Table 1_Species delimitation in the Populus laurifolia complex (Salicaceae) based on phylogenetic and morphometric evidence.docx

Due to significant morphological differences and extensive interspecific hybridization, there are numerous species complexes with taxonomic challenges in the genus Populus. Integrative taxonomy, which combines evidence of morphology, molecular phylogeny, niche differentiation, and reproductive isolation, provides the most effective approaches for species delimitation. The Populus laurifolia complex, which belongs to Populus subg. Tacamahaca (Salicaceae), is distributed in the Altai Mountains and Tianshan Mountains. This complex exhibits morphological variability, making species delimitation challenging. Due to limited sampling and systematic studies, its taxonomy has remained unresolved. In this study, 337 specimens, along with online digital samples representing nearly all wild populations, were collected. Morphological analyses were performed to evaluate key traits and clarify species boundaries. Phylogenetic relationships were reconstructed using concatenation and coalescent methods based on 566,375 nuclear single-nucleotide polymorphisms (SNPs). Ecological niche differentiation was assessed, and ABBA–BABA analysis was used to examine interspecific hybridization. The results revealed that this complex, based on a series of significant character states, could be morphologically distinguished into three species—P. laurifolia (Populus pilosa considered a synonym of P. laurifolia), Populus talassica, and Populus pamirica—which also correspond to three well-supported clades in the phylogenetic trees. P. pamirica exhibits some degree of ecological niche differentiation from P. talassica and P. laurifolia, whereas the latter two show minimal differentiation. Gene flow within the complex remains limited. This research underscores the importance of integrating multiple lines of evidence in the classification of Populus, providing a framework for future taxonomic studies.

由于显著的形态差异与广泛的种间杂交现象，杨属（Populus）内存在诸多存在分类学难题的物种复合群。整合分类学结合形态学、分子系统发育、生态位分化与生殖隔离等多维度证据，是物种界定的最有效手段。劳氏杨（Populus laurifolia）复合群隶属于杨柳科（Salicaceae）杨属胡杨亚属（subg. Tacamahaca），分布于阿尔泰山脉与天山山脉。该复合群表现出显著的形态变异，导致物种界定难度极大；加之采样与系统学研究较为匮乏，其分类学问题至今尚未解决。本研究共采集337份标本，以及覆盖该类群几乎所有野生种群的在线数字化样本。通过形态学分析评估关键性状以厘清物种边界；基于566375个核单核苷酸多态性（single-nucleotide polymorphisms, SNPs），分别采用串联法与溯祖法重建系统发育关系；开展生态位分化分析，并通过ABBA-BABA分析检测种间杂交事件。研究结果显示，基于一系列显著的性状特征，该复合群可在形态上划分为3个物种：劳氏杨（P. laurifolia，其中毛杨Populus pilosa被视为劳氏杨的异名）、塔拉斯杨（Populus talassica）与帕米尔杨（Populus pamirica）；这三个物种同时对应系统发育树中3个支持度极高的演化支。帕米尔杨与塔拉斯杨、劳氏杨存在一定程度的生态位分化，而后两者之间的分化程度极低。该复合群内部的基因流仍较为有限。本研究强调了整合多维度证据开展杨属分类研究的重要性，为后续分类学研究提供了参考框架。