Impacts of tree mixtures on understory plant diversity in China

Understory plants are one of the important components of forest biological diversity and play important roles in forest function. Although convincing evidence exists that mixed-species plantations are more conducive to increase the productivity and stability of forest ecosystems than monocultures, the effect of mixed-species plantations on the understory plant diversity (UPD) remains uncertain. This study conducted a meta-analysis based on 205 paired observations of plant species mixtures and corresponding monocultures from 76 peer-reviewed studies to assess the impact of tree mixtures on the UPD in China. The results showed that the UPD was on average 18.2 % higher in mixed-species plantations than in monocultures. This positive mixture effect increased over time, but it would take at least ten years for the effect size of UPD to change from negative to positive. In terms of different mixed forest types, tree-shrub mixtures were more beneficial to maintain the UPD than treetree mixtures, and this positive effect was more significant over time. In addition, the response ratio of UPD decreased with mean annual temperature (MAT) and precipitation (MAP), but this relationship was not significant. Therefore, our results suggested that planting mixed-species plantations was a more effective approach to enhance the UPD than monocultures in China. This study revealed the characteristics of UPD under different afforestation modes and could provide a scientific basis for forest management.