比較咸豐草三個變種的生物特徵以了解大花咸豐草在臺灣的入侵優勢

Abstract

外來物種的入侵是造成生物多樣性降低的重要原因之一。根據臺灣植物誌的記載，臺灣有三種咸豐草變種：小白花鬼針、白花鬼針與大花咸豐草。這三者之中，大花咸豐草最晚在臺灣開始有採集紀錄。然而，自從大花咸豐草被引入臺灣後，在短短三十年內，已成為臺灣中、低海拔路邊、廢耕地或荒地中最常見的野草。為了了解究竟是什麼樣的生物特徵，使大花咸豐草被引入後，在台灣的低海拔地區，比小白花鬼針與白花鬼針優勢，並成為臺灣嚴重的入侵植物，本研究比較此三種咸豐草變種的生長、繁育系統與生活史特徵。 在生物量的累積上，大花咸豐草、小白花鬼針與白花鬼針三者在夏季的相對生長速率均較秋季高，顯示高溫高光的夏季較適合此三種咸豐草變種生長。在夏季時，大花咸豐草的生長速率顯著高於另外兩變種；但在秋季時，三變種有相似的生長速率。由此推測：大花咸豐草相較於另外兩變種的生長優勢僅發生於高光高溫的環境中。此外，與另外兩變種相比，大花咸豐草具有較高的根部生物量比例、側枝的生長角度較為水平（較接近地面）、與較好的不定根生長能力，並具有二回側枝，這些特徵顯示大花咸豐草的營養繁殖潛力勝過另外兩變種。 研究三種咸豐草變種的開花生物學，發現大花咸豐草、小白花鬼針與白花鬼針三者均有次級花粉呈現與不完全雄先熟的現象，但在溫室中栽植時，只有大花咸豐草不會結瘦果，因此進行套袋實驗以了解其繁育系統。實驗結果確認大花咸豐草無法自交結瘦果，其繁育系統為自交不親和；而小白花鬼針與白花鬼針能於套袋中自交結瘦果，為自交親和。大花咸豐草有許多特徵於個體間有顯著差異，顯示大花咸豐草的高度異質性，可能是完全異交所造成的結果；而此特性也有助於大花咸豐草在不同的環境中生存、生長。 相較於另外兩變種，大花咸豐草雖然較晚開花，瘦果產量於實驗初期較低，但因小白花鬼針和白花鬼針植株在開完花後即死亡，為一年生植物，而部分大花咸豐草個體存活期間較長，並可透過無性繁殖的方式，產生大量分株，主幹在開花結果後，雖然會乾枯，但其分株可繼續生長、開花並結果，使其個體之生命週期可延長超過一年以上，並在栽植一年後，累積的瘦果產量超過另外兩變種。雖然小白花鬼針與白花鬼針所結瘦果較大花咸豐草瘦果重、發芽快，但三者最終發芽率並無顯著差異；且因大花咸豐草的瘦果較輕、較多，會比其它兩變種更有機會被動物攜帶、散播到遠方。 綜合實驗結果，我推論：大花咸豐草經由有性繁殖可以產生大量瘦果，這些瘦果因為有芒，容易吸附在動物身上，讓大花咸豐草可以擴散至四處。因為具完全異交的繁育系統，使得大花咸豐草具有高度異質性，能適應不同的環境，在許多不同棲地生存下來。而當大花咸豐草拓殖到一個新的棲地後，能透過無性繁殖，產生大量分株，使其在該地區能快速且持續地增加覆蓋面積。大花咸豐草兼具無性繁殖和完全異交的有性繁殖系統特徵，加上臺灣溫暖高光的氣候環境、土地利用的變遷，使得大花咸豐草能優於小白花鬼針與白花鬼針，成為臺灣嚴重的入侵植物之一。

The invasion of exotic species has become one of the most important factors reducing biodiversity. Three varieties of Bidens pilosa: var. minor, var. pilosa and var. radiata, were listed in the Flora of Taiwan. Among the three varieties, the first record of var. radiata in Taiwan was later than those of the other two varieties. However, after being introduced into Taiwan, var. radiata has become a common weed on roadsides, uncultivated field and waste grounds from low to middle elevations in the past three decades. In order to identify what traits let var. radaita dominant over the other two varieties and become one of serious invasive plants in Taiwan lowland after its introduction, I compared the growth, breeding system and life history traits of these three varieties. The relative growth rates of the three varieties were higher in summer than in fall. The result suggested that summer with high light and temperature was more suitable for the growth of all three varieties than fall. In summer, var. radaita grew faster and accumulated more biomass than the other two varieties. Thus, var. radaita had growth advantage over the other two varieties in regions with high light and high temperature conditions. In addition, var. radaita had higher root/biomass ratio, more horizontally oriented lateral shoots, and grew more adventitious roots than the other two varieties. Among the three varieties, only var. radiata had secondary axillary shoots. With these characters, var. radiata had a greater potential of vegetative reproduction than the other two varieties. It was found that all three varieties had secondary pollen presentation and were incompletely protandrous. However, when these three varieties grew sympatrically in a greenhouse, var. minor and var. pilosa produced achenes while var. radiata had no achene set. Bagging experiments were conducted to explain the phenomenon. From the results of bagging treatment, I confirmed that self-incompatibility was the key mechanism preventing selfing in var. radiata, while var. minor and var. pilosa were self-compatible and were capable of producing achenes in bags by selfing. Significant differences were found in many traits among individuals of var. radiata. The result indicated that high heterogeneities in many traits of var. raidata might result from its obligate xenogamous breeding system. Although var. radiata flowered later and set less achenes than the other two varieties, half individuals of var. radiata lived longer and formed ramets by vegetative reproduction. These ramets flowered and set achenes when matured. Consequently, var. radiata accumulated significantly more achenes than the other two varieties after being planted for one year. In addition, although achenes of var. minor and var. pilosa were heavier and germinated earlier than those of var. radiata, the final germination percentage was not different among the three varieties. Having more and lighter achenes, var. radiata would be dispersed by animals more easily than the other two varieties. In conclusion, massive achenes produced by sexual reproduction potentially allow var. radiata dispersing into far-reaching habitats. High heterogeneities were found in many traits of var. radiata, which might allow var. radiata to have widely fundamental niche. After established in a new habitat, var. radiata could generate many ramets by vegetative reproduction, expand horizontally, eventually occupied the area and became dominant. In summary, the breeding system and life history traits of var. radiata, in combination with the warm and high light climate and land use change in Taiwan confer var. radiata advantage over var. minor and var. pilosa and become one of serious invasive plants in Taiwan.