Data from: Positive shrub-tree interactions facilitate woody encroachment in boreal peatlands

Boreal ecosystems are warming roughly twice as fast as the global average, resulting in woody expansion that could further speed up the climate warming. Boreal peatbogs are waterlogged systems that store more than 30% of the global soil carbon. Facilitative effects of shrubs and trees on the establishment of new individuals could increase tree cover with profound consequences for the structure and functioning of boreal peatbogs, carbon sequestration and climate. We conducted two field experiments in boreal peatbogs to assess the mechanisms that explain tree seedling recruitment and to estimate the strength of positive feedbacks between shrubs and trees. We planted seeds and seedlings of Pinus sylvestris in microsites with contrasting water-tables and woody cover and manipulated both shrub canopy and root competition. We monitored seedling emergence, growth and survival for up to four growing seasons and assessed how seedling responses related to abiotic and biotic conditions. We found that tree recruitment is more successful in drier topographical microsites with deeper water-tables. On these hummocks, shrubs have both positive and negative effects on tree seedling establishment. Shrub cover improved tree seedling condition, growth and survival during the warmest growing season. In turn, higher tree basal area correlates positively with soil nutrient availability, shrub biomass and abundance of tree juveniles. Synthesis. Our results suggest that shrubs facilitate tree colonization of peatbogs which further increases shrub growth. These facilitative effects seem to be stronger under warmer conditions suggesting that a higher frequency of warmer and dry summers may lead to stronger positive interactions between shrubs and trees that could eventually facilitate a shift from moss to tree-dominated systems.北方针叶林生态系统的升温速率约为全球平均水平的两倍，由此引发的木本植物扩张可能进一步加剧气候变暖。北方泥炭沼泽（boreal peatbogs）是积水型生态系统，其储存的土壤碳占全球土壤碳总量的30%以上。灌木与乔木对新个体定植的促进作用，可能提升乔木盖度，进而对北方泥炭沼泽的结构与功能、碳固存过程及全球气候产生深远影响。 本研究在北方泥炭沼泽中开展了两项野外实验，旨在解析乔木幼苗定植的潜在机制，并量化灌木与乔木之间的正反馈强度。我们将欧洲赤松（Pinus sylvestris）的种子与幼苗移栽至地下水位与木本盖度存在显著差异的微生境中，并对灌木冠层及根系竞争进行了人为操控。实验持续监测了长达4个生长季的幼苗出苗、生长与存活情况，并分析了幼苗响应与非生物、生物环境条件之间的关联。 研究结果表明，在地下水位更深、更为干燥的地形微生境中，乔木定植成功率更高。在这类泥炭丘上，灌木对乔木幼苗定植同时存在正负两方面的影响：在最热的生长季，灌木盖度可改善乔木幼苗的健康状况、促进其生长并提升存活率。反之，乔木胸高断面积的增加与土壤养分有效性提升、灌木生物量增加及乔木幼株丰度提升呈显著正相关。 综合分析表明，灌木可促进乔木在泥炭沼泽中的定植，而这一过程又会进一步推动灌木的生长。这类促进效应在温度更高的环境下更为显著，这意味着暖干夏季频率的提升可能会强化灌木与乔木之间的正相互作用，最终促使泥炭沼泽从以苔藓为主的生态系统向以乔木为主的生态系统转变。