Germination and seedling survival in fens undergoing succession

We studied mechanisms of vegetation change in fens subject to succession from open water to floating mats and finally herbaceous rich-fens. Earlier research showed that these systems are characterized by transient seed banks. Our main question was whether seedlings of later successional fen stages are already present in earlier stages, remaining subordinate in the vegetation until conditions become suitable for them. If, however, conditions during succession change in a way that only a limited set of species can survive as seedlings during each of the successional stages, no seedling bank will exist. The transient character of the seeds would then imply that seeds will not germinate and will subsequently die and that seeds that have germinated in the wrong stage will not become established. We hypothesized that: (1) germination and seedling survival of fen species are significantly better in the successional fen stage for which these species are characteristic, (2) as a consequence no seedling banks occur in these fens. In a field experiment, seeds of five characteristic fen species in the standing vegetation of three successional fen stages i.e. raft fen, quaking fen and rich fen were sown in each of these stages in a turf pond in the Tienhoven area, The Netherlands. Germination and seedling survival were measured over two growing seasons together with environmental variables. Germination was higher in the own stage for all species groups as was survival for quaking fen species and rich fen species. For both these stages, percentage of germination and survival of four out of five characteristic species were significantly higher in the own stage. Germination and survival can be considered stage-dependent and it was concluded that seedling banks do not exist in these fens. Site-specific environmental variables act as a sieve and differentiate on species presence already during early life history stages. We found clues that the environmental sieve acts at the level of nutrient availability, tolerance for high sulphide concentration and light climate. Because of the transient seed bank and absence of a seedling bank in these fen wetlands, successful establishment of species necessitates a continuous dispersal of characteristic species until the environmental conditions permit establishment. This also implies that species of the whole successional sere should be present within dispersal distance.