Whitebark pine (Pinus albicaulis) (PIAL) is a proposed threatened species that plays a keystone ecological role in the Greater Yellowstone Ecosystem (GYE). Its population response to climate change is of high interest to managers because climate-induced declines may adversely affect critical ecosystem services that this species provides. While previous studies of reproductive size classes of the species have projected dramatic reductions in area of suitable habitat under climate warming scenarios, it has been suggested that the species can tolerate warmer and drier conditions if seedlings and saplings are not competitively excluded by other conifer species. Thus, we asked if juvenile-sized PIAL are found in warmer and drier locations than larger individuals, under the assumption that competitive exclusion would require several years to decades to influence the distribution of regenerating PIAL. We used a new genetic technique to distinguish non-cone bearing PIAL from the more warm-dry tolerant limber pine (P. flexilis) among samples collected along transects extending from lower treeline to the subalpine around the GYE. Predictor data on climate and water balance were obtained from a 250-m spatially explicit data product. We used a stochastic gradient boosting model to predict probability of presence of PIAL < 1 cm dbh (diameter at breast height) and >=1 cm dbh as a function of these predictors. We discovered that smaller diameter PIAL were not proportionally more abundant at lower elevations, suggesting that competitive exclusion may not be the primary mechanism limiting this species' low elevation distribution. In contrast, the small size class PIAL were slightly less warm-dry tolerant than larger individuals. This suggests that the zone of regeneration of PIAL has shifted upwards in elevation in recent decades, perhaps associated with the observed warming in the GYE. In comparison to a previous study of reproductive-sized trees (>20 cm dbh) from a coarser (1.6 km) sampling frame, however, the predicted zone of suitable habitat of PIAL (<1 cm dbh) was 122 m lower in elevation. We conclude that consideration of the fine-scale distribution of PIAL near lower treeline suggests that the tree species is slightly less sensitive to climate warming than found by previous studies of reproductive-sized trees, but, nonetheless, large range contractions of PIAL in GYE are likely under projected future climates.

白皮松（Pinus albicaulis（PIAL）是拟议中的受威胁物种，在大黄石生态系统（GYE）中起着关键的生态作用。它的种群对气候变化的反应引起管理人员的高度关注，因为气候导致的衰退可能会对该物种提供的关键生态系统服务产生不利影响。虽然先前有关该物种生殖大小等级的研究预测，在气候变暖的情况下，合适栖息地的面积将大大减少，但有人提出，如果幼苗和幼树不能被其他针叶树种竞争性地排斥，则该物种可以忍受变暖和干燥的条件。因此，我们问是否在比较大的人更温暖和更干燥的地方发现了少年大小的PIAL，假设竞争排斥将需要数年至数十年才能影响再生PIAL的分布。我们使用了一种新的遗传技术来将非锥状PIAL与更温干燥的耐弯松（P.弹性）在沿样条线收集的样本中，这些样条线从下林线延伸到GYE周围的亚高山。气候和水平衡的预测数据是从250米的空间明确数据产品中获得的。我们使用随机梯度增强模型来预测PIAL <1 cm dbh（乳房高度处的直径）和> = 1 cm dbh的可能性，作为这些预测器的函数。我们发现较小直径的PIAL在较低的海拔高度上并没有成比例地增加，这表明竞争排斥可能不是限制该物种低海拔高度分布的主要机制。相比之下，小尺寸PIAL的耐干热性稍差于较大的个人。这表明，最近几十年来，PIAL的再生区已经向上抬升，可能与观测到的GYE变暖有关。与先前从较粗（1.6 km）的采样框架中繁殖树（> 20 cm dbh）的研究相比，PIAL适宜生境（<1 cm dbh）的预测区域海拔低了122 m。我们得出的结论是，考虑到PIAL的小尺度分布在较低的树线附近，这表明该树种对气候变暖的敏感性比以前对繁殖树种的研究发现的要小，但是，尽管如此，在GYE中PIAL的大范围收缩还是可能在预计的未来气候下。