Characterizing the pathway and rate of salt marsh vegetation dynamics: A multivariate approach

Abstract

The ability of plants to enhance sedimentation is a critical factor in modeling the evolution and fate of salt marshes under future scenarios of climate change. Most eco-geomorphic models have been developed based on the changing biomass of a single species (e.g., Spartina alterniflora) through time; therefore, it still remains a challenge to predict how a vegetation cover consisting of multiple species will change through interspecific competition and facilitation under sea-level variations. In a temperate marsh of the Danish Wadden Sea, the plant species composition across a total of 402 quadrats (1 m(2)) was compared between 2006 and 2012 using multivariate ordination techniques. At low-elevation sites (< 0.8 m Danish Ordnance Zero), where many stress-tolerant species coexisted in 2006, the direction of vegetation changes was dominantly progressive, indicating decreases in stress-tolerant plants and increases in high-marsh competitors over the 6-year study interval. In contrast, whenever the competitive shrubby species Atriplex portulacoides was dominant in 2006 (> 80% relative proportion), the rate of vegetation change was nearly zero, due to little encroachment of other species into this already stable, dense matrix. These discussions imply that the direction and rate of multi-species interactions can be predicted by the initial environmental (i.e., marsh surface elevation, soil bulk density, distance from tidal creek) and biological conditions (i.e., plant species richness and abundance). Based on these findings, it is proposed that the evolution of salt marshes will be better understood by more explicit incorporation of multi-species interactions into future eco-geomorphic modeling efforts.