Bedrock plays a crucial role in regulating the process of landscape evolution and biological colonization. Experts in geosciences, botany, and ecology have been interested in understanding how bedrock influences regional plant diversity levels. This research aimed to study the natural secondary forests in two typical karst areas of Guanyan and Gongcheng located in northern Guangxi. The study focused on the forests that grew on three bedrocks namely limestone, clastic rock, and limestone sandwiched with dolomite. By analyzing the field-measured data of the sample plot, the study aimed to identify the plant diversity characteristics, both alpha and beta, and the environmental factors that influence them in the karst area. The research results showed that: (1) The study area recorded 86 trees, 93 shrubs, and 56 herbs, revealing a relatively affluent species count. The plant alpha diversity was observed to have a noticeable variation across different types of bedrock. The research findings indicate that the highest plant diversity was found on limestone bedrock, hosting a rich and diverse range of plant species, while clastic rock also displayed a significant level of plant diversity. However, the lowest plant diversity was recorded on limestone bedrocks sandwiched with dolomite. (2) The plant communities in different and same bedrock areas showed a considerable difference in their composition. The variation in the beta diversity was significantly influenced by the species turnover component, which accounted for 82.86% to 84.49% of the beta diversity, rather than the species richness component, which accounted for only 15.51% to 17.14%. (3) The statistical analysis conducted using the Pearson correlation and Mantel test methods indicated that bedrock, bedrock exposure rate, and soil thickness significantly correlated with plant composition and diversity in the study area (P<0.05). To summarize, the karst forests located in Northern Guangxi exhibit a high degree of heterogeneity. It has been observed that plant richness in carbonate-mixed sediments is significantly lower than on pure limestone and clastic rock. Bedrock, bedrock exposure, and soil thickness are the main factors influencing plant compositional diversity, which depends mainly on species turnover processes. This study evaluates the plant diversity characteristics and identifies the main influencing factors of karst forests from the perspective of bedrock. It highlights the importance of these factors in shaping the local plant community and emphasizes the need for future research to investigate the mechanisms behind their impact further. This information could be valuable for informing conservation efforts and forest management practices in the karst region of northern Guangxi and even southwest China.