台北縣三芝鄉淺水灣西側海岸沙丘地形及植群分布關係之研究

Abstract

海岸沙丘為動態變化相當快的風成地形，風、地形和植群在此系統內交互作用，地形會影響植群受風、波浪、鹽霧影響的程度及獲取資源的可及性，進而影響植群型的分布。本研究進行植物調查以了解沙丘植物社會組成，並配合地形的現地量測生產出DEMs，進行東北季風前、後期的地形調查，了解沙丘的地形特徵及短期的高程變遷。環境變數選擇高程、與海距離、坡度、坡向、沙粒移動性、沙粒粒徑等地形參數，樣區植物資料以TWINSPAN、CCA分別進行植群分類及排序，並以統計方法分析不同植群型與地形參數的關係，釐清淺水灣西側海岸沙丘的地形特徵對植群分布的重要性。 研究結果顯示，海岸沙丘地形大致可區分成圓頂前丘區、沙丘平台區、縱丘脊區、沙丘斜坡區四類地形單元。其中，沙丘高程變動最大的地區為最靠海前端的圓頂丘、侵蝕坡及斜坡區的坡腳處，由於近海的位置有豐富的沙源供給而使沙丘堆積，但也易受波浪、風力的侵蝕而加速風蝕窪地的形成。在坡度驟升之處因氣流受到擠壓而朝上方加速而侵蝕，使迎風坡的坡腳處侵蝕量最多。 193個植物樣區共記錄到29科53種植物，其中出現最多的前三科為菊科、禾本科和旋花科，物種以白茅及濱艾為優勢。植物社會的分類結果顯示，共有狗牙根-裂葉月見草型、濱刺草-天蓬草舅型、白茅-濱艾型、大花咸豐草-濱豇豆型、槭葉牽牛-雞屎藤型、林投-海桐型六種。CCA及統計檢定的結果認為高程、距海遠近為主要影響植群變異的因子，植群大致有由海岸線往內陸及受高程變化影響而呈現出帶狀分布類型，其他地形參數對植群分布則有不同程度的影響。 圓頂前丘區主要為濱刺草、蔓荊生長，因為不連續的植物覆蓋，促進沙粒堆積形成圓頂丘。而在另一區，白茅-濱艾型的植物大範圍地生長，刺激沙粒堆積成沙丘平台，沙丘平台區及縱丘脊區為較平緩的地勢，其最前端的沙丘易受波浪、風等切割成侵蝕坡，濱刺草-天蓬草舅型為最能適應海岸前端擾動大、沙粒移動性高的環境，可以穩定侵蝕坡、增加植物覆蓋，一旦侵蝕坡的植被遭破壞，可能持續侵蝕成風蝕窪地，而此類低凹地由於較接近地下水面，潮濕且遮蔽的環境較能供應以木本植物為優勢的林投-海桐型的生長。當氣流沿風口入侵，使風蝕窪地加長、加深，甚至吹蝕成拋物線形沙丘。而南側斜坡區由於前方植被較稀疏，沙粒移動性最大，在此出現數道拋物線形沙丘，白茅-濱艾型的植物僅在脊部生長；北側斜坡因坡度較陡，攀緣藤本莖部柔軟，較能適應地形較陡峭且迎風的斜坡脊部，以槭葉牽牛-雞屎藤型為主。而斜坡區最高處的平台則為大花咸豐草-濱豇豆型的分布，分布位置最高且離海較遠。狗牙根-裂葉月見草型主要出現在路徑兩旁，屬於偏好出現在人為干擾較頻繁的一型。 整體而言，海岸沙丘植群在不同地形因子影響下，植群結構、組成、功能、多樣性等產生些許差異，且在不同地形環境中亦可發現有不盡相同的優勢族群。海岸沙丘生態系統是複雜的，需要有更多的環境因子解釋系統的運作，更有助於瞭解植群的空間分布。

Coastal dunes are very dynamic landforms where wind, geomorphology and vegetation interact within the system. Geomorphology affects exposure to wind, waves, salt spray, and resource available for vegetation. The aims of this study were to explore the geomorphology characteristics that affect coastal dune vegetation distribution in the western Chianshuiwan, Sanjhih of Taipei County. The field surveys included collecting geomorphology and vegetation data. The geomorphological characteristics were identified by establishing DEMs before and after the northeastern monsoon, and short-term changes were obtained by a subtraction between two DEMs. The six environmental variables were elevation, slope, aspect, distance from shoreline, sand movement and grain size. Classification of the vegetation was based on TWINSPAN, while CCA was used to identify environmental gradients linked to community distribution, and statistical analysis were used to compare geomorphological parameters between different vegetation types. After a survey of contour maps, four geomorphological units were distinguished: frontal hummocks, dune platforms, longitudinal dune ridges, and dune slopes. Where elevation change most were frontal hummocks, erosion scarps and the foot of slope. In spite of abundant sand supply, wave and wind erosion happened frequently in frontal dunes, then irritated the formation of blowouts. On the margin of windward slopes, airflow was compressed both laterally and vertically, and the deflation became concentrated toward the crest. Fifty three species in twenty nine familes were recorded from 193 quadrats located within the study area. Asteraceae, Poaceae and Convolvulaceae were the most frequent families, and Imperata cylindrica var. major, Artemisia fukudo were dominant species. The samples were classified into six vegetation types including Cynodon dactylon - Oenothera laciniata, Spinifex littoreus - Wedelia prostrate var. prostrate, Imperata cylindrica var. major - Artemisia fukudo, Bidens pilosa var. radiata- Vigna marina, Ipomoea cairica- Paederia foetida, and Pandanus odoratissimus- Pittosporum tobira. The main gradients, identified by CCA, were elevation and distance from shoreline. The vegetation zones were not only parallel to shoreline, but also varied with landform characteristics. The effect of other variables on vegetation distribution was different. The discontinuous plant growth irritated sand accretion to form frontal hummocks, dominated by Spinifex littoreus and Vitex rotundifolia. Plants encouraged sand deposition over a wide range, leading to the development of sand platforms. Imperata cylindrica var. major - Artemisia fukudo extensively established on sand platforms and longitudinal dune ridges. The frontal dunes where Spinifex littoreus - Wedelia prostrate var. prostrate dominated were scarped by wind and waves. Because Spinifex littoreus - Wedelia prostrate var. prostrate was adapted to frequent disturbances and high sand movement, and could stabilize the foot of slopes. Once the vegetation were destroyed by disturbances, and this would led to the formation of blowouts. Then the airflow moving through gaps into the dunes made blowouts larger and deeper, until closer to the water table. The blowouts and slacks which were wetter and more sheltered could supply Pandanus odoratissimus- Pittosporum tobira shrubs with more water and nutrients. While airflow continued to erosion, the blowouts were elongated to form parabolic dunes. The largest amount of sand movement was found on southern slopes where the rake-like parabolic dunes were formed, and only Imperata cylindrica var. major - Artemisia fukudo grew on the ridges. The northern slopes were steepest in the study area, and Ipomoea cairica- Paederia foetida vines with flexile stems usually climbed upward on the steep and windward slopes. Bidens pilosa var. radiata- Vigna marina was dominant in the farthest upland platforms and closer to inland. In conclusion, the geomorphological characteristics led some difference in coastal dune vegetation construction, composition, function and species diversity, and had different dominant communities. Coastal dune systems were too complex to understand, and needed more factors to interpret processes and vegetation distribution.