Response of dune-building grasses to summer precipitation

Coastal ecosystems are vulnerable to climate change, with rising sea levels and increased anthropogenic pressure constraining space for natural processes. Nature based solutions using sediments rather than hard surfaces in coastal defense may offer an alternative that both creates new habitats and offer a flexible protection against flooding.

In contrast to hard infrastructure, the topography of dunes depends on the highly dynamic processes of wind and waves and the resistance to them offered by dune vegetation. Perennial grass species such as marram grass (Ammophila arenaria) and sand couch (Elytrigia juncea) play a key-role for topographic stability and the development and shape of coastal dune forms. This is usually attributed to their dense cover which effectively traps sand as well as their positive growth response to burial by sediments. Therefore, species like marram grass have been used as ecosystem engineers in both past and recent coastal dune restoration projects.

Whether this solution will be applicable in the future depends on climate change. Coastal vegetation is vulnerable to climate change due to its susceptibility to changes in growing conditions (e.g. Temperature, Precipitation). Especially at the dry-beach section where the influence of groundwater is limited, a change in growing season precipitation could potentially affect the cover of dune grasses. Past research was already able to establish a general link between dune development and growth in function of precipitation. However, to this date direct responses of dune vegetation to precipitation has not been quantified.

We explored the response of dune building grasses to summer precipitation and its implication for the future dune building in a two-step approach. We used a greenhouse-experiment to derive species growth relationships with water availability for marram grass and sand couch. In a second step we used these relationships to explore the impact of potential changes in summer precipitation on the growth of these species. We found that both marram grass and sand couch were equally sensitive to changes in water availability and responded positively to an increase in it. Comparing soil moisture from the field to the greenhouse, showed that field water availability tended to be on the lower end of ranges in the greenhouse. This suggests that dune vegetation in the field is susceptible to drought effects. Exploring these results further using climate scenarios, we found that plant growth was increased by 1.3 % (experimental period) – 1.8 % (extrapolated) under the most recent RCP 4.5 IPCC projection and by 9.6 – 13 % for an extremely wet year. In contrast, for an extremely dry year plant growth could decrease by 6.2 – 8.2 %.

While changes of < 2 % in plant growth might have limited implications for dune development and stability, years of extreme climate conditions show a bigger range in plant growth (- 8 % - + 13 %) which is more likely to also have direct consequences for dune growth and development. Incorporating these relationships between plant growth and climate in models of coastal dune development should improve predictions of climate change impacts.