Relationships between ecosystems above and below ground including forest structure, herb diversity and soil properties in the mountainous area of Northern China

摘要

One hundred field plots of 20?m?×?20?m were established in a typical secondary forest of Populus davidiana and Betula platyphylla Suk. in the mountainous area of northern China. The aim was to explore quantitative tree–herb–soil relationships at three different soil depths, using a structural equation model (SEM) approach to reveal the connection between the above and below ground ecosystems. The corrected model (ratio of the chi-square degree of freedom 3, p- value 0.05, the root meant square error of approximation 0.05, and all the fix indexes were from 0.9 to 1.0) confirmed that the total effects between tree canopy stand structure and understory herb diversity were negative and significant, with standard path coefficients (Std. PCs) of ?0.87, ?0.84, and ?0.98 from the topsoil down to the deepest layer. The positive relationship of soil properties with stand structure (Std. PCs: 0.90, 0.30, and 0.01, respectively) and herb diversity (Std. PCs: 0.54, 0.44, and 0.32, respectively) decreased with soil depth, and the effect was greatest in the 0–20?cm layer. Average DBH, stand density, and Simpson index were the major drivers of structure and diversity, with path coefficients more than 0.8?at all the soil depths. Total phosphorus (Std. PC: 0.83?at 0–20?cm), total nitrogen (Std. PC: 0.62?at 20–40?cm), and available potassium and organic matter (Std. PCs: 0.86 and 0.77, respectively at 40–60?cm) were significant to soil properties, and even to trees and herbs. Hence, management to improve both the overstory and understory of forest ecosystems in similar mountainous area of northern China should be more targeted towards optimization of soil properties, stand structure, and herb diversity simultaneously, by increasing litter thickness as well as soil microbial activity of the topsoil, optimizing stand density, and planting different grass seeds.