A 5000-Year Pollen Record From Chitsai Lake , Central Taiwan

A 3.5 m core taken from Chitsai Lake (altitude 2890 m) of central Tai­ wan shows the vegetational and enVironmental changes in a Tsuga forest since 5000 yr BP. During about 5300-4840 yr BP (Zone 4), the Tsuga forest experienced a clear fluctuation toward a Tsuga-Cyclobalanopsis mixed forest. The high percentages of Cyclobalanopsis in the lower half of this zone (Sub­ zone 4b) indicate the last episode of the mid Holocene warm interval. Tsuga forest was well established in the later half of this zone. The extremely abun· dant spores . as well as the finest sediments together indicate high lake level and moist climatic conditions. Between about 4840 and 3730 yr BP (Zone 3), a warmer episode occurred in which Cyclobalanopsis increased slightly compared with Subzone 4a. Following this, during about 3730-2030 yr BP (Zone 2), the relatively high percentages of Abies, Ericaceae and Tsuga as well as lower percentages of Alnus and Pinus indicate cooler but stable en­ vironmental conditions. Zone 1 represents vegetational changes during the last 2000 years. The increase in Cyclobalanopsis indicates warmer conditions than before. The remarkable decline of Tsuga and Ericaceae accompanying the increase in the secondary forest elements such as Alnus and Pinus may indicate a change in environment. The larger grain size of silt in Zone 1 than in Zone 4, a higher erosion which may be due to stronger precipitation seasonality in a warmer climate. The upper 15 cm shows rapid increase in secondary forest elements such as Pinus and Alnus. Whether it is related to the increasing atmQsphere C02 is worthy of further investigation. (


INTRODUCTION
Over the last decade, there has been an increasing interest in understanding the global climatic changes.An alpine lake which is immune from anthropogenic influence would be an ideal site to provide proxy records for natural climatic changes.Located at an altitude of 2890 m above mean sea level in the Central Range of central• Taiwan (L at.23° 45' 10" N 11nd Long.121° 14' 10" E) (Figure 1), Chitsai Lake is topographically open with an are .a of 2.2 ha.The peat-bearing, 3.5 m-thick sediment taken by hand auger is suitable for C 14 dating and palynological study.There have been several pollen diagrams illustrating the pollen stratigraphic changes in lake sediments (Tsukada, 1967;Huang and Huang, 1977a;1977b) and fluvial -lacustrine sediments (Chen and Liew, 1990) as useful indicators for the late Pleistocene paleoenvironment of Taiwan.Yet, high-resolution records for the past several thousand years are badly needed to differentiate between natural variations and those attributed to human impact.Here we report a high-resolution, detailed pollen record of the past 5000 years obtained from Chitsai Lake.The modem vegetation around this lake now belongs to the Tsuga-Picea Zone of the mountain vegetation (2500-3100 mi n altitude), but down core variation in pollen should reflect past changes resulting from possible vertical displacements of vegetation during the last 5000 years.Samples were taken at 3 cm interval to achieve a temporal resolution of approximately 30-50 years.
An influential framework of Holocene subdivision in Europe is the Blyt-Semander model which is originally based on peat stratigraphy and fitted by the following pollen works (Roberts, 1989).It shows the periods of Pre-Boreal (10000-9500 yr BP); Boreal (9500-7000 yr BP); Atlantic (7000-5000 yr BP); Sub-Boreal (5000-2500 yr BP) and Sub-Atlantic (2500 yr BP to present).But it has become clear that Holocene vegetational changes were time transgression� (Wendland and Bryson, 1974).Thus, the observed data from each part of the world should be established.In Taiwan, an available pollen diagram from Taipei Basin (Chen and Liew, 1990) shows wanner conditions of middle Holocene at approximately 9000-6000 yr BP and cooler conditions there after.But further interpretation is impossible due to inadequate age control in that study, especially the recent half of the Ho.Iocene.This study may provide the possible climatic changes of this stage.

