Abstract:

The study was carried out within Yingde Loblolly Pine Seed Orchard. Trees were planted in 1986, with three classes of planting density, which was 6 m×6 m,8 m×8 m and 10 m×10 m respectively. Randomized Complete Block Design was used with square plot, three replications. Area and tree number of plot was 0.673 hm² and 187, 0.665 hm² and 104, 0.769 hm² and 77 respectively for the three classes of planting density. Clones involved in different classes of planting density were all the same. All there were 20 clones from 9-year-old superior trees in Pengchang Forest Farm, Hubei Province. Because the tree number within plot was different for planting density, the clones were deployed by their remats with equal rate. Measurement was carried out at 7 and 9-year-old and sampling trees at 13-years-old. Studied traits include height, diameter at breast height(DBH) and crown width. From the 7^th to 9^th year, cones were collected and weighed by plot in autumn each year.;The results of 13 years showed that there were significant differences between classes of planting density, for the growth in height, DBH and crown width. The density of 8 m×8 m brought the functions of both the individuals and the population into full play, gave the highest cone yield per unit area. It was followed by the density of 6 m×6 m. The density of 10 m×10 m gave the lowest yield. Cone yield per hectare for the three classes of density, at the age of 13, were 470.98, 422.4 and 307.1 kg respectively. The responding mean yield of single tree was 3.93, 1.88 and 4.11 kg respectively. For the density of 6 m×6 m, trees had grown well until the 7^th year and then came to close gradually(8～10 a) and competition had become violent. On the other hand, for the density of 10 m×10 m, DBH and crown width had the highest growth. At the age of 13, degree of closing was only 0.57, that was most favorable for fruitage. Competition still didn't happen between trees. Our results also revealed that the degree of closing around 0.6 was most favorable for loblolly pine to fruit, with the highest cone yield per unit area. When the degree up to 0.8, it restrain cone yield obviously. For the planting density of 6 m×6 m, roguing should be implemented at the age of 8～9. For the density of 8 m×8 m, time for roguing come to the 10～11^th year. It was concluded that the planting density of 6 m×6 m was suitable for high seed yield in early stage. Meanwhile, the density of 8 m×8 m was suitable for middle to long-term management with high yield. To have both their advantages, the density of 4 m×8 m or 4.5 m×9 m can be used. These two classes of density would give the high cone yield in the early stage,and were convenient for mechanical cultivation.

Key words: Loblolly pine, Seed orchard, Planting density, Degree of closing, Cone yield