Pollen Stratigraphy, Vegetation and Environment of the Last Glacial and Holocene—A Record from Toushe Basin, Central Taiwan
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ARTICLE IN PRESS Quaternary International 147 (2006) 16–33 Pollen stratigraphy, vegetation and environment of the last glacial and Holocene—A record from Toushe Basin, central Taiwan Ping-Mei Liewa,Ã, Shu-Yue Huangb, Chao-Ming Kuoc aDepartment of Geosciences, National Taiwan University, 1, Sec. 4, Roosevelt Rd. Taipei 106, Taiwan, ROC bDepartment of Life Science, National Taiwan University, 1, Sec. 4, Roosevelt Rd. Taipei 106, Taiwan, ROC cChinese Petroleum Corporation, 3, Sung Ren Rd, Taipei 11010, Taiwan, ROC Available online 9 November 2005 Abstract The pollen record from the Toushe Basin (231490N; 1201530E; 650 m above sea level), a peat bog of central Taiwan, displays a continuous vegetation history of the past 96,000 yr BP of monsoon Asia. Instead of today’s closed subtropical evergreen broadleaved forest dominated by Machilus–Castanopsis surrounding the basin, temperate deciduous forest predominated during most of the last glacial. In early MIS 4, Alnus reaches the highest value of the whole sequence (60–70%) representing temperate deciduous forest and relatively cold and arid conditions. Following this stadial, Alnus and herbs (mainly Cyperaceae) dominated alternately, with a minor increase of Castanopsis. Peaks of monolete spores between cal. 42.2 and 37.0 kyr BP (kyr BP represent calibrated years) indicate episodic wet conditions. The later glacial, especially between 23.2 and 18.7 kyr BP, shows a high percentage of Gramineae, indicating dry and possibly sometimes cold conditions. The late glacial shows a remarkable increase of warm-temperate to temperate forest elements, such as Ilex, Cyclobalanopsis and Symplocos. At about 15.1 kyr BP a peak of monolete spores indicates wet–warm conditions. A subsequent sharp increase of Salix and then Gramineae between 13.0 and 11.6 kyr BP corresponds to the Younger Dryas. A warming event at 11.5 kyr BP is also evident. The Holocene is characterized by warm–wet conditions of the overwhelmingly abundant monolete spores since 10.7 kyr BP and the prominent increase of Castanopsis. r 2005 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction warm (Huang et al., 1997) as previously estimated by CLIMAP (1981). This is also shown for other low-latitude Toushe Basin (231490N; 1201530E; 650 m above sea level), terrestrial areas (Flenley, 1979; Hooghiemstra, 1989; a 1.75 km2 desiccated peat bog in the low hills of central Farrera et al., 1999). On the other hand, a chronological Taiwan, is one of a series of north–south trending middle discrepancy of climatic events between the two poles is Pleistocene tectonic basins along the island’s backbone recognized (Sowers and Bender, 1995). Thus, the latitu- (Fig. 1). To the north lies Sun-Moon Lake (or Jih-Yueh dinal variations of regional climates should be better Tan, 750 m above sea level) where a pollen sequence understood before attempting to interpret the global covering time since the last glacial has been reported by features of climate change, even though the climate Tsukada (1967). Mountains surrounding these basins range conditions of higher latitude since the last glacial have in altitude from 700 m to higher than 1000 m eastward. The been well documented. The tropical–subtropical record is fluvial and lake deposits in Toushe Basin are about 80 m crucial in understanding the driving force of climate change thick. The bog became desiccated at about 1.8 kyr BP. from a global point of view (Stock, 1999). Recent studies In the context of global climate, changes in low-latitude from the stalagmites of Hulu Cave (China) show that the areas have the same importance as those at high latitudes. timing of changes in the monsoon generally agrees with the Marine records of the last glacial from offshore Taiwan timing of temperature changes from the Greenland ice core show that low-latitude sea-surface temperature is not as GISP2 (Wang et al., 2001). This indicates that the East Asian monsoon is integral to millennial-scale changes in ÃCorresponding author. Tel.: +886 2 33662932; fax: +886 2 23636095. atmosphere/oceanic circulation patterns and is affected E-mail address: [email protected] (P.-M. Liew). by orbitally induced insolation variations. Remarkable 1040-6182/$ - see front matter r 2005 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2005.09.003 ARTICLE IN PRESS P.