重複性火燒對淨初級生產量、土壤呼吸及土壤微生物的影響

Abstract

火燒是造成陸域生態系儲存碳量釋放最大的原因之一，近年來隨著全球氣候變遷，高溫及長期乾旱使森林火燒的強度、面積、及頻率皆更為加劇。一般而言，火燒頻率增加常伴隨著草類的入侵，而這些入侵草類在雨季萌發、乾季死亡並成為下一次火燒的燃料來源。如此正向回饋循環機制的結果，導致重複性火燒 (repeated fire) 的發生。台灣中部的大肚山台地保安林，原有植被為相思樹 (Acacia confusa)，但因大黍 (Panicum maximum) 入侵引起的重複火燒，並造成相思樹死亡，導致現有植群生態隨火燒次數不同大致可分為：(1)未發生火燒之相思樹林 (AC)、(2)大黍已入侵地被植物但仍未明顯受火燒的相思樹林 (IV)、(3)經一次或兩次火燒後殘存相思樹枯立木的草原 (SD) 以及(4)長期處在高火燒頻率的草生地 (GR)。我們以大肚山台地為例，探討高頻率火燒對淨初級生產量 (net primary production, NPP)、淨生態系生產量 (net ecosystem production, NEP)、土壤微生物量及族群的影響。研究結果顯示，重複性火燒明顯造成生態系統NPP的下降，SD與GR樣區之NPP僅為原有森林NPP的64%與30%，且NEP皆呈現負值 (-3.16及-4.76 Mg C ha-1 yr-1)，屬碳排放狀態，其中又以發生火燒次數最多之GR樣區的NEP最低，顯示高頻率火燒對碳循環帶來顯著的負向影響。另外，火燒也嚴重影響了土壤微生物，SD和GR微生物族群結構明顯與AC、IV不同，呈現AC與IV、SD、GR等三類分布差異。重複性火燒不僅造成GR微生物生物碳量最低，而且是唯一微生物族群的養分來源分配比例明顯呈現季節差異的樣區，這再次證明了多次重覆火燒是造成微生物差異的重要關鍵。

Fire is one of the most important factors that reduce terrestrial carbon stocks. Recently, the global climate change results in higher temperature and long-term drought, which increase the intensity, burning area, and frequency of forest fire. In general, rising fire frequency would promote the grass encroachment. The invaded grasses sprout quickly in rainy season and become fuels at the next fire event. This grass-fire positive feedback cycle usually leads to repeated fire. Take the protection forest at Dadu tableland in Taichung for example. The original dominate vegetation there was Acacia confuse. The repeated fire caused tree mortality and induced the invasion of the pioneer grasses such as Panicum maximum. Passing through about 30 years repeated fire, the vegetation type of Dadu tableland includes: (1) unburned forest (Acacia confuse, AC) (2) grass invaded forest (IV) (3) standing dead with grass (SD) (4) grassland (GR). We studied how the high-frequency repeated fire influences net primary production (NPP), net ecosystem production (NEP), soil microbial biomass, and microbial community. The results suggested that NPP and NEP were higher at AC and IV, and both NPP and NEP decreased significantly due to fire. The SD and GR sites only remained 64% and 30% NPP of original forest respectively, and became carbon source (NEP were -3.16 and -4.76 Mg C ha-1 yr-1, respectively). These data indicated that repeated fire has a quiet negative impact on carbon cycle. Besides, fire also influenced soil microorganism obviously. Principal component analysis (PCA) separated the microbial communities by times of fire (GR; SD; AC and IV). Microbial biomass carbon of GR, the site being burned most of times, was significant lowest. In addition, GR was the only site that showed seasonal fluctuation in C3 and C4 plant contribution to microbe. Therefore, we considered that the high-frequency repeated fire certainly plays a key role in microorganisms.