Features of pollen assemblages in topsoil as affected by land use in the karst valley area, Qingmuguan Town, Chongqing

This study performed a palynological analysis on 33 surface soil and moss samples collected from eight land-use types in the Qingmuguan karst valley(29°48″~29°46′29″N, 106°17′12″~106°19′45″E), Chongqing. By applying diversity indices and multivariate statistical methods, we investigated the relationships between surface soil pollen assemblages and vegetation, characterized the composition and variability of pollen assemblages across different land-use types, and evaluated their indicative significance for vegetation conditions. Additionally, the impacts of human activities on the fragile karst ecosystem were assessed, offering a scientific reference for rocky desertification control and paleoenvironmental reconstruction. The results demonstrate that: (1)Pollen assemblage analysis in the study area revealed that herbaceous plants constituted the dominant group(averaging 39.4 %), followed by trees and shrubs(33.8 %)and fern spores(26.8 %). This herb-dominated community structure markedly diverges from the zonal features typical of subtropical evergreen broad-leaved forests at this latitude, indicating possible vegetation degradation in the region. (2)Crops such as Poaceae and Brassicaceae dominated the pollen spectra in farmland samples, whereas stress-tolerant taxa like Chenopodiaceae and Urtica were enriched in abandoned fields. Pioneer fern species, including Dicranopteris and Pteris, were notably prevalent in reclaimed farmland. These distinctive pollen assemblage patterns serve as effective indicators for identifying human activities and habitat characteristics in karst valley areas. (3)Surface soil pollen assemblages displayed clear differentiation among land-use types, generally aligning with actual vegetation conditions and reflecting the current state of modern vegetation in karst desertification zones. Both cultivated land and abandoned fields exhibited high Simpson dominance and evenness indices, suggesting community assembly dominated by a limited number of stress-tolerant species(e.g., pioneer herbs and crops)under disturbed conditions. Meanwhile, the highest Shannon-Wiener indices were recorded in orchards and cultivated land(2.443 and 2.409), indicating complex herbaceous layer composition at moderate species richness levels and highlighting the multidimensional effects of human disturbance intensity and habitat stress on pollen community structure.