Below- and above-ground vertebrate herbivory and abiotic factors alternate in shaping salt-marsh plant communities

Small vertebrate herbivores retard vegetation succession in young stages of barrier-island salt-marsh succession. They cannot do so at later successional stages due to increasing soil productivity by the sedimentation of nitrogen containing clay. At later, productive stages of succession, grazing by livestock can reduce the dominance of late succession perennial plant species. As an exception, the vegetation at the island of Langli in the Danish Wadden Sea has a low dominance of late successional species (Elytrigia atherica and Atriplex portulacoides) despite decades without livestock grazing. We manipulated both above- (hares and geese) and below-ground herbivory (water vole), and applied nitrogen fertilization to test their role in controlling vegetation succession. Additionally, we measured surface elevation change to test whether sediment inputs are low. Below-ground herbivores and fertilization affected vegetation composition but their effects were highly variable between years. Below-ground herbivory decreased the dominance of E. atherica at the high marsh. N-fertilization interacted with below-ground herbivory to slow down the increase of A. portulacoides at the low marsh. Nitrogen was not limiting the speed of succession. The effects of herbivory and fertilization were overruled by the onetime event of high sediment accumulation in 2007 which created favorable conditions for the annual plant species Atriplex littoralis. Its invasion in all plots, irrespective of treatment, caused the late successional perennials E. atherica to be outcompeted at the high marsh, and slowed down the increase of A. portulacoides at the low marsh. The study showed that herbivory and abiotic factors alternated in shaping long-term vegetation dynamics, which could be a common feature of dynamic ecosystems.

► We tested effects of above- and below-ground herbivory on salt-marsh vegetation.
► Additionally, we applied nitrogen fertilization and estimated sediment input.
► Effects of below-ground herbivores and fertilization varied between years.
► Both factors were overruled by sporadic high sediment accumulation.
► Effects of herbivory and abiotic factors alternated in shaping vegetation dynamics.