LONG-TERM CHANGES (1953-1990) IN THE SALT-MARSH VEGETATION AT THE BOSCHPLAAT ON TERSCHELLING IN RELATION TO SEDIMENTATION AND FLOODING

Long-term vegetation changes in permanent plots along two transects fr om the low to the high salt marsh at the Boschplaat on Terschelling be tween 1953 and 1990, were analysed in relation to sedimentation and fl ooding. In both transects, the number of plant species decreased betwe en 1953 and 1990. In lower parts of the two transects the dominant pla nt species of the vegetation changed from Puccinellia maritima in 1953 /54 to Limonium vulgare in 1980 to Atriplex portulacoides in 1990. The increase in A. portulacoides between 1980 and 1990 coincided with an increase in the thickness of the silt layer, caused by a sedimentation rate of about 5 mm per year. This sedimentation led to an increase in elevation but not to large changes in flooding frequency between 1953 and 1990, probably due to a sea level rise during the same period. In the higher parts of the salt-marsh transects Elymus athericus strongl y increased and this species became dominant in 1980 and 1990. In thes e parts of the salt marsh, a small increase in elevation, in thickness of the silt layer and in flooding frequency occurred between 1953 and 1990. The described changes in vegetation are discussed in terms of g eneral succession of salt-marsh vegetation in relation to flooding and sedimentation.