Interaction of climate change and nitrogen deposition on subalpine pastures

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 base‐poor 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. The AlpGrass transplant experiment in the European Alps showed that in subalpine grassland, N addition and temperature interact to alter plant functional group cover. Whereas under current conditions sedges profit from increased N inputs, this effect disappears with increasing temperatures. Grasses gain from the increased N with increasing temperature. Current N critical load limits will remain valid in a future climate.