南仁山欖仁溪樣區木本植物社會15年期動態

Abstract

中文摘要 本研究於2005年進行欖仁溪三公頃樣區的第三次木本植物調查，目的在瞭解南仁山欖仁溪三公頃樣區從1997年迄今，共8年期森林的動態變化，並與前一次調查間期(1990~1997)的森林動態相互比較，探討兩期之間森林組成結構、新增率、死亡率變動趨勢的異同，及其可能原因。此外，對於不同生育地分布類型的樹種，比較其在不同生育地的新增、死亡表現，以瞭解單一樹種在特定的生育地有較高的密度時，是否在該生育地上的新增、死亡表現也會較佳，而其在該生育地擁有較高密度的情形是否能維持。 樣區在2005年共調查到31,662棵植株個體，分屬42科84屬122種。八年間移出5種、移入3種；株數成長12%，底面積減少4.16%。株數減少最多的徑級以2~4cm最多，1~2cm徑級最少，僅8株。樣區內四種生育地的植株數量及底面積均呈現減少，減少幅度以迎風區最大，依序向緩風區、背風區遞減，至溪谷區則變動幅度最小，株數僅減少5%，底面積有微幅的正成長。76種多於50株的樹種中，有63種出現株數負成長，48種出現底面積負成長，有45種植株數量和底面積同時呈現負成長。樣區前20名重要值優勢種，株數減少的有18種，底面積減少的有9種。 兩期之間，樣區的年新增率從2.23%下降到1.51%、年死亡率從2.41%上升到3.16%，是第二期株數大幅下降的主因。各徑級均有新增率下降、死亡率上升的情形。生長速率從1.14(mm/yr)下降為0.62(mm/yr)。四種生育地，也都出現新增率下降、死亡率上升及生長速率趨緩的情形，變動幅度也都以迎風區最大，溪谷區最小。 數量及底面積兩期的變動趨勢並不相同，而兩種介量都有轉變成負成長及負成長幅度加大的趨勢。第一期植株數僅微幅減少488株(1.23%)，但第二期大幅減少4,459株(12%)。底面積第一期大幅正成長12.34m2(10%)，但第二期負成長5.62 m2(4.16%)。從樹種數量來看，第一期數量負成長的樹種有41種，底面積負成長的樹種有10種，至第二期，數量和底面積負成長的樹種都變多了(63、48種)。 樹種在各生育地的新增、死亡表現，多無法呈現出『樹種在其密度高的生育地有較好的表現』的預期。新增率方面，廣泛分布種及偏迎風型的樹種都在迎風區有最低的新增率、越往溪谷新增率有增加的趨勢。此種表現大致符合整個森林迎風區新增率低、溪谷區新增率高的排序，顯示樹種在不同生育地新增率的表現與生育地的環境條件較有關係。死亡率方面大多數樹種的表現趨勢較不一致。以迎風型樹種為例，有符合『樹種若在該生育地數量較多，則在該生育地表現較佳』預測的樹種，如嶺南椆、大葉羅漢松、恆春山茶，亦有符合生育地整體死亡率高低排序的樹種，如松田冬青、小葉赤楠，也有一部份的樹種沒有任何趨勢可言。 從樹種在各生育地的株數變動來看，大多數的樹種，在迎風區的數量衰退幅度較大，而往緩風區、背風區則衰退幅度減緩，甚至有少量的株數正成長。上述數量變動趨勢以廣泛分布種、和偏向迎風型的樹種最為常見，緩風背風型、及溪谷型樹種則多無此趨勢。 綜合前述整體森林、生育地、樹種三個層級的各項變動來看，在此做一假設，『樣區的物理環境條件在15年之間有所改變，此環境因子有可能是颱風及東北季風的年間變動(可能是變的較惡劣)』，造成森林株數減少、底面積下降、生長趨緩。因為各生育地受風程度不同，而受到的影響也不同。受風最強的迎風區株數減少、底面積下降、生長速率趨緩的幅度最高，最避風的溪谷則維持穩定。樹種的新增、死亡表現，甚至植株數量，受到環境變動的影響，在迎風區可能有較大的存活壓力，因而死亡率較高、新增率較低、株數減少最多，而少數仍能在迎風區維持族群數量的迎風型樹種，在緩風區則有比較大幅的成長。 然而，如以颱風侵襲數量，東北季風的強風天數當做環境惡劣程度的指標時，第一期的環境反而比較惡劣，與我們的預期不符。可能的解釋為颱風或東北季風造成植物的死亡是具有時間差而不是立即性的。另外，也有可能是風力環境緩和後，樹木間的競爭變強而導致森林的自我疏伐，而在原來密度高的地區(迎風區)，則產生較強的疏伐程度。然而，此推測無法解釋為何生理及型態上較為適應強風的樹種如嶺南椆、大葉羅漢松、竹柏可以在迎風區維持其族群，生理及形態上較不適應強風的樹種如奧氏虎皮楠、紅花八角在迎風區大幅的衰退。 8年間，我們觀察到森林的植株數量有所改變，而這樣的變動程度有多大？與國內外採用相同調查方式的樣區相比，第二期欖仁溪森林的株數變動幅度比國內另外兩個恆春半島樣區還大，然而與其它熱帶樣區比起來，大致上介於熱帶雨林和乾燥的熱帶森林之間，這些森林各自都有或大或小不同的干擾機制(disturbance regime)。要對預測未來森林的動向，仍需進一步探討此樣區森林特殊的環境因子，以及環境因子變動下各樹種的生理表現。

Abstract In 1990, a 3-hectare permanent forest plot was established in Nanjenshan. All free-standing woody plants ≧1cm dbh were identified to species, measured, and mapped. The second and third censuses were completed in 1997 and 2005. Over the three censuses, 131 species and 45,317 individuals were recorded. In this thesis, forest-wide recruitment, mortality, growth, population size and basal area change were compared in both census intervals. This comparison was also conducted in habitat and species levels in order to elucidate the changing pattern of this forest. The population was continuously decreased in both census interval, but the degree was much higher in the second interval (-1.65%yr-1) than the first interval (-0.18%yr-1). The basal area increased 10% in the first interval, but decreased 4% in the later one. The annul recruitment rate decreased and the annul mortality rate was elevated at the same time, both kind of change contributed to the high-degree population decreasing in second interval. The growth rate was lower in the second interval than in the first interval. The forest-wide changing patterns were also appeared in four kind of habitat type which were assumed to be different in wind exposure extent. Nevertheless the changing pattern was more obvious in Windward habitat which exposed more to north-east monsoon wind, and became obscure according to the wind exposure extent. So, we suggest the windy environment maybe became worse and it might be one of the possible explanation of population decreasing. In species level, most species have the same changing patterns with total forest. 61 of 76 species had lower recruitment rate in the second census interval, 57 species had higher mortality rate in the second interval. There were more species that had negative population change in second interval than first. And the basal area change shared the same trend with population change. In 15 years, most species had higher degree of population decreasing in Windward habitat, include species that distributed only in Windward habitat like Ilex lonicerifolia, but had lower degree of population decreasing in Intermediate habitat. Moreover, some species had population increasing in Leeward habitat. Species that sustained or expanded their population size in Windward habitat were species that have more tough leaf and were more suitable for windy environment in physiology like Podocarpaceae nageia, Podocarpaceae podocarpus, Cyclobalanopsis championii. Windward Specialists which had better wind-resist ability underwent population growth, and less wind-resist Windward specialists underwent population shrinkage. The different performance of Windward specialists support the assumption that windy environment became worse.