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，要求种群补充量超出当前分布边界。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监测数据与空间显式随机种群模型（spatially explicit stochastic population models），探讨了气候控制实验中观测到的种群补充气候敏感性，是否足以改变两种广泛分布的北美高海拔针叶树的种群动态与海拔分布范围。
3. 观测到的经验数据显示，气候变暖驱动的种群补充量下降，会导致亚高山森林低海拔“暖边界”处的模拟种群快速衰退，而高海拔“冷边界”以外区域的种群定植则进展缓慢。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，两种针叶树均出现了分布范围收缩。其中恩氏云杉（Engelmann spruce）的范围收缩在模拟时间范围内呈永久性，即使在水分增加的条件下亦是如此。而柔枝松（limber pine）对气候变暖的敏感性较低，这使其能够在低海拔区域持续存活——尤其在水分增加的条件下——并能在林木线（treeline）以上区域快速完成定植，最终实现向高山生境的扩张。
4. 综合分析。假设21世纪持续气候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出现暂时性分布范围收缩，而恩氏云杉则可能发生永久性分布范围收缩。由此可见，气候变暖下的幼苗补充限制，会制约亚高山树木的分布范围迁移速率。