Saltmarsh boundary modulates dispersal of mangrove propagules: implications for mangrove migration with sea-level rise

Abstract

Few studies have empirically examined the suite of mechanisms that underlie the distributional shifts displayed by organisms in response to changing climatic condition. Mangrove forests are expected to move inland as sea-level rises, encroaching on saltmarsh plants inhabiting higher elevations. Mangrove propagules are transported by tidal waters and propagule dispersal is likely modified upon encountering the mangrove-saltmarsh ecotone, the implications of which are poorly known. Here, using an experimental approach, we record landward and seaward dispersal and subsequent establishment of mangrove propagules that encounter biotic boundaries composed of two types of saltmarsh taxa: succulents and grasses. Our findings revealed that propagules emplaced within saltmarsh vegetation immediately landward of the extant mangrove fringe boundary frequently dispersed in the seaward direction. However, propagules moved seaward less frequently and over shorter distances upon encountering boundaries composed of saltmarsh grasses versus succulents. We uniquely confirmed that the small subset of propagules dispersing landward displayed proportionately higher establishment success than those transported seaward. Although impacts of ecotones on plant dispersal have rarely been investigated in situ, our experimental results indicate that the interplay between tidal transport and physical attributes of saltmarsh vegetation influence boundary permeability to propagules, thereby directing the initial phase of shifting mangrove distributions. The incorporation of tidal inundation information and detailed data on landscape features, such as the structure of saltmarsh vegetation at mangrove boundaries, should improve the accuracy of models that are being developed to forecast mangrove distributional shifts in response to sea-level rise.

Fig 1. Study site diagram and experimental design.

a) Schematic diagram (not to scale) of main plant taxa along tidal gradient at study site on Cannon Island. b) Design of experimental plots at succulent-grass ecotone with initial position of propagules; shading indicates saltmarsh composition: (S. virginicus = gray; succulent plants = white) for saltmarsh grass monoculture (SV) and succulent-grass ecotone (SUC) treatments.

Fig 2. Spatial dispersion of A. germinans relative to initial position.

Box-plot with data indicating the distance from initial position (0 m) at which marked A. germinans were recovered for saltmarsh grass monoculture (SV = gray) and succulent-grass ecotone (SUC = white) treatments. Data include all A. germinans regardless of rooting status. For each box-plot, median distance (bolded line) and outliers (circles) are shown.

Fig 3. Distance moved by A. germinans propagules that dispersed seaward.

Mean distance ± se (cm) seaward of initial position at which A. germinans propagules were recovered for saltmarsh grass monoculture (SV) or succulent-grass ecotone (SUC) treatments on three dates. Different letters above bars represent significantly different distances (RMANOVA).

Fig 4. Establishment of A. germinans on 9 November 2012.

Initial establishment success was evaluated by categorizing the rooting status and leaf development of A. germinans six weeks after propagule emplacement into two saltmarsh treatment a) saltmarsh grass monoculture (SV) and b) succulent-grass ecotone (SUC). The percentages of A. germinans propagules in the three categories of seedlings establishment at three positions relative to the starting position (0 m) within the saltmarsh are presented for each of the saltmarsh treatments.