Foredune cross-section schematics.

(A) Foredunes contain both spatial and temporal gradients along their cross-sections. Foredune consists of toe, crest, and heel moving from left to right. For the spatial dune gradient, quadrats were assigned values between-1 and 1 for their placement along the dune cross-section. Chronosequence age increased from the dune toe to heel (see main text for details). Shoreline and backdune are shown for orientation. (B) Historical dune consisting of only Ammophila arenaria (gray). (C) Invasion of historical dune by A. breviligulata (black) through preemptive colonization of a new dune. Ammophila arenaria retains its original population, and a backdune invasion boundary delineates the two species’ distributions along the dune. (D) Invasion of historical dune by A. breviligulata through displacement of A. arenaria. Note that there is no longer a backdune invasion boundary because the A. arenaria has been locally extirpated.