Growth responses of Carex ramenskii to defoliation, salinity, and nitrogen availability: Implications for geese-ecosystem dynamics in western Alaska

Abstract

:The Yukon-Kuskokwim River Delta in western Alaska is the principal nesting area for several species of geese, including Pacific black brant. Grazing by geese on Carex ramenskii, one of the most abundant plant species throughout much of this region, appears to have increased in recent years. The purpose of this study was (i) to evaluate the effects of early-season defoliation and fertilization on plant growth and nutrient cycling processes within field plots of C. ramenskii over a 3-year period, and (ii) to study the interactive effects of defoliation, N availability, and salinity stress on growth, and biomass and N allocation in C. ramenskii under controlled, greenhouse conditions. Relative to control plots, clipped-fertilized plots showed significant increases in aboveground net primary production (AGNPP) and leaf N concentration, resulting in significant increases in offtake biomass and offtake N during both 1991 and 1992. In the greenhouse, total production of clipped and unclipped plants did not differ, but clipped plants had significantly higher offtake biomass, and biomass and N allocation to offtake compared to unclipped plants. Both field and laboratory experiments found that rapid regrowth following defoliation was dependent on soil nutrient availability. Fertilization increased soil respiration rates each year, but tended to decrease rates of net N mineralization, indicating that the soil microbial biomass is a strong nutrient sink in this ecosystem. In addition to the direct positive effects that goose feces have on plant growth, nitrogen recycled through feces may be an important source of nitrogen contributing to salinity tolerance in C. ramenskii. Our results also suggest that the observed increase in grazing pressure on patches of C. ramenskii early in the growing season may increase forage quality and quantity within these swards, and have important implications for geese-ecosystem interactions at a time of rapid goose population increase.