Interspecific interactions between dune grasses: landscape formation by sand couch negatively affects marram grass’ establishment

Ecosystem engineers are often suggested to drive early vegetation succession by modifying environmental conditions. Along the Northwestern European coast, dune formation is classically believed to start with sand couch (Elytrigia juncea), followed by marram grass (Ammophila arenaria) once sufficiently large dunes have formed. However, these ideas lack experimental validation, leaving the specific relationship between sand couch, the landscape they form, and marram grass establishment unknown. Here, we experimentally investigated this relationship by planting 975 young marram grass plants in two distinct, sand couch dominated, embryonic dune systems. Using structural equation models, we examined the effect of sand couch and its landscape attributes (elevation, distance to sea and elevation change) on marram grass establishment. Results show an indirect negative effect of sand couch via landscape modifications on survival of establishing marram grass. Specifically, sand couch elevated the environment and elevated areas in turn eroded more in winter, which was the key factor reducing marram grass survival. Moreover, we observed the highest survival in the lower areas surrounded by sand couch dunes. While the indirect negative effect of sand couch through elevation was independent of location, the direct effects of sand couch on survival (i.e., neutral in exposed and positive in sheltered conditions) and growth (i.e., negative in exposed and neutral in sheltered conditions) were location and thus context dependent. Contrary to the idea that created elevation is a prerequisite, we hypothesize that dune formation by sand couch benefits marram grass establishment by creating relatively low-lying, stable and sheltered microsites.