The lasting impact of highly resistant pioneer vegetation on the development of a scaled experimental salt marsh with multiple species in a tilting flume

Abstract

Landscape-scale experiments of erosive self-forming tidal basins were carried out in the Metronome tidal flume, to investigate biogeomorphic feedback in salt marshes. Live seeds of analog species were introduced into the flow at the tidal inlet to simulate real-world marsh plants. The present study is a follow-up on an earlier experiment, in which a single species (Lotus pedunculatus) was used. An assemblage of three species was selected, with differing physical properties, resulting in differences in resistance and resilience. The idea was that through feedbacks such as facilitation, different patterns of vegetation colonization would arise compared to the single-species experiment, which would also bring about morphological differences between the two systems. It was expected that more surface area would be colonized than in the single-species experiment, as the more resilient Medicago sativa (alfalfa) could act as a pioneer in areas with high flow velocities, and by its physical presence create conditions suitable for the other, less resistant and resilienct species (Lotus pedunculatus and Veronica beccabunga) to settle. Instead, the result for the multi-species experiment was a cover rate of only 2.5% against 8% in the single-species experiment. This disparity was attributed to the very rapid and localized colonization by Medicago, which modified flow patterns in a way that reduced the chances of seeds reaching intertidal areas from the tidal inlet. Typical flow velocities around the Medicago patch only slowly destroyed it. While local facilitation at the Medicago patch was observed, other areas remained relatively bare. On a system scale, there was no positive feedback for the vegetation. The results are not reflective of the natural state of salt marshes, and lessons learned mostly apply to future flume studies with live vegetation. Resistance and resilience must be scaled to flow conditions, such that vegetation is dynamic on a timescale relevant to the experiment. Similar conditions to the experiment could hypothetically occur in real salt marshes as a result of human interference, such as artificial planting of non-native species, to the detrement of long-term salt marsh survival.