Abstract
Taiwan fir (Taiwania cryptomerioides) plantation trees of 26-year-old were treaded from various plots of different thinning intensities in which established in 1998. Investigation and three treatments were conducted : the control zone (no thinning) of which 1,044 trees/ha retained, lightly thinned zone (with thinning intensity of 20%) of 556 trees/ha retained and heavily thinned zone (wity intensity of 40% ) of 378 trees/ha were retained,in 2012. As compare to 2015, only 778,522 and 311 trees/ha were surveyed in those contol, lightly-thinned and heavily-thinned zones, respectively. The tree no. were decresed everyyear except in the lightly-thinned zone. In 2013, the measurements showed that dbh of unthinned control was 27.2±7.0 cm; lightly thinnd plot was 33.7±6.0 cm; and heavily thinned plot was 35.0±7.8 cm. In 2014, the measurements showed dbh growth of the control plot was 27.7±6.9 cm; and those of lightly and heavily thinned plots were respectively 33.7±6.0 cm and 35.3±7.7 cm. Whenever in 2015, the measurements showed dbh growth of the control plot was 27.5±7.2 cm and tree height 13.7±2.0 m ; and those of lightly and heavily thinned plots were respectively 32.8±7.3 cm with tree height 19.2±1.3 m,35.3 ±8.3 cm with tree height 15.0±1.3 m. Thus, the results of yearly variation of dbh growth indicated that part of the 42-year-old plantation trees showed growth stagnation due to growth competition, disturbances like insect and disease infestations as well as typhoons assault,which caused tree bole broken then to shorten tree height .As crown closure of plots were disturbed cause light environment change to increase light penetrate into forest floor that can influence the composition and distribution of regenerate vegetation. In 2015, number of regenerate species appeared in the plantation among different thinned intensity plots,showed the rate of occurance of predominant species keep steadily,while the rate of occurance of liana and herbaceous plant communities were reduced over to 5% . The seedings occurance of sub-canopy tree species and understory shrubs of the Taiwan fir plantation plots of different thinning intensities increased apparently.The population fluxion of seedings occurance of Ardisia quinquegona of Araucariaceae ,Litsea acuminata of Lauraceae and Randia cochinchinensis of Rubiaceae affected natural regeneration dynamics of Tawania fir forest deeply. The investigation of richness and abundance of natural hardwood forest neayby the plantation plots, showed 61 species and 445 trees appear in a 900m2 plots .There are 13 species and 227 trees belong to Lauraceae and 8 species 61 trees belong to Fagaceae . From the seedling numbers of both order increased in the plantations ,the natural forest accounts for more regenerate materials facilitating the increase number of invading predominant tree species entering into plantations.