Effects of long-term grazing on sediment deposition and salt-marsh accretion rates

Many studies have attempted to predict whether coastal marshes will be able to keep up with future acceleration of sea-level rise by estimating marsh accretion rates. However, there are few studies focussing on the long-term effects of herbivores on vegetation structure and subsequent effects on marsh accretion. Deposition of fine-grained, mineral sediment during tidal inundations, together with organic matter accumulation from the local vegetation, positively affects accretion rates of marsh surfaces. Tall vegetation can enhance sediment deposition by reducing current flow and wave action. Herbivores shorten vegetation height and this could potentially reduce sediment deposition. This study estimated the effects of herbivores on 1) vegetation height, 2) sediment deposition and 3) resulting marsh accretion after long-term (at least 16 years) herbivore exclusion of both small (i.e. hare and goose) and large grazers (i.e. cattle) for marshes of different ages. Our results firstly showed that both small and large herbivores can have a major impact on vegetation height. Secondly, grazing processes did not affect sediment deposition. Finally, trampling by large grazers affected marsh accretion rates by compacting the soil. In many European marshes, grazing is used as a tool in nature management as well as for agricultural purposes. Thus, we propose that soil compaction by large grazers should be taken in account when estimating the ability of coastal systems to cope with an accelerating sea-level rise.