Data from: Limited stand expansion by a long-lived conifer at a leading northern range edge, despite available habitat

1. In an era of rapid climate change, understanding the natural capacity of species’ ranges to track shifting climatic niches is a critical research and conservation need. Because species do not move across the landscape through empty space, but instead have to migrate through existing biotic communities, basic dispersal ecology and biotic interactions are important considerations beyond simple climate niche tracking.
2. Yellow-cedar (Callitropsis nootkatensis), a long-lived conifer of the North Pacific coastal temperate rainforest region, is thought to be undergoing a continued natural range expansion in southeast Alaska. At the same time, yellow-cedar’s trailing edge is approaching its leading edge in the region, due to climate-induced root injury leading to widespread mortality over the past century. To examine the current dispersal capacity of yellow-cedar at its leading range edge, and potential for the species’ leading edge to stay ahead of its trailing edge, we characterized recent yellow-cedar stand development near Juneau, Alaska, and surveyed the spread of yellow-cedar seedlings just beyond existing stand boundaries.
3. Despite suitable habitat beyond stand edges, stand expansion appears limited in recent decades to centuries. Large quantities of seed are germinating within stands and just beyond boundaries, but seedlings are not developing to maturity. Furthermore, ~100-200-year-old yellow-cedar trees are located abruptly at stand boundaries, indicating stand expansion is in a period of stasis with a last pulse at the end of the Little Ice Age climate period.
4. Vegetative regeneration is common across stands and may be an adaptive strategy for this long-lived tree to persist on the landscape until conditions are favorable for successful seedling recruitment, leading to an overall punctuated migration and colonization of new landscapes.
5. Synthesis. Species ranges do not always respond linearly to shifting climatic conditions. Instead, successful colonization of new habitat may be tied to episodic, threshold-related landscape phenomena, dispersal ability, and competition with existing plant communities.

1. 在气候快速变化的时代，明晰物种分布区追踪变动气候生态位（climatic niche）的自然能力，是兼具重要研究价值与保护意义的核心需求。由于物种并非穿越空旷空间完成扩散，而是必须依托现存生物群落进行迁移，因此除单纯的气候生态位追踪外，基础扩散生态学（dispersal ecology）与生物相互作用（biotic interactions）亦是不容忽视的关键考量因素。
2. 黄扁柏（Yellow-cedar, *Callitropsis nootkatensis*）是北太平洋沿岸温带雨林区域的长寿针叶树种，学界普遍认为其目前在阿拉斯加东南部持续开展自然分布区扩张。与此同时，受过去一个世纪以来气候诱导的根系损伤引发大范围死亡的影响，该区域内黄扁柏的分布后缘（trailing edge）正逐渐迫近分布前缘（leading range edge）。为探究黄扁柏在分布前缘的当前扩散能力，以及该物种分布前缘能否持续领先于分布后缘的可能性，我们对阿拉斯加朱诺附近近期形成的黄扁柏林分（stand）开展了特征刻画，并针对现有林分边界外侧的黄扁柏幼苗扩散情况进行了调查。
3. 尽管林分边界外侧存在适宜栖息地，但近数十至数百年间，林分扩张似乎已陷入停滞。林分内部及边界外侧均有大量种子萌发，但幼苗无法发育至成熟阶段。此外，林分边界处突兀分布着树龄约100至200年的黄扁柏个体，这表明林分扩张正处于停滞期，最后一次扩张脉冲发生于小冰期（Little Ice Age）气候时期结束之际。
4. 营养繁殖（vegetative regeneration）在各林分中均较为常见，这或是该长寿针叶树的适应性生存策略，使其能够在景观中持续存续，直至出现利于幼苗成功定植（seedling recruitment）的环境条件，最终整体呈现出间断式迁移（punctuated migration）与新景观定植的动态模式。
5. 综合分析表明，物种分布区并非始终对气候变化呈线性响应。与之相反，新栖息地的成功定植可能与阶段性、阈值相关的景观现象、扩散能力以及与现存植物群落的种间竞争密切相关。