Fertilization Alleviates Multi-Layer Competition and Reshapes Biomass Allocation Strategies in the Herbaceous Layer of a Subtropical Moist Forest in Southwestern China

The herbaceous layer in subtropical moist forests often faces multiple concurrent growth and development constraints, yet the relative importance of these constraints and their responses to nutrient addition remain poorly quantified. We conducted a three-year factorial experiment in a montane moist secondary evergreen forest in central Yunnan of southwestern China, manipulating fertilization (control vs. fertilization), plant root competition (roots present vs. excluded), and forest litter (litter present vs. removed) in a fully crossed design. We investigated plant species richness, number of individuals, height, biomass, water content, and root-to-shoot (R/S) ratio of the herbaceous layer. The study results revealed a consistent hierarchy of inhibitory effects: root competition suppressed herb layer growth more strongly than litter layer inhibition, and their combined effect was severe enough to eliminate all herbaceous plants in the unfertilized control (i.e., R+L+ treatment). Fertilization significantly increased community-level plant species richness, number of individuals, height, biomass, and water content but substantially reduced R/S ratios in three of the four treatment combinations where comparisons were valid (R+L−, R−L+, and R−L−). This pattern indicates a shift in community biomass allocation toward aboveground organs under nutrient enrichment, although species turnover and intraspecific plasticity could both contribute to this pattern. In the R+L+ treatment, no plants were present in the control, so fertilization enabled the establishment of a plant community with an R/S ratio of 1.18 ± 0.08. The R/S ratios responded hierarchically to both competition and fertilization: it was highest under combined root + litter competition, intermediate under single-factor competition, and lowest when both competition sources were removed. Fertilization consistently lowered R/S ratios in the three treatment combinations where valid comparisons were possible (R+L−, R−L+, and R−L−). Our findings suggest that fertilization can alleviate multi-layer competition and promote herb layer development in this subtropical forest.