Wind-Driven Sea-Level Variation Influences Dynamics of Salt Marsh Vegetation
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Long-term variation of mean sea level has been considered the primary exogenous factor of vegetation dynamics in salt marshes. In this study we address the importance of short-term wind-induced rise of the sea surface in such biogeographic changes. There was an unusual opportunity for examining field data on plant species frequency sea-level variation and sedimentation acquired from the Skallingen salt marsh in Denmark since the 1930s. The environmental and floristic history of Skallingen was summarized as (1) continuous sea-level rise with temporal variability (2.3-5.0 mm yr-1) (2) continuous sedimentation with spatial variability (2.0-4.0 mm yr-1) (3) increased frequency of over-marsh flooding events and (4) contemporary dominance of Halimione portulacoides indicating little progressive succession toward a later phase. Conventionally recent eustatic sea-level rise was believed to drive the increased frequency of flooding and such retarded succession. Skallingen however has showed more or less equilibrated yearly rates between sea-level rise and surface accretion. This implies that the long-term gradual sea-level rise alone might not be enough to explain the increased inundation frequency across the marsh. Here we suggest an alternative chain: Recent trends in the North Atlantic Oscillation index toward its positive phase have led to increased storminess and wind tides on the ocean surface resulting in increased frequency duration and depth of submergence and hence waterlogging of marsh soils which has retarded ecological succession. To conclude we stress the need for a multitemporal perspective that recognizes the significance of short-term sea-level fluctuations nested within long-term trends. 2011 by Association of American Geographers.
