| ABSTRACTTwo main conifers in North China, Pinus tabulaeformis Carr and Larix principis-rupprechtii Mayr,, were used as the objects of the study. Litter decomposition dynamics of these two coniferous, as well as effects of different precommercial thinning (PCT) intensities on the decomposition process were explored. The study used the litterbag technique to quantify the dynamics of organic and inorganic nutrients in leaves of same-age-class Pinus tabulaeformis and Larix principis-rupprechtii plantations under four different PCT treatments with plots allocated in the middle hilly lands of Yingpan, Yanqing County, Bejing. Comprehensively, the effects of PCT and understorey vegetation (shrubs and herbs) & soil properties changed by PCT on decomposition rate were analyzed and evaluated by Principal Components Analysis (PCA) to find the dominant factor controlling the coniferous litter decomposition, as well as the optimal density of coniferous plantations for the litter decomposition. The results showed that:(1) Decomposition cycle of coniferous litter for Pinus tabulaeformis was 11.77-17.69 years and for Larix principis-rupprechtii was 9.01-13.25 years. The decomposition rate reached the peak value in the autumn. The initial N content and initial C:N ratio were the main factors influence litter decomposition rates in early stage.(2) The effects of four different PCT intensities were different in the initial quality of the coniferous litter. The more initial litter nutrient content can shorten the time of elements release, especially nitrogen. Thinning effects on litter decomposition showed that with thinning intensity increase, the accumulation and release of nutrient fluctuations was more obvious, which in summer-autumn performance was the most significant.(3) Correlated analysis showed that the decomposition rate was significantly correlated with biodiversity of understory vegetation (shrubs and herbs) and the retained amount of nutrients. The layer of forest litter and decomposition rate was significantly negatively correlated, the correlation coefficient was-0.621 in the Pinus tabulaeformis plantations, and was-0.758 in the Larix principis-rupprechtii plantations.(4) The activities of four soil enzymes were most active in summer. Different degrees of correlations (positive or negative) were discovered between the returned amount of five nutrients and the activities of soil enzymes in the process of litter decomposition. At the same time, the retained amounts and returned amounts of five nutrients also interacted with each other.(5) The PCA was used among all shaping factors in the data process, the results showed the quality of coniferous litter (initial C and N content) of 24-year-old and 32-year-old Pinus tabulaeformis plantations, biodiversity of understory vegetation and undergrowth dead litter played stronger role in the decomposition of coniferous litter. In contrast, the highest contributing ratio of 40-year-old Pinus tabulaeformis plantations was the factor of litter quality, Larix principis-rupprechtii (index III)summarized the effects of coarse fat, coarse protein, total nitrogen content, C/N in leaves of Larix principis-rupprechtii, Simpon index of shurb and soil moisture content on the litter decomposition. Larix principis-rupprechtii (indexâ…£)mainly integrated the information of lignin content in leaves, indexes of shrub layer and depth of litter layer.(6) The shaping factors on litter decomposition were analyzed and determined to reveal the total role played by plantations under different PCT treatments. It was observed that PCT treatment III was the optimal density for 24-year-old and 32-year-old Pinus tabulaeformis plantations, while PCT treatmentâ…£was the best for 40-year-old Pinus tabulaeformis plantations. For Larix principis-rupprechtii plantations in two kinds of site conditions, PCT treatmentâ…£played the strongest role but PCT treatment I poorly worked on the coniferous litter decomposition. It might suggest that appropriate PCT intensities accelerate and promote the coniferous litter decomposition in conifer plantations.
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