Vegetation succession and herbivory in a salt marsh: changes induced by sea level rise and silt deposition along an elevated gradient

The relationships between soil development, vertical vegetation zonation, vegetation succession and herbivory by Brent geese, Branta bernicla, were studied in a coastal salt marsh. We were able to analyse up to 100 years of salt marsh development by comparing sites where vegetation succession had progressed for various periods of time. These data were related to a continuous daily record of high water levels measured since 1824. Most elevational variation in edaphic conditions (and therefore vertical vegetation zonation) could be attributed to variation in height of the sandy subsoil, as rapid dune formation occurs on the beaches early in succession-In the intermediate part of this elevational gradient, the maximum annual increase of 1.2 mm of silt corresponded to an annual increase of 5.6 g N m-2 the topsoil (0-50 cm). The average sea level rise in this area over the last 170 years was 0.63 mm year-1. A sedimentation model suggests that this has had strong effects on sedimentation and the annual inundation frequency in the mid-part of the elevational gradient, thus affecting vegetation zonation on the salt marsh. For the major part of the investigated transects, sea level rise has probably speeded up succession due to an increased rate of sedimentation. The occurrence and dominance of all plant species were recorded in 3927 plots, and and for the 11 most common species response surfaces were calculated for their dependence on elevation and transect age. Most plant species were clearly separated along these axes. Most halophytic species, which were preferred by the geese, occurred early in succession and low on the gradient, where we observed the highest densities of Brent geese grazing. Forage quality of Festuca rubra increased towards the lower salt marsh. Other preferred forage species (Puccinellia maritima and Plantago maritima) were gradually displaced during succession by the tall grass Elymus athericus, especially in the mid- and upper salt marsh. Few geese grazed in areas where Elymus was dominant. Herbivores first increased in numbers but then declined along a gradient of primary productivity. We propose that declining forage quality (due to changing vegetation composition during succession) is a better explanation for this pattern than the classic explanation of predator control of herbivores at high levels of primary productivity. This quality threshold hypothesis, as an alternative explanation of the exploitation ecosystem hypothesis, is expected to hold especially where smaller (quality-sensitive) herbivores such as geese are present. Grazing by cattle in a 200-year-old part of the salt marsh led to the disappearance of Elymus athericus, to establishment of early successional halophytes and a return of Brent geese. Grazing by a larger herbivore therefore facilitated conditions for smaller herbivores by preventing the dominance of plant species that were good light competitors, and thus improved forage quality. Populations of these small herbivores could then become regulated by predators, although none was present at our site.