Evergreen shrubs dominate responses to experimental summer warming and fertilization in Canadian mesic low arctic tundra

Climate change in arctic tundra is projected to increase soil fertility, which may alter plant community composition and ecosystem processes by shifting niche space to favour particular species' life-history strategies. The rate and magnitude of change in soil fertility may be critical to determining plant community responses, and so effects of slow increases in nutrient availability due to climate warming may differ substantially from those of chronic high-level fertilizer additions. We investigated above- and below-ground plant biomass responses to experimental summer warming and above-ground responses to nutrient additions (low-level N and factorial N and P) in a mesic birch hummock tundra community in the central Canadian Low Arctic after eight years of experimental treatment. Plant community biomass responses to experimental warming were fundamentally different from those of high-level N and/or P additions, mainly due to opposing effects on the evergreen shrubs. Evergreen shrub above-ground biomass increased 66% with greenhouse warming, but decreased on average 70% with high-level N and/or P additions, driven by the strong responses of Rhododendron subarcticum. Because of this evergreen response, greenhouse-warming increased total above-ground biomass by 32% and total below-ground biomass by 70%, but did not significantly change the total above-ground/below-ground biomass ratio. However, warming increased the shoot/root ratio of Betula glandulosa threefold. Increased soil fertility created interactions between N and P availability, whereby increased P availability led to a substantial increase in inorganic N availability. Meanwhile, the growth of several species that span a range of different functional groups was stimulated by the separate N and P additions. These factorial fertilization results highlight the importance of understanding climate warming impacts on availability of both of these nutrients in order to predict plant community responses. Synthesis. Our results strongly suggest that the trajectory of mesic tundra vegetation change with warming depends critically on the rate of increase in soil fertility. The relatively large greenhouse-induced biomass increase in evergreen compared to deciduous shrubs suggests that carbon balance and albedo feedbacks to warming will be restricted in mesic tundra ecosystems, at least in their early responses to climate change.