Effects of experimental small-scale habitat fragmentation on above- and below-ground plant biomass in calcareous grasslands

The effect of small-scale fragmentation on the above-ground plant biomass was examined in nutrient-poor calcareous grasslands in the northern Swiss Jura mountains in three successive years. In the same field experiment, the fragmentation effect on the below-ground plant biomass was evaluated in 1 year. Additionally, species richness and abundance of plants were recorded. The experimental set-up consisted of 12 blocks with 48 fragments (24 small, 12 medium and 12 large) and 48 corresponding control plots distributed over three study sites. Fragmentation was maintained by frequently mowing the area between the fragments. Three to 5 years after the beginning of the experiment, plant biomass was harvested in all fragments and control plots in late autumn every year. In general, fragments contained more above- and below-ground plant biomass than control plots. The increase in above-ground plant biomass was partly due to an increased density of plants, and partly due to a change in species composition in fragments. In particular, a competitive shift of dominance towards tall grasses and forbs at the expense of smaller forbs was observed. Beside the main fragmentation effect on above-ground plant biomass, interactive effects with year of investigation, study site and plot size were found. This indicates that the observed fragmentation effect is a combined result of various factors acting to a different extent at different sites and plot sizes. The fragmentation-related increase in plant productivity could affect higher trophic levels such as herbivore and predator communities.