Sheep versus donkey grazing or mixed treatment:: results from a 4-year field experiment in Armerio-Festucetum trachyphyllae sand vegetation

The abandonment of former land-use often leads to an increasing grass encroachment and a related decrease in plant species richness. Extensive grazing can be an adequate management method, but the effects on the vegetation depend on the livestock species. We investigated two different subtypes of the Armerio-Festucetum trachyphyllae Hohenester 1960 (Armerion elongatae Poetsch 1962) where the Armerio-Festucetum trachyphyllae typicum is characterised by a stronger topsoil acidification and a denser vegetation sward than the Festuca duvalii subtype. The investigation area is a dry and nutrient-poor inland sand ecosystem located in the Upper Rhine Valley (Germany). Therefore we chose local sheep breeds and donkeys as grazing animals which are adapted to these conditions. In four vegetation periods (2002-2005), the influence of sheep grazing (s), donkey grazing (d) and successive sheep and donkey grazing (s+d) on the floristic structure of the plant communities was investigated. For both vegetation subtypes, every treatment was replicated thrice on paddocks 700 m(2) in area. The vegetation was recorded by 1200 releves (2002-2005: s, d) and by further 429 releves (2003-2005: s+d). Additionally, we obtained data on species composition, nitrogen input and phytomass at special donkey-generated microsites. The factors year and vegetational subtype had a strong influence on vegetation parameters. For all investigated treatments, nature management goals such as an increase of herbs and a decrease of grasses and litter are reached. Species number, especially therophytes, was higher for donkey grazing than for sheep grazing. But since donkey grazing produced a nitrogen input of 223 kg N * ha(-1) at the donkeys' faeces accumulation sites, ungrazed areas covering 4% of the paddock area, the combination of sheep and donkeys can also become an important management tool because with the mixed grazing regime no ungrazed areas occur.