Population genetics analyses of larch tree stands in north-central Siberia

Arctic treelines are facing a strong temperature increase as a result of recent global warming, causing possible changes in forest extent, which will alter vegetation-climate feedbacks. However, the mode and strength of the response is rather unclear, as potential changes are happening in areas that are very remote and difficult to access, and empirical data are still largely lacking. Here, we assessed the current population structure and genetic differentiation of Larix Mill. tree stands within the northernmost latitudinal treeline reaching ~72° N in the southern lowlands of the Taymyr Peninsula (~100° E). We sampled 743 individuals belonging to different height classes (seedlings, saplings, trees) at eleven locations along a gradient from 'single tree' tundra over 'forest line' to 'dense forest' stands and conducted investigations applying eight highly polymorphic nuclear microsatellites. Results suggest a high diversity within subpopulations (HE=0.826-0.893), coupled, however, with heterozygote deficits in all subpopulations, but pronounced in 'forest line' stands. Overall, genetic differentiation of subpopulations is low (FST=0.005), indicating a region-wide high gene flow, although 'forest line' stands harbour few rare and private alleles, likely indicating greater local reproduction. 'Single tree' stands, located beyond the northern forest line, are currently not involved in treeline expansion, but show signs of a long-term refuge, namely asexual reproduction and change of growth-form from erect to creeping growth, possibly having persisted for thousands of years. The lack of differentiation between the subpopulations points to a sufficiently high dispersal potential, and thus a rapid northward migration of the Siberian arctic treeline under recent global warming seems potentially unconstrained, but observations show it to be unexpectedly slow.北极林线（Arctic treeline）近期因全球变暖正经历显著升温，可能引发森林范围变化，进而改变植被-气候反馈机制。然而，由于此类潜在变化发生在极为偏远、难以抵达的区域，且目前仍大幅缺乏实测数据，学界对林线响应的模式与强度尚不明晰。 本研究针对泰梅尔半岛（Taymyr Peninsula）南部低地（约100°E）内达到约72°N的最北纬度林线区域的落叶松属（Larix Mill.）林分，评估了其当前的种群结构与遗传分化情况。我们沿从“单株树”苔原、“林线”到“密集森林”林分的梯度，在11个样点采集了743个隶属于不同高度级（幼苗、幼树、成树）的个体，并利用8个高度多态性核微卫星（nuclear microsatellites）开展了相关检测分析。 研究结果显示，各亚种群内部具有较高的遗传多样性（HE=0.826~0.893），但所有亚种群均存在杂合子缺失现象，其中“林线”林分的杂合子缺失尤为显著。整体而言，亚种群间的遗传分化程度较低（FST=0.005），表明区域内存在高强度的基因流；不过“林线”林分仅携带少量稀有等位基因与私有等位基因，这可能暗示其局部繁殖占比更高。位于北部林线之外的“单株树”林分目前未参与林线扩张，但呈现出长期避难所的特征，即无性繁殖以及生长型从直立向匍匐生长的转变，这种状态可能已存续数千年。 亚种群间缺乏遗传分化表明其扩散潜力足够高，因此近期全球变暖背景下西伯利亚北极林线的快速北移似乎本应不受限制，但实际观测结果却显示其迁移速度异常缓慢。