Avoidance of hydrological disturbance by aquatic vegetation in the floodplain of a large upland river

Abstract

Most studies suggest that floods remove substantial plant biomass due to mechanical forces applied during peak flows, thus contributing to the structuring of riverine vegetation. Effects on biomass were tested via an in situ experiment in the backwater of a large upland river in Scotland where frequency of connection to the main channel during floods controlled exposure to potential mechanical disturbance. Four macrophyte species (Potamogeton natans L., Myriophyllum alterniflorum DC., Ranunculus flammula L. and Mentha aquatica L.) were grown in trays and exposed to floods of different amplitude. Trays were distributed between an adjacent non-flooded control and the intermittently flooded backwater that differed principally in exposure to floods. The four taxa combined post-flooding biomass was surprisingly insensitive to floods, including two large events with recurrence intervals of 10–30 years. The four species showed different responses to flooding but only M. aquatica experienced a significant but small biomass reduction relative to control. Differences in biomass between control and backwater were mostly non-significant and did not vary with flood amplitude or spatially within the flooded backwater, with the exception of amphibious species that were disproportionately affected in the most disturbed upstream part. The macrophyte assemblage was generally more sensitive to winter than summer floods. This study indicates that macrophytes can limit significant biomass loss during major floods, and that this as with other disturbances, is likely to promote species coexistence.