Zonation of plant communities caused by hydrological stresses in headwater riparian wetlands

Background/Question/Methods

Species composition and physical structure of riparian plant communities along low-order streams greatly affect downstream water quality in large order rivers.  Most headwater streams have short intermittent flood-pulsing, but impacts of this flood regime on plant community structure in riparian wetlands are poorly understood.  We tested how flood-pulsing caused changes in plant communities by controlling the duration and frequency of inundation in wetland mesocosms (10 m x 20 m).   In a Flood pulse treatment, water levels fluctuated naturally in response to floods from storm events.  In a Static treatment, mesocosms were held at stable water levels.  We sampled riparian plants growing on the mesocosm banks in Low, Mid and High elevation zones (0, 66, and 132 cm above the baseline water levels, respectively) in each treatment.  Plant species assemblages were compared among treatments and elevation zones with NMDS ordination.  We also compared community attributes (total biomass, species richness, percent cover of common species, wetland indicator status and bare ground) with 2-way ANOVAs. 

Results/Conclusions

Differences in hydrology strongly affected plant community structure in all elevation zones.  There was more bare ground and lower richness in Flood pulse wetlands than Static wetlands, because many species that did not tolerate stochastic flooding were suppressed. Flood pulse wetlands were dominated by Obligate wetland plants and Static wetlands were dominated by Facultative Wetland, Facultative and Upland plants.  Distinct species assemblages occurred in each elevation zone in Flood pulse wetlands but not in Static wetlands.  Overall, the most flood-tolerant species were found in Flood Pulse / Low and Flood Pulse / Mid elevation zones, and the least flood-tolerant species were in the Static / High elevation zone.  Plant biomass was highest in Low elevation zone, which received frequent flooding. This study illustrates that even short and stochastic flooding found in headwater creeks can strongly affect riparian plant community structure.  Furthermore, the subtle differences in hydrology along elevational zones resulted in distinctly different plant communities.  We also found a high abundance of non-native wetland species in Static wetlands, supporting the importance of maintaining natural flooding regimes in headwater ecosystems.