MULTIPLE STATES IN THE SEA-LEVEL INDUCED TRANSITION FROM TERRESTRIAL FOREST TO ESTUARY

In this paper we provide a conceptual model to examine changes in ecos ystem state during the transition from terrestrial forest to shallow e stuarine environments for coastal mainland marshes at the Virginia Coa st Reserve (VCR), United States of America. Ecosystem states are chara cterized by plant community dominants and soil/sediment characteristic s. The five states considered are upland or wetland forest, organic hi gh marsh, intertidal mineral low marsh, autotrophic benthic with or wi thout submersed aquatic vascular plants, and heterotrophic benthic (es tuarine bottom). Transitions between states are described from the per spective of a fixed forest location undergoing transition from one eco system state to another. Rising sea level is acknowledged as the maste r variable that forces the process of change overall. Each state is hy pothesized to have self-maintaining properties and thus is resistant t o change from rising sea level; alternatively, transitions between sta tes are facilitated by disturbance or exposure to acute stress. For ch ange to occur, resistance must be overcome by events that are more abr upt than rising sea level and that appear as accentuated pulsings, whi ch result in another self-maintaining and resistant state. Such events facilitate plant species replacement and alter sediment conditions. M echanisms responsible for causing a state to cross a threshold are uni que for each transition type and include brackish-water intrusion (osm otic stress and sulfide toxicity), tidal creek encroachment (redistrib ution of sediments), erosive currents and waves (resuspension of sedim ents, which increases light extinction), and increasing water depth (l eads to greater bottom shading). Field experiments relevant to scales at which pulsings occur are not abundant in coastal marshes.