Aims: Nitrogen (N) deposition causes compositional shifts in subalpine grassland, which can threaten their high plant species diversity. The effects of the interaction of N deposition and climate change are unknown. What are the independent and joint effects of N deposition, warming and water stress on plant functional group cover in subalpine grassland? Do these results apply to the main grassland types of this region typical of base-poor and -rich soils? What vegetation changes can we expect with the current N critical load limits in a warmer future? Location: Central Alps, southeast Switzerland. Methods: A fully-factorial three-way transplant experiment was established, with a water treatment and three-level N addition (+0, +3, +15 kg N·ha−1·year−1) along an elevation gradient representing temperature changes of −1.4 to +3.0°C. In all, 216 monoliths, sourced from six summer pastures and with vegetation typical of basepoor and base-rich soils, were grown in the experimental site for five years. The cover of functional groups (grasses, sedges, forbs and legumes) was estimated annually. Results: Nitrogen addition increased sedge cover only at cooler sites. Grass cover was higher at warmer compared to cooler sites, but the watering treatment reversed this pattern. N addition increased grass cover at warmer sites. Legumes showed a unimodal temperature response with maximum cover at +0.7 to +1.8°C. Further warming decreased their cover and also that of forbs. Watering increased the cover of the latter two functional groups at warmer sites especially. Conclusions: In subalpine grassland, the stimulating effect of increased N on sedge cover is dampened by increased temperatures in both grassland types. Within the current critical N load limits, however, a distinct compositional change — increased grass cover — is expected in a warmer climate. Forb and legume cover decreases with temperature increases greater than 2°C, attributable to both water deficit and temperature.
