Divergent responses of leaf traits to latitudinal gradients in natural and planted forests

Leaf functional traits are key indicators of plant resource use and environmental adaptation, playing a crucial role in regulating carbon cycling and ecosystem stability. However, how leaf traits respond to latitudinal gradients in natural and planted forests remains insufficiently understood. Based on 482 forest plots across China (105 natural and 377 planted forests) surveyed from 2008 to 2020, latitudinal variation in specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen (LN), and leaf phosphorus (LP) were examined using quadratic polynomial fitting, variance partitioning, CatBoost analysis, and structural equation modeling (SEM). Natural and planted forests exhibited marked structural differences: natural forests had higher species richness and high stand diversity, whereas planted forests were structurally simplified, younger, and strongly shaped by management. Planted forests showed pronounced non-monotonic variation along latitude, with SLA and LP peaking at mid-latitudes, whereas natural forests exhibited weaker and more gradual latitudinal changes. Climatic and soil factors jointly dominated trait variation in natural forests, while latitude and stand structure were the primary determinants in planted forests. SEM further revealed that latitude affected leaf traits through indirect pathways mediated by climate, soil, and stand factors, with opposite effects between forest types. Natural forests showed consistent and climate-dominated trait responses, with soil properties mediating these effects in predictable ways, reflecting long-term environmental filtering. In contrast, planted forests exhibited greater short-term environmental plasticity. These findings highlight divergent mechanisms of trait-environment relationships between natural and planted forests and underscore the importance of integrating stand structure and climate matching in planted forests management to enhance ecological resilience and carbon sequestration under global change.