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Community Field Guide

Scientific Name:
Populus balsamifera ssp. trichocarpa / Cornus sericea Forest

Common Name:
Black cottonwood / red-oiser dogwood Forest

Community Description

This association has been documented from Washington south to northern California and eastward to Idaho and all of Montana west of the Continental Divide, as well as central Montana. In Montana alone it occurs over a broad elevation range of 610-2010 m (2000-6600 feet) where Populus balsamifera ssp. trichocarpa is the dominant cottonwood at elevations considered relatively low- to mid-gradient; in Idaho it ranges to 2135 m (7000 feet). This forest type occupies alluvial terraces of major rivers and streams, point bars, side bars, mid-channel bars, delta bars, an occasional lake or pond margin, and even creeps onto footslopes and lower subirrigated slopes of hilly or mountainous terrain. Many of these sites are flooded in the spring and dry deeply by summer's end; capillary action keeps upper portions of soil profile moist. Other sites are merely subirrigated. Populus balsamifera ssp. trichocarpa dominates the overstory with cover values ranging from approximately 12-90%, though the modal range, at least in Montana is 40-60%. Populus angustifolia is a subordinate canopy species in the eastern portion of the range, and Populus tremuloides and Betula papyrifera occur as subordinates in the western portion. The shrub layer comprises at least 25% cover with Cornus sericea diagnostic for the type and having anywhere from 1-90% cover; other shrub taxa with high constancy include Symphoricarpos spp., Rosa spp., Salix spp., Crataegus spp., Amelanchier alnifolia, and Alnus incana. There are no graminoids exhibiting high constancy, though any one of a number of disturbance-associated exotics can manifest high coverages. Maianthemum stellatum, Galium triflorum, Solidago canadensis, and Equisetum spp. are the only forbs that exhibit even relatively high constancy across the range of the type. This is a successional community that colonizes moist, newly deposited alluvium exposed to full sunlight; in the absence of fluvial disturbance it is capable of developing into conifer-dominated communities belonging to alliances as diverse as Thuja plicata, Picea spp. and Juniperus scopulorum. Adjacent wetter sites are dominated by a suite of wetland Salix spp., Alnus incana, wetland-associated Carex spp. often including Carex utriculata, Carex aquatilis and Carex buxbaumii or Typha latifolia-dominated communities. Adjacent drier sites are dominated by Populus balsamifera ssp. trichocarpa or Populus tremuloides types or any of a vast array of conifer-dominated types that are capable of growing within the elevational zone occupied by the Populus balsamifera ssp. trichocarpa / Cornus sericea Forest (CEGL000672).

Sites occur on alluvial terraces of major streams and rivers, point bars, side bars, mid channel bars, delta bars, islands, and occasionally around lakes and ponds. Soil textures vary from loam to coarse sand, and are generally well drained with a low available water holding capacity. These sites are often flooded in the spring with water tables lowering to 3 or more feet below the soil surface at the end of summer; upper soil profiles remain moist due to capillary action. Coarse textured soils, moderate stream gradients, and high coarse fragment contents throughout the soil profile provide an environment that produces a rapid movement of highly aerated groundwater. Redox concentrations (mottles) are common as evidence of a fluctuating water table (Kovalchik et al. 1993, and Hansen et al. 1995).

This community type is characterized by an overstory dominated by, Populus balsamifera ssp trichocarpa (25 -85% cover) with Populus angustifolia, sometimes occurring as subordinates in the eastern portion of the range. Betula papyrifera and Populus tremuloides occurr as subordinates in the western portion of the range. The dense shrub layer is diverse and dominated by Cornus sericea (20-90% cover). Amelanchier alnifolia, Symphoricarpos oreophilus, Alnus incana, Rosa woodsii, Salix exigua and other Salix species are often present. Smilacina stellata and Equisetum arvense are often present along with graminoids, none of which have high constancy.

Populus balsamifera ssp. trichocarpa / Cornus sericea community type occurs in Montana, Washington, Idaho, and Oregon.

The erosion and depositional pattern of a river helps maintain diversity of plant communities of this type on the floodplain. The distribution of communities depends on the way the river meanders. In turn, the rate of meandering determines the seral stage of the communities. Where the river meanders frequently, few stands progress to later successional stages. Near the outer edges of the floodplain, the effect of the river is less pronounced, allowing later successional stages to develop (Hansen et al. 1995 and Boggs et al. 1990).

Populus balsamifera ssp. trichocarpa is a pioneering species that requires moist, barren newly deposited alluvium exposed to full sunlight for regeneration. In the absence of fluvial disturbance, succession continues to a variety of conifer dominated habitat types such as Pinus ponderosa, Pseudotsuga menziesii, Abies grandis, Picea, Thuja plicata, Tsuga heterophylla, Abies lasiocarpa, or Juniperus scopulorum. If conifers are absent, shrubs and herbaceous species that formed the former undergrowth may persist. In other instances, this community type may be successional to the Salix geyeriana / Calamagrostis canadensis habitat type or the Salix lutea / Calamagrostis canadensis habitat type, depending upon elevation. If disturbance is severe enough, all shrubs can be eliminated and the understory will be converted to a herbaceous one dominated by species such as Poa pratensis, Phleum pratensis, Bromus inermis, and Centaurea maculosa (Hansen et al. 1995).

Because of its close proximity to streams and rivers and the flat topography, recreational developments and transportation corridors are common within this type; care must be taken when locating structures in the floodplain to avoid damage or loss by floods. Dams that limit peak flows, can lead to the gradual disappearance of mature cottonwood forest. Due to the lack of sediment deposition for seedbeds, periodic floods are necessary for continued cottonwood recruitment (Merigliano 1996). Although streambank erosion is a naturally occurring process, attempts to stabilize streambanks using riprap can lead to increased erosion downstream, thus speeding the loss of cottonwood forest in some cases. Poorly managed livestock grazing can lead to loss of understory shrubs and decreased recruitment of cottonwoods. Management should emphasize the importance of the understory shrub layer in streambank stabilization; a buffer strip of the Populus trichocarpa dominated community types should be maintained adjacent to rivers and streams. Under certain conditions, fire may be used as a tool to extend the life span or rehabilitate a stand (Hansen et al. 1995 and Boggs et al. 1990).

Global Rank: G3? State Rank: S3?

Global Rank Comments:
Association rank has been changed from G3? to G3G4 on the basis of this type's broad geographic distribution and the fact that within local landscapes, say western Montana, it is relatively common. In Montana alone, this type is represented by 26 plots, and this is for the case of the most restrictive or conservative interpretation of criteria for the association. A thorough crosswalk of this type across its range of distribution is needed; there may be local variations in composition and ecology that would bear recognition of separate associations. Threats to this type include floodplain harvesting of cottonwood and over-browsing from livestock and wildlife, both entities of which find Cornus sericea extremely palatable to the point of extirpating it from local floodplain landscapes. The more serious over-browsing consequence is reduced diversity, the introduction of weedy species and the increase in unpalatable native taxa such as Symphoricarpos occidentalis, Ribes spp. and Urtica dioica.

Community References


95-08-07 / L. WILLIAMS

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This information is from the:
Montana Natural Heritage Program
Montana State Library--Natural Resource Information System
1515 East Sixth Ave., Helena, MT 59620-1800
406 444-5354