臺灣中部蓮華池亞熱帶森林木本植物多樣性與共存之研究

Abstract

群聚生態學家常探討為何在熱帶森林中樹木多樣性如此高，以及不同物種在森林裡如何共存的問題。影響森林中物種在空間中的分布及共存，主要受棲位過程以及散佈過程的影響。近年來史密森熱帶森林研究中心在全世界設立了許多大型森林動態樣區，都不似台灣為一個亞熱帶多高山的島嶼，地形變化劇烈，且每年遭受颱風的侵襲。因此於2008年根據史密森熱帶森林研究中心森林動態樣區設立的方式，在台灣中部蓮華池試驗林建立了一座25公頃的森林動態樣區，除對於樣區內所有的樹木皆進行精確的測量及標記位置外，更收集了樣區內的地形、土壤及以干擾等環境因子資料，本研究的目的為探討棲位過程及散布過程如何影響台灣亞熱帶森林木本植物之多樣性以及共存；是否與取樣地形變化平緩且干擾較少的熱帶雨林研究所得到結果相似？ 研究結果顯示，蓮華池森林動態樣區是一個以樟科和殼斗科為優勢的楠櫧森林。分析各種類的徑級結構，顯示大部份的物種以小徑级的幼樹為主，表示此森林更新狀況良好。以alpha多樣性而言，與台灣其他低海拔動態樣區比較，蓮華池動態樣區為最為豐富，但較世界上其他熱帶的森林動態樣區低。 以植物社會而言，蓮華池森林可以區分為四個植物社會類型:南投石櫟-茜草樹型，主要分布於山頂陵線，海拔最高、植株密度最大；白匏子-黃杞型，分布於中上坡，植株密度、胸高斷面積次之；山紅柿-厚殼桂型，主要分布在中下坡、溪谷，植株密度較少；以及大葉楠-山龍眼型，分布於樣區西側溪谷附近，植株密度及胸高斷面積皆為最低。四型植物社會的分化及分布皆受到微地形梯度的變化所影響。 此外，比較兩個生態過程對於影響物種組成在空間中的分布時，當環境因子僅有地形因子時，散布過程為主要的影響因素，但如果加入土壤的因子的作用時，棲位過程較散布過程更顯得重要。除了地形以及土壤因子外，另在環境因子中又加入了干擾因子，比較兩個生態過程，對於影響先驅以及非先驅幼樹組成在空間中的分布，棲位過程仍為主要的影響。然而，比較散布過程的作用，主要對於非先驅幼樹在空間上的分布影響大於先驅的幼樹。 本研究同時說明了地形因子主要影響非先驅幼樹的分布，干擾因子則影響先驅幼樹的分布，土壤因子雖然對於森林中全部物種的分布，以及先驅及非先驅的幼樹分布都相當重要，但由於植物、地形以及干擾因子都會反應於土壤的物理或化學成分之中，故對於利用土壤因子解釋物種的分布，不能過度放大其重要性。再分解多尺度的純空間變量後顯示，地形因子表現解釋反應在大尺度的空間變量，土壤及干擾因子則為增加中尺度及小尺度空間變量的重要性。總體而言，在地形變化劇烈且多干擾的蓮華池亞熱帶森林中，棲位過程主要維持了蓮華池森林樹木的多樣性與共存，並影響著植物社會的分化。

Why tree diversity is so high and what drives coexistence of species in a tropical forest is the central question that community ecologists concern. Niche- and dispersal-based processes are two main processes which affect distribution pattern and coexistence of trees in a forest. Recently, forest dynamics plots (FDP) established by the Smithsonian Institution Center for Tropical Forest Science (CTFS). However, none of the above is the same as in Taiwan, a subtropical and mountainous island with rough terrain and frequent disturbance. Therefore, in 2008, I used the standard protocol as the Smithsonian CTFS’s to establish a 25-ha FDP in the Lienhuachih Experimental Forest (23o54’49”N, 120o52’43”E) in Central Taiwan. Expect precise stem mapping and topography measurement, I also collected soil and disturbance data to assess how niche- and dispersal-based processes affect coexistence of species and diversity of a subtropical forest and whether these were the same as the tropical FDPs with relative flat terrains and little disturbance. My results show the Lienhuachih FDP is dominated by Fagaceae and Lauraceae and which is characteristic of the Machilus-Castanopsis forest zone of Taiwan. The size-class structure of trees show most tree species have plenty saplings, which display good recruitment in the forest. Fisher’s alpha diversity of the Lienhuachih FDP is the highest among low-elevation FDPs in Taiwan, but lower than which of other FDPs in tropical plots. On the other hand, the Lienhuachih forest could be divided to four types, these are Pasania nantoensis - Randia cochinchinensis type locates on the ridge and the highest elevation was with the highest stem density, Mallotus paniculatus - Engelhardtia roxburghiana type locates on the upper slope was with the middle stem density and basal area, Diospyros morrisiana - Cryptocarya chinensis type locates on the lower slope and stream side was with lower stem density and Machilus japonica var. kusanoi - Helicia formosana type locates on west stream side was with the lowest stem density and basal area. All four plant communities and species composition varied across micro-topographic gradients. In addition, both niche process and dispersal process work together to assemble the Lienhuachih tree communities. When comparing above two processes affect community assembly, if only topography is included, dispersal-based processes prevail. But if including soil variables along with topography variables into variation partitioning, the result reverses. Furthermore, including disturbance variables with topography and soil variables, niche-based processes still prevail for both pioneer and non-pioneer saplings. However, dispersal limitation is also an important process influencing the spatial distribution of species diversity for both functional saplings, especially for the non-pioneer saplings. My study also demonstrates that topography affects distribution of non-pioneer saplings, whereas disturbance affects distribution of pioneer saplings. Although soil is an important factor to explain distribution of both pioneer and non-pioneer saplings, it is also evident that plant species, geomorphic processes, and disturbance have different effects on the physical and chemical properties of soils. Soil and disturbance effects contribute to meso- and fine-scale spatial variations for distribution of species, whereas topographic effects contribute to broad-scale spatial variations for distribution of species. To conclude, in a rough terrain and highly disturbance area, niche-based process is the main ecological process to maintain the coexistence and diversity, and which also affect vegetation classification in the Lienhuachih subtropical forest.