Shifts within the carbon cycle in response to the absence of keystone herbivore Ovibos moschatus in a high arctic mire

Abstract: Tundra ecosystems are generally recognized as globally important carbon sinks, yet the knowledge on threats to such ecosystems is narrow. Climate change is predicted to reduce the number of muskox (Ovibos moschatus) in the arctic, it is therefore essential to understand the effect herbivores induce on the arctic ecosystem. The muskox is known as a keystone species in the arctic tundra. Such keystone herbivores can induce strong top-down control, resulting in a transformed landscape when not present. Little is known about how herbivores impact the terrestrial carbon cycle, especially in arctic mires. Within this study, I evaluate whether the absence of grazing and trampling by the large herbivore muskox influences vegetation density and composition, subsequently altering the exchange of greenhouse gases between the atmosphere and tundra. During July 2016, CO2 and CH4 fluxes were measured in experimental muskox exclosure and control plots at Zackenberg, north-east Greenland. Vegetation and core samples were analysed for variances in composition and density. Furthermore, environmental parameters including soil temperature, water table depth, active layer and photosynthetic active radiation were measured. A significant change in the composition of vascular plants occurred and lower number of tillers of Eriophorum scheuchzeri and Dupontia psilosantha were recorded in exclosure plots. This change in vegetation resulted in no significant difference between net ecosystem exchange, gross primary production and ecosystem respiration with no below-ground root biomass variance. However, significantly lower CH4 fluxes were recorded within the exclosure plots. I speculate that the difference between treatments is an ongoing divergence from a vascular plant community towards a less diverse moss dominated layer. As time goes on more structural differences below ground are predicted to occur. While it is uncertain if herbivory in the arctic will increase or decrease, this experiment indicates potentially significant effects on the vegetation composition and its subsequent impact on fluxes of the greenhouse gas CH4.