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）处幼苗向乔木的转变是一个受温度限制的过程，该过程最终在全球尺度上决定了林线的海拔高度。就此而言，林线或可成为全球变暖对物种分布影响的关键生物测定指标（bioassays）。若要实现学界预测的林线向上迁移，必须存在充足的幼苗种群。我们首次在全球尺度上，探究了林线处幼苗可获得性的海拔分布格局及其驱动因子。 我们在覆盖五大山地区域的10个样点开展研究，包括干旱安第斯山脉、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，这些样点具有不同的林线形态（急变型（abrupt）与扩散型（diffuse）），且由不同的乔木类群主导，涵盖阔叶树与针叶树。我们旨在解答如下问题：幼苗丰度如何受海拔（作为温度的粗粒度替代指标）、光照暴露度（样地上方的开阔程度）或风力暴露度（水平方向开阔度的量化指数），或是这些因子的组合影响？各因子的相对重要性又如何？ 我们针对1056个不同大小的幼苗丰度（S）采样样地（样本量N=1056），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，我们检验了以下驱动因子的可能组合：温度限制（T）、作为资源的光照（光照L）与作为辐射胁迫的光照（低温下的高光辐射R）、作为林线胁迫因子的风力暴露度（W），以及林线形态（粗尺度检验：急变型vs扩散型D）。 研究结果显示，我们评估的风力暴露度、辐射胁迫以及海拔相关温度，分别对幼苗丰度产生强烈、中等和微弱的负向影响。此外，在林线生境中，样地所在林线的扩散程度对幼苗丰度具有正向调控作用。有两种截然不同的促进机制可能提升了林线海拔处的幼苗丰度：邻体树木的挡风庇护效应，以及上方树木的部分遮阴作用。 综合分析表明，高山林线处的幼苗丰度受多种同时存在的因子共同限制，其中随海拔升高而降低的温度仅发挥相对次要的作用。据此我们指出，若全球变暖缓解了限制幼苗向成年乔木转化的温度阈值，那么林线的向上迁移将取决于幼苗是否能获得充足的庇护位点。