Data from: Above and belowground responses of four tundra plant functional types to deep soil heating and surface soil fertilization

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 1) accelerated nutrient mobilization in the surface soil layers, and 2) 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 belowground responses of four plant functional types (grasses, sedges, deciduous shrubs and evergreen shrubs). Seasonal thawing was accelerated with heating cables at ~15 cm depth without warming the surface soil, whereas nutrient availability was increased in the surface soil by adding slow-release NPK fertilizer at ~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 aboveground 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 aboveground biomass of the two dwarf shrub functional types, which both had very shallow root systems. Grasses showed the strongest response to fertilization, both above and belowground. 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.

1. 北极地区的气候变暖速率高于全球平均水平。气候变暖将通过两种途径提升苔原（tundra）土壤的养分有效性：一是加速表层土壤层的养分活化，二是增加生长季的解冻深度，从而提升深层土壤层养分的可获取性。这两种过程均可能引发苔原植被组成的转变，因此厘清二者对苔原植物功能型（plant functional types）的影响至关重要。
2. 本研究在东北西伯利亚苔原样地中设置了土壤解冻深度与养分有效性的操控实验，以探究二者对4种植物功能型（禾本科植物、莎草科植物、落叶灌木和常绿灌木）地上与地下响应的影响。实验通过在约15 cm深度铺设加热电缆来加速季节性解冻，且未对表层土壤进行增温；同时通过在约5 cm深度施加缓释氮磷钾（NPK）复合肥来提升表层土壤的养分有效性。此外还设置了两种处理的组合组。本实验是首个专门针对苔原生态系统中加速解冻效应开展的野外控制实验。
3. 深层土壤增温提升了本研究中根系最深的植物功能型——莎草科植物的地上生物量，但对其余三类植物功能型的生物量无显著影响。与之相反，施肥处理提升了两种根系极浅的矮灌木功能型的地上生物量。禾本科植物对施肥的地上与地下响应均最为强烈。禾本科植物为深根型，其垂直根系分布具有最高的可塑性：在深层土壤增温时，其根系分布向深层土壤偏移；而在表层土壤施肥时，根系分布则向表层土壤偏移。
4. 综合分析——本研究结果表明，解冻深度增加仅能使深根型莎草科植物获益，而浅根型矮灌木与根系可塑性较强的禾本科植物则可利用上层土壤提升的养分有效性。研究结果显示，禾本科植物拥有最高的根系可塑性，这使其在快速变化的环境中更具竞争优势。综上，在北极苔原生态系统中，根系垂直分布策略是植被响应气候诱导的土壤养分有效性提升的关键因素；未来植被组成的转变将取决于解冻深度变化与表层土壤养分有效性变化之间的平衡。