Goose grubbing and warming suppress summer net ecosystem CO uptake differentially across high-Arctic tundra habitats.

Environmental changes, such as climate warming and higher herbivory pressure, are altering the carbon balance of Arctic ecosystems; yet, how these drivers modify the carbon balance among different habitats remains uncertain. This hampers our ability to predict changes in the carbon sink strength of tundra ecosystems. We investigated how spring goose grubbing and summer warming-two key environmental-change drivers in the Arctic-alter CO fluxes in three tundra habitats varying in soil moisture and plant-community composition. In a full-factorial experiment in high-Arctic Svalbard, we simulated grubbing and warming over two years and determined summer net ecosystem exchange (NEE) alongside its components: gross ecosystem productivity (GEP) and ecosystem respiration (ER). After two years, we found net CO uptake to be suppressed by both drivers depending on habitat. CO uptake was reduced by warming in mesic habitats, by warming and grubbing in moist habitats, and by grubbing in wet habitats. In mesic habitats, warming stimulated ER (+75%) more than GEP (+30%), leading to a 7.5-fold increase in their CO source strength. In moist habitats, grubbing decreased GEP and ER by ~55%, while warming increased them by ~35%, with no changes in summer-long NEE. Nevertheless, grubbing offset peak summer CO uptake and warming led to a twofold increase in late summer CO source strength. In wet habitats, grubbing reduced GEP (-40%) more than ER (-30%), weakening their CO sink strength by 70%. One-year CO-flux responses were similar to two-year responses, and the effect of simulated grubbing was consistent with that of natural grubbing. CO-flux rates were positively related to aboveground net primary productivity and temperature. Net ecosystem CO uptake started occurring above ~70% soil moisture content, primarily due to a decline in ER. Herein, we reveal that key environmental-change drivers-goose grubbing by decreasing GEP more than ER and warming by enhancing ER more than GEP-consistently suppress net tundra CO uptake, although their relative strength differs among habitats. By identifying how and where grubbing and higher temperatures alter CO fluxes across the heterogeneous Arctic landscape, our results have implications for predicting the tundra carbon balance under increasing numbers of geese in a warmer Arctic.