Competition as a factor structuring species zonation in riparian fens - a transplantation experiment

Question: What is the relative importance of competition and physical factors in restricting the occurrence of different fen species to certain zones of the riparian landscape?

Location: Biebrza National Park, NE Poland.

Methods: We carried out a two-season reciprocal transplantation experiment: sod-blocks were replaced between a low-productive groundwater-fed small-sedge fen and a high-productive fluviogenous tall-sedge fen. Five treatments were applied to the transplanted sods: (1) no manipulation; (2) fertilization with NPK to exclude the effect of nutrient limitation; (3) clipping vegetation around the transplant to exclude competition for light; (4) clipping + fertilizing; (5) sods were also transplanted within their own environment.

Results: After two seasons, the small-sedge sods transplanted into the high-productive zone had a biomass two times lower than that of the control transplants, mainly due to a decrease in small sedges and bryophytes. Tall sedges of the high-productive zone did not respond significantly to transplantation in the low-productive zone. Removal of the surrounding vegetation largely enhanced the growth of small sedges and bryophytes and, to a lesser degree, of tall sedges. Fertilization, on the other hand, resulted in increased growth of tall sedges, grasses and non-graminoid herbs.

Conclusion: Species characteristic of low-productive fen communities are competitively excluded from the high-productive zone through light competition. In contrast, the performance of tall sedges in the low-productive zone is lowered by nutrient limitation. In the long run this may lead to a complete disappearance of these species from this zone. We did not find evidence that the physical stress of flooding has a direct effect on the performance and distribution of species. Results from the experiment suggest that productivity gradients and their influence on competition intensity are of primary importance for structuring vegetation patterns in lowland riparian fens.