Biomass difference between mixed plantations and monocultures and its influencing factors: A meta-analysis

Establishing mixed-species plantations has become the most promising planting method to provide various goods and improve multiple environmental services. Although mixed planting has the potential to increase tree biomass, the divergent findings make it challenging to understand the mixing effects on whole forest (trees, shrubs, herbs, and litter) biomass. Therefore, we conducted a meta-analysis compiling 156 studies published from 1982 to 2021 across 128 sites in China. We aimed to quantify the mixing effects on forest biomass between mixed plantations and monocultures and disentangle the driving factors. Our results showed that mixed planting significantly increased the total biomass, biomass of tree, and shrub layer by 14.07%, 22.49%, and 34.84%, respectively, when compared with monocultures. The effects of mixed planting on biomass were more pronounced in plantations with both nitrogen-fixing trees and non-nitrogen-fixing trees, plantations with both coniferous and broad-leaved trees, as well as uneven-aged plantations containing young- and middle-aged species. Furthermore, our results demonstrated the effects of mixed planting were predominantly governed by climatic conditions and woodland characteristics. The effect size of total biomass exhibited a significant positive correlation with mean annual temperature and precipitation, while the shrub and herb layers showed a significant negative relationship with these climatic variables. Our findings highlighted the importance of complementarity effects in mixed plantations, especially establishing uneven-aged plantations with different leaf morphology types and N acquisition strategies. Overall, our meta-analysis could provide strategies for future sustainable ecosystem management and improving the terrestrial carbon sink.