Wie rasch kehrt die Vielfalt in artenarme Wiesenfuchsschwanz- Wiesen zurück? Ergebnisse aus Dauerflächenuntersuchungen zur Extensivierung des Grünlandes

Abstract

Schmidt, W.: How long does it take for diversity to return to species-poor Alopecurus pratensis meadows? Results of permanent plot experiments following reduction of agricultural use intensity. – Hercynia N.F. 40 (2007): 111–132.With the increasingly intensive use of agricultural areas, the once species-rich, mesic to moist meadow communities of Arrhenatherum elatius have increasingly given way to species-poor monocultures of Alopecurus pratensis. Whether or not the development can be reversed and if so, how long this takes, has been the question behind a permanent plot experiment carried out in a meadow that was subjected to intensive use 25 years ago. As of 2001, the meadow was no longer fertilised, but continued to be mowed regularly for silage making. Data on biomass production and vegetation development revealed surprisingly low productivity already in the first year without fertilisation. The productivity from then on was determined to a large extent by annual precipitation patterns. With the reduction in intensity of use, the proportion of dominant grasses (Alopecurus pratensis, Elymus repens, Poa trivialis) began a continuous decline. Compared to the declining species (of which there were 8 between 2001 and 2006), there is a markedly larger share of species which increased in cover, a total of 42 species. Species with strong quantitative increases include mainly the class character species of the Molinio-Arrhenatheretea (e.g. Trifolium repens, Arrhenatherum elatius, Poa pratensis). The share of grasses dropped from 75 % to less than 50 %. At the same time, the proportion of herbs rose from 25 % to 45 %, and that of legumes from less than 0.1 % to 11 %. In terms of the ecological indicator values of Ellenberg, the reduction of use intensity is reflected most strongly in the reduction of the average N-value. All diversity measures (species richness, Shannon-Index, evenness) exhibit a clear increase from 2001 to 2006. Compared with many other meadow restoration projects in which fertilisation has been stopped, the species-poor Alopecurus pratensis meadow in this case developed surprisingly quickly into a species-rich meadow. The following primary factors responsible for this are discussed: 1) the rapid drop in productivity to 3.5 t/hectare in the first year after cessation of fertilisation; 2) the associated gradual decline of competitive species, which altered the light regime and thereby provided favourable conditions for weakly competitive species present in small quantities and made possible the establishment of new species; 3) the arrival of new species primarily dependent on microhabitats, especially those that are created by soil disturbance (e.g., burrowing animals, disturbance by machines); 4) since the ability of the seedbank to contribute to the recolonisation is limited in meadows that have undergone a long period of intensive use, the introduction of off-site colonisers from the surrounding landscape is of paramount importance in reclaiming species richness. The latter requires a landscape in which residual stands with higher plant diversity are still present. Important dispersal vectors can be agricultural machinery, provided they drive through patches with higher diversity in the vicinity. Both conditions (residual patches and machinery) were fulfilled in the meadow studied here.