Long-term management is needed for conserving plant diversity in a Wadden Sea salt marsh

Evaluation of long-term management regimes is important for guiding biodiversity conservation in salt marshes. However, such long-term experiments are sparse. Using a 46-year experiment in a salt marsh, we evaluated long-term effects of eight different management regimes (treatments; control, grazing, mowing, and their combinations) on the expansion of a late successional plant species (Elytrigia atherica), plant species richness and diversity, and community composition (species identities and dominance structure). Results show that E. atherica expanded strongly over time in the control treatment (without grazing or mowing) while plant species richness and diversity declined substantially. By contrast, E. atherica was greatly suppressed while plant species richness and diversity remained relatively unchanged in all other treatments except for the mowing, where species richness declined in the late season mowing treatment and plant diversity declined after 17 years in the both early and late season mowing treatment. Therefore, all management types except for the mowing were effective in conserving plant diversity. The trends for change in species identities reversed: change in species identities accumulated in the control treatment and exceeded that of other treatments 15 years after the start of the experiment. This suggests that results based on shorter-term (<15 years) experiments may provide misleading conservation recommendations. Also, trends for change in dominance structure (taking abundance into account) were substantially different from those for species identities. Our results highlight the importance of long-term monitoring for guiding conservation management, and that monitoring should not only focus on the number of species but also community composition, to fully identify critical changes.