Stability of a subarctic saltmarsh: Plant community resistance to tidal inundation

Abstract

We investigated the effects of tidal inundation and grazing pressure on soil carbon and nitrogen mineralization and plant community characteristics in the predominant plant communities used by breeding black brant (Branta bernicla nigricans) in southwestern Alaska. Three plant communities dominate the outer coastal zone, the lowest in elevation being Carex subspathacea grazing lawns, the next being C. ramenskii meadows, and the highest being a slough levee community. We flooded each of these communities in the field with tidal waters (mean total dissolved salts 28.3 ± 0.7 g L-1) at different frequencies for two growing seasons. Relative to controls, treatments that were flooded weekly had soils that had two-, three-, and six-fold increases in salinity in the C. subspathacea, C. ramenskii, and slough levee communities, respectively. We detected no effect of flooding on either soil carbon or nitrogen mineralization, nor any effect on live plant standing biomass and net aboveground primary productivity in the most coastal communities, those dominated by C. subspathacea and C. ramenskii. Biomass in the slough levee community was lower in plots that had soil salinities six times higher than the ambient soil salinity. This effect resulted from an 83% decline in woody vegetation. Grass biomass, predominately Poa arctica and Deschampsia caespitosa, increased in these plots, however, and maintained the productivity of this community relative to controls. Geese reduced the biomass of a preferred forage species (Triglochin palustris) by 50% over two growing seasons within the slough levee community. Carex subspathacea compensated for the loss of over 95% of its production by geese, even in soils that had twice the ambient salinity. The C. subspathacea and C. ramenskii communities are resistant to short-term tidal inundation and elevated soil salinities, and grazing by geese is responsible for maintaining their community boundaries. Elevated soil salinity and tidal inundation restricts the development of woody vegetation in the slough levee community, but graminoids compensate for the reduction in woody biomass and maintain the overall productivity of this community.