Relationships between flood frequency and riparian plant community structure in mountain streams of western Montana

Flooding is the dominant factor structuring riparian plant communities along large, low elevation streams, but it is less clear what role flooding plays in the structure and composition of riparian plant communities along mountain streams. In the first part of this study, I examined the influence of four environmental variables (flood frequency, microtopography, light availability, and soil texture) and spatial heterogeneity on riparian plant community structure in seven study areas along mountain streams in western Montana, USA. Multivariate and spatial statistics were used to determine the relative strength of each set of factors and their interactions in explaining riparian plant community structure. Flooding influenced vegetation characteristics by an indirect pathway through microtopography in all study areas, whereas the direct influence of flooding was found commonly only in three study areas. Other consistent direct influences included light (60%) and space (87%). Direct and indirect influences of flood frequency through microtopography, and the direct influences of light and vegetation patch structure should be included in predictive models of riparian plant community structure in mountain streams. Boundary analysis showed that microtopography was an important factor for boundary maintenance of riparian plant communities. In the second part of the study, the effective flood frequency was determined initially by partial Mantel tests between inundation extent associated with each flood frequency and plant communities, and the results were compared to floristic characteristics derived from DCA and Indicator Species Analysis. Flood frequencies ranging from one to eight years were most strongly associated with the understory and overall plant community structure, whereas overstory communities were influenced by both frequent (¡Ü 9 year) and large infrequent (¡Ý35 year) floods. Riparian plant communities in mountain streams are mostly influenced by frequent flooding but large floods are also important for maintaining spatial heterogeneity