Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 2: Spartina Distribution and SpreadCOMPETITION AMONG MARSH MACROPHYTES BY MEANS OF VERTICALGEOMORPHOLOGICAL DISPLACEMENTJ.T. MORRISBelle W. Baruch Institute for Marine & Coastal Sciences, University <strong>of</strong> South Carolina, Columbia, SC 29208This paper describes a <strong>the</strong>ory <strong>of</strong> biogeomorphology that addresses how intertidal macrophytes canmodify landscape elevation. Competitive interactions among marsh plant species are mediated by<strong>the</strong> influence <strong>of</strong> vegetation on sediment accretion and <strong>the</strong>ir modification <strong>of</strong> <strong>the</strong> relative elevation <strong>of</strong><strong>the</strong> marsh surface. A model described here demonstrates <strong>the</strong> effects <strong>of</strong> feedback between physicalprocesses like sediment accretion and biological processes such as those that determine specieszonation patterns. Changes in geomorphology, primary productivity and <strong>the</strong> spatial distribution <strong>of</strong>plant species are explained by competitive interactions and by interactions among <strong>the</strong> tides, biomassdensity, and sediment accretion that move marsh elevation towards an equilibrium with mean sealevel (MSL). This equilibrium is affected positively (relative elevation <strong>of</strong> <strong>the</strong> marsh surface increases)by <strong>the</strong> biomass density <strong>of</strong> emergent, salt marsh macrophytes and negatively by <strong>the</strong> rate <strong>of</strong> sea-levelrise. It was demonstrated that a dominant, invading species is able to modify its environment toexclude competitively inferior species. However, <strong>the</strong> outcome depends on a number <strong>of</strong> variablesincluding <strong>the</strong> rate <strong>of</strong> sea-level rise and <strong>the</strong> fundamental biomass distributions <strong>of</strong> species across <strong>the</strong>intertidal gradient. The model predicts that a marsh can move toward alternative states, dependingon <strong>the</strong> rate <strong>of</strong> sea-level rise and species biomass distributions within <strong>the</strong> tidal frame.Keywords: sedimentation, marshes, Spartina, sea level, models, geomorphology, competitionINTRODUCTIONPhysical ecosystem engineering is a process commonto organisms that possess <strong>the</strong> ability to physically modify<strong>the</strong>ir habitats (Jones et al. 1997). This trait is common tointertidal marsh macrophytes that have <strong>the</strong> potential toraise <strong>the</strong> relative elevation <strong>of</strong> <strong>the</strong>ir habitat and modify <strong>the</strong>geomorphology <strong>of</strong> <strong>the</strong> coastal landscape, potentially to <strong>the</strong>benefit and detriment <strong>of</strong> o<strong>the</strong>r species (Zedler & Kercher2004). Historically, coastal wetlands have maintained anelevation in equilibrium with mean sea level by <strong>the</strong> accumulation<strong>of</strong> mineral sediment or organic matter (Redfield1972; Stevenson et al. 1986). Commonly, stable intertidalsalt marshes occupy a broad, flat expanse <strong>of</strong> landscape <strong>of</strong>tenreferred to as <strong>the</strong> marsh platform at an elevation within<strong>the</strong> intertidal zone that approximates that <strong>of</strong> local meanhigh water (MHW) (Krone 1985). Marsh species typicallysegregate along gently sloping gradients across <strong>the</strong> marshplatform (Hacker & Bertness 1999; Silvestri et al. 20005).The elevations <strong>of</strong> <strong>the</strong> marsh platform relative to sea leveldetermine inundation frequency, duration and, consequently,wetland productivity and species distributions.Recent work in a North Inlet, SC marsh has shown that<strong>the</strong> relative elevation <strong>of</strong> <strong>the</strong> sediment surface is a criticallyimportant variable that ultimately controls <strong>the</strong> productivity<strong>of</strong> <strong>the</strong> salt marsh plant community (Morris et al. 2002).Productivity has a positive feedback on <strong>the</strong> rate <strong>of</strong> accretion<strong>of</strong> <strong>the</strong> marsh surface. This feedback is key to predicting <strong>the</strong>responses <strong>of</strong> coastal wetlands to rising sea level, includingchanges to <strong>the</strong> geometry <strong>of</strong> <strong>the</strong> land margin and to <strong>the</strong> totalarea <strong>of</strong> wetland habitat. It is also fundamental to understanding<strong>the</strong> spread <strong>of</strong> invasive marsh macrophytes, such asSpartina hybrids in San Francisco Bay, that have <strong>the</strong> abilityto physically alter <strong>the</strong>ir environment in a manner that benefits<strong>the</strong> invading species (Cuddington & Hastings 2004).The objective <strong>of</strong> <strong>the</strong> study reported here is to explore <strong>the</strong>behavior <strong>of</strong> a model that explicitly treats <strong>the</strong> modification <strong>of</strong>habitat elevation by a hypo<strong>the</strong>tical group <strong>of</strong> marsh macrophytescompeting for habitat space within <strong>the</strong> intertidal zone.The model accounts for <strong>the</strong> species-specific effect <strong>of</strong> marshvegetation on mineral sediment accretion, and for feedbacksamong sea-level rise, relative elevation, species replacement,and primary production. From this work it should be possibleto generalize about <strong>the</strong> types <strong>of</strong> adaptations that enable macrophytespecies to exploit a particular range <strong>of</strong> habitat within<strong>the</strong> intertidal zone and that endow some species with superiorcompetitive abilities. The concept <strong>of</strong> geomorphologicaldisplacement is described whereby one species displacesano<strong>the</strong>r by modifying <strong>the</strong> relative elevation <strong>of</strong> its habitat.MODEL DESCRIPTIONThe model described here is based on fieldwork thatwas carried out in a salt marsh at North Inlet, South Carolina(Morris et al. 2002). The model was initially developedand calibrated for a single species, Spartina alterniflora,which forms a monoculture over <strong>the</strong> majority <strong>of</strong> this marshwithin a narrow range between 0.22 and 0.481 m relative- 109 -