Long-term low-level factorial nitrogen and phosphorus additions significantly impact a low arctic mesic tundra plant community, but species responses differ from high-level fertilization: Implications for predicting climate warming effects

Arctic climate warming is expected to enhance plant growth-limiting nutrients in tundra soils, and thereby affect community composition. Much of our understanding of nutrient influences on tundra plants is derived from very high level fertilization experiments. Here, we report effects of 11-19 years of low-level factorial annual nitrogen (N) and phosphorus (P) additions that more realistically simulate warming-induced nutrient increases, on aboveground biomass of all species in a mesic birch hummock community, and on birch shoot extension. Only the low N treatment had significant effects, enhancing biomass of the moss group and some infrequent vascular species, reducing others, and having no effects on the dominant shrubs (including Birch). Hence, overall community composition was altered, but in markedly different ways to classic high-level fertilization responses. However, where birch was also measured in much larger sampling areas (9 m2), its new shoot extension was strongly stimulated by the separate low-level N and P additions, and even more so by their combination, indicating that its growth was NP co-limited. Overall, our results demonstrate that this tundra plant community is sensitive to low-level (climatically-realistic) increases in nutrient availability, and suggest that changes will be slow (multiple decades), and likely to favour species whose growth is primarily N-limited.