Sand burial has more negative impacts than interspecific competition on a riparian pioneer plant species: the Dwarf bulrush (Typha minima Hoppe)

Both biotic interactions and abiotic processes are important drivers of changes in riparian communities. However, little is known about how their effects could lead to the riparian pioneer species exclusion over time. We therefore conducted an ex-situ experiment in the Isère valley (France) to disentangle the effects of interspecific competition and sediment burial on the survival, growth and reproduction strategies of an endangered pioneer herbaceous plant species: Typha minima Hoppe. We crossed two factors with two modalities each: with the presence or absence of Salix alba L., (a common pioneer riparian tree) and in buried (with 10 cm of silty and -sandy alluvial sediments) or unburied conditions. The plants were placed in individual containers with 12 replicates for each combination of modalities. During the second vegetation season, we monitored mortality rates and traits, related to competitive vigor, energy storage ability and spatial exploration. Our analysis showed that interspecific competition had little effect on T. minima: it only reduced inflorescence and stem number. On the contrary, sediment burial induced a high mortality rate and strongly decreased all the monitored traits related to growth and reproduction strategies. When applied together, competition and sediment burial induced reduced leaf length and rhizome biomass. Our results highlight that T. minima is more resilient to interspecific competition than to sediment burial. To better understand alpine population trends, future research should be focused on other abiotic factors such as water resource availability, disturbance regimes and habitat disconnection induced by bar aggradation processes.