Latitudinal differentiation and patterns of temperate and subtropical plants in the Qinling-Daba Mountains

Geographically, the Qinling-Daba Mountains serve as the main body of the north-south transitional zone of China. However, the transitional patterns of their plant species still need to be clarified. This study analyzed latitudinal variations of plant species richness, relative importance values (RIV), and plant species abundance based on plant community field survey data for 163 sample sites along three north-south transect lines in the eastern, middle, and western parts of the study areas. The difference in RIV between subtropical and temperate species (SND-RIV) was selected to reveal the latitudinal interlacing pattern of northern and southern plant species. Along the eastern (Sanmenxia-Yichang), middle (Xi'an-Dazhou), and western (Tianshui-Guangyuan) transects, the richness and RIV of subtropical plant species increased while those of temperate plant species decreased from north to south. In the eastern transect, temperate plant species richness and RIV were the highest at Shennongjia and Funiu Mountain, respectively, because of their high elevations. In the middle transect, subtropical plant species richness and RIV were the highest in the Daba Mountains. In the western transect, richness and RIV were higher for subtropical than temperate plant species from the south of Longnan. The crisscrossing areas of northern and southern plant species were similar to 180 km, similar to 100 km, and similar to 60 km wide for the eastern, middle, and western transects, respectively, showing a narrowing trend from east to west. For the eastern and western transects, decreases in subtropical plant species distribution from south to north could be attributed to a decrease in mean annual precipitation in the same direction. However, for the middle transect, mean annual temperature had a slightly greater influence on plant species' latitudinal distribution than the moisture index. This study provides a more solid scientific basis for future investigations of this key geographical boundary in China.