Stand characteristics and ecological benefits of Chinese Fir, Chinese Cedar, and mixed plantations in the mountainous areas of the Sichuan Basin

Many studies of biodiversity-ecosystem functioning (BEF) relationships indicate that mixed plantations can enhance ecological benefits by enabling ecological niche differentiation between species. However, few studies explore multi-dimensional evidence to fully evaluate the integrated ecological benefits of mixed plantations. This paper aims to assess the stand characteristics and ecological benefits of different forest types, drawing on multiple lines of evidence (trees, shrubs, herbs, soil fauna, and soils) to understand the effects of mixed plantations on integrated ecological benefits. Our study focused on pure Cunninghamia lanceolata (Chinese Fir) plantations (PCL), pure Cryptomeria fortunei (Chinese Cedar) plantations (PCF), and mixed C. lanceolata-C. fortunei plantations (MCC), which are common in the mountainous areas of the Sichuan Basin in southwestern China. We analysed the growth status, spatial structure, understory vegetation diversity, soil fauna characteristics, and physical and chemical soil properties found in plots representing these three forest types. The results showed that productivity was significantly higher in the MCC than in PCL or PCF, by 39.5% and 18%, respectively. Mixed plantations showed higher stand growth rates, but this effect was limited by interspecific competition. The results further showed that mixed plantations promote forest growth by allocating spatial locations between different tree species. In addition, the spatial stand structure in the mixed plantations was more stable than that in the two pure plantations, and the management potential of the mixed plantations was greater. Understory vegetation in MCC was significantly more diverse than in PCL and PCF, by 15.8–20% and 13.2–35.7%, respectively. Moreover, mixed plantations performed better in terms of soil water and soil fertility. Interestingly, we found no difference in soil fauna diversity between mixed and pure plantations. We conclude that, overall, planting mixed plantations can enhance the integrated ecological benefits of afforestation and may be an essential strategy for ensuring that plantations can cope with climate extremes.