Changing phenology is an important consequence of climate change because asynchrony in changes at different trophic levels can have implications for ecosystem processes including biogeochemical cycling and regional greenhouse gas (GHG) emissions. In subarctic regions in particular, both the timing of primary productivity and influence of herbivores are important controls on biogeochemical cycling, and therefore the interacting effects of these processes may influence regional GHG budgets. We conducted research in a wetland tundra ecosystem in subarctic Alaska where climate change is creating the potential for a trophic mismatch between the timing of maximum forage quality and maximum nutritional demand by migratory herbivores. We evaluated the effects of this mismatch on regional GHG emissions through two studies. First, we determined the magnitude and variability of GHG emissions across multiple vegetation communities including those used and avoided by herbivores. Second, we investigated the effects of a trophic mismatch on GHG emissions from the communities used by herbivores through a replicated factorial experiment that manipulated the onset of the growing season (advanced vs. ambient) and timing of herbivore arrival (early, typical, late or no grazing).
In the first experiment, we found that grazing affected regional GHG emissions by altering the drivers of GHG exchange. Grazed vegetation communities had lower net GHG emissions than ungrazed communities, but they had higher rates of CH4 (14.0 ± 6.56 nmol CH4 m-2 s-1) emissions. Grazed communities also had the highest sensitivity to temperature suggesting they could experience the largest changes in emissions with future warming. In the second experiment, we found that altering the timing of grazing had a larger effect on GHG emissions than altering the timing of the growing season. Timing of grazing was particularly important for determining CH4 and N2O emissions, whereas CO2 exchange was controlled by both timing of grazing and the growing season. Delaying grazing altered GHG emissions more than advancing grazing, but the effects were different on CO2 and CH4 (increased net CO2 uptake; -2.47 ± 4.5 µmol m-2 s-1, and increased CH4 emissions; 19.17 ± 18.6 nmol CH4 m-2 s-1). A trophic mismatch that delays the timing of grazing relative to plant productivity in this region will likely lower net GHG emission to the atmosphere, but the net result will be determined by opposing effects on two major GHGs (CO2 and CH4).