Millions of migratory geese rely on the wetlands of the Yukon-Kuskokwim Delta in western Alaska for breeding and molting. Historically, migrations were timed to take advantage of optimal forage quality, but increasing temperatures are resulting in earlier plant growth and altered nutritional quality. It is not known whether geese will change the timing of their migration to match plant phenology and how possible trophic mismatches may affect the quantity and quality of forage and nutrients stored in this important ecosystem. In 2014, we started a 2 x 4 factorial experiment examining the effects of an advanced growing season and goose (Black Brant) grazing times on vegetation biomass, and foliar and soil C and N. Each of six blocks contains eight treatments and one control. Half the plots received early season passive warming treatments to advance the growing season while half were under ambient conditions. This treatment was crossed with grazing treatments that simulate early, typical, late, and no arrival by geese. For two seasons, from early June to late August, we measured above- and belowground biomass, leaf tissue C and N concentration, and inorganic N availability in soil.
Results/Conclusions
Our results suggest that if the growing season advances and geese do not arrive early, aboveground biomass and C:N ratios in foliage will increase by roughly 50% and inorganic N in the soil will decrease by roughly 50%. Interestingly, an advanced growing season did not affect belowground biomass, but late arrival by geese increased belowground biomass by 80%. In summary, both an advanced growing season and delayed arrival by geese increased plant growth and the amount carbon fixed by the plant community, but reduced overall quality of forage. Because forage quality is highly related to goose fitness, this is likely to affect goose populations negatively. Furthermore, because we measured stronger effects of our treatments in the second year, results suggest that multiple years of advanced green-up and delays in goose arrival may have lasting repercussions for C-storage and N-cycling in this important coastal ecosystem.