淡水河系紅樹林分布之環境因子研究

Abstract

淡水河流域紅樹林及其在沿岸構成的沼澤濕地自1970年代後期成為保育標的開始，持續有許多動物、植物、生態、地理學等領域的研究以此為研究對象。紅樹林生長於水域與陸域的交界地帶，其植群分布是受自然環境風貌影響的結果，量化描述物理環境空間變異與特性可以藉助力學與數學工具來進行研討。本文由水利工程領域中河川水理、感潮鹽分入侵、河床演變等觀點出發，嘗試進行淡水河系水筆仔紅樹林枝環境影響因子研究。 研究中以水筆仔分布條件為基礎，經由現地調查，取得淡水河系水筆仔紅樹林分布上界與其分布密度之空間資料；利用NETSTARS數值模式模擬流域之潮流分布，配合鹽分入侵分布，進行潮流與鹽份影響分析；收集與演算流域河床型態及幾何特性，探討灘地形成之地形特徵，歸納出淡水河流域各河段影響與限制水筆仔紅樹林分布之環境因子。 研究結果顯示，水筆仔幼苗潮流傳播範圍大於目前分布上界，潮流環境因子並非影響目前水筆仔紅樹林片狀分布及限制其分布界限之環境因子。鹽分相關分析發現，水筆仔分布上界與鹽份入侵終點在流域中的邊界位置相互吻合，感潮鹽分限制了淡水河流域水筆仔成株分布之上界。然而在鹽水界內，變異數分析結果顯示鹽分變動無法完全解釋水筆仔植株密度的分布差異，鹽分濃度此一環境因子並非造成水筆仔紅樹林呈不連續片狀分布的主因。河川幾何特徵與河床特性分析結果認為水筆仔紅樹林在鹽水界內淡水河主流之分布可以寬深比值為一判斷參考，除特殊平面變化之地形，河段內適宜水筆仔生長之斷面寬深比值有一下限值，150。對於鹽分可達的感潮河段而言，不同河川型態的幾何特徵與河床特性才是致使水筆仔紅樹林呈現不連續片狀分布的主因。

Since the late 1970s, many zoological, botanical, ecological and geography research took the mangroves in Tanshui River as the object of study. In order to describe the relationship between the distribution of mangrove and the physical environment in quantitative methods, this study try to analyze the environmental factors of mangrove distribution in the viewpoints of hydraulics and river morphology. The investigations of mangrove distribution and collections of hydraulic characteristics were evaluated and calculated. The average velocity during the flood tide, tidal salt intrusion and river channel geometry were used to analyze the relationship between the mangrove distribution and the differences of its habitat environment factors along the river. The research result shows that the flood tide didn’t limit the drifting and expansion of mangroves in Tanshui River. However, the growth and embedding of mangroves were restrained by the salinity. In the salt water sector, channel geometry and characteristics effect the formation of the sands as the habitat of mangroves. The depositions were mainly caused by the suddenly contraction and gradually expansion of the channel, which could be found out with the width to depth ratio and contraction ratio as reference indexes. For the tide-effected reaches of the Tanshui River, the patchy distribution of mangrove is the result of the geometric patterns of different characteristics of the river.