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Community Field Guide

Scientific Name:
Populus deltoides / Symphoricarpos occidentalis Woodland

Common Name:
Plains cottonwood / western snowberry Woodland


Photo by Marc Jones

Community Description

Summary:
This riparian woodland community is found in the northwestern Great Plains of the United States on medium- to coarse-textured alluvial soils on the floodplains of major rivers. The floodplains are both seasonally inundated and sub-irrigated. It is found more rarely at higher elevations in the mountains of eastern Wyoming and western South Dakota. This community is dominated by a single deciduous tree species, Populus deltoides. In some stands other species, such as Acer negundo (box elder) and Fraxinus pennsylvanica (green ash), may contribute to the canopy. The tallest trees exceed 15 meters. The shrub layer is typically 0.5-1 m tall. It is dominated by Symphoricarpos occidentalis and commonly includes Juniperus scopulorum (Rocky Mountain juniper) and Rosa (rose) spp. In Wyoming, Ericameria nauseosa (=Chrysothamnus nauseosus), common rabbitbrush) is present and increases with heavy grazing. The herbaceous layer usually includes Pascopyrum smithii (western wheatgrass). Weedy species such as Melilotus officinalis (yellow sweet clover), Taraxacum officinale (common dandelion), and Poa secunda (Sandberg’s bluegrass) are very common, especially in the presence of grazing. Maianthemum stellatum (starry Solomon’s seal) is abundant only where grazing is absent.

Environment:
This community is found on medium- to coarse-textured alluvial soils on the floodplains of major rivers. The floodplains are both seasonally inundated and subirrigated (Thilenius et al. 1995). The meandering erosional and depositional pattern of rivers maintains and influences this community along rivers (Hanson 1990). It is rarely found at higher elevations in the mountains of eastern Wyoming and western South Dakota (Johnston 1987).

In Montana, this plant association occurs on major river systems of the eastern plains, with a flood cycle driven by mountain run-off and vegetation succession linked to intact hydrological processes. In Powder River County, this plant association is found in Riparian subirrigated Ecological Sites along the Powder and Little Powder Rivers.

Vegetation:
This community is typically dominated by a single deciduous tree species, Populus deltoides. In some stands other species, such as Acer negundo and Fraxinus pennsylvanica, may contribute to the canopy. The tallest trees exceed 15 meters. Populus deltoides is a pioneer species that requires moist, sparsely vegetated alluvium to become established from seed; therefore stands of this community may be considered seral, but the stage is long persistent (up to 100 years) (Girard et al. 1989). The shrub layer is typically 0.5-1 m tall. It is dominated by Symphoricarpos occidentalis and commonly includes Juniperus scopulorum and Rosa spp. In Wyoming, Ericameria nauseosa is present and increases with heavy grazing (Thilenius et al. 1995). The herbaceous layer usually includes Pascopyrum smithii and Elymus trachycaulus. Weedy or increaser species such as Cirsium arvense, Melilotus officinalis, Taraxacum officinale, Bromus inermis (smooth brome), Poa pratensis (Kentucky bluegrass), Agropyron repens (quackgrass) and Poa secunda are very common, especially in the presence of grazing (Jones and Walford 1995, Thilenius et al. 1995). Maianthemum stellatum is abundant only where grazing is absent.

In Montana, stands are typically heterogeneous in vegetation age and structure as well as microtopography, because of the small-scale patterns of alluvial deposition and erosion operating over the years. In Powder River County, cottonwood communities are represented by all seral stages, from seedlings up to trees greater than 18 inches circumference. The latter stands are widely spaced, with Artemisia cana establishment if the understory is intact. In the study area, Symphoricarpos occidentalis was typically present, but was there was not a distinct shrub-layer dominant. The dominant grass was Pascopyrum smithii, followed by Nassella viridula, and replaced by Poa pratensis under disturbance. Other characteristic species include Rosa woodsii, Elymus canadensis and such generalists as Achillea millefolium. There is widespread invasion by Euphorbia esula, which has become locally abundant in many stands. Tamegrasses have been seeded or accidentally introduced into some stands, including Agropyron repens and Bromis inermis.

Range:
This riparian woodland community is found in floodplains of the northwestern Great Plains of the United States, ranging from eastern Montana to North Dakota and south to Colorado

Dynamics:
This type is found closest to the river on young, unstabilized floodplains, where it colonizes the freshly deposited alluvial substrates on the meanders of the streams and rivers. From extreme eastern Montana and further eastward the following successional model is operative: Proceeding away from the river, later successional stages may include a Populus deltoides - Fraxinus pennsylvanica community type (CEGL000658) and a Fraxinus pennsylvanica - (Ulmus americana) / Symphoricarpos occidentalis forest (CEGL002088). As the stream continues to move away from the more recent deposits, the stand may eventually succeed to the Fraxinus pennsylvanica type, a process that could take 100 years (Girard et al. 1989). Within the Bitter Creek / Frenchman Creek study west of the distribution of Fraxinus pennsylvanica and Ulmus americana (American elm) these Populus deltoides stands do not succeed due to the presence of these more shade tolerant trees. Rather, the Populus deltoides component tends to slowly die out until a Symphoricarpos occidentalis - dominated stand is the consequence.

In Montana, the climate is too dry for succession to the Fraxinus pennsylvanica type. Decadent stands may succeed to Artemisia cana (Boggs 1984).

Global Rank: G2G3 State Rank: S2S3

Global Rank Comments:
The total number of occurrences is unknown. Thirteen have been documented in North Dakota, where the community is ranked S1S2?. Although no other occurrences have been documented, the community is also reported from Wyoming (S2), Colorado (S2) and may occur in South Dakota (SP). It is found in three northern Great Plains ecoregional sections. The community occurs on medium- to coarse-textured soils on the floodplains of major rivers.

Community References

Identifier:
CEGL000660

Author:
Drake, J., MWCS; modified Cooper, S.V. and C. Jean, MTNHP

Citations:
Girard et al. 1989, Hansen et al. 1990, Hansen et al. 1995, Johnson et al. 1990a, Johnston 1987, Jones and Walford 1995, Kartesz 1999, Kittel et al. 1999, Thilenius 1970, Thilenius and Brown 1990, Thilenius and Smith 1985, Thilenius et al. 1995

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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
http://mtnhp.org
mtnhp@mt.gov