Structural Dynamic Changes of the Diameter at Breast Height and Basal Area in a VID Thinned Pinus yunnanensis Plantation

In order to understand the dynamic effects of thinned Pinus yunnanensis plantation on diameter at breast height (DBH) and their basal area (BA) structures for being thinned at middle aged stage (25-year-old), the method of variable-density thinning (VDT) was employed to be thinned 4 sampling plots (SPs) of P.yunnanensis plantation with densities of 7 311, 3 660, 4 612, and 3 045 stem/hm² of the species, respectively, which never implemented any tending approaches after established stand by seeding method at Xujiashan Forest Zone in Lufengcun Forestry Centre, Yiliang County, Kunming. The thinning intensities were 0.0% (ck), 25.8%, 31.5% and 27.7% for abovementioned SPs. Dynamic changes of DBH, BA and their important value (Ⅳ) of stand trees were analyzed based on 2 cm acted as one DBH grade for the period of before stand thinned, after moment and for 5 years thinned. The results showed that the 6 and 8 cm were dominant DBH grade of standing-stocks, however, the 6-12 cm grade was dominaant BA with 77.5%, 70.4%, 74.6% and 72.1% of total BA for the 4 SPs, respectively, before stand thinned. After the forest thinned, excepting the proportion varied, the 6-12 cm grade structured primary the BA (which were 73.9%, 73.7%, and 69.3% of total Bas for the SP 2-4, respectively) also. After thinned for 5 years, stand trees composed of the BA was forward to 8-14 cm DBH grade, and there were great proportion increment of the 12 and 14 grade trees with 23.5%, 21.5%, 24.4% and 32.8% of those two DBH grade in total stand trees, respectively. Moreover, the total BA of the SP 2-4 were from 13.86, 28.31 and 24.24 m²/hm² after thinned moment increased to 24.02, 36.43 and 26.33 m²/hm² after 5 years thinned, while which of natural thinning SP 1 was from 39.21, 26.33 m² decreased to 29.07, 26.33 m² at the same period with the SP 2-4. Simultaneously, the total BA of the SP 2-4 increased by 73.3%, 28.7% and 8.5% respectively after thinning, the control stand (SP 1) decreased by 25.8%. Stand tree IVs of dominant structures also moved forward to bigger DBH grade orientation with tree growth after stand thinned.