Habitat and Canopy Position Influence Leaf Traits and Trait-Associations of a Large-Sized Leguminous Herb (Crotalaria spectabilis)

<i>C. spectabilis</i> (<i>Crotalaria spectabilis</i>), a large leguminous herb species, is widely distributed in tropical and subtropical regions, and it has important ecological and economic values. However, the ecological adaptation of the major leaf functional traits of the species across different habitats and canopy positions remains poorly understood. To address this gap, we sampled leaves from the upper, middle, and lower canopy positions in two common habitats-forest understory and exposed land-and quantified key leaf traits as well as trait-trait relationships to assess differences. The results showed that irradiance and air temperature were significantly lower in the understory than in exposed land, whereas soil moisture and relative humidity were higher, indicating that habitat exerted a stronger influence on leaf traits than canopy position. Canopy position also significantly affected most traits and showed significant interactions with habitat. In exposed land, middle plants exhibited higher individual leaf dry mass (180.049 ± 68.480 mg), larger vein diameter (1.692 ± 0.288 mm), and longer petioles (5.406 ± 0.940 mm). These traits were accompanied by a higher morphology-based leaf dry matter accumulation rate and greater stability of the leaf-trait network, reflecting an adaptive strategy characterized by increased structural investment. In contrast, understory middle leaves were generally longer (13.361 ± 2.714 cm) and wider (7.005 ± 1.464 cm), along with lower photosynthate accumulation rates and weaker trait-network stability, indicating a strategy that enhances light-use efficiency under low-light conditions. In both habitats, leaves from the middle canopy position generally exhibited the highest values for most measured traits. Overall, leaf traits of <i>C. spectabilis</i> and their interrelationships showed considerable plasticity in response to external environmental pressures, primarily differences in light availability. However, from a practical production perspective, minimizing shading is recommended to maximize its ecological benefits.