The “Tanaka Line” and its application as a biogeographic floristic line

Japanese scholar Tanaka imagined a straight line starting from the crossing point at 28°N, 98°E in northwest Yunnan running diagonally southward to 18°45'N or 19°N, 108°E, which was suggested as the "Tanaka Line of Citrus distribution" ("Tanaka Line" (TL)). Later, Chinese scholars considered the TL to have biogeographical significance in distinguishing Sino-Japanese and Sino-Himalayan genera, and combined with the distribution of some orchid genera, proposed the Tanaka-Kaiyong floristic line (TKL) between the Sino-Japanese and Sino-Himalayan floristic sub-kingdoms of the East Asian floristic kingdom. Several studies have revealed that the TKL is meaningful in regard to the phylogeography, genetic isolation, and population differentiation of certain plant species, but mainly due to its role as a climatically-and topographically-driven barrier to present-day plant dispersal, not as a classical vicariant-biogeographic boundary confined to a geo-block. However, the KL is not supported by phylogenetic studies on Citrus or related genera. Furthermore, floristic divergence and regionalization do not correspond with the TL in Yunnan. Obviously, climate, topography, and geological history have had comprehensive effects on biogeographical divergence in Yunnan. Thus, as a biogeographical boundary, the TL is inconsistent with recent studies.