Photo by Michael Mancuso CLICK PHOTO FOR A LARGER IMAGE

Populus trichocarpa/Cornus sericea

black cottonwood/red-osier dogwood

Vegetation Composition: The Populus trichocarpa/Cornus sericea plant association is characterized by an overstory dominated by Populus trichocarpa (25-85% cover) with Populus angustifolia and Populus balsamifera sometimes occurring as subordinates in the eastern portion of the range and Betula papyrifera and Populus tremuloides occurring 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 spp., Alnus incana, Rosa spp., and Salix spp. Maianthemum stellatum, Galium triflorum, Solidago canadensis, and Equisetum spp. are often present along with graminoids, none of which have high constancy.

Classification Comments: There is considerable variability in defining this community at both the alliance (based on tree species composition) and association levels (based on undergrowth species). Some authors taking a habitat type approach have considered any stands with conifers represented (even in the seedling/sapling size classes) to be members of the alliance (series in habitat type idiom) of the most shade-tolerant conifer represented on site and not the alliance of the Populus balsamifera spp. trichocarpa, or other deciduous trees dominant on the site. It is also a highly debatable point as to whether stands containing Populus balsamifera spp. trichocarpa and Populus tremuloides should be allocated to the Populus tremuloides Forest Alliance regardless of its cover value (as some authors have advocated) when in fact Populus balsamifera spp. trichocarpa generally has a narrower ecological amplitude and better serves as a diagnostic species. Another troubling observation is that more than half of the identified stands have less than 60% tree canopy cover, which means that a significant portion of this association qualifies physiognomically as woodland, rather than as forest as currently classified. There are strongly discordant criteria as to how much Cornus sericea cover should be represented (ranging from 1-25%) for a stand to be considered a member of this association (NatureServe Explorer 2001).

Photo by Michael Mancuso CLICK PHOTO FOR A LARGER IMAGE

Environment: Populus trichocarpa is a pioneering species that requires moist, barren, newly deposited alluvium exposed to full sunlight for regeneration. This plant association 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 foot slopes 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 (NatureServe Explorer 2001).

Soils: 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; Hansen et al. 1995).

Management Considerations: 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 damaged by floods or loss. Management should emphasize the importance of the understory shrub layer in streambank stabilization; a buffer strip of the Populus trichocarpa dominated plant associations 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; Boggs et al. 1990).

Successional Dynamics: The erosional and depositional pattern of a river helps maintain diversity of plant communities 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; Boggs et al. 1990). 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 engelmannii, Thuja plicata, Tsuga heterophylla, Abies lasiocarpa, or Juniperus scopulorum or types dominated by sagebrush. If conifers are absent, shrubs and herbaceous species that formed the former undergrowth may persist. Stands in moister regions are successional to habitat types from the Populus tremuloides, Thuja plicata series, and the Picea engelmannii/Cornus sericea habitat types. In other instances, this plant association 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 pratense, Bromus inermis, and Centaurea maculosa (Hansen et al. 1995).

Wildlife Functions: This plant association provides valuable cover, shade, and food for a variety of species. Big game use may be high, depending upon the time of year. The spreading crown of Populus trichocarpa provides nesting sites for bald eagles, osprey, and great blue heron. Woodpeckers, great horned owls, wood ducks, and raccoons nest in trunk cavities. Beaver use both the cottonwood and dogwood vegetation for food and building material. Understory species provide food and cover for a variety of waterfowl, small birds, and mammals. The streamside location of this plant association is very important in providing thermal cover, debris recruitment, and streambank stability for fish habitat (Hansen et al. 1995).

Distribution: 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 spp. trichocarpa is the dominant cottonwood at elevations considered relatively low- to mid-gradient; in Idaho it ranges to 2135 m (7000 feet) (NatureServe Explorer 2001).

References:

Boggs, K., P. Hansen, R. Pfister, and J Joy. 1990. Classification and management of riparian and wetland sites in northwestern Montana. University of Montana, Montana Forest and Conservation Experiment Station, School of Forestry, Missoula. 217 pp. Draft Version 1.

Hansen, P. L., R. D. Pfister, K. Boggs, B. J. Cook, J. Joy, and D. K. Hinckley. 1995. Classification and Management of Montana's Riparian and Wetland Sites. Montana Forest and Conservation Experiment Station, School of Forestry, Missoula, MT. 646 pp.

Kovalchik, B. L. 1993. Riparian plant associations on the national forests of eastern Washington-Draft version 1. USDA Forest Service, Colville National Forest, Colville, WA. 203 pp.

NatureServe Explorer: An online encyclopedia of life [web application]. 2001. Version 1.6 . Arlington, Virginia, USA: NatureServe. Available: http://www.natureserve.org/explorer. (Accessed: February 21, 2002 ).

Author/Date(Update): L. Williams/1995-08-07(2002-02-21)