Populus deltoides / Symphoricarpos occidentalis Community Type 



POPDEL / SYMOCC; plains cottonwood / western snowberry 

 MTNHP rank: G2G3/S? 



Environment: Cottonwood stands are found on alluvial bottoms, and there are few of these setting 

 for cottonwood communities on BLM land in Carter County. The possible exceptions are small 

 holdings along the Little Missouri River and lower Cottonwood Creek, which were not surveyed. 

 Most riparian zones along the larger watercourses are private property and cottonwoods along 

 smaller watercourses are scattered or comprise a very narrow corridor without distinctive woodland 

 community structure. In addition, these habitats are usually altered in their vegetation. One small 

 stand of POPDEL/S YMOCC in relatively good condition was located and sampled on BLM along 

 North Cottonwood Creek. Fencing is in place, and the stand appears to be far less impacted by 

 livestock compared to surrounding habitat. It is located on a floodplain with a meandering stream 

 and is probably both seasonally inundated and subirrigated; higher terraces and surrounding low 

 relief rolling uplands are occupied mostly by sagebrush steppe (ARTTSW/ELYLAN) and grasslands 

 (ELYLAN/STIVIR). 



Vegetation: Populus deltoides is the only tree present in the sampled stand. Trees are large and 

 provide a tall canopy with about 50% cover. The shrub layer is about 1 m tall and provides nearly 

 complete cover, mostly by Symphoricarpos occidentalis, but Rosa arkansana is also common. Two 

 rhizomatous grasses are well represented, the native Pascopyrum smithii (tall, with wide glaucous 

 leaves in this mesic habitat), and the exotic Poapratensis. Forb cover is also significant with exotic 

 species {Cirsium arvense, Medicago lupulina, Medicago sativa, Taraxacum officinale, Tragopogon 

 dubius) prominent, but the highest cover is attained by the natives Aster falcatus and Ratibida 

 columnifera. 



Cottonwood stands are serai (community type), becoming established in response to deposition by 

 shifting stream channels. They normally do not regenerate at the same spot except as exposed river 

 channels adjoin the stand. Hansen et al. (1995) described POPDEL/SYMOCC as a grazing induced 

 secondary serai stage of the mid-seral POPDEL/CORSTO community type, but in Wyoming, the 

 type was seen as indicative of low levels of grazing (Thilenius et al. 1995). Hansen et al. (1995) 

 report the latter from narrow bands on the Little Missouri River in the county, where it probably 

 depends on stable soil moisture. Exclusion of cattle from our sampled stand appears to have 

 preserved or allowed reestablishment of high cover by shrubs, but there is no evidence of 

 progression towards POPDEL/CORSTO or any particular climax type (there are not shrubs of 

 Cornus stolonifera or tree species besides Populus deltoides in the vicinity). 



Comments: POPDEL/SYMOCC is considered a rare plant community (G2G3) as it occurs in 

 Wyoming, western South Dakota, and western North Dakota (Faber-Langendoen et al. 1997). In 

 central and eastern Montana. Hansen et al. (1995) characterize it as a major low-elevation serai 

 community type of older alluvial bars and terraces. The discrepancy may be attributable to Hansen 

 et al. (1995) not weighing the condition of the stands and the ongoing threats to their composition. 

 Hansen et al (1995) speculate POPDEL / SYMOCC is a grazing-induced form of POPDEL / 

 CORSTO (wherein the tall shrubs have been eliminated). Our sampled stand is similar floristically 

 to 8 stands sampled in eastern Wyoming (Thilenius et al. 1995) in the importance value of 



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