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Summary 1.Climate warming is faster in the Arctic than the global average. Nutrient availability in the tundra soil is expected to increase by climate warming through (i) accelerated nutrient mobilization in the surface soil layers, and (ii) increased thawing depths during the growing season which increases accessibility of nutrients in the deeper soil layers. Both processes may initiate shifts in tundra vegetation composition. It is important to understand the effects of these two processes on tundra plant functional types. 2. We manipulated soil thawing depth and nutrient availability at a Northeast-Siberian tundra site to investigate their effects on above-and below-ground responses of four plant functional types (grasses, sedges, deciduous shrubs and evergreen shrubs). Seasonal thawing was accelerated with heating cables at c. 15 cm depth without warming the surface soil, whereas nutrient availability was increased in the surface soil by adding slow-release NPK fertilizer at c. 5 cm depth. A combination of these two treatments was also included. This is the first field experiment specifically investigating the effects of accelerated thawing in tundra ecosystems. 3. Deep soil heating increased the above-ground biomass of sedges, the deepest rooted plant functional type in our study, but did not affect biomass of the other plant functional types. In contrast, fertilization increased above-ground biomass of the two dwarf shrub functional types, both of which had very shallow root systems. Grasses showed the strongest response to fertilization, both aboveand below-ground. Grasses were deep-rooted, and they showed the highest plasticity in terms of vertical root distribution, as grass root distribution shifted to deep and surface soil in response to deep soil heating and surface soil fertilization respectively. 4. Synthesis. Our results indicate that increased thawing depth can only benefit deep-rooted sedges, while the shallow-rooted dwarf shrubs, as well as flexible-rooted grasses, take advantage of increased nutrient availability in the upper soil layers. Our results suggest that grasses have the highest root plasticity, which enables them to be more competitive in rapidly changing environments. We conclude that root vertical distribution strategies are important for vegetation responses to climate-induced increases in soil nutrient availability in Arctic tundra, and that future shifts in vegetation composition will depend on the balance between changes in thawing depth and nutrient availability in the surface soil.

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Questions:The rapid climate warming in tundra ecosystems can increase nutrient availability in the soil, which may initiate shifts in vegetation composition. The direction in which the vegetation shifts will co-determine whether Arctic warming is mitigated or accelerated, making the understanding of successional trajectories urgent.One of the key factors influencing the competitive relationships between plant species is their access to nutrients, depending on the depth where they take up most nutrients. However, nutrient uptake at different soil depths by tundra plant species that differ in rooting depth is unclear.Location: Kytalyk Nature Reserve, northeast Siberia, Russia. Methods:We injected 15 N to 5 cm, 15 cm and the thaw front of the soil in a moist tussock tundra. The absorption of 15 N by grasses, sedges, deciduous shrubs and evergreen shrubs from the three depths was compared. Results:The results clearly show a vertical differentiation of N uptake by these plant functional types, corresponding to their rooting strategy. Shallow-rooting dwarf shrubs were more capable of absorbing nutrients from the upper soil than from deeper soil. Deep-rooting grasses and sedges were more capable of absorbing nutrients from deeper soil than the dwarf shrubs. The natural 15 N abundances in control plants also indicate that graminoids can absorb more nutrients from the deeper soil than dwarf shrubs. Conclusions:Our results show that graminoids and shrubs in the Arctic differ in their N uptake strategies, with graminoids profiting from nutrients released at the thaw front, while shrubs mainly forage in upper soil layers. Our results suggest that tundra vegetation will become graminoid-dominated as permafrost thaw progresses and nutrient availability increases in the deep soil. K E Y W O R D S 15 N, Arctic tundra, dwarf shrubs, graminoids, niche differentiation, nutrient uptake, plant functional types, rooting depth, soil depth | 35 Journal of Vegetation Science WANG et Al. SUPPORTING INFORMATIONAdditional Supporting Information may be found online in the supporting information tab for this article. Appendix S1 Overview of species presence in each plotHow to cite this article: Wang P, Limpens J, Nauta A, et al.Depth-based differentiation in nitrogen uptake between graminoids and shrubs in an Arctic tundra plant community.