Altitudinal Patterns of Species Richness of Seed Plants in Qingling Mountains: A Test of Rapoport’s Rule

Understanding patterns of species richness along an altitudinal gradient is one of the central focuses of ecological research and can offer us insights into the mechanisms of species diversity. However, few studies have focused on a large scale. Based on the distribution data of 3538 species collected from the local flora of the entire Qinling Mountains, the patterns of seed plant richness along the altitude were analyzed; three hypotheses proposed to explain the pattern were tested, and five methods verifying the applicability of Rapoport’s rule were examined. The results of the research were as follows: species richness of seed plants along an altitudinal gradient in the Qinling Mountains displayed a single peak, occurring at 1000–1500 m (R2 = 0.67, p < 0.001), and similar patterns were also observed at the genus and family level; The proportion and density of endemic species were increased monotonously with altitude; the altitudinal patterns of species richness in the Qinling Mountains were not fully explained by a single hypothesis such as species area relationship (SAR), mid-domain effect (MDE) and ecological metabolism theory (MTE), while the interpretation amount of SAR and MDE was 80.4%, indicating that area and mid-domain effect significantly affect the patterns of plant species richness in the Qinling Mountains; only the Pagel upper-bound method supported Rapoport’s rule. However, all species were grouped according to their altitude distribution; most of results of the species-by-species method, Rohde midpoint method, and Stevens method conformed to Rapoport’s rule. The test of Rapoport’s rule is obviously affected by the test method, so more advanced and universal methods are expected to be developed in future.