Plant community responses to 15 years of nitrogen and phosphorus fertilization along an elevational gradient in the subarctic tundra

Abstract: Climate change in the Arctic has profound effects on tundra vegetation, directly through increasing temperatures and indirectly through changes in plant community composition and nutrient cycling. Fertilization experiments are widely used to study the effects of increased nutrient availability on arctic tundra vegetation. This study investigated the effects of 15 years of nitrogen (N) and phosphorus (P) addition along an elevational gradient in subarctic heath and meadow vegetation in northern Sweden. It shows that vegetation type strongly interacted with elevation, time, N and P addition to influence plant communities and hence that different vegetation types can have highly different responses to increased nutrient availability over time. In both vegetation types, N was the main limiting nutrient, but combined fertilization with N and P generated the greatest change in plant community composition, whereby slow-growing species were replaced by one or two dominant graminoids, which was associated with a decrease in species richness, Simpson’s diversity and Shannon diversity. The effect of N addition on plant community composition was stronger in the nutrient-poor heath, whereas species richness declined more in the nutrient-rich meadow. During 15 years of nutrient addition, the fastest and greatest change in plant community composition, species richness and diversity occurred at the warmer sites in the heath and the colder sites in the meadow, but after ten years of fertilization, the colder sites in the heath also responded to increased nutrient availability, implying that temperature mediates the effect of fertilization on heath vegetation. The results of this study highlight the complex responses of different vegetation types in the tundra to increased nutrient availability over time and show that short-term responses to fertilization may persist on the long-term, but additional changes may occur after more than ten years of nutrient addition in more temperature and nutrient limited plant communities.