A comparison of the nutrient status of Molinia caerulea and neighbouring vegetation in a rich fen.

In the absence of management and with sustained non-point source pollution, the diversity of European low productive herbaceous vegetation is under threat. In many cases dominant grasses exclude other species. Molinia caerulea is such a potential dominant species. In a Campine rich fen near Mol, aerial photos show progressive invasion by M. caerulea tus socks. Marginal zones are completely closed by M. caerulea, while in ground water discharge zones open low growing vegetation persists mixed with small M. caerulea tussocks or non-tus sock individuals. During the tussock development process, M. caerulea shows a preference for the slightly drier marginal sites, contrary to the wettest ground-water discharge flushes where it is present as non-tussock individual shoots. This contrast was used to make a comparative study between zones and growth form of M. caerulea and its relation with neighbouring species. We investigate the hypothesis that differential M. caerulea performance is caused by differences in nutrient availability. Biomass and nutrient concentrations of M. caerulea shoots were investigated in relation to their location (marginal / ground-water discharge) and in rela tion to their growth form (spread non-tussock individual shoots / tussock shoots). M. caerulea shoots from marginal sites grew larger than shoots from central sites and tussock shoots proved superior to individual shoots. From the nutrient concentrations and the known P-limitation of the site, it is concluded that the P-availability is responsible. It appears that M. caerulea tussocks set up an efficient retranslocation of P. The mechanism is also present in non-tussock individuals, where it proved more efficient in the marginal sites. The biomass and N- and P concentration in M. caerulea leaves from non-tussock individuals and tussocks of increasing height in the ground-water discharge sites were investigated. June leaf P-concentrations were unexpectedly low in shoots from small (10-30 cm) tussocks as compared to non-tussock shoots. This invalidates a straightforward relation between P-availability and M. caerulea development in the wettest sites. Possible explanations are discussed. Overall the set-up of an efficient retranslocation of seems crucial in the development of M. caerulea dominance in this type of fens.