*4.2. Characteristics of the B-Diversity Pattern of the Altitudinal Gradient*

Patterns of species turnover are vital to the geography of biodiversity [49]. The Cody index for elevations above 2000 m decreased sharply, indicating that the rate of replacement of plant communities decreased to a relatively low degree. Studies show that areas with lower species richness are more easily invaded by exotic species [50], and thus the plant community structure of a low richness area is more easily disturbed. According to this hypothesis, the results in Figure 2 indicated that the community structure of the subalpine zone with a lower plant diversity richness was more unstable in the central Taihang Mountain region.

While the processes of biodiversity maintenance and species coexistence are the focus of ecological studies, those of community construction and succession along altitudinal gradients still remain unclear. Ecological niche theories are increasingly becoming a

mainstream issue in community construction and species distribution. Species diversity increases with increasing heterogeneity of the environment along vertical or horizontal gradients [6,47]. In β-diversity research, numerous methods of measurement have been proposed. Using combined β-diversity, gradient analysis, and ecological niche modeling, significant and novel insights are made into biological diversity patterns [51]. Among these, similarly and dissimilarity indices are widely used. The application of additive decomposition in β-diversity better reveals the processes of community construction along elevation gradients. In this study, we mainly focused on the Jaccard and Cody indices, which can give insight into the plant community structure change and species turnover rate along the elevational gradient. As mentioned at the beginning, the Taihang Mountain range is a mountainous region that has been overexploited for a long period; thus, human disturbances have created a lasting pressure on the ecosystem of the Taihang Mountains. The altitudinal pattern of α-diversity was found to be relatively complicated, and two peaks of plant richness appeared at the elevation range of 600–900 m and 1500–1900 m, which reflected more suitable natural conditions and less anthropogenic disturbance of the plant diversity. Characteristics of the β-diversity at the elevational gradients with higher richness showed that the plant community had more active succession capacity and a higher species turnover rate. Inner succession activity, natural conditions, and human disturbance formed the current plant altitudinal distribution pattern.

In summary, the altitudinal distribution pattern and species diversity of plant communities on the sunny slopes of the central Taihang Mountain region were a result of the joint actions of community succession and natural and anthropogenic disturbances.
