Species mixing increases biomass and alters architecture of fine roots, but does not affect their morphological traits: A meta-analysis of forest plantations in China.

Multispecies plantations, compared to monocultures, have more effective nutrient acquisition strategies and greater productivity, and consequently, make significant contributions to enhancing ecosystem services and mitigating climate change. However, the responses of fine root architectural, morphological, and chemical traits associated with belowground resource acquisition to species mixing remain elusive. Here, we conducted a meta-analysis of 839 observations from 65 studies across China to evaluate the effects of species mixtures on fine root biomass and traits, and identify the key driving factors. We found that species mixtures significantly increased fine root biomass, root length density, surface area density, and root nitrogen content, but had no significant impact on specific root length and specific surface area. The positive mixing effects on fine root biomass and traits were highly dependent on plant functional types, climatic variables, planting density, and stand age. Compared to multispecies plantations with similar functional types, multispecies plantations with different leaf morphologies exhibited higher fine root biomass, even under higher planting densities and in deeper soil layers, and their positive effects remained relatively more stable across different stand ages. Similarly, multispecies plantations composed of species with different mycorrhizal types or nitrogen acquisition strategies showed higher root nitrogen content. Overall, our results suggest that species mixing enhances fine root spatial distribution and nutrient uptake capacity, while not altering their morphological traits. Our results highlight the importance of selecting species with complementary functional types and appropriate planting densities to establish mixed plantations in future afforestation projects.