Expansion of acidophytic late-successional bryophytes in Dutch fens between 1940 and 2000

Question How did frequency and local abundance of key bryophytes in Dutch fens change between 1940 and 2000? Location The Netherlands. Methods A total of 1939 vegetation relevés, objectively assigned to the class Parvocaricetea and dating from the period 1940–1999 inclusive, were extracted from a national Netherlands database. From this relevé set, two additional data sets were derived; these were syntaxonomically homogeneous at the association level. One contained 418 Scorpidio-Caricetum diandrae relevés and the other 956 Pallavicinio-Sphagnetum relevés. The following analyses were done for these three data sets: frequency and local abundance change of 13 bryophyte species over three 20-yr periods were described using logistic regression and the nonparametric Jonckheere–Terpstra test, respectively. Results Acidophytic, late-successional species, notably Sphagnum palustre and Polytrichum uliginosum, have replaced relatively early-successional species such as brown mosses, Sphagnum subnitens and Sphagnum squarrosum during the studied period. The results were mostly consistent across the three data sets. Local abundance patterns generally paralleled those of frequency, although cover trends were often less robust. Conclusions Drastic changes have taken place in the bryophyte layer of Dutch fens between 1940 and 2000. Our study has shown that acidophytic, late-successional bryophytes have replaced relatively early-successional brown moss and Sphagnum species. Both hydrological changes and nutrient shifts (in particular phosphorus enrichment and ammonium toxicity) appear to have been responsible for the observed rapid shifts in the bryophyte layer of Dutch fens.