Hypotheses regarding phenological mismatches emerged from observations of rapidly changing timing of spring budbust, insect emergence, and avian migration. Mismatch is the natural extension of observations of different phenological response to changing climate by plants and herbivores or by predators and prey, leading to altered timing of interactions between species. Studies demonstrate negative consequences of mismatch for various organisms, yet the impact of mismatches on ecosystem processes has been largely overlooked. We conducted an experiment manipulating the timing of spring green-up and migratory herbivore arrival while focusing on ecosystem C and N processes as the response. In the Yukon-Kuskokwim Delta of western Alaska, we used passive open-top chambers to advance the growing season ca. three weeks compared to ambient while altering the timing of grazing by Pacific Black Brant early, typical, and late relative to their historic arrival date in this coastal wetland. We also completely excluded herbivory and monitored ‘background’ plots without manipulation. We measured ecosystem C flux continuously in one replicate block, and measured C flux twice weekly in five other blocks. In all six blocks, we used ion exchange resins and soil water samples to monitor organic and inorganic N forms. Aboveground and belowground growth were quantified.
Results/Conclusions
Altered timing of plant-herbivore interactions significantly influenced C and N processes in this coastal Alaskan wetland. Our design allowed us to examine the influence of spring green-up and timing of grazing independently as well as in interaction and we observed that altering the timing of grazing was a considerably larger effect than an earlier spring. While advancing the spring resulted in more rapid growth and larger plants by mid-summer, the effect on C flux was minor, increasing ecosystem respiration (RE) slightly with no effect on gross primary productivity (GPP). Advanced spring increased NH4+ in soil solution by 26% but had no effect on NO3- or organic N. Alternatively, early grazing substantially reduced aboveground and belowground biomass, suppressed GPP by 35%, and more than doubled NH4+ and NO3-. Delayed grazing had the opposite effect on growth and C flux, resulting in more biomass, a 15% increase in GPP, and reduced exchangeable NH4+ and NO3- by ca. 16%. These results highlight the importance of examining mismatch effects on ecosystem processes. Future studies should emphasize how changes in C and N cycles act as positive or negatives feedbacks on ecological communities and these critical interactions between species.