Are shade tolerance and canopy gradient affecting twig-leaf trait scaling relationships in temperate forests?

Abstract Scaling relationships among twig size, leaf size and leafing intensity is pivotal in understanding plant resource allocation and carbon investment strategies. However, it remained unclear how these relationships might maintain stability across genetic traits (shade tolerance) and canopy gradients (microclimates). We investigated eight different shade-tolerant tree species within five mixed broad-leaved-Korean pine (Pinus koraiensis) forests in Northeast China. Employing linear mixed-effects models and phylogenetically independent contrasts, we examined the scaling relationships between twig-leaf size and leafing intensity. Shade tolerance altered the scaling relationships between twig and leaf size, as well as leafing intensity. We discovered that the scaling relationships between twig cross-sectional area and individual leaf area, leafing intensity and between individual leaf mass and leafing intensity were allometric (slope ≠ −1 or 1). However, the relationship between individual leaf area and individual leaf mass was isometric (slope = 1). Moreover, these scaling relationships exhibited consistent trends across canopy gradients, with shade tolerance playing a critical role in the coordinated evolution of twigs and leaves across these gradients. These results emphasized the significant role of shade tolerance in coordinating the covariation patterns between plant leaves and twigs, adopting conservative strategies in heterogeneous microclimates.