The effect of a climatic compound drought and heatwave event on the dune-building grass Elytrigia juncea

Abstract Background and aims Coastal dunes provide vital ecosystem services, including flood protection and freshwater storage. These ecosystems are shaped by clonally-growing dune grasses that trap sediment as patch size increases, enabling the grasses to avoid stress from seawater flooding and freshwater scarcity. However, it remains poorly understood how increasing climate extremes will impact the establishment and survival dynamics of this vegetation. This study investigated the effects of an experimental climatic compound drought and heatwave (CDHW) on Elytrigia juncea in an embryonic dune field. Methods Over a four-week field experiment, we examined two plant-patch sizes (0.014 m 2 vs. 1.953 m 2 ) and two climatic treatments (ambient vs. CDHW), monitoring plant response and soil moisture. We hypothesized that larger patches would better resist the CDHW due to their enhanced freshwater storage within larger dune bodies. Results Contrary to our expectations, E. juncea exhibited a positive response to CDHW, with longer shoots in both patch sizes. Initial soil moisture profiles remained similar across patch sizes throughout the experiment. Moreover, soil moisture profiles indicated a substantial freshwater source within reach of roots in all plots, explaining the absence of drought stress. Conclusion Our findings suggest that pioneer dune grasses can be highly resistant to climatic CDHW when roots can reach fresh groundwater, a condition which can occur when a thin freshwater lens is present in proximity to larger dune complexes. For establishing dune grasses, it is not merely dune formation on a local scale but rather on a landscape scale that is crucial for coping with extreme climatic events.