六龜試驗林人工林更新動態之研究Study on regeneration dynamics of manmade forest at Liukuei experimental forest

針對六龜試驗林1998年建立之26年生台灣杉人工林疏伐處理之樣區，其中對照區為未疏伐處理區，輕度疏伐區則保留800株/公頃，疏伐強度為20%；強度疏伐區為700株/公頃，疏伐強度近40%。至2015年之調查存活之台灣杉造林木株數，對照區每公頃株數為778株/ha、輕度疏伐區為522株/ha、強度疏伐區為311株/ha。存活株數除了輕度疏伐區，其餘兩區皆在持續減少中。台灣杉不同疏伐處理間之胸徑與樹高生長表現，2013年調查分別為對照區27.2±7.0cm、輕度疏伐區為33.7±6.0cm、強度疏伐區為35.0±7.8cm。2014年為對照區胸徑為27.7±6.9cm、輕度疏伐區胸徑為33.7±6.0cm、強度疏伐區胸徑為35.3±7.7cm。至2015 年為對照區胸徑為27.5±7.2cm樹高13.7±2.0m、輕度疏伐區胸徑為32.8±7.3cm樹高19.2±1.3m、強度疏伐區胸徑為35.3±8.3cm樹高15.0±1.3m，年度間之胸徑生長變化，顯示此42年生之台灣杉造林木因彼此間生長競爭、病蟲害或受颱風等干擾，林分生長漸趨停滯，更因樹梢風折造成平均樹高的降低。由於樣區之冠層鬱閉連續性明顯受到干擾造成林下光環境改變後，便進入林下之光度增加，進而影響林下天然更新植群之種類與分布。 2015年調查之不同疏伐處理台灣杉樣區林下更新物種出現之優勢冠層樹種發生比率維持不變，而藤本與草本發生比率減少5%以上，次冠層樹種與下層灌木則明顯增加，強度疏伐區次冠層樹種發生比率增加6%。不同疏伐處理台灣杉林下更新物種數量數量最多前三大科的分佈，2015年對照區仍以紫金牛科佔82%最多，輕度疏伐區亦以紫金牛科佔48%最多。強度疏伐區則以樟科與紫金牛科為主各佔34%。比較2014與2015年疏伐處理六龜試驗林鳳崗山台灣杉樣區顯示，主要還是紫金牛科之小葉樹杞、樟科之長葉木薑子以及茜草科之琉球雞屎樹族群族群消長，影響了人工林下天然更新植群之動態變化。調查台灣杉樣區附近天然林樣區中的樹種多樣性，共出現樹種61種、445株，其中樟科共發生13種227株，殼斗科8種61株，可提供鄰近針葉人工林林下物種更新之更新材料來源，此從台灣杉人工林試區樟科及殼斗科林木數量的增加，可見更新材料的不虞匱乏。
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 heavilythinned 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 in tensities in creased 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.