Desmoschoenus spiralis displacement by Ammophila arenaria: the role of drought

The exotic sand-binder Ammophila arenaria (marram grass) has displaced the native sedge Desmoschoenus spiralis (pingao or pikao) from many of New Zealand’s coastal dunes. This study explores the possible role of drought as a mechanism promoting marram invasion and pingao displacement. The response of the two sandbinders to conditions of increasing soil water deficit was compared in a four-week pot trial. Water potential, relative water content and stomatal conductance were measured every 3–4 days on randomly selected individuals from a control and two drought treatments. Desmoschoenus exhibited significant signs of water stress within eight days, while Ammophila did not show significant signs of stress until day 18. Only 5% of Desmoschoenus individuals recovered after four weeks without water, compared with 80% of Ammophila. Although pot trials have limited applicability, these results suggest that Ammophila is relatively tolerant of desiccation compared with Desmoschoenus. This may give Ammophila a competitive advantage over Desmoschoenus during drought events, potentially resulting in the displacement of the native sedge.