Studies On Thinning Effects Of Different Aged Pinus Tabulaeformis Plantations

Chinese pine (Pinus tabulaeformis Carr.) is one important afforestation tree species in the warm temperature humid and semi-humid climatic region of north China. Thinning is an important method to optimize and manipulate stand structure. It is of vital importance to study the thinning effects on Chinese pine plantations for sustainable management. A thinning trial initiated in artificially regenerated Chinese pine plantations and produced 12 monument-sample plots under conditions of site class Ⅳ. Accordingly, the trial created three series of thinning intensity as followed:27a Chinese pine plantations (0%,23.3%,31.7% and 52.5%),35a Chinese pine plantations (0%,14.4%,21.2% and 30.3%) and 42a Chinese pine plantations (0%,14.4%,21.2% and 27.3%). The study objected the 27a,35a and 42a Chinese pine plantations and combined situ sample survey, temporary sample survey and indoor analysis to quantify the comprehensive thinning effects on arbor growth, understory vegetation diversity, litter decomposition and forest soil quality of different aged Chinese pine plantations from ecosystem management aspect. The principal component analysis (PCA) was employed to better unravel best optimized thinning intensity and to quantitatively reveal the comprehensive thinning effects and their key contributing factors. The results and conclusions showed:(1) Grading evaluation system on comprehensive thinning effects of different aged (27a,35a and 42a) Pinus tabulaeformis plantations were established based on quantitative study. The quantitative grading and quantitative evaluation of comprehensive thinning effects showed from the system.(2) 27a Chinese pine plantations, thinned at 18a with per-thinned density of 3000 stem·hm-2, the favorable thinning intensity with the best comprehensive thinning effects at post-thinned three years, six years and nine years was moderate intensity Ⅲ (31.7%).(3) 35a Chinese pine plantations, thinned at 26a with per-thinned density of 3300 stem-hm-2, the best comprehensive thinning effects increased with the thinning intensity post-thinned three years, six years and nine years. The favorable thinning intensity with the best comprehensive thinning effects was strong intensity Ⅷ (30.3%).(4) 42a Chinese pine plantations, thinned at 33a with per-thinned density of 3300 stem·hm-2, the favorable thinning intensity with the best comprehensive thinning effects at post-thinned three years, six years and nine years was moderate intensity Ⅺ (21.2%).(5) Thinning interval period for 27a and 35a Chinese pine plantations were six years, while thinning interval period for 42a Chinese pine plantations was at least nine years. The final thinning intensity was 22.11% .(6) 27a Chinese pine plantations had key indicators of height, diameter at breast height and shrub diversity for the best comprehensive thinning effects. Key determinants of undergrowth diversity, soil bulk density, litter K element content, undergrowth diversity, height and diameter at breast height contributed to the best synthetic thinning effects for 35a Chinese pine plantation. Specifically, the key contributing factors of 42a Chinese pine plantation with best comprehensive thinning effect were canopy density, diversity and nutrients accumulated of undergrowth.(7) Individually, the favorable thinning intensities of weak intensity Ⅱ (23.3%), moderate intensity Ⅶ (21.2%) and moderate intensity Ⅺ (21.2%) with best arbor growth responses to thinning for 27a, 35a and 42a plantations. It was found that negative correlation between shrub storey coverage and canopy density and between herb storey coverage and canopy density. It found that positive correlation occurred between shrub storey coverage and herb storey coverage, and the positive correlation increased with stand age. Thinning enhanced understory vegetation diversity by increasing the number of dicotyledon. Nutrients accumulated in shrub, undergrowth biomass and dominant species were altered by thinning. Litter nutrients showed elements release, namely the return of nutrients to the forest soil. Typically, the ratio of litter C to N and ratio of cellulose to litter K element content is the key factor restricting litter decomposition. Soil enzymes were more sensitive to thinning disturbance.Generally, the favorable thinning intensity was confirmed moderate intensity Ⅲ (31.7%), strong intensity Ⅷ (30.3%) and moderate intensity Ⅺ (27.3%) from the comprehensive thinning effects aspect for 27a,35a and 42a Chinese pine plantations.