台灣南部南仁山低地雨林樹木的更新：林隙與地被植物莎勒竹的影響

Abstract

林隙(forest gap)為森林中樹木枯立、傾倒或斷折而形成樹冠層破空的區域。林隙釋放了森林中的生長空間和光量等環境資源，提供森林下層物種新個體建立與更新的機會。對於需光量(high light-demanding)高的物種，其建立與更新更需依賴林隙的高光量。此外，不同面積和形成時間的林隙常導致森林裡資源及環境的異質性，有助於增加森林物種的多樣性。 台灣南部墾丁國家公園的南仁山低地雨林為一受颱風干擾頻繁且樹冠層破碎的森林。在1993年、2000年與2008年，已完成三次的每木調查。 為了解南仁山低地雨林的林隙動態與此環境下森林樹木的組成、結構和更新、與在林隙中大量覆蓋的地被植物莎勒竹(Schizostachyum diffusum)對林隙更新的影響，本研究於2008年完成第三次的林隙調查與第二次莎勒竹覆蓋的調查。 結果顯示，研究樣區的樹冠層動態為先鬱閉再開闊，總林隙面積在2008年達到最大，其中包括一面積超過1000 m2的林隙。單一林隙面積在三次調查期間多小於100 m2，在15年期間，林隙維持區莎勒竹的覆蓋度較樹冠鬱閉區為高；組成物種中有較高比例的非耐蔭性樹種；植株新增率高，死亡率也高，且死亡植株的徑級較大。此外，莎勒竹覆蓋越高時，耐蔭性樹種的存活也越低。南仁山低地雨林的林隙未恢復成鬱閉樹冠，除了與大徑級樹木的高死亡率有關外，也與莎勒竹的覆蓋有關。

Gaps are openings in forest canopy which are caused by broken or fallen dead trees. Space and resources release by the formation of forest gaps can provide opportunities for understory growth and establishment. Especially, the growth of high light-demanding species relies on the sunlight under these canopy openings. Furthermore, gaps with different sites and ages may create heterogeneous habitats, and increase species diversity. Lowland rainforests in Nanjenshan are characterized by frequent wind disturbances and broken openings in the forest canopy. This study examined species composition and forest structure under forest gaps and compared with those of adjacent forests with intact canopies. In addition, the effect of the climbing bamboo (Schizostachyum diffusum) cover on regeneration of forest was assessed. The analyses were based on data from three tree-by-tree censuses in 1993, 2000 and 2008. The coverage of the climbing bamboo and forest gaps were measured in 2008. The results showed that there was a notable change in canopy structure from closed in 1993 to more open condition in 2008. The average size of a gap was less than 100 m2. The biggest gap size was found to be over 1000 m2 in 2008. The coverage of climbing bamboo and the proportion of shade-intolerant species in the gaps were both high. The recruitment and mortality rates of individuals in the gaps were also high, and the size of dead trees was large. Besides, the survival of shade-tolerant species was negatively correlated with the coverage of climbing bamboo. The persistence of canopy gaps during the past 15 years may be caused by large tree-fall and a high coverage of climbing bamboo in Nanjenshan forest.