Threatened inland sand vegetation in the temperate zone under different types of abiotic and biotic disturbances during a ten-year period

In sandy grassland vegetation of Central Europe, extensive grazing is often a management tool to maintain and improve nature conservation value. As part of a long-term field experiment lasting 10 years we investigated the effects of sheep grazing as a type of moderate disturbance. As experiments by nature happened in this time period we were also able to study severe biotic (rabbit grazing) and abiotic (drought) disturbances, examining the recovery and resilience of the system after these severe disturbances. Within a six-fold replicated randomised split-plot design, 25-m(2) plots of Armerio-Festucetum trachyphyllae vegetation were studied from 2000 to 2009 in the northern upper Rhine valley (Germany). We analysed sheep-grazed and non-grazed plots (both with very low rabbit impact until the first half of 2005). In 2005 we established an additional plot type without rabbit and sheep grazing. Severe rabbit impact was studied in detail from 2006 to 2008 concerning flower resources, seed production, endozoochorous seed dispersal and phytomass extraction. Data were analysed by ordination methods and mixed linear models. In times of low rabbit densities moderate disturbance by sheep grazing maintained species diversity. Severe drought in 2003 resulted in a marked decrease of species numbers, but the system recovered already in 2004. The subsequently analysed high rabbit impact resulted in decreases of species numbers, cover of bryophytes, flower quantities and seed content in faeces. Species numbers also declined on plots without rabbit grazing, but for different reasons. Shifts in phenological phases due to rabbit grazing were not observed, but the sum of flowers and number of flowering species decreased. After the decline of the rabbit population, species numbers slightly increased in 2009. Non-grazed plots showed an increase of litter. Despite severe disturbances by rabbits or drought, the community structure was relatively stable, especially in case of non-ruderalised plots and showed only slight changes. We developed a conceptual model, showing the high potential of recovery and resilience even in the case of severe disturbances. (C) 2011 Elsevier GmbH. All rights reserved.