Herbivory and warming have opposing short-term effects on plant-community nutrient levels across high-Arctic tundra habitats

1. Environmental changes can rapidly alter standing biomass in tundra plant communities; yet, to what extent can they modify plant-community nutrient levels? Nutrient levels and their changes can affect biomass production, nutrient cycling rates and nutrient availability to herbivores. We examined how environmental perturbations alter Arctic plant-community leaf nutrient concentrations (percentage of dry mass, i.e. resource quality) and nutrient pools (absolute mass per unit area, i.e. resource quantity). 2. We experimentally imposed two different types of environmental perturbations in a high-Arctic ecosystem in Svalbard, spanning three habitats differing in soil moisture and plant-community composition. We mimicked both a pulse perturbation (a grubbing event by geese in spring) and a press perturbation (a constant level of summer warming). 3. After 2 years of perturbations, we quantified peak-season nitrogen and phosphorus concentrations in 1268 leaf samples from the most abundant vascular plant species. We derived community-weighted nutrient concentrations and total amount of nutrients (pools) for whole plant communities and individual plant functional types (PFTs). 4. Spring grubbing increased plant-community nutrient concentrations in mesic (+13%) and wet (+8%), but not moist, habitats, and reduced nutrient pools in all habitats (moist: -49%; wet, mesic: -31% to -37%). Conversely, summer warming reduced plant-community nutrient concentrations in mesic and moist (-10% to -12%), but not wet, habitats and increased nutrient pools in moist habitats (+50%). 5. Fast-growing PFTs exhibited nutrient-concentration responses, while slow-growing PFTs generally did not. Grubbing enhanced nutrient concentrations of forbs and grasses in wet habitats (+20%) and of horsetails and grasses in mesic habitats (+19-23%). Conversely, warming decreased nutrient concentrations of horsetails in wet habitats (-15%) and of grasses, horsetails and forbs in moist habitats (-12% to -15%). Nutrient pools held by each PFT were less affected, although the most abundant PFTs responded to perturbations. 6. Synthesis. Arctic plant-community nutrient levels can be rapidly altered by environmental changes, with consequences for short-term process rates and plant-herbivore interactions. Community-level responses in nutrient concentrations and pools were opposing and differed among habitats and PFTs. Our findings have implications for how we understand herbivory-and warming-induced shifts in the fine-scaled distribution of resource quality and quantity within and across tundra habitats.