臺灣中南部楠梓仙溪上游集水區闊葉森林的植被類型及物種組成

Vegetation Pattern and Woody Species Composition of a Broad-Leaved Forest at the Upstream Basin of Nantzuhsienhsi in Mid-southern Taiwan

為了瞭解中海拔闊葉森林的組成、結構及物種多樣性，本研究於臺灣中南部楠梓仙溪上游集水區設立一處8.37公頃的森林動態樣區，樣區內所有胸高直徑大於1公分之木本植物都登錄其物種、胸高直徑、位置，並給予編號。結果紀錄了27科64種，共18,790棵木本植物。其中三個優勢的科別為樟科、殼斗科與茶科，共佔了總數的78.8%；樣區中優勢的物種分別為長尾栲、假長葉楠、長葉木薑子及狹葉櫟；森林結構方面，樹冠層以長尾栲為最優勢種類，次冠層以長葉木薑子為最優勢，灌木層則為假長葉楠與長葉木薑子。將資料以雙向指標種分析法（TWINSPAN）進行植群分類後，呈現出四型植物社會：假長葉楠型位於樣區東面及西面的底坡谷地，以及部分的下坡處；假長葉楠-長尾栲型位於樣區中至下坡處，以及樣區中央乾涸的底坡谷地；木荷-長尾栲型出現在東側的稜脊與上坡的區域，這三型屬於常綠闊葉森林，另外尚有臺灣赤楊型的落葉森林，位於樣區中偏西側的稜脊與西南角的稜脊上。樣區三種常綠闊葉林型的分佈與地形有密切相關，且各類型的平均物種數、密度及底面積，從稜脊到底坡谷地呈現逐漸下降的趨勢。顯示在楠溪樣區中，地形是影響常綠闊葉樹種分佈的重要因素。

A 8.37-ha plot of the broad-leaved forest at an elevation of about 2000 m in the upstream basin of the Nantzuhsienhsi River in mid-southern Taiwan was set up for long-term monitoring of forest dynamics. All stems with diameter at breast height (d.b.h.) ≧ 1 cm were identified, measured, tagged, and mapped to analyze the forest composition, structure and species diversity of the plot. A total of 18,790 woody plant individuals, belonging to 64 species in 27 families, were recorded. The dominant families were Lauraceae, Fagaceae, and Theaceae, accounting for 78.8% of total individuals. The dominant species were Castanopsis carlesii, Machilus japonica, Listea acuminate, and Cyclobalanopsis stenophylloides. The most abundant species in the canopy layer was Castanopsis carlesii, in the subcanopy layer was Listea acuminate, and in the shrub layer were Machilus japonica and Listea acuminata. Four plant communities were identified based on Two-way Indicator Species Analysis (TWINSPAN) classification, including three evergreen forest types and one deciduous forest type. The three evergreen types are Machilus japonica type, locating on the east and west valleys and partial lower slopes, Machilus japonica-Castanopsis carlesii type, locating on middle to lower slopes and the central dry valley, and Schima superba-Castanopsis carlesii type, locating on eastern ridge and upper slopes. The deciduous type is Alnus formosana forest which is distributed on mid-west and southwestern ridges. The means of species number, density and basal area for different forest types declined gradually from ridge to valley habitats. These results reveal that topography is an important factor which is closely related to the distribution of evergreen broad-leaved forest types in the plot.