Interactions between winter and summer herbivory affect spatial and temporal plant nutrient dynamics in tundra grassland communities

In the long-term, herbivores can alter nutrient dynamics in terrestrial ecosystems by changing the functional composition of plant communities. Here, we ask to what extent herbivores can affect plant-community nutrient dynamics in the short-term. We provide theoretical expectations for immediate effects of herbivores on tundra-grassland plant-community nutrient levels throughout a single growing season and empirically evaluate these predictions. We established an experiment within two forb-dominated and two grass-dominated tundra-grassland communities. We selected tundra-patches disturbed by small rodents during the previous winter, and neighbouring undisturbed tundra-patches. Within each tundra-patch, we set up a reindeer-open and a reindeer-exclusion plot. Throughout the summer, we randomly collected over 2800 leaf samples from 34 vascular plant species/genera and analysed their nitrogen and phosphorus contents. Plant-community nutrient levels were consistently higher in tundra-patches affected by small rodents, both across tundra-grassland types and throughout the growing season. Forbs and grasses growing in small-rodent disturbed tundra-patches had 11% and 25% higher nutrient content, respectively, compared to undisturbed tundra-patches. Reindeer affected only grasses growing in grass-dominated tundra-grasslands and the outcome was dependent on small-rodent winter disturbance. Reindeer increased grass nitrogen content in undisturbed tundra-patches (+7%) and weakened the positive effects of small rodents in disturbed tundra-patches (from 25% to 15% higher nutrient content [both nitrogen and phosphorus]). By enhancing plant nutrient levels throughout a single growing season, herbivores were key, immediate modifiers of plant-community nutrient dynamics in tundra-grasslands. Higher nutrient contents still detected in senescent leaves at the end of the summer in herbivore-affected tundra suggest that herbivory is accelerating short-term tundra-grassland nutrient cycling rates. Our findings from tundra-grassland communities align with theoretical expectations of positive herbivore effects on nutrient cycling in relatively productive ecosystems.