The primacy of species turnover over intraspecific variation in the environmental filtering of understory ferns.

Introduction: Quantifying community-level trait shifts, driven by species turnover and intraspecific trait variation (ITV), is essential for understanding environmental filtering and elucidating community assembly and species coexistence. While well-studied in seed plants, the relative roles of these processes in ferns-a key component of forest understories-remain poorly understood. Methods: Here, we evaluated how topographic, soil, and overstory biotic factors influence the functional traits of understory fern communities at a local scale in a subtropical forest. We measured six key functional traits across 45 fern species in 121 plots of 10 m × 10 m. Results: We found that trait-environment models based on species turnover alone (CWM_fixed) had consistently higher explanatory power than models that included ITV (CWM_specific) (mean pseudo-R² = 0.56 vs. 0.23). Variance partitioning revealed that trait-environment relationships were primarily driven by the unique effects of environmental factors rather than their shared variance, identifying soil properties and overstory biotic structure as distinct, independent drivers of community functional composition (explaining 23.0% and 17.7% of variance for plant growth and resource-use strategies, respectively). Discussion: Our results highlight two key insights: (1) the understory fern community responds to environmental filters primarily through species turnover (compositional shifts) rather than widespread intraspecific trait variation; (2) soil phosphorus and forest structure act as critical filters that together shape community-level functional traits of ferns.