Data from: Alternative tree-cover states of the boreal ecosystem: a conceptual model

Aim: Previous analyses of remotely-sensed data detected the multimodality of the tree-cover distribution of the boreal forest, and identified areas with potentially alternative tree-cover states. This paper aims at investigating the causes of multimodality and multistability of the boreal forest, their influence on the asymmetric tree-species distribution between Eurasia and North America, and whether multistability could be associated with recent greening trends in LAI/NDVI. Location: Eurasian and North American boreal forests. Time period: 2000--2010. Major taxa studied: Boreal forest plant functional types. Methods: We employ a conceptual model based on tree-species competition to simulate the sensitivity of tree cover to stochastic disturbances and to changes in environmental factors. We include different plant functional types based on survival adaptations, and force the model with remotely-sensed environmental data. We analyse the model as a dynamical system. We use metrics from statistics and information theory to compare the detection of alternative tree-cover states and greening trends in LAI/NDVI. Results: We find that multimodality and multistability can emerge through competition between different plant functional types. Additionally, our model is able to reproduce the asymmetry in tree-species distribution between Eurasia and North America. Moreover, changes in permafrost distribution can be associated with phenomenological bifurcation points of the model. Finally, we find that the detection of multistable areas is not affected by recent vegetation trends, whereas shifts between alternative states could have affected the greening trends. Main conclusions: Tree-cover multistability in the boreal region can emerge through competition between species subject to periodic disturbances. Changes in permafrost thaw and distribution could be responsible for the asymmetry in tree-species distribution between North America and Eurasia. Climate change and permafrost degradation could cause shifts in tree-cover states and dominant species. Recent vegetation greening trends in multistable areas could have been affected by shifts between alternative states.

研究目的：过往基于遥感数据的相关分析已探明北方针叶林（boreal forest）林木覆盖分布的多模态性，并识别出存在潜在替代林木覆盖状态的区域。本文旨在探究北方针叶林多模态性与多稳定性的成因、其对欧亚大陆与北美之间林木物种分布不对称性的影响，以及多稳定性是否与叶面积指数（Leaf Area Index, LAI）/归一化植被指数（Normalized Difference Vegetation Index, NDVI）的近期绿化趋势存在关联。
研究区域：欧亚大陆与北美北方针叶林。
时间范围：2000—2010年。
研究类群：北方针叶林植物功能型（plant functional types）。
研究方法：本文采用基于树种竞争的概念模型，模拟林木覆盖对随机干扰及环境因子变化的敏感性。模型纳入了基于生存适应性划分的多种植物功能型，并以遥感环境数据驱动模型运行；同时将模型作为动力系统进行分析，并借助统计学与信息论指标，对比分析替代林木覆盖状态的检测结果与LAI/NDVI的绿化趋势。
研究结果：结果表明，不同植物功能型之间的种间竞争可产生多模态性与多稳定性。此外，本模型能够复现欧亚大陆与北美之间林木物种分布的不对称性。进一步分析发现，多年冻土分布的变化可与模型的现象学分岔点建立关联。最后，研究发现多稳定区域的检测结果不受近期植被趋势的影响，而不同替代状态间的转换则可能对绿化趋势产生影响。
主要结论：北方针叶林区域的林木覆盖多稳定性，可通过受周期性干扰的物种种间竞争产生。多年冻土融化与分布的变化，可能是造成欧亚大陆与北美之间林木物种分布不对称性的原因。气候变化与多年冻土退化，可能引发林木覆盖状态与优势物种的转变。多稳定区域的近期植被绿化趋势，或受到不同替代状态间转换的影响。