Measuring Diversity in Plant Communities with Mosaic Spatial Patterns: Danish Coastal Dunes

Hierarchical pin-point data from 5316 plots from 73 Danish coastal dune sites were analysed in order to describe the species diversity in dune plant communities on a regional scale. Due to the mosaic spatial pattern of the dune communities, it was decided to describe the spatial structure of each plant species in each community using a vegetation type conditioned approach, where the hierarchical pin-point data were fitted to a zero-inflated generalised binomial distribution. Furthermore, summary statistics of the Lorenz curve of the regional estimates of species abundance are suggested in order to describe inequality of species abundance and to test for a possible log-normal species abundance distribution. The mean plant cover and the degree of spatial aggregation were estimated for all species found in six dune communities. Most plant species had a significant aggregated spatial distribution, and there was a significant positive correlation between the mean plant cover and the degree of aggregation. Species abundance did not depart from a log- normal species abundance distribution in any of the investigated dune community types. A vegetation type conditioned approach was found to be appropriate for analysing vegetation data of mosaic vegetation at a regional scale, and it is expected that the introduced method of measuring the direction of the deviation from a log-normal distribution will be important for interpreting the underlying cause of observed departures from log-normally distributed abundance curves.