Ecological Mechanisms Underlying the Effects of Forest–Medicinal Plant Agroforestry Systems on Soil Quality and the Understory Environment

Long-term monoculture management of plantation forests has caused declining soil fertility and biodiversity loss, a key bottleneck for sustainable forestry. Understory cultivation of medicinal plants, forming a multi-layered vertical structure, offers a promising pathway to improve land-use efficiency while balancing ecological protection and economic benefit. Using the framework “plant functional traits–soil processes–ecosystem functions,” this paper examines how forest–medicinal plant agroforestry systems affect soil quality and the understory environment. Aboveground traits (specific leaf area, leaf dry matter content) regulate soil carbon stability via litter input, while belowground traits (root architecture, exudates) drive soil structure and nutrient cycling through rhizosphere effects. These systems optimize soil pore structure through root penetration, influence organic matter accumulation via litter decomposition and root carbon input, and enhance nutrient availability via mycorrhizal symbiosis and organic acid secretion. However, poor long-term management may cause acidification, nutrient imbalance, and replant problems. Diversified carbon inputs also reshape microbial community structure, functional genes, and enzyme activities, while enhanced plant diversity regulates understory microclimate (light, temperature, moisture). Finally, the paper outlines future research directions and proposes a core “context-dependency” hypothesis, emphasizing that effect direction and intensity are nonlinearly modulated by forest type, medicinal species, and management duration.