地景對台灣亞熱帶山地雲霧森林木本狹幅種之局域物種豐富度影響

Abstract

亞熱帶山地雲霧森林（subtropical montane cloud forests, SMCFs）因其獨特之地理與氣候條件，使其終年雲霧繚繞。特殊的環境使其棲地與周圍的棲地大不相同，加上台灣複雜的地形，造就了此棲地分布不連續，形成棲地區塊。過去研究顯示，地景構型在此種不連續的棲地區塊中對於局域植物群集的組成及豐富度有著重要的影響，且對於僅適合生存於該棲地之狹幅種尤為重要。這些地景構型因子包含棲地的面積效應、棲地內環境異質性以及棲地的邊緣效應（edge effect），這些因子可直接影響局域物種豐富度，也可以藉由影響區域物種庫再經由物種庫效應（species pool effect）間接影響局域物種豐富度。根據理論，對於狹幅種而言，其局域物種豐富度與棲地面積效應以及棲地內環境異質性呈現正相關，與棲地之邊際效應呈現負相關。本研究以分佈於台海拔1400-2600 公尺之SMCFs為例，針對僅適合生存於雲霧林帶的雲霧森林木本狹幅種，旨在探討其局域物種豐富度是否受其周圍之地景構型影響。 首先，使用植物標本館資料庫中，每種植物之出現點位及其點位對應之雲霧頻度定義雲霧森林木本狹幅種。接著，從臺灣國家植群資料庫中，篩選出 108 個 20 m × 20 m 的樣區，並計算每個樣區中雲霧森林木本狹幅種之物種豐富度。而後，使用以雲霧頻度及植群資料預測之台灣雲霧森林分佈圖，以樣區為中心，分別以 500、750 及 1000 公尺之半徑，向外畫出圓形緩衝帶，並定義緩衝帶包含之雲霧森林為影響雲霧森林木本狹幅種之物種豐富度的棲地區塊，進而計算棲地區塊之地景構型參數值。本研究使用棲地區塊之地景構型參數包含了面積效應（SMCFs於緩衝帶面積比例以及連接度）、邊緣效應（樣區距離SMCFs邊緣的最近距離及緩衝帶內SMCFs 之邊緣比例）和環境異質性（地形異質性及植群型多樣性）相關參數。最後使用結構方程模型（structural equation modeling, SEM）進行分析。 結果顯示，500 及 750 公尺半徑的緩衝帶之資料有較佳的模型適配度。SMCFs之邊緣比例對於樣區中木本狹幅種之局域物種豐富度有顯著且直接的負影響，而樣區距離SMCFs邊緣的最近距離及面積效應與SMCFs之邊緣比例有顯著負相關。根據結果，我們推測，若樣區距離邊緣越近，SMCFs 面積及連接度越小，邊緣比例越高，木本狹幅種之局域物種豐富度越低，反之，若樣區距離邊緣越遠，SMCFs 面積及連接度越大，邊緣比例越低，木本狹幅種之局域物種豐富度越高。在SMCFs中，木本狹幅種之局域物種豐富度受到面積效應及邊緣效應的影響。相關研究顯示，臺灣的SMCFs在近年將面臨棲地面積減少的威脅，基於本研究結果，建議未來保育雲霧森林木本狹幅種之局域物種豐富度可優先保護大面積且較少邊緣之棲地。

Subtropical montane cloud forests (SMCFs) are frequently immersed in fog. In Taiwan, their distribution is restricted in a narrow elevation band between 1400 and 2600 m a.s.l., and due to complex topography is considerably patchy. Within this kind of discontinuous habitat, we hypothesize that landscape structure is an important factor that affects richness of local plant communities (defined as the number of species per fixed area plot). Here we specifically focus on specialists of SMCFs, i.e., species preferring to grow in the SMCFs habitat, for which surrounding habitats are unsuitable. The area, the edge and environmental heterogeneity are major landscape structures of SMCFs habitat patches that affect local species richness, either directly or indirectly through the species pool effect. We hypothesize that for cloud forest specialists, the area and environmental heterogeneity have a positive effect on local species richness, while the edge effect has a negative effect. This study focuses on SMCFs distributed in Taiwan and aims to explore how the landscape structure of the surrounding SMCFs habitat patch affects the local species richness of woody specialists in vegetation plots located within SMCFs habitat patches. Firstly, we used occurrence data from several major herbaria databases to identify specialists of SMCFs in Taiwan. Secondly, we selected 108 20 m × 20 m forest vegetation plots from the National Vegetation Database of Taiwan, and calculated the richness of woody specialists in each of them. Thirdly, we used a published predicted MCFs distribution map to estimate the landscape context of SMCFs around each vegetation plot within circular buffer zones of increasing radius (500, 750 and 1000 m). The variables of landscape structure include 1) the area of SMCFs habitat and its connectivity, both related to the area effect, 2) the nearest distance to the edge from the plot, related to the edge effect, 3) the proportion of edges, related to both area effect and edge effect, and 4) vegetation diversity and topographical heterogeneity, both related to the environmental heterogeneity effect. We used structural equation model to investigate which of the three mechanisms of landscape context has the strongest effect on the local richness of specialists. Results show that only the proportion of edges has a direct significant negative effect on local species richness of specialists in SMCFs. The latent variable of area effect and nearest distance to the edge also show significant relationship to the proportion of edges. We infer that higher edge proportion within the habitat patch, lower patch area effect and lower nearest distance of the plot to the edge correspond with lower local species richness in a vegetation plot. Both edge effect and area effect are important to the local species richness of woody specialists in SMCFs in Taiwan. SMCFs in Taiwan have been predicted to undergo habitat loss with ongoing climate change within relatively near future. If the decision about establishing protected areas within SMCF is focused on the protection of the richness of woody specialists, it will be better to choose larger and less discontinuous patches of SMCFs.