Extreme heat and drought did not affect interspecific interactions between dune grasses

The frequency of extreme climatic events, such as storm and heatwaves, is predicted to increase because of climate change. Understanding interactions between species in environmental extremes plays a vital role in predicting ecosystem resilience. In this study, we examined how heat and drought combined with interspecific interactions between pioneer dune builder sand couch (Elytrigia juncea) and primary foredune builder marram grass (Ammophila arenaria) affected growth and survival of the latter species in an embryonic dune system. In a 4-week field experiment, we transplanted marram grass within sand couch patches or on bare sediment. This plant interaction treatment was combined with a compound heat and drought treatment that was simulated with greenhouses that inhibited rainfall and increased temperatures (average daily maximum temperature +4°C). Results show that the presence of sand couch significantly reduced growth (i.e., formation of new shoots, shoot and root length and aboveground biomass) of marram grass. By contrast, the heat and drought treatment had no significant effects on growth or survival of marram grass, irrespective of species interactions. The neutral response suggests that even in its early establishment marram grass is highly heat and drought resistant. Since the competitive interaction between sand couch and establishing marram grass did not change under pressure of an extreme heat and drought event, we expect that these factors do not affect embryonic dune development.