Study On The Effect Of Green-Pruning To Improve The Wood Quality Of Chinese Fir

The development trends of current forest silviculture are to intensively manage plantation, adopt reasonable cultivation measures, effectively control the wood formation and improve the quality of timber, and achieve the goal of high yielding and quality. For the purpose of producing high-quality large-diameter clear wood, the author studied the effects of density control and green-pruning on the wood quality of Chinese fir (Cunninghamia lanceolata) and the measures to improve the timber quality and productivity of Chinese fir which planted the largest area in South China.The sample site is at the Chinese fir pure plantation established in 1996 located at the Yangkou State-owned Forest Farm in Fujian Province. The Chinese fir trees were pruned artificially with 4 intensities :6 cm, 8 cm, 10 cm and 12 cm, and 3 stand densities:900 tree?hm-2, 1 200 tree?hm-2 and 1 800 tree?hm-2 with non-pruned Chinese fir trees as the contract. The influences of green-pruning on tree growth and productivity, wood physical properties and defects, and on the forest environment conditions, such as understory vegetation and forest soil, were studied. By integrating the wood quantity, quality and utilization and other indicators, the reasonable stand density and pruning intensity beneficial to improving the timber quality of Chinese fir were determined. Some silviculture measures about initial time, frequency and time interval of pruning, and stand soil management were suggested so as to provide a scientific basis for produce large-diameter clear wood of Chinese fir.The results of the study showed that green-pruning played a positive role in improving Chinese fir’s wood quality. 10 years after pruning, the DBH of Chinese fir increased by 20%, the height and stand volume increased by 4%, the wood quantity maturity came earlier, the productivity increased by more than 50%. Green-pruning could also effectively control the length of knots on wood, reduce tree taper, improve the proportion of clear wood and the under branch height, so that to improve the stem form and quality. The proportion of clear wood increased to 20% to 25%, nearly 5 times that of non-pruned Chinese fir tree. The coverage, species richness and diversity of understory vegetation were greatly improved after pruning. Green-pruning played a positive role in improving the forest stability and helping the dominant species’change. Green-pruning could also improve the wood properties by reducing the proportion of juvenile wood, and improve wood dimensional stability. After pruning, the proportion of juvenile wood of Chinese fir dropped by 40% or more. However,green-pruning could also bring some negative impacts. Pruning in the dense stands and using heavier intensity would restrict the increments of tree height, diameter and volume, delay the quantity maturation. In addition, pruning would degrade some physical and mechanical properties of wood such as density and bending strength. And more soil fertility will also be consumed. So it was necessary to strength artificial tending and to maintain the site quality to yield the maximum volume increment.The study showed that, for cultivating knot-free,large-diameter Chinese fir wood with stand density of 1 200 tree?hm-2, it was appropriate to prune the 5-year-old young Chinese fir stands with pruning intensity of 10 cm. Using tree diameter as a measure of pruning intensity could effectively control the wood defects and could be free from the influences of tree growth conditions. Also, it was more operational compared with the method of tree height controlling. 10-years after pruning, the tree height of Chinese fir was close to 15 m, DBH up to 22 cm, the average volume up to 0.24 m3. The fourth to tenth year was the fast growth period for Chinese fir trees, when the trees were 7 to 9 years-old, the growth rate was the fastest. Chinese fir could not reach the quantity maturity until 16-years-old. 10 years after pruning, the moisture content of wood was 8%, air-dry density was 0.34 g?cm-3, MOE was 11.9 MPa, bending strength was 65.8 MPa, and the toughness was 15.58 kJ?m-2. The coverage of understory vegetation of pruned Chinese fir plantation was more than 40% while that without pruning was only 2%. The dominant understory herb, shrub and liana species were Miscanthus floridulus, Cyclosorus acuminatus, Dicranopteris linearis, Ficus hirta, Maesa japonica, Pleioblastus amarus, Tetrastlgma hemsleyanum, Blumea riparia var. megacephala and Mussaenda pubescens.By CT scanning, the internal structure information of wood was obtained. It effectively solved the problem that the traditional dissection methods could not determine the internal defects of wood. The results of CT scanning showed that the whorled branches of Chinese fir were 4 to 5 rounds with round spacing of 0.22 m, and the knots number was 20 per meter. The knot number of each round was about 4-5 and was symmetrical distributed. For the knots on the trunk, the higher they locate, the less the number was, the length became shorter while the diameter became larger. Plant growth model was used to simulate the structure of branches, which could provide effective informations for wood processing and utilization by visually presenting the structure and distribution of internal wood defects, so the timber utilization would be greatly improved.