Contrasting pathways of light and soil impacts on understory shrub diversity via functional traits in plantation and natural forests

The mechanisms by which environmental conditions in plantations and mixed natural forests drive changes in understory diversity through functional traits remain unclear, particularly regarding their dynamic effects during stand development. In subtropical montane regions, three stand-age classes of two plantation types and two mixed natural forests were selected to examine understory shrub diversity and the responses of shrub functional traits to variations in understory light conditions and soil factors. Results show that, in Pinus massoniana plantations, understory photosynthetically active radiation (PAR) increased with stand age, whereas soil nutrient availability declined significantly. In young stands, light mainly regulated leaf traits and soil nutrients mainly regulated branch traits, but later shrub responses shifted to light regulation of leaf traits, causing only a slight reduction in shrub species diversity in mature stands. In Cunninghamia lanceolata plantations, PAR declined sharply during the transition from young to intermediate stands, while soil nutrient availability was significantly highest in intermediate stands. Light and soil factors jointly shaped shrub leaf and branch trait space in young stands, whereas sharply reduced PAR in intermediate stands intensified environmental filtering and concentrated shrub responses in leaf economic traits, leading to rapid declines in shrub species and functional diversity that persisted into mature stands. In contrast, soil nutrient availability in mixed natural forests significantly increased with stand age, while understory light conditions remained generally stable. The relative contributions of soil and light to shaping shrub functional traits varied among stand ages, while shrub diversity remained relatively high across all stand ages. Overall, the mechanisms maintaining high understory species diversity in mixed natural forests can serve as an important reference for near-natural management and the conservation of understory diversity in subtropical plantations.