椴树属的地理分布

椴树属Tilia是椴树科一个形态特殊且唯一的北温带分布属，分布于亚洲、欧洲和北美，构成典型的北温带分布格局，三个分离的分布区之间缺乏共有种。本文对各分布区的种类进行重新评价，确认全属25种。其中东亚17种，占68％，包含了现存种类各个演化阶段的类群，是现代分布中心；欧洲-西西伯利亚6种，属于木果组及壳果组；北美2种，均为木果组成员。化石分布与现代地理分布格局基本相似，但分布纬度较现代分布偏北，达到北纬80°附近，且还出现于现今无椴树分布的亚洲大陆腹地，北美西部椴树至第三纪末完全绝迹，而东部到第四纪才有化石记录。根据现代地理分布，结合化石证据、地质历史、气候变迁及形态演化推测，椴树属可能在白垩纪晚期起源于中国东部亚热带山地，至少到始新世之前已散布至欧洲和北美西部。渐新世之后的全球降温和更新世大冰期对椴树属现代地理分布格局的形成起着至关重要的作用。

GEOGRAPHICAL DISTRIBUTION OF TILIA LINN.

Tilia is morphologically well characterized by its unique inflorescence bracts. It is the sole genus of the Tiliaceae in the north temperate, occurring discontinuously in East Asia, Europe-West Siberia and North America, with no common species between these regions. A taxonomical review is made and 25 species are recognized, of which 17 species are in East Asia, six in Europe-West Siberia and two in North America. The northernmost occurrence of Tilia is represented by T, cordata in north Scandinavia Peninsula at 66. 5N, and the southernmost by T. mexicana at 17N in south Mexico. In Europe and East Asia all the species occurring at the northernmost limit belong to Sect. Tilia, and those at the southernmost are members of Sect. Lindnera. An infrageneric system of three sections, i. e. , Sect. Trichophilyra, Sect. Lindnera and Sect Tilia, is followed. A morphological analysis reveals that Sect. Trichophilyra is the most primitive, and Sect. Tilia is the most advanced, and this presumption is also strongly supported by fossil records. According to Takhtajan's regionalization of world flora, East Asia is the richest in both species and sections with 17 species in all the three sections, followed by the Circumboreal region with four species in Sect. Lindnera and two in Sect. Tilia. Accordingly the East Asia region is considered to be the centre of geographical distribution. The fossil distribution is also reviewed. The authentic megafossils of Tilia have been found since Eocene onwards. The earliest megafossils of Tilia are similar to the existing species of Sect. Trichophilyra and are found from Spitzber-gen, west North America and East Asia. The distribution pattern of fossils is similar to that of existing species. Based on phylogenetic analysis and fossil findings, the genus Tilia is considered to have originated in subtropical mountains in East Asia in late Cretaceous, and had extended to north Europe and west North America before Eocene. The temperature decline since Oligocene and the glaciation in the Pleistocene have been dominant factors on the development of the present distribution pattern of Tilia.