擎天崗地區植群構造在森林「邊緣-內部」梯度上的變異

Abstract

本研究主要探討植群構造在森林「邊緣-內部」梯度上的變異。研究地點為擎天崗地區，以類地毯草(Axonopus affinis)草原以及水牛（Bubalus bubalis）放牧為特殊景觀，草原的成因不只與天然的氣候因素相關，也與當地的干擾（放牧及人為干擾）有關。研究方法主要由兩個部份來進行，（一）利用多個時期的航照資料進行比對，設法找出邊緣形成的年代以及邊緣變化的情形；（二）實地取樣調查，劃設12條不同的穿越帶，進行森林邊緣到森林內部在植物組成、密度、徑級、多樣性等調查。 分析結果顯示大尖山區草原於民國72年時已經存在，迄今至少有20餘年之久，而石北區之正射圖由於雲層遮蓋僅能追溯至民國83年，迄今有10餘年。針對草原分布之變遷可發現，大尖山區草原在這20年來其覆蓋範圍有漸漸縮小之勢，而石北區從民國83年至今其改變則不甚明顯。 研究結果共紀錄75種植物，以樟科（Lauraceae）植物為主要優勢。樹木族群結構之分析大多為反J型（40％）及L型（50％），顯示大部分族群呈現小苗或小徑級數量充足，而大徑級數量逐漸減少之趨勢，表示族群有連續的繁殖與更新。針對植群結構之各個項目與距邊緣距離進行two-phase linear regression分析結果顯示，實生苗及小型小桿材的密度及胸高斷面積總和在迴歸分析上皆未達到顯著水準；大型小桿材的密度及胸高斷面積總和、樹木之密度及胸高斷面積總和、林冠鬱閉度、樹冠高度及物種豐富度等皆呈現正相關，顯示愈往森林內部愈有增加之趨勢；陽性樹種的重要值則與距邊緣距離呈現負相關，顯示其愈往森林內部愈少。本研究利用物種分布取向分析發現假柃木（Eurya crenatifolia）等三個邊緣物種（edge-orientated species）及米碎柃木（Eurya chinensis）等五個內部物種（interior-orientated species）。邊緣影響距離（distance of edge influence）計算之結果大多落在35 m或65 m處。植群分析方面，綜合群團分析（cluster analysis）及降趨對應分析（detrended correspondence analysis）之結果將本研究樣區分成兩植群型，分別為尖葉槭-假柃木林型及紅楠-牛奶榕林型，前者樣區多位於森林邊緣，後者則多位於森林內部。以上結果使得本區植群結構的相關生態資料更為詳盡，且可作為未來相關研究之參考依據。

The objective of this study was to analyze variations in vegetation structure along forest edge-to-interior gradient in Cingtiangang area, northern Taiwan. Chingtienkang area was charactered by its Axonopus affinis pasture that was mainly resulted from local climate, grazing cattle (Bubalus bubalis) and human activities. To analyze variations in vegetation structure (plant composition, species richness, canopy coverage et al. ), 12 transects were established from edge into interior of forest, and aerial photographs were used to detect the age of forest edge and temporal dynamic of pasture distribution. Results showed that pasture in Dajianshan area had existed for at least 20 years, and, due to cloud contamination, another pasture in Shibai area could only be traced back to 1994. The area of pasture in Dajianshan area had reduced during the past 20 years, but I found no significant change in area of the pasture in Shibai area during the past 10 years. A total of 75 seed plant species, dominated by Lauraceae, was recorded in this study. Inverse J-shaped (40％) and L-Shaped (50％) were the main population structure types of plant species in study sites, indicating most plant species were continuously regenerated in study site. Two-phase regression was used to analyze trends of vegetation structure along forest edge-to-interior gradient in this study. Density and total basal area of seedlings and small saplings showed no significant trend along this gradient, whereas density and total basal area of large sapling and trees, canopy coverage, and plant species richness were significantly greater in the forest interiors. Importance value index of pioneer plants increased from the forest interiors towards the edges. Three edge-orientated species (e.g. Eurya crenatifolia) and five interior-orientated species (e.g. Eurya chinensis) had been found in this study. The distance of edge influence on various variables of vegetation structure mostly fell between 35 m and 65 m from the edge. Based on cluster analysis and detrended correspondence analysis, vegetation of study site was classified into Acer kawakamii - Eurya crenatifolia type (mainly occurred near forest edges) and Machilus thunbergii - Ficus erecta var. beecheyana type (mainly distributed in forest interiors). Results of this study not only provide detailed ecological information of vegetation structure in Chingtienkang area but also have important implications for future researches.