利用穩定碳同位素與黑炭推估臺灣塔塔加地區主要植群型可能之變遷

Abstract

暸解植群改變與演替機制有助於推測植群在環境變遷時的適應 狀態，可為森林經營提供重要理論依據。臺灣二葉松被認為是以林火來維持演替狀態，但缺乏直接證據證明。選擇臺灣中部塔塔加鞍部地區觀察草原與森林植群的演變遷過程，本研究目的即透過穩定碳同位素推估植群之變遷，並探討植群改變是否對土壤有機碳的分解與組成造成影響，又是否曾有林火發生的黑炭存在。 本研究樣區為塔塔加鞍部北面集水區，包括各植群與其相鄰的推移帶：二葉松疏林區、草原區、草原與鐵杉森林推移帶、鐵杉森區、鐵杉與雲杉林推移帶及雲杉森林區，並於所規劃集水區外選取兩參考樣點：二葉松森林與麟趾山草原區。研究分析土壤基本理化質、並分別將植體、土壤有機碳、以及分解試驗後的物質進行穩定碳同位素比值分析（GIRMS）與碳同位素核磁共振分析（13C CPMAS NMR）。運用穩定碳同位素比值辨識 C3 與 C4 植物，進行土壤分解試驗來探討土壤性質的轉變以及黑炭存在與否。 土壤基本理化性質顯示土壤 pH 值為 3.1~ 5.6，除鐵杉與雲杉推移帶區（EHS）因位於小型崩塌區， pH 值約在 6.8~ 7.6。總碳量皆在1.4~ 19.0 %間，惟參考樣點麟趾山草原（G）總碳量皆較??，約12.7~31.62%。此外，陽離子交換容量為 32~ 4.9 cmol(+)kg-1、可交換陽離子含量在 12 cmol(+)kg-1 以下，鹽基飽和度則約在 6.04% 以下，除雲杉、雲杉與鐵杉間推移帶（EHS）約在 12~ 60%。 植物穩定碳同位素分析結果顯示，當地主要的 C4 型植物為高山 芒， C3 植物則包括二葉松、鐵杉、雲杉、玉山箭竹等。研究地區各剖面的深層土壤顯示 C4 植物所佔比例在 28 ~ 49% 之間，淺層土壤則顯示有 C3植物增加之趨勢：其中二葉松疏林與草原區早期 C3、 C4 植物比例相當，土壤碳同位素比值（δ13C）相近（-17.45 ~ -24.66 permil），但 C4 植物漸漸成為主要優勢物（~60%）。此外，塔塔加草原地區在民國 50 年曾發生林火，又試驗前有機碳可能已初步進行分解，土壤酸解分析結果則推測可能有黑炭存在。 然而，鐵杉與草原推移帶、鐵杉森林、雲杉森林與其推移帶則顯示 C3 植物維持優勢，且變動較小， δ13C 約 -23 ~ -26 permil（C4 植物所佔比例約在 20~ 30%）。 土壤分解試驗可了解有機碳的分解動態，結果顯示仍殘存有不溶於酸之烷基碳以及難以分解的芳香基碳，而土壤在剖面深度增加時，土壤中芳香基碳的比例相對增加；兩者說明，烷基碳可抵抗酸性 化學分解，而芳香基碳則具有化學與微生物分解抵抗力與穩定性。 由於土壤分解試驗無法完全分解除了黑炭之外的有機物質，無法直接顯示是否存在黑炭；因此不適宜作為分析黑炭定量之試驗，相關方法尚須進一步探討。 又高山芒為此樣區中最主要的 C4 植物，若能暸解高山芒生態適應 與競爭優勢，有助於推測植群變遷之機制。

To understand vegetation change and succession is helpful for predicting vegetation adaptation to climate change and providing an important theoretical basis for forest management. It is believed that pine (Pinus taiwanensis) forest keeps its succession through wildfires, but yet no direct evidence is proposed to verify such a mechanism. Therefore, our objectives were to infer possible vegetation changes at Tatachia Long Term Ecological Research area using carbon stable isotope and black carbon. And the soil organic carbon hydrolysis method was used to infer the existence of black carbon and the effect of vegetation change on soil carbon dynamics. In this study, the northern catchment of the research site was selected. The main vegetation types included pine woodland (P), meadow (M), ecotone between hemlock forest and meadow (EHM), hemlock forest (H), ecotone between hemlock and spruce forest (EHS), and spruce forest (S). Additionally, two reference sites were chosen at a pine forest (P0) and grassland (G) outside the catchment. Analyses included soil basic physicochemical properties, δ13C and 13C NMR of soil and plant samples, HCl-insoluble organic carbon hydrolysis. δ13C can be used to identify vegetation composition of C3 and C4 plants. HCl-insoluble organic carbon hydrolysis combined with 13C NMR can indicate black carbon produced by fires in soil. According to the analysis of soil basic physicochemical properties, soil pH ranges from 3.1 to 5.6 except for EHS (6.8 -7.6). The total carbon content ranges from 1.4 to 19.0% except for G that has 12.7 -31.6%. Moreover, the capacity of exchangeable cations is 4.9-32 cmol(+)kg-1. The content of the exchangeable cations of all samples is below 12 cmol(+)kg-1 and the base saturation is below 6.04 % except for EHS and S that range from 12 to 60%.

Results of plant δ 13C analysis showed that only Miscanthus transmorrisonensis is a C4 plant among the collected samples while C3 plants include pine, hemlock, spruce, and bamboo. In the deep part of soil profiles, the percentage of C4 plant in soil organic carbon of all samples is 28- 49%. However, in the surface part the C4 plant ratio increases with increasing soil depth. In the P and M pedons, both have a similar δ13C distribution that ranges from -17.45 to -24.66?. The distribution of δ13C indicates C4 plants increases with time to 60%. Additionally, it was once reported that a fire occurred in the M plot in 1961. Results obtained from carbon stable isotope, HCl-insoluble organic carbon and 13C NMR analysis demonstrated that soil organic matter decomposed by fire at 10– 15 cm depth and resulted in the increase of alkyl-C and aromatic-C, suggesting black carbon was present in the M plot possibly due to wild fires. However, the δ13C values range from -23 to -26 ? in the plots of EHM, H, S as well as EHS, suggesting C3 plant dominated in those plots with little variations. The dynamics of soil organic carbon can be understood by soil acidhydrolysis method. The results in this study showed that HCl-insoluble alkyl-C and aromatic-C remained after acid digestion. In the soil profile, the ratio of aromatic carbon increased with increasing soil depth. Synthetically,the alkyl-C is refractory to chemical hydrolysis while aromatic-C is stable and resistant to chemical and biological degradation. After the hydrolysis with 6M HCl, the samples were not completely digested according to the 13C NMR results. Except for black carbon, alkyl-C was still present after the digestion. Thus the black carbon identification method should be improved. Since Miscanthus transmorrisonensis is the dominate C4 plant, it will be helpful in understanding the mechanism of vegetation change through the investigations of its ecological adaptation and competition in this study area.