Home/Ecosystems/ Western Boreal River Floodplain Forest

Western Boreal River Floodplain Forest

EVT 7272Eastern Boreal Floodplain Shrubland

CES105.117GNRShrubRiparian

Summary

This system includes large floodplains associated with high-volume interior rivers (such as the Yukon, Kuskokwim, Koyukuk, and Tanana rivers). Flooding regime is characterized by large spring floods at ice break-up. Young successional stages are dominated by willow and alder followed by balsam poplar and/or white spruce. Wetland development in abandoned channels is intermixed with succession on more mesic sites [see description for Western North American Boreal Shrub and Herbaceous Floodplain Wetland (CES105.118)]. The active flooding zone is often several km wide. Permafrost is usually absent.

Source: NatureServe Explorer

Vegetation

Primary succession on floodplains begins when new alluvial surfaces are colonized by tree, shrub, and herbaceous species. Common woody species include Populus balsamifera, Picea glauca, Alnus viridis ssp. sinuata, Alnus incana ssp. tenuifolia, Salix barclayi, and Salix alaxensis (Viereck 1966, Scott 1974, Thilenius 1990, Shephard 1995, Boggs 2000). Common early-seral herbaceous species may include Lupinus spp., Hedysarum spp., and Equisetum spp. The next seral stage includes communities dominated by Populus balsamifera and/or Picea glauca with an understory of Alnus viridis ssp. sinuata, Salix spp., and bryophytes. The tall-shrub component of the early-successional stages diminishes rapidly, probably because of decreased light from the dense tree overstory. Populus balsamifera does not regenerate in the understory, and consequently, Picea glauca gains dominance in the overstory within 150 years. On older surfaces common shrubs include Rosa acicularis, Viburnum edule, and Linnaea borealis, and common herbs include Pyrola ssp. and Cornus canadensis. Feathermosses and lichens such as Peltigera spp. occur on older surfaces.

Source: NatureServe Explorer

Environment

This system includes large floodplains associated with high-volume interior rivers (such as the Yukon, Kuskokwim, Koyukuk, and Tanana rivers). Flooding regime is characterized by large spring floods at ice break-up. Wetland development in abandoned channels is intermixed with succession on more mesic sites. The active flooding zone is often several km wide. Permafrost is usually absent. Ice-scour and ice dams are important dynamics that may cause regeneration of willow carrs where scoured by ice.

Source: NatureServe Explorer

Dynamics

Flooding can be caused by snowmelt, precipitation, ice jams and glacial runoff. Different rivers or portions of rivers may be more prone to certain types of flooding. Frequent flooding and channel migration create a pattern of gravel bars and early-successional stages across the valley bottom. Sediment deposition raises the surface of the floodplain over time. As the terrace becomes farther removed from the channel, flooding becomes less frequent. Water availability on terraces plays a major role in community structure and composition. Water inputs are from overbank flow (flooding), groundwater, and precipitation. Fine sediments are trapped when the floodwaters recede; this ongoing sediment input maintains high productivity.

Fire frequency in floodplain systems is less than that of the surrounding terrain because channels can act as firebreaks and early-seral vegetation is less flammable than mature boreal forest.

Source: NatureServe Explorer

Distribution

This system is found in the boreal region of Alaska.

Source: NatureServe Explorer

Ecologically Associated Plant Species

Plant species that characterize this ecosystem type, organized by vegetation stratum. These are species ecologically associated with the ecosystem, not confirmed present in any specific area.

Tree canopy

Betula papyrifera, Picea glauca, Populus balsamifera

Shrub/sapling (tall & short)

Alnus incana ssp. tenuifolia, Alnus viridis ssp. sinuata, Rubus arcticus, Salix alaxensis, Salix barclayi, Viburnum edule

Short shrub/sapling

Linnaea borealis, Rosa acicularis

Herb (field)

Calamagrostis canadensis, Cornus canadensis, Mertensia paniculata, Trientalis europaea

Source: NatureServe Ecological System assessment

State Conservation Ranks (1)

Subnational conservation status ranks (S-ranks) assigned by Natural Heritage Programs in each state where this ecosystem occurs. S1 indicates critically imperiled at the state level, S2 imperiled, S3 vulnerable, S4 apparently secure, and S5 secure. An ecosystem may be globally secure but imperiled in specific states at the edge of its range.

State	S-Rank
AK	SNR

Methodology and Data Sources

Ecosystem classification: Ecosystems are classified using the LANDFIRE 2024 Existing Vegetation Type (EVT) layer, mapped to NatureServe Terrestrial Ecological Systems via a curated crosswalk. Each EVT is linked to the USNVC (U.S. National Vegetation Classification) hierarchy through pixel-level co-occurrence analysis of LANDFIRE EVT and NatureServe IVC Group rasters across all roadless areas.

Vegetation coverage: Coverage percentages and hectares are derived from zonal statistics of the LANDFIRE 2024 EVT raster intersected with roadless area boundaries.

Ecosystem narratives and community species: Sourced from the NatureServe Explorer API, representing professional ecological assessments of vegetation composition, environmental setting, dynamics, threats, and characteristic species assemblages.

IVC hierarchy: The International Vegetation Classification hierarchy is sourced from the USNVC v3.0 Catalog, providing the full classification from Biome through Association levels.

Component associations: Plant community associations listed as components of each NatureServe Ecological System. Association data from the NatureServe Explorer API.

State ranks: Conservation status ranks assigned by NatureServe member programs in each state where the ecosystem occurs.