重複火燒對土壤養分、叢枝菌根菌及入侵植物生長 表現的影響─以台灣中部大肚山台地為例

Abstract

入侵植物─火燒境況循環所引起的重複火燒 (repeated fire)，已成為近年來重要的生態問題。重複火燒不僅造成植群的改變，也可能影響原有的土壤有效性養分及叢枝菌根菌 (arbuscular mycorrhizal fungi)的族群或群落，因而進一步改變生態功能以及植物間的競爭優勢。過去研究雖已顯示入侵植物─火燒境況循環對植群的影響，然而，較少研究去探討重複火燒對土壤養分與叢枝菌根菌的影響，以及其背後的機制和對入侵植物優勢的影響。本研究以台灣中部大肚山台地的大黍 (Panicum maximum) 入侵相思樹 (Acacia confusa) 保安林所引起的重複火燒為例，探討在重複火燒對土壤養分、叢枝菌根菌及入侵植物生長的影響。研究目的包括： (1) 瞭解重複火燒對土壤養分、叢枝菌根菌孢子豐度、群落組成及根內感染率 (percentage of root length colonized, RLC) 的影響；(2) 瞭解重複火燒影響叢枝菌根菌的機制；(3) 比較不同火燒序列階段，大黍與相思樹的生長表現的差異，並瞭解大黍生長表現與土壤養分、叢枝菌根菌的關係。研究結果顯示，重複火燒顯著改變土壤養分，經多次火燒的大黍草原土壤有較高的有效性鉀、鈣、鎂及pH值，相對的，其土壤交換性無機態氮及有效性磷卻通常顯著最低。重複火燒也明顯降低叢枝菌根菌的孢子豐度，並改變群落的組成，使群落組成從Glomus為主，逐漸變成Glomus和Acaulospora並存的情況。然而，孢子豐度及群落的差異並不總是顯著。我們推測菌根菌的火燒適應能力、沖蝕、土壤pH值及氮有效性是孢子豐度減少的可能原因，而群落組成的變化則與火燒次數及氮有效性有關。大黍的RLC也隨長期的重複火燒而明顯減少 (僅於Aug 2012顯著)。叢枝菌根菌群落及RLC的改變，皆暗示長期重複火燒之下，叢枝菌根的共生功能已逐漸改變。盆缽試驗進一步顯示，大黍在長時間重複火燒土壤的生長表現，顯著低於短時間重複火燒土壤或原生相思樹的土壤。施加氮肥能明顯提高大黍的生物量，但對各土壤的大黍生長表現的影響並無差異。然而，偏低的RLC以及菌根抑制實驗 (施加免賴得)，暗示叢枝菌根對於大黍的重要性可能低於土壤養分。相思樹盆缽試驗顯示，相思樹不論在長期重複火燒土壤或相思樹土壤均有一致的生長表現，暗示相思樹並不受到土壤養分及叢枝菌根改變的影響。總結來說，長期的重複火燒，明顯改變大肚山台地地區的土壤養分性質、叢枝菌根菌的孢子豐度、群落組成及RLC，而土壤氮、磷肥力的劣化可能已減弱入侵植物─火燒境況正回饋循環，並降低了大黍的入侵優勢。

Repeated fire, caused by invasive plant - fire regime positive feedback cycle, has become an ecological issue in recent years. In addition to the obvious vegetation conversion, repeated fire may also affect soil available nutrients and arbuscular mycorrhizal fungi (AMF), and subsequently alter soil function and plant competition. Studies have shown the effects of invasive plant – fire regime on plant community composition. However, few studies aimed to investigate the repeated fire effects on soil available nutrients and AMF, and even fewer studies tried to unravel the underlying mechanism and their roles in plant competition. We took the Acacia protected forest (Acacia confusa) which underwent repeated fire initiated by Guinea grass (Panicum maximum) invasion at Dadu tableland as example, and aimed to investigate the repeated fire effects on soil nutrients, AMF and invasive plant growth. Our study aimed (1) to understand how repeated fire affected soil nutrients, and AMF spore abundance, community composition and percentage of root length colonized (RLC); (2) to unravel the mechanism underlying repeated fire to affect AMF; (3) to compare the growth performance of Guinea grass and Taiwanese Acacia at different fire sequential stages, and to understand the influence of soil nutrients and AMF on Guinea grass growth performance. Results indicated repeated fire significantly altered soil nutrients. Under long-term repeated fire, soil available K, Ca, Mg and pH elevated significantly, but in contrast, exchangeable inorganic N and available P decrease significantly at Guinea grassland. Repeated fire also decreased AMF spore abundance, and altered AMF community composition significantly. Along with repeated fire fire, AMF community has shifted from Glomus dominated composition to Glomus and Acaulsopora coexistence. We suggested that fire adaption ability, erosion, soil pH and N availability were the underlying mechanisms to decrease AMF spore abundance, and the changes in AMF community were related to times of fire and N availability. Pot experiment demonstrated, Guinea grass growth performance at long-term repeated burnt soil, was significantly lower than at short-term repeated burnt soil or unburnt forest soil. Nitrogen fertilization could significantly improve Guinea grass growth performance, but its effect on Guinea grass grown in all sequential soil had no difference. The low RLC and mycorrhiza inhibition experiment (benomyl addition) implied the AMF is less important for Guinea grass growth than soil nutrients. Acacia pot experiment, which indicated Taiwanese Acacia seedlings growth was similar at all fire sequential stage soil, implied Taiwanese Acacia weren’t affected by changes in soil nutrients and AMF. To sum up, long-term repeated fire has dramatically changed soil nutrients, AMF spore abundance, community composition and RLC at Dadu tableland. Furthermore, the depletion in soil N and P may reduce the invasive plant - fire regime positive feedback cycle, and in turn reduce the dominance of Guinea grass.