Grasslands cover approximately 40% of the global land surface and provide essential ecosystem services and biodiversity. Disturbance-dependent old-growth grasslands are particularly characterized by high biodiversity, but they are rapidly disappearing due to land-use changes and management abandonment. While restoration assessments often focus on species richness, this metric provides limited insight into the recovery of ecosystem functioning. In contrast, functional diversity—the range, dominance, and distribution of morphological and physiological strategies— is a stronger predictor of ecosystem processes because it reflects how species acquire resources and interact with their environment through niche complementarity and mass ratio effects. Yet, few studies have quantified functional diversity to evaluate whether young secondary grasslands have regained the functional integrity of old-growth grasslands.

　To address this gap, we investigated 20 grassland sites with contrasting management histories in the Sugadaira Highlands, Central Japan: 10 Old-Growth (managed for over 300 years) and 10 Secondary (restored 35 to 86 years ago following deforestation) sites. We measured a comprehensive suite of 23 leaf and root traits, representing species accounting for 75% of total community abundance, to capture resource acquisition strategies. We analyzed functional diversity metrics (Functional Richness [FRic], Functional Divergence [FDiv]) and Community Weighted Means (CWM) to detect shifts in dominant ecological strategies between Old-Growth and Secondary grasslands.

　Distinct functional signatures were associated with management history and duration. Old-Growth communities maintained a "Conservative Strategy," characterized by tough leaves and roots adapted for persistence in nutrient-poor environments, likely due to centuries of aboveground biomass removal. In contrast, secondary grasslands contained a various types of dominant species ranging from acquisitive species with higher leaf and root nitrogen concentrations to conservative species along with management duration. This suggests that higher soil nutrient legacies from the pre-existing forest continue to drive community assembly, favoring fast-growing species at younger secondary grasslands even in decades after deforestation. Furthermore, Old-Growth sites exhibited significantly higher FRic, supported by functionally diverse native perennials. Conversely, Secondary sites showed signs of functional homogenization at younger grasslands; the invasion of alien Poaceae reduced FDiv by crowding out functionally distinct native species.

　Despite decades of succession, some secondary grasslands have not recovered the functional integrity of ancient systems. Instead of regaining the conservative strategies typical of old-growth grasslands, younger secondary communities remain in an acquisitive state, likely driven by persistent fertile soil. Therefore, passive succession is insufficient; active management to mitigate soil eutrophication is required to restore the functional diversity of ancient grasslands.