一、緣起
銀合歡為恆春半島入侵面積最大的外來種植物，墾丁國家公園範圍內亦無法倖免，受到影響的區域達數千公頃，已嚴重危害到自然生態環境。銀合歡的萌糵能力強、能產生大量的種子庫及擁有植物相剋作用等特性，使得其防治工作不易成功。保持完整的原生植物生態系與生物多樣性是防止銀合歡等外來種入侵的有效方法，然對於已被銀合歡入侵之區域，則往往需要採取長期的復舊處理以控制銀合歡的蔓延更新。外來種入侵地的生態復舊工作，應同時包括外來種移除及原生植群的重建兩部分，而新植群的重建需在短期內使林分樹冠層形成鬱閉，日後可形成多層次的森林。生態復舊作業中，造林樹種的選擇及作業方式的實施，為整體生態復舊工作成敗重要的關鍵，需要以實地試驗才能獲得經驗，供後續實作之參考。
二、方法及過程
本研究分別於恆春龜山、石珠、大圓山及後灣地區，設置生態復舊試驗地，將外來種銀合歡移除後，栽植原生樹種的樹苗。每年分別於乾季4月及雨季10月，量測栽植樹苗的高生長、樹冠幅、地徑及胸高直徑，並統計存活率，同時記錄前生樹的種類，測天然更新樹苗的覆蓋度，前生稚樹及成樹亦量測樹高、胸高直徑及樹冠幅。對於外來種銀合歡，則監測其小苗及根株萌蘖的數量、高度及覆蓋度。於2011年2月起，每隔2個月至各試驗地測定林下光環境及林冠魚眼鏡頭攝影，分析樹冠層的鬱閉度及葉面積指數等項目，並可評估颱風的擾動對樹冠層的影響。本研究也監測各試驗地的林地土壤水分狀態，以土壤水分計測定表層土，並藉測定構樹葉部黎明前水勢，得知一年間土壤水分在乾季及雨季的變化。本研究也在4處試驗地架設自計式溫濕度記錄器，監測每日林內溫濕度的變化。
本研究另一項目探討海岸林原生樹種之生態生理學特性，為各樹種在田間試驗可能產生之適應性問題提供科學證據，並與生態復舊試驗相互搭配，篩選適用於此區生態復舊作業之樹種。於石珠海岸選擇15種天然更新的海岸林樹種，每月測定其葉部黎明前水勢及光合作用潛力，以瞭解不同樹種在乾季及雨季植物體水分狀態，以及光合潛力的變化。最後另於屏東科技大學森林系苗圃，搭建植物根系觀察箱，觀察台灣海桐、樹青、紅柴、月橘、黃槿、恆春厚殼樹、臭娘子、蓮葉桐、稜果榕、血桐、繖楊及蟲屎，共12種海岸原生樹種根系的分布樣式。針對外來種銀合歡，本研究在恆春西海岸現地觀察其物候現象，記錄從花苞至開花、從開花至結果、從結果至成熟所需要的時間，以及一年結果的次數等項目。
三、重要發現
石珠試驗地栽植苗生長39個月後整體成活率達49.2%，其中以繖楊(86%)及海檬果(81%)的成活率最高；龜山及大圓山兩處樹苗栽植27個月後，整體成活率分別僅有24.5及35.2%。龜山栽植的恆春厚殼樹成活率最高(63%)，而大圓山以繖楊(69%)及水黃皮(60%)成活率較高。栽植苗生長方面，石珠試驗地栽植的血桐及繖楊平均樹高分別為269及222 cm，龜山的構樹高度已達285 cm，大圓山的構樹及血桐高度在240 cm以上，高生長快速。試驗地砍伐銀合歡後，當地原有的前生樹的種類、株數、樹冠幅面積等均增加，尤其是石珠試驗地，前生樹成樹密度及樹冠幅都較復舊作業前提高60%以上。石珠試驗地生態復舊作業對抑制銀合歡的成效顯著，已可將銀合歡的密度控制在穩定程度。此抑制效果應是前生樹樹冠提供有效的遮陰，在雨季時林下光量可低至4~8%，令銀合歡生長受抑制。龜山及大圓山的前生樹的樹冠覆蓋程度較不足，8月雨季時林內光量分別仍有61%及24%，不足以抑制銀合歡。在2011年8月下旬，強烈颱風南瑪都侵襲恆春地區，石珠及後灣的森林受擾動，葉面積指數降低20%，林內光量提高約6%，但颱風過後50天樹冠層枝葉已恢復生長，葉面積指數已比颱風前更高。本研究將12種海岸原生樹種苗木栽植在深度1 m介質為砂土的容器，觀察根系分佈，結果發現植株體型越大者，根系垂直或水平延伸長度也越大。然而，高度僅93 cm體型雖不大的黃槿，單株根系水平分佈面積可達3.22 m，根系生長極為旺盛。整體而言，石珠試驗地生態復育的成效最佳，大圓山及龜山試驗地前生樹較少，尚不足以抑制銀合歡，復舊的目標尚未達成。
計畫類別：[植物生態]
英文摘要：Abstract
Ecological restorations were conducted at three experimental sites, namely Shiju, Kuashan, and Dayuanshan, in the raining season of 2008 and 2009. We cut out Leucaena leucocephala and alternatively planted with several coastal native tree species. At Shiju site, the overall survival rate of planted seedlings after 39 months’ growth was 49.2% with Thespesia populnea (86%) and Cerbera manghas (81%) showed the highest survival rate. At Kuaishan, and Dayuanshan sites, the overall survival rate of planted seedlings after 27 months’ growth was only 24.5% and 35.2%, respectively. Ehretia resinosa (63%) had the highest survival rate among species planted at Kuashan site, while T. populnea (69%) and Millettia pinnata (60%) had higher survival rate at Dayuanshan site. For the growth of planted seedlings, the