-M. Liew et al. / Quaternary International 147 (2006) 16–33 17 Fig. 1. Location map of the studied site with the climate conditions of Sun-Moon Lake. vegetation and climate changes during glacial time in the whole island is generally humid. According to the subtropical island of Taiwan were found in a previous meteorological data near the study site (Fig. 1), the Sun- study of Sun-Moon Lake (Tsukada, 1967). According to Moon Lake Station (altitude 1014 m) immediately north of his study, by excluding Alnus pollen, an early stadial at the Toushe peat bog, mean annual rainfall is 2341 mm, some time subsequent to 60,000–50,000 BP contains pre- annual evaporation is 1098 mm and mean annual tempera- dominantly boreal conifers and pine and a low percentage ture is 19.2 1C. The coldest month has an average of temperate elements, indicating a temperature decrease of temperature of 13.9 1C, whereas the warmest month is 8–11 1C; the period from about 50,000 to 10,000 BP is 23.6 1C, with an average of 155.6 rainy days. The estimated dominated by cool-temperate species. However, there are mean annual temperature of Toushe is 21.2 1C, the lapse only four radiocarbon dates in the core and a detailed rate being 0.54 1C/100 m. The present vegetation surround- record of climate change is not available. It is important to ing the study area belongs to the subtropical evergreen obtain a high-resolution pollen record with good age Lauro-Fagaceae forest. This forest consists mainly of control from the last glacial in Taiwan so as to reveal the Machilus kasanoi, M. zuihoensis, Beilschmiedia erythrofolia, synchronism (or not) of global climate events. Thus, the Phoebe formosana, Sapium discolor, Michelia formosana, Toushe Basin was chosen for a study of its palynostrati- Cyclobalanopsis flauca, Pasania uraiana, P. konishii, graphy. We use biomization of fossil pollen assemblages as P. ternaticupula, P. brebicaudata, Ardisia sieboldii, Zelkova proposed by Prentice et al. (1992, 1996) and used for the fomosana, Engelhardtia roxburghiana, Glochidion hongkon- reconstruction of vegetation in Europe, Africa and Asia to gensis, Trema orientalis, Liquidambar formosana, Rhus discuss the climatic conditions of the stadials and succedanea, Schefflera octophylla, Castanopsis hystrix, interstadials of the last glacial and Holocene based on Quercus variabilis, Fraxinus formosana, Lagerstroemia the record of surface pollen assemblages of the natural subcostata, Symplocos theophrastaefolia and Sapindus forests nearby in the Salixian area (Jolly et al., 1998; mukorosii among others (Lin et al., 1968). Tarasov et al., 1998; Yu et al., 1998; Allen et al., 2000; Tsukada (1967) described the vegetation of mountain Takahara et al., 2000; Gotanda et al., 2002). We aim to forests above the nearby subtropical forest as follows: describe the vegetational changes quantitatively so as to interpret the magnitude of possible environmental Warm-temperate forest (ca. 500–1800 m): Dominated changes. The simplified pollen diagram of the upper 17 m by Castanopsis, Lithocarpus, Cyclobalanopsis and Cin- of this site has been published previously (Liew et al., 1998; namomum with other broadleaved species and with Kuo and Liew, 2000), but data to 39.5 m depth will be conifers such as Keteleeria and Podocarpus species. The described here. undergrowth is crowded with ferns and mosses. Cool-temperate forest (ca. 1800–2400 m): Composed of 2. Modern vegetation and climate of the study area deciduous hardwood species of Cyclobalanopsis, Ulmus, Zelkova, Juglans, Carpinus and others, mixed with Taiwan is a subtropical mountain island whose climate is conifers including Chamaecyparis. Cyclobalanopsis and dominated by the East Asian monsoon. Warm–wet the Chamaecyparis species form two separate associa- summers and cool-relatively dry winters prevail and the tions but both groups occupy the misty climate belt. ARTICLE IN PRESS 18 P.-M. Liew et al. / Quaternary International 147 (2006) 16–33 Subalpine coniferous (or boreal) forest (ca. 2400–3500 m): C. stenophylloides, Trochodendron and Castanopsis Composed of Tsuga chinensis, Abies kawakami and carlesii. Picea morrisonicola mixed with Pinus armandii. Above The Upper Quercus Zone is often mixed with montane about 3300 m, alpine shrubs and herbs occur. mixed coniferous forest including Chamaecyparis, Pinaceae and Taxodiaceae, although they may separate Su (1984) studied the vegetation in the mountains of into associations. When local conditions are less humid, central Taiwan and identified the following altitudinal montane deciduous broadleaved forest appears in zones (Fig. 2) with annual temperature range: Quercus Zone including species of Acer, Juglans, Ulmus, Carpinus, Platycarya and Quercus. When (1) Ficus–Machilus Zone (below altitude 500 m; 23–26 1C; aridity increases Alnus formosana prevails in this zone tropical): Lowland evergreen broadleaved forest in- although this tree may appear at altitudes between 900 cluding species of Ficus and Machilus. and 2600 m. However, Alnus with Salix, Carpinus and (2) Machilus–Castanopsis Zone (500–1500 m; 17–23 1C; Acer frequently appear near 2000 m, whereas Alnus subtropical), submontane evergreen broadleaved forest associated with Urticaceae usually occurs below this with two major types: 1. Castanopsis type: mainly altitude. In addition, Pinus exists in the still drier composed of Castanopsis hystrix, C. Kawakamii, conditions of this zone. Schima superiba, Engelhardtia, Lithocarpus and 2. (5) Tsuga–Picea Zone (2500–3100 m; 8–11 1C; cool tempe- Machilus type: mainly Machilus japonica, M. kusanoi, rate), with major components Tsuga chinensis, Picea Ficus, Lagerstroemia and tree fern species. morrisonicola and Pinus armandii mastersiana.