What Drives Understory Plant Functional Diversity in Karst Plantations? Critical Roles of Canopy Gap and Soil Nitrogen

Establishing plantations in areas characterized by fragile natural vegetation emerges as a crucial approach to ecological restoration in karst regions. However, strong environmental filtering in karst regions constrains the ecological functions of understory plant communities. As the key stand structure and soil environmental factors driving community assembly remain elusive, variations in functional diversity are highly unpredictable, hindering the full realization of community functional potential. Based on a two-year investigation across 3 plantation types (36 plots) in the Liujiang River basin, we recorded stand structure and understory species abundances, quantified soil physicochemical properties, measured 8 leaf traits of understory plants encompassing leaf morphological, physiological, and chemical traits, and calculated community-weighted mean trait values to represent community-level leaf traits. Mixed-effects models were employed to elucidate the key drivers of understory leaf functional diversity. Results showed that Eucalyptus grandis × urophylla plantations supported the highest understory species diversity (10 species), temporally increased community-level leaf traits, and increased function diversity over other stands by 20.53%. In Pinus massoniana plantations, community-level leaf traits remained stable, but functional dispersion significantly increased by 24.47% temporally. In Bauhinia purpurea plantations, understory plants exhibited thicker, smaller leaves and significantly higher C:N ratios, accompanied by a 37.77% decline in functional evenness and an 11.42% decline in dispersion. Canopy gap and soil total nitrogen were the critical drivers for understory leaf functional diversity. Under light-nitrogen co-limitation, intensified environmental filtering suppressed relative abundance of subordinate species. Conversely, leveraging high trait plasticity, Bidens pilosa and Miscanthus floridulus dominated community-level trait trade-offs, determining functional variation approaches. Therefore, a synergistic management strategy, optimizing aboveground light and supplementing belowground nitrogen, should be considered to break single-species dominance and maintain functional diversification.