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.北方生态系统（Boreal ecosystems）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速气候变暖。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覆盖度，这将对北方泥炭沼泽的结构与功能、碳固存及气候产生深远影响。本研究在北方泥炭沼泽中开展了两项野外实验，旨在阐明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。研究人员在水位与木本覆盖度存在差异的微生境中种植了欧洲赤松（Pinus sylvestris）的种子与幼苗，并对灌木冠层与根系竞争进行了操控处理。本研究对幼苗的萌发、生长与存活情况进行了长达4个生长季的监测，并分析了幼苗响应与非生物及生物环境条件的关联。研究发现，在地势较干燥、水位较深的微生境中，乔木定植成功率更高。在这些泥炭丘中，灌木对乔木幼苗定植同时存在正向与负向调控作用。在最温暖的生长季中，灌木覆盖度的提升可改善乔木幼苗的健康状况、促进其生长并提高存活率。反之，乔木基径面积更高的区域，其土壤养分有效性、灌木生物量以及乔木幼株丰度均呈正相关关系。综合讨论：本研究结果表明，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。此类促进作用在温度更高的环境中更为显著，这意味着暖干夏季频率的提升可能强化灌木与乔木之间的正相互作用，最终推动生态系统从以苔藓为主导向以乔木为主导转变。