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

Arctic climate warming is expected to enhance the soil’s supply of
growth-limiting nutrients, resulting in changes in plant community
composition. Much of our understanding of nutrient influences on tundra
plants is derived from experiments with very high levels of fertilization.
Here, aboveground biomass of all species in a mesic birch hummock
community, and shoot extension of Betula glandulosa, were measured in
response to 11-19 years of factorial annual low-level nitrogen (N) and
phosphorus (P) additions that simulate climatically realistic anticipated
increases in soil fertility. Only the low N addition treatment had
significant effects, enhancing the mosses and some infrequent vascular
species’ biomass, reducing others, while the dominant shrubs were
unaffected. Hence, overall community composition was altered, but in
markedly different ways than classic high-level fertilization responses.
By contrast, when measured in much larger sampling areas (9 m2), birch
shrub new shoot extension was clearly 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.