Long-term changes of rock grassland communities in Hungary

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Abstract
This paper demonstrates a possible application of large historical vegetation data sets as reference to reveal natural trends. Phytosociological releves re-sampled after 3–6 decades were used to detect and interpret long-term plant compositional changes of seven rock grassland communities in Hungary. Altogether 151 re-established plots were subject of the study. Data analyses were designed to minimize the negative effects arising from the application of historical information. Principal coordinates analysis was used to discover general compositional changes. With the help of ecological indicator values and species attributes, vegetation state trends were evaluated. Principal coordinates analysis reveals a uniform displacement of plot averages in the ordination space. Ecological indicator values for nitrogen requirement of vascular plants show a significant increase in mesotrophic categories. Occurrences of species typical of rock grasslands decreased significantly. Natural pioneers, disturbance-tolerant and weed species increased in number. Nevertheless, their amount is relatively low and the natural constituents of the communities still dominate, which is a sign of only a minor disturbance. Considering the wide geographical distribution of the sample plots, general changes seem to indicate pressures operating at large scales. These include elevated nitrogen deposition, increased rates of erosion and trampling caused by overpopulated ungulates and more frequent summer drought events. Acidification only occurs in silicate grasslands as calcareous soil types have higher buffering capacities. The global tendency of biotic homogenisation with the increase of common species was also detected over time. The study showed that the use of historical vegetation data enables us to estimate long-term trends in vegetation state