| Summary: | 博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 99 === The pollen and diatom preserved in lacustrine sediment are the good indicators for reconstructing the paleo-environment. The successions of pollen and diatom assemblages respectively infer to the vegetation history and aquatic condition as well as elaborate the climate changes. In order to reconstruct the nature history of East Taiwan, the sediments from tree lakes were used namely Liyu Lake of Hualien County, Tsuifong Lake of Ilan County and Dahu Lake of Ilan County.
Liyu Lake is a lowland lake situated in Hualien valley, from which a 658 sediment core (LYHL-B) was taken. The sediments covered the record form 7020 cal BP to present. However, the lacustrine sediment appeared after 2480 cal BP. Over 2480-2090 cal BP, a considerable amount of the aquatic plants pollen was found, suggesting an expansion of the swamp. Then, during 2090-1500 cal BP, it was exhibited by stable occurrences of water plant, Myriophllum spicatum and fluviatile diatoms, Fragillaria construens and Cymbella spp., suggesting a persistant occurance of the shallow water environment. During 1500-330 cal BP, a significant destitution of shallow water habitat indicator pollen, in association with lowering in fluviatile diatoms and increase in planktonic species, suggested the possible formation of inland lake condition. After 850 years BP, the co-occurrence of the cultivated Poacese and the charcoal suggest increased agriculture activity. Over last 330 years, increased frequencies of fern spore and euplanktonic diatom, Discostela stelligera, suggested the elevated monsoon precipitation.
Tsuifong Lake is a subalpine lake, from which a 55.5-cm sediment core (TF-2) was taken. It covered a time period of recent 1460 years. Over the early period, the climate was warm and dry and the lake was acidic and oligotrophic and remained little changes. A remarkable change to cooler climate was traced back to 630 cal BP (AD 1320), which was corresponding to the time of the Little Ice Age (LIA). It was indicated by the slight increase of Tsuga and changes in pollen assemblages. The climate remained dry in early Little Ice Age. However, the increases of alkaliphilous diatom assemblage, soil input and the erosions after ca. 470 cal BP (AD 1480) was possibly a result of elevated rainfall caused by the intense East Asia monsoon. Over last century, a decline in pollen influx was attributed to deforestation. This was associated with increases dominance of acidophilous diatoms, possibly as a consequence of acid rain.
A 3486 cm sediment core of Dahu Lake, a lowland lake, covered a time period of recent 7750 years. Over 7550-530 cal BP, the higher percentages of pollen or diatoms in the sediments indicated 8 traceable drought events, occurred at 7300-7280, 6630-6590, 4940, 4790- 4770, 4140-3690, 2040-2010, 1000-950, 530 cal BP. In ca. 2000 cal BP, the increase of lowland plant pollen as well as the appearance of wetland arboreal pollen, Barringtonia, indicated the possible beginning of the formation of the recent floodplain. Over ca. 310-130 cal BP an increase of euplanktonic diatoms indicated the elevated precipitation event related to East Asia monsoon. Moreover, increases in the amount of cultivated Poaceae pollen and open land pollen in recent 200 year were the indication of agricultural activity over that time.
Finally, we synthesized the fossil pollen data from 11 lakes and 2 land cores to integrate the alteration of forest covered areas in northern Taiwan with special respect to changes in humidity and temperature over the last two millennia. Our results suggested that the climate in northern Taiwan was stably cool and dry during 1500-1000 cal BP, but changed to wet and warm during 1000-500 cal BP, which corresponded to the Medieval Warm Period (MWP). In addition, an increased density and dispersal of Tsuga pollen corresponding to 500-200 cal BP was observed, which corresponded to the LIA. In recent decades, a decline in Tsuga pollen density and increased fern spore density indicated an elevation in temperature associated with increased precipitation. The results revealed that the temporal and spatial climate changes could be inferred from the palynological GIS mapping method, and that the fluctuations in temperature over time matched well with the global climate events, including MWP, LIA and recent warming.
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