Studies On Understory Vegetation And Soil Properties Affected By Tree Density In Coniferous Plantations

As the main afforestation tree species in the warm temperature humid and semi-humid climatic region in China, it is important to study the soil quality and understory vegetation from the aspect of sustainable management for Pinus tabulaeformis and Larix principis-rupprechtii plantations. In 2001, four treatments of different thinning intensities were carried out on the 18-, 25- and 32-year-old Pinus tabulaeformis plantations in the similar site condition, respectively. And in 2000, four treatments of different thinning intensities were manupliated in Larix principis-rupprechtii stands of 15-, 23-year-old in IV site condition and 15-year-old plantations in III site condition, respectively. In 2005, the coverage, diversity and biomass of forest undergrowth, accumulation of litter, decomposition rate and chemical composition of leaf litter, physiochemical properties and enzymes of soil were examined and compared. Based on those data, arbor density, vegetation and soil were evaluated comprehensively by Principal Components Analysis (PCA) and then analyzed by general correlation method to study quantitatively the dynamic process of interactions among the three components. The results are as follows: compared with the young platations, soil quality of both middle, near ripen Pinus tabulaeformis and Larix principis-rupprechtii plantations were deteriorated, and Pinus tabulaeformis were more severe because of its elder age. Thinning with reasonable intensity was an ideal method to relieve the soil fertility degradation for the plantation of Pinus tabulaeformis and Larix principis-rupprechtii, being more outstanding for their young stands. The main reasons were that the diversity, coverage and biamass of undergrowth were enhanced and the structure of two types of platantions was optimized, the ratio of lignin to N in the leaf litter was decreased sharply and so the decomposition of leaf litter and nutrient turnover were accelerated and the soil fertility was elevated correspondingly. Specifically, the mash content in the leaf litter was increased simultaneously in the Larix principis-rupprechtii plantations with ideal density and so the pH of their soil was enhanced slightly. Integrated analysis showed that arbor density was negatively correlated with understory vegetation and soil fertility, while the the vegetation was positively correlated with soil fertility mostly. Also, the Shannon-wiener index and biomass of undergrowth, soil organic C and soil enzymes could be percieved as indicators to assess soil quality. Typically, the ratio of lignin/N was linked to the decomposition rate of leaf litter and so it could be regarded as the key determinant to confirm which thinning intensity was best. Generally, reasonable thinning intensity for the forest with high canopy density was benefit to the healthy development of the vegetation and so can be an effective way for the sustainable management of plantations.