GENERAL DESCRIPTION OF THE STUDY SITE
This lake is surrounded by schists of the metamorphic belt of the Central Range.It is actually composed of two lakes: a big one (2 ha) and a small one (6000 m 2 ) about 60 m apart.A 3.5 m core was taken from the small one which has gradually desiccated recently.
The vegetation in the Central Range of central Taiwan has been divided into the following zones along altitude by Su (1984) Abies-Zone (3100-3700 m; 5-8 OC) According to the lake's altitude, the study site is within the Tsuga-Picea vegetation zone.It has been documented (Su;1984) that in the mountain area of central Taiwan, pure stands of Abies descend downward to about 3100 m and merge into the Tsuga forests.In the Tsuga-Picea zone, the upper montane coniferous forest is largely represented by the forest of Tsuga chinensis.Pure stands of Tsuga are very common while Picea morrisonicola are not frequent.They can grow" at lower elevations but are mixed with other conifers and hardwoods.In the Central Range at about 1500-2500 m the broad leaved forest is dominated by the forest with an association with oaks.However, the evergreen oaks have been named under genus Cyclobalanopsis.The Quercus zone, is divided into the upper and lower belts based on tree composition and ground vegetation types.In the upper belt, the dominant trees are Cyclobalanopsis morii; Cyclobalanopsis stenophylloides, Trochodendron ar alioides and Castanopsis carlesii; the ground vegetation is characterized by large colony of Plagiogyria glauca philippinensis and /or Yushania niitakayamensis.The lower belt is dominated by Cyclobalanopsis longinux and Cyclobalanopsis gilva, while the ground flora is rich in Dryopteris, Arachniodes, Selaginella and Monachosorum.At the submontane zone below 1500 m, the vegetation is Laurel-oak forests.Well drained upper slopes are dominated by Castanopsis whereas middle or lower slopes are characterized by Machilus.
The extant vegetation has been disturbed by fires during the past several decades.Thus, around the lake, a mixed forest of Pinus and other small trees indicates a. secondary suc cession after fire.Tsuga chinensis, Pinus, Juniperus formosana, Rhododendron are common in moderate relief area.Rubus hirsutopungens, Osmanthus heterophyllus, and Ranunculus matsudai are the low plants (Hsu and Chen, 1992).Yushania niitakayamen sis is widely distributed.Gramineae or Poaceae prevail including Deschampsia caespitosa, Deschampsia fl.exuosa, Miscanthus fl.avidus; Yushania niitakayamensis in which the Yushania niitakayamensis is most common.In addition, sorrie aged Tsuga and Pinus are also present among the Poaceae.

METHOD
The upper (0-100 cm) and the lower parts (317-350 cm) of the core consist of silt while the middle part (100-317 cm) is peat (Figure 2).In addition to pollen and algae, size analysis of peat-free sediments is carried out .Extraction of pollen fossil of most samples follows the standard method, except that HF is added for the lower clay.Calibrated C 14 ages in depths 312-317 cm; 220-230 cm; 170 cm and 35 cm are 4840±50, 3467±40; 2763±60 and 946±90 yr BP (Figure 2).The core thus represents the last 5000 years record.This interval records the vegetational changes since the beginning of the cooling phase during the late Holocene.

RESULTS OF POLLEN ANALYSIS
At least 500 grains of pollen fossil are counted for each sample.Pollen percentages were calculated on the basis of a pollen sum including all pollen grains, while percentages of spores were based on the sum of all pollen and spores.Ratios of pollen to spore are also tabulated.Based on the variation patterns of the main taxa, the following zones and subzones are delineated from the bottom upwards (Figure 2): This zone is defined by the relatively low percentages of Pinus (<5%) and the high frequences of spores (>25%, occasionally even >50% ).Cyclobalanopsis and Tsuga fluctuate abruptly.!lex is present.It is further divided into 2 subzones.

