Leaf Tissue C:N is Modified by Growing Season and Goose Grazing Phenology in a Sub-Arctic Coastal Wetland of Western Alaska

There has been an advancement of spring conditions in coastal sub-arctic wetlands of the Yukon-Kuskokwim Delta. Pacific black brant (Branta bernicla nigricans) are dominant grazers in these coastal systems, and feed heavily on Carex subspathacea sedge lawns. Sedges have their highest nitrogen (N) concentration shortly following springtime emergence; however the greatest availability of N in the system occurs after hatch when nutrient demand for N by growing gosling and molting adults is greatest. We examine the influence of advanced growing season and different goose arrival times on vegetation biomass and C:N ratio. We set up fenced exclosures to control for grazing. Using a flock of captive wild brant, two plots received early, typical, late, or no grazing treatments to simulate different arrival times. To simulate an early growing season, we used fiberglass open-top chambers (OTCs) from May 1 to July 1. Half of our plots received the advanced growing treatment, while the remainder was exposed to ambient conditions. We found that grazing had a greater effect on plants than an advanced warming treatment. Early season grazing heavily reduced above- and belowground plant biomass. Plants responded to grazing by producing new leaves with higher N concentrations, lower C:N, and increased δ15N. An advanced growing season increased overall plant biomass, decreased C:N in plat tissue, and increased δ13C. While increased plant biomass might provide more forage, this might not be advantageous for geese if plants have lower N concentrations. Thus, the timing of the growing season and grazing both have important implications for C- and N-cycling and nutrient availability for geese in this system