The Effect of Mixed Plantations on Chinese Fir Productivity: A Meta-Analysis

Mixed plantation of Chinese fir (Cunninghamia lanceolata) is an effective artificial forest management for tree productivity. However, the mixing strategies, site conditions, and subsurface properties that affect tree productivity are not yet fully understood. In this study, we conducted a meta-analysis of 96 publications to consolidate insights on the effects of mixing strategies (e.g., planting density, mixing proportion, mixed species, and tree age), site conditions (e.g., mean annual precipitation (MAP), mean annual temperature (MAT), elevation, and total nitrogen (TN) or total phosphorus (TP) of sample sites), and subsurface properties (e.g., soil characteristics, microbial communities, and extracellular enzyme activity) on tree height, diameter at breast height, and individual volume of Chinese fir. We used the Web of Science and China National Knowledge Infrastructure for searching peer-reviewed papers, and the searching words were: ("Cunninghamia lanceolata" OR "Chinese fir") AND "mix*". Following the data screening process, the natural logarithm of the response ratio (lnRR) was computed for subsequent analysis. The results showed that introduced companion species generally increased the individual volume of Chinese fir by an average of 20%. Densities ranging from 1200 to 2000 trees per hectare and moderate mixing proportions (1:1 to 3:1) optimized individual tree growth and thereby boosted productivity. Broadleaf species may be beneficial companions, and trees aged 10 to 20 years grew fastest. At sites with low MAT and high MAP, mixed plantations enhanced the tree productivity of Chinese fir. The optimal elevation range for mixed plantations may be 200 to 600 m. Further, mixed plantations significantly changed soil properties by improving soil structure, increasing soil pH and soil water content, and soil total and available N and P, which were crucial for boosting the productivity of Chinese fir. Soil microbial biomass and enzyme activities were also significantly increased by mixed plantations. Overall, these findings highlight the importance of mixing strategies and site conditions in increasing tree productivity of Chinese fir by improving soil physicochemical characteristics, increasing resource availability, and reducing interspecific and intraspecific competition through niche separation.