In highly seasonal environments, organisms time their breeding to coincide with the period of highest resource availability. The sub-arctic coastal wetlands of the Yukon-Kuskokwim Delta in western Alaska are critical breeding habitat for millions of migratory geese. Geese time their long-distance migrations to take advantage of optimal forage quality; however, these breeding grounds are experiencing rapid increases in annual temperature and an advancing growing season. Increased variability in the timing of the growing season and the timing of goose arrival can have cascading effects on the quantity and quality of forage as well as nutrient storage in this system. We predict that an advanced growing season will cause plants to have greater biomass and higher foliar C:N, making plants less palatable, while also increasing soil N through warmer temperatures and increased N cycling. Delayed goose arrival and grazing will compound the effects of an advanced growing season, while earlier goose arrival may nullify them. We conducted a three-year experiment where we manipulated the start of the growing season and the arrival time of geese (natural grazing) to examine how the timing of these events influences above- and belowground vegetation biomass, foliar quality, and inorganic soil N.
We found that the timing of grazing had a stronger effect on vegetation than an advanced growing season, with late goose arrival increasing aboveground biomass by 190% and foliar C:N by 35%. Advancing the growing season increased season-long biomass by 35% and foliar C:N by 5%, but this effect was nullified by grazing in early arrival plots. In years 2 and 3, delayed goose arrival led to a 10% increase in biomass accumulation and a 10% reduction in forage quality. Because forage quality is strongly linked to goose fitness, this is likely to negatively affect goose populations and gosling survival. Additionally, delayed goose arrival resulted in depletion of available soil inorganic NH4 by 15%, while early goose arrival increased it by 25%. These findings indicate that if climate change causes multiple years of advanced green-up or delays in goose arrival, the impacts may have lasting repercussions for C-storage and N-cycling and reduce habitat quality in this important coastal ecosystem.