Competitive hierarchy, but no competitive exclusions in experiments with rich fen bryophytes

摘要

Competitive exclusion among species with high niche overlap has been questioned by plant ecologists over the last decades. In this study, we used a greenhouse experiment to test the role of competition for community dynamics and composition among bryophytes in boreal rich (calcareous) fens. We evaluate the rate and direction of competition between three of the most common bryophytes in these wetlands, Campylium stellatum, Scorpidium cossonii and Scorpidium scorpioides. A pair-wise experimental setupwith equal amounts of initial plant material of the species was used and the experiment continued for two years. To assess the competitive outcome we measured the area expansion of one species into the part of the experimental containers covered by its competitor, and the height growth of each species. Differences were observed between tested species for total spatial expansion into the area of its competitor. C. stellatum and S. cossonii showed similar total spatial expansion, while S. scorpioides showed very low spatial expansion. The spatial expansion also depended on the identity of the other species in the combination. All three pair-wise interactions were strongly asymmetric, and in qualitative terms a competitive hierarchy could be established.However, this did not lead to competitive exclusions, even though the experimental containers were small and the two years of continuous growth in the greenhouse would correspond to several years of growth under field conditions. The results suggest thatin the natural habitat, where interactions are further weakened by microtopographic niche separation and interrupted by fine-scale disturbances, competitive exclusions are probably rare, if they occur at all.