Bryophyte diversity, distribution pattern and their environmental drivers in the forest along the vertical gradient of a mature tiankeng, China

Tiankeng, as a distinctive karst landform, maintains exceptional biodiversity and serves as crucial refugia for extant organisms. In this study, we investigated bryophyte distribution patterns along the vertical gradient of Lanjiawan Tiankeng in Guangxi province, China, through quadrat surveys of ground and epiphytic communities. Alpha and beta diversity of bryophyte communities at the top, middle, and bottom levels were evaluated. Beta diversity was decomposed into species turnover (β jtu ) and nestedness (β jne ) components. Non-metric multidimensional scaling (NMDS) and canonical correspondence analysis (CCA) were utilized to explore the vertical distribution patterns and identify the environmental drivers of bryophyte communities, respectively. Eighty-nine bryophyte species belonging to 68 genera and 33 families were recorded in the study site. Among these, 31 species occurred in both ground and epiphytic habitats, while 30 were ground-exclusive and 28 were tree-exclusiv specialists. The total species richness follows a top- > bottom- > middle-level vertical distribution pattern for epiphytic communities. However, ground bryophyte diversity decreased with the decreasing vertical gradient. Along the vertical gradient, β diversity was dominated by β jtu (contribution >85%) rather than β jne (contribution <15%) across ground and epiphytic communities, and the bryophyte communities in both habitats did not exhibit clear stratification. Top-level communities were shaped by synergistic climate-forest interactions, while mid-bottom distributions were constrained by microclimatic thresholds (temperature, humidity, light intensity). From the top to the bottom level of tiankeng, humidity showed a rapid increase, whereas temperature and light showed a rapid decrease. This research pioneers in elucidating how the tiankeng's unique “cold-wet island” microclimate orchestrates bryophyte community assembly through vertical environmental filtering. The demonstrated refuge capacity of a tiankeng ecosystem provides critical insights for conserving the karst biodiversity hotspots, particularly in mitigating climate change impacts through microclimate conservation. • Tiankeng offers a crucial refugium for conserving bryophyte diversity in Karst landforms. • Vertical distribution of bryophyte communities along the tiankeng is driven by environmental filtering. • The cold-wet island effect of tiankeng distinguishes bryophyte community compositions along the vertical gradient.