Analysis of spatial distribution patterns of major tree species in natural secondary forests of subalpine western Sichuan and their association with soil factors

This study focused on typical natural secondary forests in the subalpine areas of western Sichuan, primarily composed of Betula albosinensis Burkill and Abies fargesii var. faxoniana (Rehder & E. H. Wilson) T. S. Liu as the dominant species. Using an adjacent grid method, a detailed survey of every tree within a 1-hectare core area was conducted. Point pattern analysis was employed to investigate the spatial distribution patterns of the three main tree species and their interspecific associations in the canopy layer. Water conservation indicators were also measured to further reveal the relationship between tree distribution and soil water holding capacity. Results showed that: (1) The relative dominance of B. albosinensis, A. fargesii var. faxoniana, and Picea asperata Mast. was high in the canopy layer of the secondary forest. The B. albosinensis population was in decline, characterized by a higher proportion of large-diameter individuals and a lack of seedlings. The A. fargesii var. faxoniana population was undergoing active regeneration, characterized by a high frequency of small-diameter individuals and seedlings, while the P. asperata population maintained low abundance across all size classes. (2) All three species and seedlings exhibited significant aggregation at fine spatial scales, transitioning towards random distribution at broader scales. (3) At small scales, A. fargesii var. faxoniana and P. asperata demonstrated facilitative interactions and jointly exerted competitive pressure on B. albosinensis. Seedlings were negatively correlated with A. fargesii var. faxoniana and P. asperata but positively correlated with B. albosinensis; however, these associations shifted to positive correlations with all three species as spatial scale increased. (4) A strong positive relationship between B. albosinensis presence and soil water retention underscored its functional importance in maintaining the hydrological stability of the secondary forest soil. These findings provide valuable insights into the ecological management and restoration of natural secondary forests in the western Sichuan region.