Data from: Bryophyte traits explain climate-warming effects on tree seedling establishment

1. Above the alpine tree-line, bryophytes cover much of the tundra soil surface in dense, often mono-specific carpets. Therefore, when climate warming enables tree seedling establishment above the tree-line, interaction with the bryophyte layer is inevitable. Bryophytes are known to modify their environment in various ways. However, little is known about to which extend and by which mechanisms bryophytes affect the response of tree seedlings to climate warming.

2. We aimed to assess and understand the importance of bryophyte species identity and traits for tree seedling performance at tree line temperatures and their response to warmer conditions. Seedlings of two common, tree-line-forming tree species (Betula pubescens and Pinus sylvestris) were planted into intact cushions of eight common tundra bryophyte species and bryophyte-free soil and grown for 18 weeks at current (7.0 °C) and near-future (30-50 year) (9.2 °C) tree-line average growing season temperatures. Seedling performance (biomass increase and N-uptake) was measured and related to bryophyte species identity and traits indicative of their impact on the environment.

3. Tree seedlings performed equally well or better in the presence of bryophytes than in bryophyte-free soil, which contrasts to their usually negative effects in milder climates. In addition, seedling performance and their response to higher temperatures depended on bryophyte species and seedlings of both species grew largest in the pan boreal and subarctic bryophyte Hylocomium splendens. However, B. pubescens seedlings showed much stronger responses to higher temperatures when grown in bryophytes than in bryophyte-free soil, while the opposite was true for P. sylvestris seedlings. For B. pubescens, but not for P. sylvestris, available organic nitrogen of the bryophyte species was the trait that best predicted seedling responses to higher temperatures, likely because these seedlings had increased N-demands.

4. Synthesis. Climatically driven changes in bryophyte species distribution may not only have knock-on effects on vascular plant establishment, but temperature effects on seedling performance are themselves moderated by bryophytes in a species-specific way. Bryophyte traits can serve as a useful tool for understanding and predicting these complex interactions.

1. 在高山林线（alpine tree-line）以上区域，苔藓植物（bryophytes）常以密集且多为单优种的垫状覆盖物占据苔原土壤的大部分表面。因此，当气候变暖促使林木幼苗在林线以上成功定植时，与苔藓植物层的相互作用不可避免。已知苔藓植物可通过多种方式改造其生长环境，但目前对苔藓植物在多大程度上、通过何种机制影响林木幼苗对气候变暖的响应仍知之甚少。
2. 本研究旨在评估并阐明苔藓植物物种身份及其功能性状，对林线温度条件下林木幼苗生长表现，以及其对升温环境响应的重要性。我们将两种常见的林线建群树种——白桦（Betula pubescens）和欧洲赤松（Pinus sylvestris）的幼苗，分别定植于8种常见苔原苔藓植物的完整垫状体以及无苔藓植物的土壤中，并分别在当前林线生长季平均温度（7.0℃）和近未来（30-50年后）林线生长季平均温度（9.2℃）下培养18周。随后测定幼苗生长表现（生物量增量与氮吸收量），并将其与苔藓植物的物种身份及指示其环境影响的功能性状相关联。
3. 与无苔藓植物的土壤相比，苔藓植物存在时林木幼苗的生长表现相当甚至更优，这与温和气候下苔藓植物通常产生的负面效应形成对比。此外，幼苗生长表现及其对高温的响应依赖于苔藓植物物种：两种幼苗在泛北方及亚北极分布的苔藓植物Hylocomium splendens中生长量最大。然而，白桦（Betula pubescens）幼苗在苔藓植物环境下对高温的响应远强于在无苔藓土壤中的响应，而欧洲赤松（Pinus sylvestris）幼苗则呈现相反的趋势。对于白桦幼苗而非欧洲赤松幼苗，苔藓植物物种的有效有机氮含量是预测其对高温响应的最佳性状，这可能是因为白桦幼苗的氮需求更高。
4. 综合分析表明，气候驱动的苔藓植物物种分布变化，不仅可能对维管植物定植产生连锁效应，温度对幼苗生长表现的影响本身也会以物种特异性的方式被苔藓植物所调节。苔藓植物功能性状可作为理解和预测这些复杂相互作用的有效工具。