Identifying the species abundance distributions (SADs) in Tiankeng forests is crucial for restoring and managing degraded karst ecosystem, whereas previous studies rarely explored the differences and response of vegetation dynamics to environmental variations. The species composition and SADs of the inner and outer fringe areas of Tiankeng forest and nearby non-Tiankeng forest were compared in Southwest China. Six models were adopted to compare SADs of three habitats. Kolmogrov–Smirnov (K–S) test was selected to compare the discrepancy between the simulated and observed SAD patterns. The Akaike Information Criterion (AIC) test was adopted to compare the models, and the best model was indicated by the lowest AIC value. The results showed that (1) the species dispersal from the inside of Tiankeng forests to the nearby non-Tiankeng forests is limited, while species have unlimited dispersal from nearby non-Tiankeng forests to the inside of Tiankeng forests via the fringe of Tiankeng forests. (2) Species abundance, species rarity, richness, and species accumulation rate in the Tiankeng forests were significant in non-Tiankeng forests (p < 0.05), and most species in inner Tiankeng forests originated from nearby non-Tiankeng forests. (3) Based on the criterion of K-S values, all models have passed the K–S test (p > 0.05), which indicated that niche processes and neutral process worked together in the maintenance of community species diversity, the community in study area is a niche-neutral continuum. (4) Considered the lowest AIC value, the neutral (△_mean AIC = 1.3) models performed better than the niche (△_mean AIC = 22.7) models and statistical (△_mean AIC = 2.7) in the Tiankeng forest, while the statistical models performed better than the niche and neutral models in the non-Tiankeng forests. The results suggested that the main driving force of Tiankeng forests is the neutral process. The negative terrain in Tiankeng restricted the species dispersal due to topographic constraints. However, the species dispersal from the nearby non-Tiankeng forests could promote the species succession in the inner Tiankeng. Therefore, we propose that nearby non-Tiankeng forests should be emphasized for protecting the biodiversity of Tiankeng forests.

Silicon (Si) plays important role in reducing the toxic effects of manganese (Mn) in plants, however, more research is needed to elucidate the photosynthetic response with different Si and Mn treatments. This study aimed to investigate the alleviating effect of Si on the growth and photosynthesis of Castanopsis hystrix (C. hystrix) under Mn stress. Seedlings were grown in pot experiments with five Mn levels (0, 200, 600, 1500, and 3000 mg·kg^-1) and four Si levels (0, 115, 230, and 460 mg·kg^-1). The results showed that three types of (positive, stagnant, and negative) growth of C. hystrix seedlings were observed among twenty treatments. Low concentrations of Si (Si ≤ 115 mg·kg^-1) and Mn treatments (Mn ≤ 600 mg·kg^-1) can stimulate a positive growth of seedlings by increasing net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs). The 230 mg·kg^-1 Si with low concentration of Mn treatments can cause a stagnant growth of seedlings by increasing Gs, Tr and maximum photochemical efficiency of PSII (Fv/Fm) but decreasing intercellular carbon dioxide concentration (Ci). High concentrations of Si (Si ≥ 460 mg·kg^-1) or Mn treatments (Mn ≥ 1500 mg·kg^-1) treatments can cause negative growth of seedlings by decreasing photochemical quenching (qP), non-photochemical quenching (qN), Gs, Pn and pigment contents. The application of Si alleviated the stress induced by Mn and promoted the growth-defense regulation mechanism of seedlings to avoid stress. Hence, we concluded that the most suitable concentration of Si was 115 mg·kg^-1 Si for C. hystrix’s growth under low Mn stress.