Is Campanula glomerata threatened by competition from expanding grasses? Results from a 5-year pot-experiment.

The disappearance of low-intensity types of agricultural land use such as mowing and grazing is a global phenomenon which leads to changes in species composition and interactions in plant communities. In central Germany, formerly species-rich, semi-natural dry grasslands have been converted into species-poor communities dominated by Festuca rupicola or Poa angustifolia. As a consequence, several forbs have become endangered, and biodiversity has decreased. In a 5-year competition experiment (replacement design; pots were placed outdoors at Halle Botanical Garden), we evaluated the performance of plant-plant interactions between the rare forb Campanula glomerata, F. rupicola and P. angustifolia, respectively. The response of C. glomerata to the presence of the two grass species were measured by stem length, number of flowers and above-ground biomass annually harvested. We did not find significant differences in C. glomerata stem lengths between years and treatments, but flower production, which was highest in the first year, strongly decreased in the subsequent years. Moreover, flower production was lower in combination with P. angustifolia than with F. rupicola. Biomass production showed significant differences between years, but not between treatments. The positive and negative interactions between C. glomerata and the two grass species was estimated by calculating Relative Neighbour Effect Index (RNE). In combination with F. rupicola, interactions switched between facilitation and competition from year to year. In the presence of P. angustifolia, competitive effects on C. glomerata were highest in the second year, but declined from year to year. Curiously, there were no significant correlations between positive and negative interactions with climatic conditions (annual temperature, precipitation and relative air humidity). Our results showed that the response of the rare forb C. glomerata varies with neighbour species, and that fecundity is more prominently affected by competition than stem height or biomass. Our data indicate that biomass removal by traditional land use methods would facilitate the restoration of species-rich dry grassland communities. [ABSTRACT FROM AUTHOR]