Does salt stress constrain spatial distribution of dune building grasses <i>Ammophila arenaria</i> and <i>Elytrichia juncea</i> on the beach?

Abstract

Rising sea levels threaten coastal safety by increasing the risk of flooding. Coastal dunes provide a natural form of coastal protection. Understanding drivers that constrain early development of dunes is necessary to assess whether dune development may keep pace with sea-level rise. In this study, we explored to what extent salt stress experienced by dune building plant species constrains their spatial distribution at the Dutch sandy coast. We conducted a field transplantation experiment and a glasshouse experiment with two dune building grasses <i>Ammophila arenaria</i> and <i>Elytrigia juncea</i>. In the field, we measured salinity and monitored growth of transplanted grasses in four vegetation zones: (I) nonvegetated beach, (II) <i>E. juncea</i> occurring, (III) both species co-occurring, and (IV) <i>A. arenaria</i> dominant. In the glasshouse, we subjected the two species to six soil salinity treatments, with and without salt spray. We monitored biomass, photosynthesis, leaf sodium, and nutrient concentrations over a growing season. The vegetation zones were weakly associated with summer soil salinity; zone I and II were significantly more saline than zones III and IV. <i>Ammophila arenaria</i> performed equally (zone II) or better (zones III, IV) than <i>E. juncea</i>, suggesting soil salinity did not limit species performance. Both species showed severe winter mortality. In the glasshouse, <i>A. arenaria</i> biomass decreased linearly with soil salinity, presumably as a result of osmotic stress. <i>Elytrigia juncea</i> showed a nonlinear response to soil salinity with an optimum at 0.75% soil salinity. Our findings suggest that soil salinity stress either takes place in winter, or that development of vegetated dunes is less sensitive to soil salinity than hitherto expected.