Plant size drives divergence in leaf economic spectrum of tropical forest understory herbaceous species

Herbaceous plants represent an understudied component of tropical forest understory diversity, and the role of plant size in mediating leaf trait variation remains unclear. In this research, we analyzed plant height (PH), leaf length (LL), and 10 leaf traits associated with the leaf economic spectrum across 23 tropical forest understory herbaceous species to explore trait responses to size and adaptation strategies. The results showed that PH significantly influenced photosynthetic nitrogen use efficiency (PNUE) and area-based nitrogen content ( N area ), with PNUE increasing and N area decreasing with height. Compared to PH, LL explained a greater number of leaf traits and a larger proportion of interspecific variation among these understory herbs. Leaf thickness ( L t ), leaf area (LA), and PNUE increased as LL increased, whereas leaf mass per area (LMA) and N area decreased with increasing LL. We further demonstrated that LL drove leaf trait variation along principal component 1 (PC1), identified as the leaf nitrogen economic spectrum (LNES), orthogonal to the carbon economic axis (PC2). The extremes of LNES were characterized by species with high PNUE, thick leaves, large area, high chlorophyll, low N area , and low LMA (long-leaved N-efficiency species) versus nitrogen-conservative strategies (short-leaved N-inefficiency species). Notably, PH and LL were unrelated to leaf carbon economics. Our results revealed that, compared with PH, LL was a more effective classifier of leaf adaptation strategies in tropical forest understory species. These findings provide novel insights into nitrogen acquisition-conservation trade-offs in forest understory ecosystems and improve mechanistic understanding of leaf trait economics in understory herbaceous plants.