Treeline, the ecotone where forest transitions to alpine or tundra ecosystems, is considered the thermal limit to tree growth and survival. Despite temperature increases across mountainous areas and high latitudes globally, there has been no ubiquitous change in treeline position. The process of range expansion must initially depend on increased recruitment at, or beyond current range limits and recruitment limitations have been hypothesized as a mechanism for the variable response of treeline position to climate warming. We conducted a unique series of observational and experimental studies to quantify early‐life stage constraints, from seed production to seedling establishment, on black spruce Picea mariana and tamarack Larix laricina recruitment at a model alpine treeline in Newfoundland, Canada. We found recruitment at treeline to be simultaneously seed and establishment limited. The treeline population produced fewer seeds than the forest population and black spruce seeds produced at treeline were less viable. Tamarack was more seed limited than black spruce where seed viability was low regardless of altitudinal position. Post‐dispersal seed predation greatly constrained recruitment across the altitudinal gradient; however, black spruce seeds experienced the lowest levels of invertebrate seed predation on the lichen mat at treeline. If seeds were not consumed, individuals at treeline were establishment limited where germination and seedling establishment was both less abundant and delayed on lichen substrate. Our study highlights the need for multiple factors to align temporally for significant recruitment at treeline to occur.

中文翻译：

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生物过滤对高山树线北针叶树募集的重要性

林线是森林过渡到高山或苔原生态系统的过渡带，被认为是树木生长和生存的热量限制。尽管全球山区和高纬度地区的气温升高，但林线位置并没有普遍变化。范围扩大的过程必须首先取决于达到或超出当前范围限制的增加招募，并且已经假设招募限制是树线位置对气候变暖的可变响应的机制。我们进行了一系列独特的观察和实验研究，以定量分析黑云杉Picea mariana和tamarack Larix laricina从种子生产到幼苗建立的早期阶段限制在加拿大纽芬兰的模型高山林线进行招聘。我们发现，在树线的招聘同时受到种子和机构的限制。林木种群产生的种子少于森林种群，在林木种群产生的黑云杉种子的生存能力较差。与黑云杉相比，Tamarack的种子限制更大，而黑云杉的种子活力无论高度如何都很低。分散后的种子捕食极大地限制了整个海拔梯度上的募集。但是，黑云杉种子在树线的地衣上的捕食水平最低。如果不消耗种子，则树的个体受到限制，其中发芽和幼苗的形成既不那么丰富，又在地衣基质上延迟。