Climate change is shifting forest tree species distributions across elevational and latitudinal gradients, and these changes are often pronounced at ecotones where species meet their climatic bounds and are replaced by other species. Using an extensive ecotone composed of lower-montane white fir (Abies concolor var. lowiana) and upper-montane red fir (Abies magnifica var. magnifica) in the central Sierra Nevada range of California, USA, we (1) examined how the demographics of the ecotone have responded to recent climate using a field observational study and a historical dataset, (2) quantified climate drivers across species life stages using contemporary demographic data, and (3) tested the potential impacts of future climate on species-specific seedling survival and growth in a fully factorial growth chamber experiment that varied temperature, growing season length, and water availability. A re-examination of the ecotone midpoint after 35 yr suggested a reduction in A. concolor sapling and tree densities and a rise in A. magnifica proportional dominance between surveys. Seedling abundances across the ecotone indicated that A. magnifica tends to dominate the regeneration layer and currently forms an important component of the seedling community at elevations below those where A. magnifica saplings or trees begin to co-dominate stands. Observational and experimental assessments suggest that temperature and precipitation serve as important drivers, differentiating A. concolor vs. A. magnifica distributions, and are primary stressors at the seedling stage. Seedlings of both species were adversely affected by experimental climate treatments, although A. concolor exhibited greater survival and a more conservative growth strategy under extreme climatic stress than A. magnifica. Projections indicate that historical climate conditions will rise by an amount greater than the ecotone’s current elevational extent by the end of the 21st century. Differential drivers of species abundances suggest that the projected climate will expand conditions that promote A. concolor abundance and impede A. magnifica abundance across the ecotone; however, disturbance activity and microclimatic conditions will also influence regeneration and overstory tree dynamics. Our study demonstrates the importance of quantifying species-specific responses to climate and indicates that widespread regeneration failure may be one possible consequence in which species exhibit strong sensitivity to projected climate conditions.

中文翻译：

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持久但脆弱：对冷杉生态系统的重新调查显示差异不大，但易受气候变化影响

气候变化正在改变海拔和纬度梯度上的森林树种分布，这些变化通常在物种满足其气候界限并被其他物种取代的交错带中表现得尤为明显。使用由下山白冷杉 ( Abies concolor var. lowiana ) 和上山红杉 ( Abies magnifica var. magnifica )组成的广泛交错带) 在美国加利福尼亚州的内华达山脉中部，我们 (1) 使用实地观察研究和历史数据集研究了交错带的人口统计学如何对最近的气候做出反应，(2) 使用当代技术量化跨物种生命阶段的气候驱动因素人口统计数据，以及 (3) 在一个全因子生长室实验中测试了未来气候对特定物种幼苗存活和生长的潜在影响，该实验改变了温度、生长季节长度和可用水量。35 年后对交错带中点的重新检查表明A. concolor树苗和树木密度的减少以及A. magnifica调查之间比例优势的增加。整个交错带的幼苗丰度表明A. magnificaA. magnifica树苗或树木开始共同占主导地位的海拔以下的海拔处，它们往往在再生层中占主导地位，并且目前形成了幼苗群落的重要组成部分。观察和实验评估表明，温度和降水是区分A. concolor与A. magnifica分布的重要驱动因素，并且是幼苗阶段的主要压力因素。尽管A. concolor在极端气候压力下比A. magnifica表现出更高的存活率和更保守的生长策略，但两种物种的幼苗都受到了实验性气候处理的不利影响. 预测表明，到 21 世纪末，历史气候条件的上升幅度将超过该交错带目前的海拔范围。物种丰度的差异驱动因素表明，预计的气候将扩大促进A. concolor丰度并阻碍A. magnifica丰度的条件在整个交错带；然而，干扰活动和小气候条件也会影响再生和覆盖层树的动态。我们的研究证明了量化物种对气候的特定反应的重要性，并表明广泛的再生失败可能是物种对预测气候条件表现出强烈敏感性的一种可能后果。