Subzone 4b: 348-337cm
This subzone marks a dramatic change in pollen assemblage: Cyc lobalanopsis increases to higher than 10%, showing a clear contrast to the other part of the pollen sequence.Tsuga is between 40-80%.Other elements are Alnus (10-20%), Polygonum, and Gramineae.

Zone 3 (316-251 cm; about 4840-3730 yr BP)
This zone is characterized by the association of Pinus (10-20%) and Tsuga (>30%).Tsuga is usually at 50% and Cyc lobalanopsis is less than 10%.Lithocarpus exists.It differs from the previous zone (Zone 4) in the abrupt increase of Pinus and more stable values of Cyclobalanopsis and Tsuga.The decrease of spores is also clear.Based on changes in these genera, Zone 3 is divided into two subzones.

Subzone 3a: 284-251 cm
This subzone differs from Subzone 3b by the small increase in Cyclobalanopsis and small decrease in Tsuga.
Pinus is usually less than 10% and Tsuga is less than 40%.On the other hand, Cyclobalanopsis is more abundant than in the Subzone 3b.Quercus increases in this interval.Ericaceae begins to become an important element from here upward to Zone 2. A rtemisia is higher than Subzone 3b.Rubus appears only in Zone 3 and 4. Tsuga attains its peak again with a range of 42-70%.To the contrary, Cyclobalanopsis is usually at about 5% only.Alnus shows the same trend as Cyclobalanopsis.Although Ericaceae is less than 5%, but are visible frequently.Pinus is at about 15% and like Tsuga, occurs at stable frequencies.Abies is more abundant here than in any other zones.This zone differs from the previous zone in having relatively high and stable values of Tsuga and Pinus as well as relatively low values of Cyclobalanopsis and Alnus.

4.4
Zone 1: (102 -0 cm; about 2030 yr BP to the present) This zone is characterized by a decrease of Tsuga to the lowest percentages (5-10%) of the sequence.Abies is also decreased except in samples at 3 and 4 cm.This zone is also characterized by the increase of Pinus, especially in the upper 15 cm, to about 30%.Cyclobalanopsis increases to higher than 10%, and Alnus to 15-30%.Castanopsis achieves its maximum value in the interval.This zone indicates the progressive change of vegetation to the present condition in which Pinus, Alnus and Cyclobalanopsis increase whereas Tsuga and Ericaceae decrease.The increasing trend of Pinus, Alnus and the decreasing trend of Tsuga are remarkable in the recent several hundred years (about the top 15 cm).After the establishment of pure Tsuga forest in Subzone 4a, Cyclobalanopsis increased again in Zone 3 although still less frequent than Subzone 4b.Quercus and Ericaceae also increased.The abrupt increase of Pinus in the beginning of Zone 3 indicates vegetational change in the area surrounding the Tsuga forest.In Subzone 3a, Cyclobalanopsis increased slightly relative to Subzone 3b, while Pinus and Tsuga, decreased slightly.This indicates Subzone 3a may be slightly warmer than 3b, although not as warm as Subzone 4b.The existence of Castanopsis is consistent with this interpretation.The appearance of Artemisia in Subzone 3a probably indicates a less humid condition too.

INT ERPRETATION OF POLLEN DIAGRAM
Zone 2 represents a relatively stable interval of Tsuga forest in which Tsuga and Pinus reach their maximum values whereas Cyclobalanopsis is at minimum values.Liquidambar occurs consistently.This indicates a relatively dry and cool interval.The beginning of Zone 2 at about 3700 yr BP is consistent in time with climatic cooling documented from the Yangtze River delta (Liu et al., 1992).It is worth noting that Chenopodium and Compositae are only found in Zones 2 and 3.
Following the relatively cool stage of Zone 2, Zone I witnesses a remarkable change of the forest in the last 2000 years during which wanner conditions occurred again.This zone clearly shows a decrease in Tsuga and Ericaceae, and an increase in Cyclobalanopsis, Castanopsis and pioneer forest elements such as Alnus and Pinus.In the middle of this zone, Castanopsis reached its highest percentages which may correspond to the Medieval Wann Period of Europe.However, a cool phase might have resumed at the 3-4 cm levels as indicated by the relatively high abundance of Abies and lower amounts of Castanopsis.

