Changes in the understory diversity of secondary Pinus tabulaeformis forests are the result of stand density and soil properties

Abstract

Although the responses of forests to the regulation of stand density are better understood, a further elucidation is required as to how it impacts understory vegetation and soil properties. The relationships between understory soil properties and vegetation also warrants attention; thus, for this study we endeavored to develop new strategies to explore changes in understory vegetation. To investigate the effects of stand density on the diversity of understory vegetation and soil properties of a secondary Pinus tabulaeformis forest, fifteen sample plots with different stand densities were established. Furthermore, the soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NN), ammonium nitrogen (AN), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) contents were determined. The Simpson, Shannon Weiner, and Pielou indices, C:N, C:P, and N:P ratios, microbial entropy carbon (qMBC), and microbial entropy nitrogen (qMBN) were calculated. It was observed that stand density substantially influenced soil attributes and the variety of understory vegetation, whereas the soil nutrient content and the diversity index of different stand density levels had significant differences. Moderate stand density promoted the retention of soil nutrients and the organic decomposition rate of the soil in a secondary Pinus tabulaeformis forest. Excessive forest stand density limited the availability of soil P; thus, microorganisms and plants transitioned to a P-restricted state, and the diversity index was reduced with higher stand densities. There was a strong correlation between soil properties and the diversity index of understory vegetation. Soil properties were one of the primary drivers toward the formation of understory vegetation diversity, whereas those of herbs were more affected by soil properties. Stand density influenced the diversity of understory vegetation by altering the nature of the soil, which was also a critical factor for how stand density influenced understory vegetation.