Restoration of degraded streams by enhancing geomorphic complexity has not always improved stream biota with catchment scale hydrology potentially more influential. Instream vegetation is a critical component of many stream ecosystems providing multiple ecological benefits. Thus, we asked: How does instream vegetation respond to (1) geomorphic complexity; (2) different flow regime components; and (3) what is the relationship between geomorphic complexity and flow in driving instream vegetation?
We surveyed instream vegetation and geomorphology along 23 lowland stream reaches. We tested for associations between six geomorphic complexity, and five flow metrics, and amphibious and aquatic richness, diversity and cover responses.
Amphibious vegetation was negatively associated with simplified channels and increasing flood frequency. Increasing depth and width variation, and variation in the timing of low flows, was positively associated with aquatic vegetation. Considering sites with complex (stable) or simplified (unstable) geomorphology separately, only vegetation at stable sites showed strong relationships to components of the flow regime, with flow flashiness negatively associated with both amphibious and aquatic vegetation, and variation in the timing of low flows positively associated with aquatic vegetation.
We identified channel complexity, flow flashiness and variation in low flow timing, as key geomorphic and flow drivers of instream vegetation. Importantly, our results suggest that instream vegetation benefits from improving geomorphic complexity are likely to be limited without also addressing catchment scale hydrology.