The wetlands of the Yukon-Kuskokwim Delta in western Alaska are important breeding areas for geese and are experiencing rapid climate change, specifically earlier onset of the growing season. Consequently, geese arrive phenologically later in the growing season than in the past, potentially setting up a mismatch with consequences for goose nutrition, plant growth, and C and N processes in the ecosystem. We examined the interactive effects between the start of the growing season and Black Brant arrival time on these processes. In 2014 we initiated a 2X4 factorial experiment with six random blocks. Each block contains eight treatments and one control including: advanced-growing-season plots which are warmed in the spring using passive open-top chambers and ambient-growing-season plots; these plots are crossed with a manipulation where geese graze in the early, typical, and late season, or not at all. Our control plot receives no season manipulation and natural season-long grazing. We monitor carbon flux continuously in one experimental block using an autochamber system, and in the remaining blocks with a portable system were plots are measured ca. twice each week. Data were collected between early June and late August in 2014 and 2015.
Results/Conclusions
Timing of grazing had greater influence on CO2 exchange than experimental advancement of the growing season in this coastal wetland. The early growing season treatment reduced net carbon uptake by ca. 50%; a change driven by an increase in the rate of ecosystem respiration (ER), but the system remained a sink for C during the growing season and the effect of season advancement diminished by mid-summer. Grazing early in the season reduced net carbon uptake by a similar amount and the interaction between the two effects largely eliminated the carbon sink. Conversely, delayed grazing enhanced the carbon sink by ca. 50% compared to typical season grazing. Here, the influence was not through ER, but through enhanced standing leaf area. When the system was grazed early, we observed little recovery of carbon exchange during the summer. Consequently, the expectation of climate change advancing the growing season relative to the arrival of migratory geese would modestly enhance carbon uptake in the system with potential negative consequences for geese that will graze phenologically older tissue. Biotic forcing in arctic tundra can thus be an important driver of ecosystem function and should be considered as tundra systems respond to changing conditions.