Effects of the expansion by Hippophae rhamnoides on plant species richness in coastal dunes

摘要

Question: Is the expansion of Hippophae rhamnoides in coastal dunes associated with a decline in plant species richness, and is this decline best described by a hump-backed relationship between species number and shrub cover? Location: Grey and yellowdunes on the East Frisian islands Spiekeroog and Norderney. Methods: Total plant species richness as well as the number of herbaceous and cryptogam species were determined in 2001 using plots of 16 m~2 size. We compared shrubland plots with varying cover of Hippophae with neighbouring dune grassland plots without shrubs as reference sites. Soil samples were collected to determine the values of some important edaphic variables (pH, organic matter, nitrogen). Results: The shrubland plots with Hippophae had or tended to have lower soil pH and C/N ratios and higher contents of organic matter and nitrogen than the grassland plots. Total species richness was marginally significantly related to the cover of Hippophae in a hump-backed manner on both islands.The pattern was more pronounced for mosses and lichens than for herbaceous species. For all species groups on Spiekeroog and for the herbaceous species on Norderney, the hump-backed relationship was much improved when using the difference in species number between shrubland and grassland plot as a dependent variable. Rel tionships could be improved by including the soil parameters as co-variables. Species richness was highest at moderate levels of shrub expansion, while it was much reduced in very denseshrubland. The decrease in species number is caused by the decline in grassland species typical of the open dunes, including some rare taxa. Conclusions: The expansion of Hippophae rhamnoides is a serious threat to the plant species richness of open coastal dunes, and needs to be counteracted by management measures.