RESULTS OF GRAIN SIZE ANALYSIS AND ALGAE STUDY
Grain size analysis of the elastic sediments are shown in• Ta ble 1.In the lower part, the values of grain size are small (6.16-18.19µ) especially in the interval below 3.29 m (Zone 4) where grain size is smaller than 10 µ.The maximum grain size (48.34 µ) occurs at the level near 1.00 m where the lithology gradually changes from peat to silt upward.
Preliminary study of algae by Dr. J. C. Wu of the Academica Sinica has shows that algae are confined to levels above 0.69 m and all of them belong to the acidic type.Zone 4 is characterized by the extremely high abundance of spores which even exceeds the total abundance of pollen.This is a common phenomenon observed at certain Holocene horizons in northern and central Taiwan (Chen and Liew, 1990;Huang and Huang, 1977).The grain size of silt sediments in the lower part of the sequence ( Zone 4) is fi ner than the silt in the upper part (Zone 1).This may indicate higher lake level and more active chemical weathering for Zone 4. The abundance of spores in this interval may indicate moist conditions.Thus, the climatic conditions might be moist and warm during the time of Subzone 4b.A few recent studies have indicated that during the Holocene there were several episodes of high precipitation (Wang, 1985;Hsieh et al., 1994).One of them occurred at about 5000 yr BP Ka (An, 1993).A warm, wet mid-Holocene has also been documented extensively in the pollen records of eastern China (Liu, 1988).The humidity of Zone 4 is probably the tail-end of this event.

On the Regression of Tsuga Forest
One of the most remarkable changes in the pollen sequence is the progressive demise of the Tsuga forest from Zone 2 to Zone l, especially in the uppermost 15 cm of Zone 1. Accompanying this is the increase of Pinus, Alnus, Cyclobalanopsis, Castanopsis, and spores.The increase of Pinus, Alnus and decrease of Tsuga indicate a more open forest than before.As aforementioned, Zone 1 is warmer than Zone 2 but whether it is also warmer than Zone 4 remains uncertain unless Castan ' opsis is determined to species level.
On ther other hand, Zone 1 has larger -grain size than Zone 4, This is consistent with the more open forest in this stage.Two possibilities are considered for the change of grain size and pollen assemblages.The vegetational change could have been resulted from the changing soil conditions due to stronger erosion, which may be caused by a change in precipitation or seasonality.Alternatively, the forest change could have been caused by more frequent fires in this interval, in that case, there should be more charcoals in the corresponding leyels.
However, no remarkable charcoal peaks indicating fire events were found at the correspond-.ing levels.Thus, climatic change perhaps involving increased precipitation seasonality seems to be a more likely explanation for the changing forests.The climatic condition of Zone l

4. 3
Zone 2: (247-102 cm; about 3730-2030 yr BP) Based on the C 14 dates, this sequence records the stage after the warm phase of the middle Holocene.The lowest part of the pollen sequence represented by Subzone 4b shows fl uctuation between Tsuga forest and the Cyclobalanopsis-Tsuga mixed forest.The bound ary between Subzones 4b and 4a indicates a vertical depression of forests belts which might respond to climatic cooling after the middle Holocene fluctuations occur abruptly.Gleiche-• niaceae, Davallia, and other monolete spores are the most common even exceeding the total number of pollen in this zone.The great abundance of spores indicates moist environmental conditions.

7 . 1
On the Paleoclimatic Conditions of Zone 4

Table 1 .
Size analysis of peat�free sediments.