大黍為何能在火燒後成功入侵大肚山地區？

Abstract

大黍 (Panicum maximum) 是一種禾本科多年生的草本植物，近年來被列為台灣危害力最高的二十種入侵植物之一。早期的研究指出：在大肚山與鐵砧山區的本地植物芒草 (Miscanthus sinensis) 為當地相思樹林反覆火燒後的最優勢種；但近年來同一地區的研究顯示：大黍已取代芒草的角色，而且發現大黍在火燒後植群覆蓋度顯著的增加，但芒草卻沒有相同的現象；反覆火燒使得大肚山地區的相思樹林逐漸轉變成以大黍為主的草生地。本研究藉由野外調查與溫室實驗，比較大黍與芒草在形態、光合作用、生長和生活史等特徵，目的在了解造成大黍成功入侵大肚山地區之因素。 野外植株測量發現：大黍比葉面積 (SLA, cm2/g) 和最大淨光合作用速率均顯著高於芒草，而光合作用氣體交換與穩定性同位素分析則顯示，兩者均為C4植物，且芒草有較高的水分使用效率。調查火燒與刈除後，經由地下根莖再萌蘗植株發現：在乾季時芒草比大黍有較好的恢復能力；但在濕季，大黍生長速率顯著增加。在生活史特徵方面，大黍有較重的種子和較長的花期與果期，而且大黍的果期主要在濕季，而芒草的果期主要在乾季，且有較高的發芽率。 溫室內生物分析實驗結果顯示，在高水分處理下，相較於芒草，大黍有顯著較高的生長速率和生物量的累積；兩者在低水分處理下生物量均顯著減少，且大黍減少的量顯著高於芒草；芒草在低水處理下會改變生物量的分配比例，大黍則無此現象。這些結果顯示芒草比大黍耐旱。栽植在不同的光照下，大黍都比芒草有更多分蘗枝和總生物量，有助於大黍有更高的覆蓋度與更多的花序生成。大黍刈除後在全光照環境下再萌蘗枝的生長比芒草顯著較優，遮蔭處理下再萌蘗植株生長也有較長的分蘗枝長，此有利於競爭光線。不管在水分或光度處理實驗都發現大黍比芒草有顯著較多的枯枝落葉，而且更容易受到乾旱壓力的影響而枯葉。 綜合以上結果顯示，由於在形態和光合作用等特徵上的優勢，再加上大肚山地區的濕季較長，大黍在濕季可以比芒草有更快速的萌蘗和生長，在與芒草競爭資源時佔優勢，使其能快速的增加覆蓋度；又大黍在濕季有很長的果期也有助於其建立更大的種子庫和更多小苗的生長；芒草雖然有適應乾旱的優勢，但因為大肚山乾季短，該優勢無法表現出來。由於大黍生物量累積顯著大於芒草，又比芒草容易受到乾旱的影響，因此在乾季時會產生較多的枯葉，而累積較多的可燃物而易於引發火災，火災的發生又會產生新的空間讓生長快速的大黍可以入侵與擴張族群覆蓋度，並導致下一個乾季有更多的易燃物且易燃物分佈的更廣。而在這樣反覆的火燒干擾下，大黍因此能逐漸取代芒草成為大肚山地區在火燒後的最優勢種。

Previous studies showed that Miscanthus sinensis, a native plant, was the dominant species of Acacia confuse forest after recurrent of burning in Dadu and Tiehchen mountain area. But recent survey reported that Panicum maximum, a perennial grass in the family of Poaceae and listed as one of the twenty most noxious invasive plants in Taiwan has replaced the role of M sinensis in those areas. As a result of recurrent burning, A. confuse forest has been gradually replaced by the grassland of P. maximum. In this study, I compared morphological, photosynthetic, growth and life history characters of field growing P. maximum and M. sinensis and conducted greenhouse experiments to understand the mechanisms contributing to the invasion of P. maximum after burning. Field growing P. maximum had significant higher specific leaf area (cm2/g) and net photosynthetic rates but lower water use efficiency than M. sinensis. In comparison of the two species, after burning or been pruned, I found that M. sinensis had better recovery ability in drought season, but P. maximum grew faster in wet season. The fruiting period of P. maximum was mainly in wet season, but M. sinensis mainly in dry season. In comparison to M. sinensis, P. maximum had significantly longer flowering and fruiting period and heavier seeds, but lower germination percentage. Greenhouse experiments revealed that under high water or high light availability P. maximum grew much faster, accumulated significantly more biomass, and produced more litters than M. sinensis. It indicates that P. maximum could have more coverage and inflorescence than M. sinensisin favorable conditions. However, under low water condition, P. maximum was more susceptible to drought than M. sinensis. P. maximum grew much better than M. sinensis in full light condition and much taller in shading environment after pruning. These results suggest that with the advantage of the morphological and photosynthetic traits in combination with longer wet season in Dadu mountain area, P. maximum can resprout and grow faster than M. sinensis in wet season which may help the species in competition for resource uptake after fire. In addition, a longer fruiting period of P. maximum might contribute to a larger seed bank which might enable the species to have more seedlings germinated than M. sinensis. Though M. sinensis is drought tolerant, the advantages of drought tolerance ability can not expressed in the area due to a short dry season. Due to its higher growth rate, P. maximum produces more litters than M. sinensis which would help inducing fire during dry season. The burning events create new space that P. maximum can easily invade again. The invasion of P. maximum makes the area prone to fire in dry season. As a result of recurrent burning, P. maximum replaces M. sinensis becoming the most dominant species in Dadu mountain area.