Size-dependent succession of microbial communities in the decaying heartwood of Populus euphratica and its association with tree architecture in the lower Tarim River, Northwest China

Elucidating microbial community succession in Populus euphratica trunks is crucial for understanding wood decay processes and maintaining tree mechanical stability in desert riparian forests. This study investigated naturally decaying P. euphratica across four diameter at breast height (DBH) classes (D 1 :15-25 cm, D 2 :25-35 cm, D 3 :35-45 cm, D 4 :>45 cm) in the lower Tarim River, with a total of 12 sample trees (n=12). Using 16S rRNA and ITS high-throughput sequencing, we analyzed heartwood bacterial and fungal communities, focusing on size-dependent successional dynamics and correlations with tree architectural traits. Results revealed a systematic shift in community composition across DBH classes, suggesting a successional trajectory associated with tree size increase.. The D 2 class (25-35 cm) appeared to be a transitional phase for community shifts, with the highest bacterial biomarker richness, indicating intense microbial turnover. Fungi were dominant in the community (average relative abundance of 67.2%) and occupied central network positions (modularity = 0.45), while bacteria played synergistic roles. Statistical analysis showed significant differences in bacterial community structure between D 1 and D 3 /D 4 (ANOSIM, p < 0.05), and fungal diversity (Shannon index) was significantly lower in D 4 than in other classes ( p < 0.05). Association analyses indicated that functional microbes are closely linked to tree architectural differentiation. Taxa positively correlated with tree size indices (DBH, crown spread) were predominantly potential pathogens (e.g., Ralstonia and Fusarium ), suggesting an accumulated pathogen load as tree size increases. Conversely, functional bacteria associated with architectural maintenance (e.g., Azoarcus ) showed distinct patterns. Ecologically, these findings suggest that heartwood microbial succession is not random but follows a predictable size-dependent trajectory, and that microbial community structure may serve as a potential bioindicator for assessing tree health and mechanical stability in desert riparian forests. This study provides evidence that size-dependent microbial succession in P. euphratica heartwood, highlighting functional microbial roles in shaping tree architecture and providing a foundation for health management of desert riparian forests. • Microbial communities in P. euphratica heartwood exhibit size-dependent succession. • The 25–35 cm DBH class is the transitional phase for community structural shifts. • Decay is driven by a fungi-dominated and bacteria-synergized microecological network. • Pathogen enrichment correlates with architectural decline like canopy dieback.