| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 93 === 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.
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