average height of Macaranga tanarius and T. populnea planted at Shiju site were 269 and 222 cm, respectively; Broussonetia papyrifera planted at Kuashan site had reached 285 cm; B. papyrifera and M. tanarius planted at Dayuanshan site were above 240 cm. After the removal of Leucaena mature trees, the vegetation composition, abundance, and the canopy coverage of the pre-existed native trees all increased subsequently. Especially at Shiju site, tree density and canopy coverage of pre-existed native trees increased more than 60% when compared to prior the restoration operations. The ecological restoration operation at Shiju site showed significant effects in inhibiting the growth of Leucaena trees, as it had maintained the density of Leucaena seedlings at a stabilized level. This inhibiting effect was probably due to the shading provided by the canopy of pre-existed trees, which greatly reduced the understory light level to as low as 4-8% in raining season such that the growth of Leucaena seedlings were inhibited. Canopy of pre-existed trees at Kuashan and Dayuanshan did not have enough coverage, where the understory light were 61% and 24%, respectively, to inhibit the growth of Leucaena seedlings. When typhoon Nanmadol hit Hengchun area in August 2011, forests of Shiju and Howon were disturbed severely that the leaf area index decreased 20% and understory light increased 6%. However, 50 days after the typhoon, tree crown had restored with leaf area index higher than before. We also observed root distribution patterns of 12 coastal native trees by planting their seedlings in 1 m-depth pots filled with sandy soil. It was found that the larger the seedling size, its roots would be longer both vertically and horizontally. However, although the average height of Hibiscus tiliaceus was only 93 cm, its root distributed over 3.22 m2 horizontally.
This research is the first project employing ecological restoration operation for Leucaena control in Taiwan. Foreign experts showed great interest as well. We suggest that the inhibiting effects of this ecological restoration operation against Leucaena should be monitored at least every three years and record the growth performances of planted trees and naturally recruited vegetation to provide long-term ecological information for restoration operation in larger area.
辦理單位：國立屏東科技大學
計畫屬性：本處委託辦理
執行年度：100B07
計畫主持人：郭耀綸
中文關鍵字：海岸林、墾丁國家公園、銀合歡、復舊、栽植苗生長
英文關鍵字：coastal forest, Kenting National Park, Leucaena, revegetation, growth of planted seedlings