Abstract
Questions What is the response of the grassland community to either increasing or decreasing productivity? Which are the possible mechanisms underlying vegetation change, addressing in particular the relative role of competition and dispersal limitation? What are the implications for grassland management? Location Mesophyte semi-natural grassland in southeast Estonia (58 degrees 06'37'' N, 27 degrees 4'14'' E). Methods We decreased grassland productivity by adding sugar and increased productivity by fertilizing in a 10-yr experiment, using four plot scales for treatments. We also conducted sowing experiment with Plantago lanceolata as a phytometer and describe the soil seed bank in experimental plots. Results We found that in comparison to control plots, fertilization decreased species richness, increased the dominance of a few species and prevented the establishment of sown P. lanceolata. Although the sugar treatment decreased the standing crop, plant communities diverged little from the control over the 10 yr. Beta-diversity decreased over time in the control and sugar-treated plots, and increased in the fertilized plots. In plots with the least fertilization, species richness began to increase from year 9, probably due to the immigration of species from outside the treatment area. There were several grassland species in the surrounding vegetation or in the seed bank, but these did not appear in the sugar-treated plots over the 10-yr experimental period. Conclusions The short-term response of grassland communities to changing productivity is driven by competition, presumably most importantly by competition for light, and the role of dispersal limitation is small. However, long-term dynamics in the community where the standing crop decreased is dispersal-limited. Given that there is a source of additional species, local immigration may reverse the decrease in diversity due to eutrophication. Biomass removal and the suppression of productivity (soil fertility) are primary goals of semi-natural grassland management and also during the creation of sown grassy strips at field margins, but in the case of grassland restoration, enhancement of long-distance arrival of grassland species should be supported.