比較生長在不同海拔高度咸豐草 (Bidens pilosa) 族群間的差異

Abstract

大花咸豐草 (Bidens pilosa L. var. radiata) 和小白花鬼針 (Bidens pilosa L. var. minor) 為Bidens pilosa的兩個變種。小白花鬼針在1928年即已出現在台灣；大花咸豐草則在1984年引進，現已成為台灣低海拔地區嚴重的入侵植物。根據前人的研究和野外觀察發現，大花咸豐草和小白花鬼針在不同海拔高度的分布有差異：大花咸豐草廣泛分布於低海拔地區，小白花鬼針在低海拔地區的數量顯著低於大花咸豐草；然而越往中高海拔山區，小白花鬼針的族群數量增加，大花咸豐草則減少。故本研究目的在探討影響兩變種在中、低海拔分布差異的可能原因和大花咸豐草入侵中高海拔的可能性，並檢驗以下假設：一、在低海拔地區，大花咸豐草有其生長優勢，使得其族群數量高於小白花鬼針；二、中、高海拔的環境因子限制大花咸豐草生長，以致海拔愈高，其族群數量愈少；三、小白花鬼針在中海拔地區已有族群特化以適應環境。研究主要比較不同光度、水分環境對兩變種的影響，又模擬不同海拔的溫度變化以比較兩變種的種子發芽和莖部不定根生長情形，以及比較生長在嘉義台18線上低海拔 (約500 m)、中海拔 (約1300~1600 m)的大花咸豐草和小白花鬼針各族群在光合作用生理、形態與繁殖特徵上的差異；並且移植生長在不同海拔的族群至台大農場，比較在同一海拔環境下，兩變種間和不同海拔族群間的差異。控制環境因子實驗結果發現，大花咸豐草在高水分和高光的環境下比小白花鬼針具有較高的側枝相對生長速率，且高溫環境下，其莖部不定根生長速率較小白花鬼針快；田間實驗發現小白花鬼針死亡率較高，為一年生的植物，這些特徵差異可能是造成在低海拔環境下，大花咸豐族群數量高於小白花鬼針的重要原因。野外植株測量和控制環境因子實驗發現，大花咸豐草中海拔族群的光合作用表現不遜於低海拔族群，但低溫會降低大花咸豐草不定根生長速率和中海拔族群種子的發芽率，因此溫度是限制大花咸豐草入侵到中高海拔的重要因子。野外植株測量和田間實驗發現小白花鬼針中海拔的族群已有適應中海拔低溫和低光照環境的特化現象。

This study investigated two varieties of Bidens pilosa which have differential distribution pattern along altitudes in Taiwan. While B. pilosa var. radiata was introduced into Taiwan in 1984 and has become an invasive plant, B. pilosa var. minor has been in Taiwan since 1928. Field observation reveals that B. pilosa var. radiata is more widely distributed than B. pilosa var. minor at lower altitudes. However, in mid altitudes, population size of B. pilosa var. minor increases while that of B. pilosa var. radiata decreases. In this study, I investigated factors affecting the differential distribution pattern of these two varieties of B. pilosa between two altitudes and tested the following hypotheses, (1) B. pilosa var. radiata has growth advantages over B. pilosa var. minor at low altitudes, (2) environmental factors limit the growth of B. pilosa var. radiata hence cause its population size to decrease in mid altitudes, and (3) populations of B. pilosa var. minor at mid altitudes have developed genetic differentiation in adaptation to mid-altitudinal environment. To test these hypotheses, I compared the responses of these two varieties grown under different irradiance and watering regimes. The growth of adventitious root and seed germination rate of two varieties at two temperatures, 30/25 ℃ (day/night temperature) and 18/13 ℃, were also examined. I also measured morphological, physiological and reproductive characteristics of these two varieties growing at low (500 m) and mid (1300~1600 m) altitudes. Finally, I transplanted these populations to a farm at National Taiwan University to compare the growth of two varieties and to examine if genetic differentiation has occurred in these populations. When two varieties were grown in NTU farm at low altitudes, B. pilosa var. radiata had higher relative growth rate in lateral branch and longer life span than B. pilosa var. minor. In addition, B. pilosa var. radiata had higher growth rate of adventitious root than B. pilosa var. minor at high temperature. These traits may enable B. pilosa var. radiata to distribute more widely than B. pilosa var. minor at lower altitudes. Low temperature significantly reduced the growth rate of adventitious root and seed germination percentage of B. pilosa var. radiata. Therefore, low temperature is an important factor limiting the invasion of B. pilosa var. radiata into higher altitudes. Results from field measurement and transplant experiment indicate that the population of B. pilosa var. minor at mid altitudes has developed genetic differentiation in adaptation to lower temperature and lower light intensity environment.