Ecological dynamics
This is a high gradient floodplain ecosite located on alluvial fans. Alluvial Fans are typically found where steep mountain drainages emerge onto flatter plains. As the stream gradient decreases, it drops its coarse-grained sediment load. This reduces the capacity of the channel to transmit water and forces the stream to change direction this is known as channel avulsion. As the channel moves back and forth across the fan depositing its load it gradually builds up a mounded, shallow conical fan shape.
Three different land form segments have been identified on this ecosite: 1) the active channel, 2) adjacent floodplain, and 3) the fan terrace. The active channels are the current conduits for the stream to flow down the fan, during periods of high rainfall or snow melt these areas may experience some localized flooding which affects the fans floodplains. The majority of the spatial extent of the alluvial fan is characterized by the fan terrace. The climax plant community on the fan terrace is a closed Sitka spruce forest with an understory of moss and forbs. Anthropogenic disturbance from forest management practices may also influence the succesional dynamics on this site.
Fan terraces are relatively free of flooding unless the channel deposition or flow become such that there is an avulsion event. At this point, water will simply find a new path down the fan and establish a new active channel and adjacent floodplain. The recently abandoned channels will begin to establish a Balsam Poplar forest with little understory cover. With time, Sitka Spruce and Western Hemlock will establish in the canopy on the abandoned channels.
Development of the new channel and floodplain will depend entirely on the size and velocity of the flooding event.
The soils on the alluvial fans are coarse grained and gravelly showing little development in the floodplains or burring older pedogenic horizons that formed on the terrace. Terrace soils are also gravelly but are usually stable for long enough periods of time to experience podzolization a process resulting from the mobilization and precipitation of dissolverd organic matter, iron and aluminum.
Community 1.1
Closed Sitka Spruce Forest
This is the reference plant community for a Maritime Forest Gravelly Floodplain, Alluvial Fan. The majority of this alluvial fan ecosite is characterized by the fan terrace, which supports a closed Picea sitchensis forest. The tree canopy cover is dominated by tall Picea sitchensis with a smaller proportion of medium to tall Tsuga heterophylla (western hemlock) and tall Populus balsamifera (balsam poplar). The understory is dominated by approximately 40% moss cover and 40% forb cover. Forb species may include Osmorhiza berteroi (sweetcicely), Athyrium filix-femina (common ladyfern), Gymnocarpium dryopteris (western oakfern), and Dryopteris expansa (spreading woodfern).
Community 1.2
Balsam Poplar-Shrub
Figure 2. community phase 1.2
This is an early sere flood sere plant community. Following flood, a Populus balsamifera forest establishes with minimal understory vegetation. The forest canopy may have up to 40% tall Populus balsamifera, with regenerating to medium sized Tsuga heterophylla, Abies lasiocarpa (subalpine fir) and Picea sitchensis. Alnus viridis ssp. Sinuata is the dominant shrub species.
Community 1.3
Balsam Poplar Regenerating Sitka Spruce-Shrub Community
Figure 3. community phase 1.3
This mid succession community phase is characterized by 25-50% forest cover. Tree species include mature Populus balsamifera and regenerating to medium sized Picea sitchensis. Below the canopy, a shrub- graminoid layer with up to 70% cover is comprised of Gymnocarpium dryopteris, Oplopanax horridus, Aruncus dioicus (bride’s feathers), and small proportion of Alnus viridis ssp. Sinuata.
Community 1.4
Sitka Spruce- Western Hemlock- Shrub
Figure 4. community phase 1.4
This late succession community phase is characterized by 40% or greater forest cover. Tree species include tall Tsuga heterophylla and Picea sitchensis with a smaller proportion of medium sized Tsuga heterophylla and Picea sitchensis. Below the canopy, shrub cover consists of Menziesia ferruginea (rusty menziesia), Vaccinium ovalifolium (oval-leaf blueberry), Viburnum edule (squashberry), Oplopanax horridus (devilsclub), and Cornus Canadensis (bunchberry dogwood). Moss cover may be up to 80%.
Community 1.5
Sitka Spruce- Balsam Poplar- Western Hemlock Forest
Figure 5. community phase 1.5
This amid succession community phase that is recovering from forest management. This plant community is characterized by approximately 50% forest cover. Tree cover is dominated by tall Picea sitchensis, few tall Populus balsamifera, and regenerating to medium sized Picea sitchensis. Below the canopy, a shrub- graminoid layer with up to 70% cover is comprised of shrubs such as Oplopanax horridus and Viburnum edule, forbs such as Gymnocarpium dryopteris. Moss cover is approximately 30%.
Community 1.6
Closed Sitka Spruce- Western Hemlock Forest
Figure 6. community phase 1.6
This late succession community phase recovering from timber management is characterized by a closed canopy of Picea sitchensis and Tsuga heterophylla. Tree species include mature Populus balsamifera and regenerating to medium sized Picea sitchensis. The understory is comprised of 60-70% moss cover with trace lichen and forbs such as Polypodium glycyrrhiza (licorice fern) and Orthilia secunda (sidebells wintergreen).
Pathway 1.1A
Community 1.1 to 1.2
Occasional and brief flooding on a high gradient floodplain. When the water flows over the floodplain channel, a new channel may form on the alluvial fan. When this happens, the understory vegetation in the climax plant community is removed, and the tree canopy structure shifts from a Sitka spruce forest to a community of mostly bare ground with Populus balsamifera and Alnus viridis ssp. Sinuata (sitka alder).
Pathway 1.1B
Community 1.1 to 1.5
Brush management and tree removal.
Pathway 1.2A
Community 1.2 to 1.3
Balsam Poplar Regenerating Sitka Spruce-Shrub Community
Time since flood.
Pathway 1.3B
Community 1.3 to 1.2
Balsam Poplar Regenerating Sitka Spruce-Shrub Community
Occasional and brief flooding. Flowing water may remove regenerating tree and understory cover. If the water flow or deposition event is great enough, the flow will breach the channel and create a new channel. See community pathway 1.1A.
Pathway 1.3A
Community 1.3 to 1.4
Balsam Poplar Regenerating Sitka Spruce-Shrub Community
Sitka Spruce- Western Hemlock- Shrub
Time since flooding.
Pathway 1.4A
Community 1.4 to 1.1
Time since flooding.
Pathway 1.4B
Community 1.4 to 1.2
Sitka Spruce- Western Hemlock- Shrub
Occasional and brief flooding on a high gradient floodplain.
Pathway 1.5A
Community 1.5 to 1.6
Sitka Spruce- Balsam Poplar- Western Hemlock Forest
Closed Sitka Spruce- Western Hemlock Forest
Time since brush management or tree removal.
Pathway 1.6A
Community 1.6 to 1.1
Time since brush or tree removal.
