Mean indicator values suggest decreasing habitat quality in Swiss dry grasslands and are robust to relocation error

Resurveys of historical plots are important to study vegetation changes over time. However, there is a high potential of relocation error, as historical plots were usually not permanently marked, which might result in misleading interpretations of changes. Quasi-permanent plots can relatively well be relocated, because of available information on their position (e.g. GPS coordinates). This minimizes relocation error. Quasi-permanent plots have thus been suggested to be suitable for vegetation resurveys. Land-use abandonment and intensification are the major drivers of habitat degradation and thereby cause biodiversity loss. To counteract the ongoing decline of biodiversity in Swiss grasslands, the dry grasslands of national importance, comprising a total of 3631 sites across Switzerland (0.5% of its total area) were legally protected in 2010. These grassland sites represent various vegetation types and cover a wide elevational gradient. From 1995 to 2006, several thousand quasi-permanent vegetation plots of 28 m(2) were surveyed in the nationally important dry grassland sites. For each plot, GPS coordinates and a metric measure of post-processed GPS inaccuracy were recorded. In a set of 384 sites, 538 of the historical plots were resurveyed between 2011 and 2017. We tested the effects of GPS inaccuracy and elevation on temporal changes of species richness, species turnover and mean ecological indicator values. Overall, mean indicator values for light and stress significantly decreased, and those for moisture, nutrients and ruderality significantly increased within only one decade. However, the temporal change of the mean indicator values for nutrients and moisture was more pronounced at higher than at lower elevation, indicating a particularly strong decrease of habitat quality at high elevation sites. While changes of in mean indicator values were largely robust to relocation error, the temporal turnover of key species significantly increased with increasing GPS inaccuracy. These results indicate that quasi-permanent plots are suitable to study changes in mean indicator values, but probably not to compare species composition and turnover across time.