Balancing deterministic and stochastic assembly during needle aging shapes phyllosphere microbial community complexity and stability.

Phyllosphere microorganisms on leaf surfaces play key roles in maintaining plant productivity and health. The composition of phyllosphere microbial communities may vary during leaf aging, leading to changes in the complexity and stability of these communities. However, the ecological mechanisms underlying these varistions, particularly the relative contributions of deterministic (i.e., environmental filtering) and stochastic assembly processes (i.e., random dispersal and ecological drift) in shaping community dynamics, remain unclear.In this study, we collected 252 needle samples corresponding to three age cohorts from three representative evergreen coniferous species in natural mixed broadleaved-Korean pine forests throughout Northeast China. Needle traits were determined, and both phyllosphere epiphytic bacterial and fungal communities were assessed using amplicon sequencing.Our results showed that stochastic assembly dominated microbial communities, and the longer immigration periods contributed to increased microbial diversity and network complexity across needle age cohorts. Deterministic assembly became stronger for both bacterial and fungal communities during needle aging, which was closely related to the increased concentrations of secondary metabolites such as flavonoids, resulting in gradually elevated community similarity. Microbial network stability decreased in older needles, primarily due to the lower modularity and fewer negative interactions among species.Our study provides comprehensive empirical evidence on the succession of phyllosphere bacterial and fungal communities across needle age cohorts, emphasizing the importance of a dynamic balance between deterministic assembly and stochastic assembly in shaping microbial communities during needle aging.