Data from: Environmental perturbation, grazing pressure and soil wetness jointly drive mountain tundra toward divergent alternative states

1. Plant communities are structured by complex interactions between multiple factors, which veil our understanding of the effects of environmental changes on communities and ecosystems. Besides the relative role of biotic and abiotic factors as community-structuring processes, addressing how they jointly affect the ecological resilience and resistance of plant communities is crucial to understand better the long-term response of communities facing global changes. 2. Here, we used the results from a long-term (23 years) perturbation experiment set up in Fennoscandian mountain tundra to test these mechanisms. The experiment consisted of a transplantation of twenty blocks of Vaccinium myrtillus heath vegetation including upper soil layer from a lower elevation tundra heath habitat to a snowbed habitat 150 m higher in elevation where V. myrtillus lies at its upper limit. In the snowbed with contrasting levels of soil wetness, half of the transplanted blocks were protected from mammalian herbivores. 3. Our results revealed that in addition to the important role of environmental conditions as a structuring force, the joint effects of multiple drivers resulted in divergent patterns in both plant functional composition and species diversity among transplanted communities. Under environmental perturbation (i.e. transplantation to snowbed), the heath vegetation was altered by grazing pressure that reduced the cover of shrubs (especially V. myrtillus). In grazed dry snowbed, a species rich community with high functional type evenness and diversity developed. Reversely, in dry exclosures, V. myrtillus gained high dominance associated with only few graminoids and forbs. In wet snowbed conditions, shrubs tended to decline both in grazed plots and exclosures whereas bryophytes attained high abundance. Grazing promoted species richness while soil waterlogging tended to promote among-plot heterogeneity (β-diversity) which was highest in wet exclosures. 4. Synthesis Our long-term experiment reveals that environmental perturbation, grazing and soil wetness exhibit joint effects that induce divergent trajectories of tundra plant communities. We suggest that a strong environmental perturbation triggers mountain tundra heath community to move away from its equilibrium state. The outcome of this shift depends on the interplay between grazing pressure and soil wetness that drive tundra plant communities toward divergent alternative states.1. 植物群落的组装由多重因素间的复杂相互作用所调控，这一过程遮蔽了我们对环境变化如何作用于群落与生态系统的认知。除明确生物因子与非生物因子在群落组装过程中的相对作用外，阐明二者如何共同影响植物群落的生态恢复力与抵抗性，对于更好地理解全球变化背景下群落的长期响应机制至关重要。2. 本研究依托在芬诺斯坎底亚山地冻原开展的为期23年的干扰实验结果，对上述机制进行验证。该实验将20个包含表层土壤的越橘（Vaccinium myrtillus）灌丛植被样方，从低海拔的冻原灌丛生境移植至海拔高出150米的积雪滞留生境——该生境恰好是越橘分布的海拔上限。在土壤湿度梯度差异显著的积雪滞留生境中，半数移植样方被设置为哺乳动物植食者排除组，以隔绝植食作用的影响。3. 研究结果表明，除环境条件作为群落组装核心调控因子的重要作用外，多重驱动因子的共同作用使得移植群落的植物功能组成与物种多样性均呈现分化格局。在环境干扰（即移植至积雪滞留生境）条件下，放牧压力改变了原有灌丛植被，显著降低了灌木（尤其是越橘）的盖度。在受放牧影响的干燥积雪生境中，形成了物种丰富度较高、功能类群均匀度与多样性俱佳的群落。与之相反，在干燥的植食者排除样方中，越橘占据绝对优势，伴生物种仅包含少量禾草类与非禾本草本植物。在湿润的积雪生境条件下，无论放牧样方还是排除样方中的灌木均呈现衰退趋势，而苔藓植物的盖度则显著升高。放牧可提升群落的物种丰富度，而土壤渍水则倾向于提升样方间的异质性（β多样性，β-diversity），该异质性在湿润的排除样方中达到最高。4. 研究总结：本项长期实验表明，环境干扰、放牧与土壤湿度三者的共同作用，驱动山地冻原灌丛植物群落呈现出分化的演替轨迹。我们认为，强烈的环境干扰会促使山地冻原灌丛群落脱离其原有平衡状态，而这一转变的最终结果取决于放牧压力与土壤湿度之间的相互作用——二者共同推动冻原植物群落走向不同的替代稳态。