Phosphorus stress disrupts leaf-root trait relationships and reduces biomass in a tropical cloud forest: Evidence from a two-year phosphorus addition experiment

Soil phosphorus, a crucial limiting nutrient in tropical forests, shapes plant morphology and functional strategies, thereby affecting overall forest productivity. However, most research to date on plant responses to phosphorus fertilization has focused only on aboveground traits, and little is known as to how trade-offs between above- and below-ground functional strategies may be affected. To address this research gap, we conducted a two-year (2022–2023) in-situ phosphorus fertilization experiment, situated in the tropical cloud forest within Bawangling, Hainan Island, South China. We tested how fertilization affected both above- and below-ground functional traits in saplings (1 cm ≤ DBH < 5 cm) belonging to five dominant tree species. The experiment included 24 fixed 10 × 10 m plots, with treatments including an unfertilized control and five phosphorus application rates (1, 2, 4, 8, and 16 g/m² yr⁻¹); low (4 g/m² yr⁻¹), medium (8 g/m² yr⁻¹), and high (16 g/m² yr⁻¹) phosphorus treatments were individually compared. When phosphorus was limiting, we found negative correlations between above- and below-ground resource-use traits (e.g., leaf dry mass and root dry mass, leaf surface area and root surface area, leaf thickness and specific root length, and specific leaf area and specific root length), suggesting that phosphorus stress leads to trade-offs between above- and below-ground traits. Increasing phosphorus significantly increased the leaf chlorophyll content and total biomass, highlighting that phosphorus limitation constrains carbon assimilation and biomass production in tropical cloud forests. In contrast, traits such as leaf thickness, root dry mass, root surface area, root volume, and specific root length generally decreased as phosphorus availability increased, suggesting a shift toward greater photosynthetic efficiency and aboveground biomass production. Comparing study years, we found evidence that root traits responded faster to fertilization, with belowground traits responding more strongly in 2022 than 2023, while aboveground traits showed the opposite pattern. Although most community-weighted mean traits showed little variation among phosphorus treatments, species-specific responses were observed, suggesting that fertilization had a greater impact at the species versus community level. Our findings highlight that phosphorus plays a fundamental role in shaping sapling development in the tropical cloud forest, underlining the need for targeted management schemes to enhance carbon storage and ecosystem stability, thereby supporting climate change mitigation efforts.