Trend monitoring of the areal extent of habitats in a subsiding coastal area by spatial probability sampling

The European Habitats Directive requires a regular reporting of areal changes of the Habitat types definedunder this Directive. To monitor changes in Habitat types in a dune and salt meadow area in the easternpart of the back-barrier island of Ameland (The Netherlands) a sampling scheme was designed suitablefor both unbiased estimation of such changes and for mapping the Habitat types. As a space–time designa supplemented panel was chosen, with a proportion of permanent plots of 0.5. Sampling plots wereselected by probability sampling, with sampling designs that spread the plots evenly over the study area.These design decisions are motivated in the paper. Eight vegetation types were distinguished, corre-sponding to six Habitat types. The areal extent of the ‘grey dunes’ type significantly decreased over theobservation period, whereas the extents of two ‘salt meadow’ types significantly increased. This has tobe considered as a loss of habitat quality. It is doubtful whether for the Natura 2000 area in its entirety,wherein we expect smaller rates of change compared to our study area, it will be possible to detect arealchanges in Habitat types at acceptable costs and within the requested six-year periods. The supplementedpanel design performed nearly equal to a pure panel design (all plots permanent) in terms of precisionof estimated linear trends, but was by far superior to an independent synchronous design with all plotschanging.