Using Areal Composition of Riparian Vegetation Communities to Identify Thresholds in Prairie Streams

Abstract

Riparian areas often have multiple plant communities that may change rapidly due to seasonal hydrological shifts, and variation of those plant communities may not necessarily indicate transitioning ecological status. Thus, while plant communities are often used to assess land ecological status in upland systems, applying that principle to riparian systems may not be appropriate. Geomorphic parameters may be better indicators of a transition in ecological status, but they cannot be understood separately from plant community dynamics. This research assessed stream geomorphology and riparian plant communities along 34 reaches of 8 streams in North Dakota. The area of three plant community components (PCCs) associated within riparian complexes were mapped, and stream cross-section and longitudinal profile data were collected to classify Rosgen stream channels into stable (E and C channels), stabilizing (B channels), and unstable (F and G channels) states. The area of PCC1, the community nearest the stream bank consisting of wetland obligate species, was similar among stable and unstable streams, but PCC2 and PCC3, transitional plant communities, had greater area along stable reaches than along unstable reaches. Thus, the proportion of PCC1 in stable reaches was much lower (≈25%) than in unstable reaches (≈75%). Entrenchment ratio was the only stream parameter that was a good predictor of PCC areas, likely due to its relationship with floodplain connectivity. These findings may be valuable in developing riparian complex ecological site descriptions, specifically identifying potential thresholds between the unstable and stable states.