Phylogenetic and functional diversity are equally important drivers of community completeness in the secondary succession of Pinus yunnanensis forest

• Community completeness in Pinus yunnanensis forests follows a hump-shaped pattern along secondary succession, peaking at the 60-year stage. • Phylogenetic community structure shifts from overdispersion in mid-successional stages to clustering in the late stage (80-year). • Phylogenetic diversity and functional diversity are identified as the primary and equally important drivers of community completeness. • The decline in community completeness at the late successional stage highlights the need for targeted management to maintain ecosystem functioning. Community completeness, an emerging concept based on dark diversity, is gaining increasing attention in ecological research. However, how community completeness changes during secondary succession remains poorly understood. This study investigated patterns of observed diversity, measured as species richness (SR), dark diversity (DD), and community completeness index (CCI), as well as phylogenetic community structure, quantified as the standardized effect size of the mean pairwise phylogenetic distance (SES.MPD) and mean nearest taxon phylogenetic distance (SES.MNTD), across different second successional stages (30-, 45-, 60-, and 80-year-old stands) in Pinus yunnanensis forests. We also examined the effects of abiotic factors (soil water content, SWC; soil pH, SpH), biodiversity metrics (SR; functional dispersion, FDis; mean nearest taxon phylogenetic distance, MNTD), and soil nutrients (soil total nitrogen, STN; soil total phosphorus, STP; soil hydrolysable nitrogen, SHN; soil available phosphorus, SAP) on CCI. The results showed that SR and CCI followed similar trends, both increasing initially and peaking at the 60-year stage, followed by a decline at the 80-year stage. In contrast, DD did not differ significantly across the four successional stages. As succession proceeded, the phylogenetic structure of the community shifted from overdispersion to clustering: stands aged 30 to 60 years exhibited significant phylogenetic overdispersion, while the 80-year-old stands showed phylogenetic clustering. Ordinary least squares (OLS) regression revealed significant positive relationships between CCI and SR, FDis, STN, STP, and SHN. In contrast, SpH and MNTD were significantly negatively related with CCI. No significant relationships were observed between CCI and SWC or SAP. The final general linear model (GLM) identified MNTD and FDis as the most important predictors of CCI, accounting for 40.61% and 44.02% of the variation, respectively. From the perspective of DD and community completeness, this study demonstrates that secondary succession in P. yunnanensis forests is generally a benign process. However, progression beyond a certain stage (e.g., 80 years) may result in a decline in community completeness, which could subsequently impair ecosystem functioning. These findings provide a scientific basis for the management and conservation of P. yunnanensis forest ecosystems.