge-grass meadows and low scrub. The transition to subalpine and alpine communities typically occurs at elevations between 1500 to 3000 feet (Boggs et al. 2010, Carstensen 2007, Martin et al. 1995), which characterize the vegetation of the Southern Alaska Coastal Mountains area.
For many decades, logging, commercial fishing, and mining have been the primary industrial land uses throughout much of the area. In recent years, changes in public interests, land use policies, and timber economics have contributed to a significant decline in the timber industry. Commercial fishing continues to be an important industry and most communities support a fleet of boats and fishing related facilities. A number of mines operate in the area and others have been prospected and proposed. Tourism and wildland recreation are becoming increasingly important. Subsistence hunting, fishing, and gathering provide food and a variety of other resources to local residents and remain the principal economy for residents of remote villages.
Ecological site concept
This site occurs in river valleys and coastal plains on high flood plains adjacent to a river or stream channel. These high flood plains are rarely to frequently flooded, and may include some closed depressions where water ponds on the surface for long durations. Soil textures are stratified silt loams to loamy sands, often with lenses of gravel and sandier textures in deeper soil horizons. Relatively small, closed depressions occur on these high flood plains and often have very deep peat deposits as a result of long duration ponding. Drainage class ranges from moderately well- to well-drained, with very poor drainage in closed depressions.
This site supports a reference state composed of three communities that reflect a disturbance regime of high-intensity flooding. A tall shrub community with abundant herbaceous understory plants is typical following severe flood events. In the absence of severe flooding, the abundance of balsam poplar trees increases, and the community resembles an open forest. If flooding becomes even less frequent, Sitka spruce becomes dominant, and understory shrubs and forbs are largely displaced by moss species.
Associated sites
| R220XY426AK |
Maritime Shrub Low Flood Plain R220XY426AK occurs downslope of this site on low flood plain positions that have more frequent and longer duration flood events. |
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Similar sites
| R220XY426AK |
Maritime Shrub Low Flood Plain Both sites occur on floodplains of rivers and streams, however, site R220XY426AK does not support tree dominance because it occurs on low flood plains with high flood frequency. By contrast, this site occurs on high flood plains and experiences less frequent flooding that enables trees to dominate over time. |
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Table 1. Dominant plant species
| Tree |
(1) Picea sitchensis |
|---|---|
| Shrub |
(1) Vaccinium ovalifolium |
| Herbaceous |
(1) Orthilia secunda |
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Ecosystem states
State 1 submodel, plant communities
| 1.1a | - | High-intensity flood events remove vegetation |
|---|---|---|
| 1.2a | - | Lack of high-intensity flood events leading to secondary succession. |
| 1.2b | - | High-intensity flood events remove vegetation |
| 1.3a | - | Lack of high-intensity flood events leading to secondary succession. |