Data from: Declines in low-elevation subalpine tree populations outpace growth in high-elevation populations with warming

1. Species distribution shifts in response to climate change require that recruitment increase beyond current range boundaries. For trees with long lifespans, the importance of climate-sensitive seedling establishment to the pace of range shifts has not been demonstrated quantitatively. 2. Using spatially explicit, stochastic population models combined with data from long-term forest surveys, we explored whether the climate-sensitivity of recruitment observed in climate manipulation experiments was sufficient to alter populations and elevation ranges of two widely distributed, high-elevation North American conifers. 3. Empirically observed, warming-driven declines in recruitment led to rapid modeled population declines at the low-elevation, “warm edge” of subalpine forest and slow emergence of populations beyond the high-elevation, “cool edge”. Because population declines in the forest occurred much faster than population emergence in the alpine, we observed range contraction for both species. For Engelmann spruce, this contraction was permanent over the modeled time horizon, even in the presence of increased moisture. For limber pine, lower sensitivity to warming may facilitate persistence at low elevations – especially in the presence of increased moisture – and rapid establishment above treeline, and, ultimately, expansion into the alpine. 4. Synthesis. Assuming 21st century warming and no additional moisture, population dynamics in high-elevation forests led to transient range contractions for limber pine and potentially permanent range contractions for Engelmann spruce. Thus, limitations to seedling recruitment with warming can constrain the pace of subalpine tree range shifts.1. 气候变化驱动的物种分布范围迁移，要求种群补充（recruitment）量突破当前分布边界。对于寿命较长的树木而言，气候敏感型幼苗建成（seedling establishment）对分布范围迁移速率的重要性尚未得到定量证实。2. 本研究结合长期森林调查数据，采用空间显式（spatially explicit）随机种群模型（stochastic population models），探究了人工气候操控实验（climate manipulation experiments）中观测到的补充量气候敏感性，是否足以改变两种广泛分布的北美高海拔针叶树的种群与海拔分布范围。3. 经验观测显示，由变暖驱动的种群补充量下降，会导致亚高山森林（subalpine forest）低海拔“暖边缘”区域的模拟种群快速衰退，而高海拔“冷边缘”以外区域的种群则缓慢定植出现。由于森林内的种群衰退速率远高于高山生境中的种群定植速率，两个物种的分布范围均出现收缩。对于恩格尔曼云杉（Engelmann spruce）而言，即使在水分供给增加的情景下，其分布范围收缩在模拟时间范围内均为永久性的。而柔枝松（limber pine）对变暖的敏感性较低，这有助于其在低海拔区域存续——尤其是在水分增加的情景下——同时能快速在林木线（treeline）以上区域定植，并最终向高山带扩张。4. 综合分析表明，假设21世纪持续变暖且无额外水分补给，高海拔森林的种群动态将导致柔枝松的分布范围出现暂时性收缩，而恩格尔曼云杉则可能出现永久性的范围收缩。由此可见，变暖背景下幼苗补充的限制，能够制约亚高山树木分布范围迁移的速率。