Interactive effects of soil moisture, vegetation canopy, plant litter and seed addition on plant diversity in a wetland community

We carried out a factorial experiment to examine how groundwater availability (low and high sites with intermediate or rare flooding), vegetation canopy, leaf litter and seed availability interacted to determine the species richness of a productive wet grassland community in Wicken Fen National Nature Reserve, Cambridgeshire, UK. Seeds of 18 species were added to half the plots in each of eight combinations of elevation, canopy and litter, and seedling emergence was observed for two growing seasons. 2 Both individual and interactive effects on plant diversity and colonization were determined for all four examined factors. Interactive effects explained 41-63% of the total variation in both species richness and numbers of individuals growing from added seeds. 3 Neither elevation nor vegetation canopy had significant individual effects on total species richness, but their interaction was significant. Litter addition limited seedling emergence at the low elevation but favoured it at the high elevation. 4 The relative importance of vegetation canopy and plant litter in affecting plant community composition varied with the community parameter considered (species richness or number of seedlings), elevation and stage of vegetation development. In general, plant litter was more important in determining species richness, whereas the vegetation canopy was more important in determining seed germination and seedling emergence. Plant litter was also more important than vegetation canopy at an early stage of vegetation development and at low elevation. 5 Seed availability was the most important factor in determining overall species richness in the studied community. The influence of the local seed bank was very limited. Seedling emergence and seedling species richness were generally enhanced by lower elevation and seed addition, but depressed by vegetation and litter addition. 6 The complex relationships observed have considerable implications for ecological modelling and ecosystem restoration. Manipulation of one factor may produce unexpected effects on other factors, which may induce a series of consequences for the whole community. Further knowledge on how natural communities are organized and maintained is needed to guide the management of ecosystems.