Effects of migratory geese on plant communities and nitrogen dynamics in an Alaskan salt marsh

Abstract

Herbivory is an integral component of ecosystems that impacts plant communities and ecosystem processes, and affects forage availability and quality for the herbivore. I investigated the effects of lesser snow geese (Anser caerulescens caerulescens) and Canada geese (Branta canadensis) on two salt marsh communities, a sedge meadow and an herb meadow, in Cook Inlet, Alaska. Geese used the marshes during spring migration for a brief period, and foraging intensity was low compared to other goose-grazing systems. Seventy percent of the snow goose diet was on belowground plant tissues, whereas 92% of the Canada goose diet was on aboveground shoots. In the sedge meadow, where feeding was primarily on aboveground shoots, there was no effect of grazing on biomass of the dominant species Carex ramenskii and Triglochin maritimum, or on shoot nitrogen concentrations in these species (an index of forage quality). An experiment with captive geese found no effect of herbivory on biomass or nitrogen concentrations at foraging intensity ten times greater than that imposed by wild geese, indicating that this community is highly resilient to herbivory. In the herb meadow, where snow geese fed on belowground tissues, biomass of Plantago maritima and Potentilla egedii was lower, and biomass of Carex ramenskii higher, on grazed compared to ungrazed plots. Plant species' response to herbivory was determined by plant growth form, the type of herbivory (above- or belowground), and competitive interactions. Light herbivore pressure in this community altered the relative abundance of forage species for geese. In the sedge meadow community, geese increased nitrogen mineralization rates by trampling litter into wet soils. Litter incorporated into soils increased organic nitrogren pool size, decreased soil C:N ratios, and facilitated the growth of nitrogen-fixing cyanobacteria, all of which led to increased mineralization rates in grazed areas. Fecal nitrogren inputs were small and did not affect nitrogen availability. A captive goose experiment found that fecal additions ten-fold larger also had no effect on nitrogen availability. In the herb meadow, geese did not affect nitrogen mineralization because soils were dry with little standing water, so that incorporation of litter into soils through trampling was less important.