Data from: Wind exposure and light exposure, more than elevation-related temperature, limit tree line seedling abundance on three continents

The transition from seedlings into trees at alpine treelines is a temperature-limited process that ultimately sets the treeline elevation at a global scale. As such, treelines may be key bioassays of global warming effects on species distributions. For global warming to promote upward treeline migration, as predicted, seedlings must be available. We examined, for the first time at a global scale, elevational patterns and drivers of seedling availability at treelines. Working at 10 sites across five mountain regions, (dry Andes, humid Andes, Patagonian Andes, Swiss Alps, and US Rocky Mountains) with different treeline forms (abrupt and diffuse) and dominated by different tree species (broadleaves and conifers), we answered the following question: how is seedling abundance affected by elevation (as a coarse grain-surrogate of temperature), light exposure (openness immediately above plots) or wind exposure (an index for openness in the horizontal direction), or combinations thereof and, what is the relative importance of each factor? We tested five biological hypotheses to determine the relative strength of these treeline drivers on variable-size sampling plots of seedling abundance (S) (N = 1056). Specifically, we tested likely combinations of temperature limitation (T), light as a resource (light, L) and as a radiation stress (via high light at low temperature, R), wind exposure as a treeline stressor (W) and treeline form (a coarse scale test: abrupt vs. diffuse, D). We found strong, moderate, and weak negative effects of our estimates of wind exposure, radiation stress, and elevation-related temperature on seedling abundance, respectively. We also found a positive effect, at treeline, for site-level treeline diffuseness. Two distinct facilitation mechanisms likely improved seedling abundance at treeline elevation: wind blockage by neighbourhood trees (the sheltering effect) and partial shading by overhead trees. Synthesis. Seedling abundance at alpine treelines is limited by multiple simultaneous factors with the temperature-decrease with elevation playing a relatively minor role. We therefore note that if the temperature threshold limiting the conversion from seedlings to adult trees is relaxed because of global warming, upward treeline migration will depend on the availability of shelter sites for seedlings.高山林线（alpine treelines）处幼苗向成树的转化是受温度限制的过程，该过程最终决定了全球尺度下林线的海拔高度。据此，林线可作为全球变暖对物种分布影响的关键生物测定指标（bioassay）。正如预测所示，若要全球变暖推动林线向上迁移，前提是幼苗种群充足。本研究首次在全球尺度下，探究了林线处幼苗充足性的海拔分布格局及其驱动因子。 本研究在五个山地区域——干旱安第斯山脉、湿润安第斯山脉、巴塔哥尼亚安第斯山脉、瑞士阿尔卑斯山脉以及美国落基山脉——的10个样点开展工作，这些样点涵盖不同林线形态（陡变型与弥散型）且以不同乔木类群（阔叶树与针叶树）为优势种，旨在解答以下问题：幼苗丰度如何受海拔（作为温度的粗尺度替代指标）、光照暴露度（样地上方的开阔度）、风暴露度（水平方向开阔度的指标）及其组合的影响？各因子的相对重要性如何？ 本研究在1056个不同面积的幼苗丰度（S）样地中，检验了5个生物学假说，以明确上述林线驱动因子的相对作用强度。具体而言，我们检验了以下因子组合的效应：温度限制（T）、作为资源的光照（光照，L）、低温高光辐射引发的辐射胁迫（R）、作为林线胁迫因子的风暴露度（W）以及林线形态（粗尺度检验：陡变型与弥散型，D）。 研究结果显示：风暴露度、辐射胁迫以及海拔相关温度对幼苗丰度分别存在显著、中等程度以及微弱的负向影响。同时，本研究发现林线处的样地水平林线弥散度对幼苗丰度存在正向影响。两种独立的促进机制可能提升了林线海拔处的幼苗丰度：邻体树木的防风阻截（庇护效应）以及冠层树木的部分遮阴。 综合分析：高山林线处的幼苗丰度受多种协同因子共同限制，其中海拔升高导致的温度降低所起的作用相对较弱。据此我们认为，若全球变暖缓解了限制幼苗向成树转化的温度阈值，那么林线的向上迁移将取决于幼苗庇护生境的可获得性。