- VEGETATION CHANGES IN EXPERIMENTALLY GRAZED AND UNGRAZED BACK-BARRIER MARSHES - 45 Applied Vegetation Science 5: 45-54, 2002 © IAVS; Opulus Press Uppsala. Printed in Sweden Abstract. Vegetation succession in three back-barrier salt marshes in the Wadden Sea was studied using a data set comprising 25 years of vegetation development recorded at permanent quadrats. The effect of livestock grazing on succes- sion was assessed by comparing quadrats where grazing was experimentally prevented or imposed. We studied changes at the species level as well as at the level of the plant community. Special attention is given to effects on plant species richness and community characteristics that are relevant for lagomorphs (hares and rabbits) and geese. Inundation frequency and graz- ing were most important in explaining the variation in species abundance data. The three marshes studied overlap in the occurrence of different plant communities and the observed patterns were consistent between them. Clear differences in frequency and abundance of plant species were observed related to grazing. Most plant species had a greater incidence in grazed treatments. Species richness increased with eleva- tion, and was 1.5 to 2 ¥ higher in the grazed salt marsh. Grazing negatively influenced Atriplex portulacoides and Elymus athericus, whereas Puccinellia maritima and Festuca rubra showed a positive response. The communities domi- nated by Elymus athericus, Artemisia maritima and Atriplex portulacoides were restricted to the ungrazed marsh. Commu- nities dominated by Puccinellia maritima, Juncus gerardi and Festuca rubra predominantly occurred at grazed sites. As small vertebrate herbivores prefer these plants and communi- ties for foraging, livestock grazing thus facilitates for them. Keywords: Correspondence analysis; Exclosure; Goose; Hare; Livestock; Salt marsh; Species richness. Nomenclature: van der Meijden (1990) and Schaminée et al. (1998). Long-term vegetation changes in experimentally grazed and ungrazed back-barrier marshes in the Wadden Sea Bos, Daan * ; Bakker, Jan P.; de Vries, Yzaak & van Lieshout, Suzan Laboratory of Plant Ecology, University of Groningen, P.O. Box 14, NL-9750 AA Haren, The Netherlands; * Corresponding author: Fax +31503632273; E-mail d.bos@biol.rug.nl Introduction Vegetation succession in salt marshes has been de- scribed by various authors for many different marsh types (Ranwell 1968; Westhoff 1987; Adam 1990). Salt marshes are suitable systems for studying vegetation succession because they are relatively simple and spe- cies-poor. Understanding the patterns of development and the processes behind them helps to gain insight into the functioning of ecosystems in general and is relevant for the sustainable management of salt marshes. Elevation and sedimentation Elevation of the marsh plays an important role in structuring salt-marsh ecosystems, as it is directly re- lated to inundation frequency and hence to sediment deposition, evaporation, aeration, nutrient status, tem- perature and salinity (Adam 1990). The distribution of salt-marsh plants is strongly related to the gradients in elevation (Sanchez et al. 1996; Olff et al. 1997). Sedi- ment type also affects vegetation composition to a large extent and is influenced by the position of the marsh in the tidal basin. Back-barrier salt marshes, or barrier- connected marshes according to de Jong et al. (1999), have developed on top of a sandy substrate, show less sediment accretion and have soils with a thinner layer of clay than more sheltered mainland salt marshes (Dijkema 1983a). Olff et al. (1997) showed that the pool of nitrogen is positively related to the thickness of the clay layer. They studied a chronosequence on one of these back- barrier marshes and they concluded that succession there is caused mainly by the continual increase in nutrient availability. In back-barrier salt marshes, most elevational variation is caused by elevational differences in the sandy subsoil (van Wijnen & Bakker 1997), and each elevational position was shown to have its own characteristic succes- sional sequence. The final stages on high and low marsh are characterized by tall-growing species (Olff et al. 1997), suggesting that light competition becomes more important with increasing nutrient availability. Similar