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Plant Silica Fossils in the Soil from Sakaerat Environmental Research Station, Northeast Thailand

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which pollens were dispersed, but does not show the his-tory of vegetation changes in the sites of researchers in-terest. In discussing vegetation changes in ecological view point, other method reconstructing past vegetation in narrow area is necessary. On-site analysis of plant sili-ca has a potential in this regards, besili-cause 1) silisili-ca con-tained in leaves is usually dispersed near the mother plants, 2) plant silica is durable and well preserved in for-est soil for long time, and 3) it enables direct analysis of the soil layers in a site. In this study, we applied this meth-od to seasonally dry forests to estimate past vegetation. 2. Materials and Methods

We selected three sites in different forests in Sakaerat, northeast Thailand (Figure 1); deciduous dipterocarp for-1. Introduction

Tropical seasonal forest widely distributed in Thailand and neighboring countries has been exposed to human impacts since the early settlers started to use the forest. Vegetation changes in the region are important in considering the influ-ence by global warming and human activities, but few studies on this topic were found except some archeological or anthro-pological work mainly based on pollen analysis of lake sedi-ments (Kealhofer and Peny, 1998; Kealhofer, 2002; White et al., 2004; Penny and Kealhofer, 2005).

Unfortunately, application of pollen analysis is restricted in tropical seasonal forests of Southeast Asia, because few natural lakes which preserve pollens in the sediment are distributed in the area. In addition, pollen diagram gives an overview of vegetation changes in the wide area in

Tropical seasonal forest widely distributed in Thailand and neighboring countries has been exposed to human impact since the early settlers started to use the forest. Vegetation changes in the region are important in considering the influ-ence by global warming and human activities. In this study, we applied on-site analysis of plant silica to seasonally dry for-ests to estimate past vegetation.

We selected three sites in different forests in Sakaerat, northeast Thailand: deciduous dipterocarp forest (DDF) and dry evergreen forest (DEF). In each site, soil samples were collected from every 10 cm up to 1m deep. Charcoal pieces in the soil layers were also collected for radio carbon dating.

Many Dipterocarpaceae species characteristic to dry forests such as Hopea ferrea, Shorea roxburghii, and S. siamen-sis produced silica in their leaves. Arundinaria pusilla, a typical bamboo species to DDF, also produced distinguishable silica. From one pit in DDF (SD3), carbon dating of charcoal pieces collected from 57 cm and 27 cm deep indicated 490± 21 and 972±21 year BP, respectively. In SD3, silica derived from current vegetation was found from the layers up to 40 cm from the surface. On the other hand in DEF, charcoal collected from 84 cm and 41 cm deep indicated 8732±34 and 937±22 year BP, respectively. Silica derived from DEF was found up to 70 cm deep except the surface layer. From the results, we concluded that 1) current DDF already existed about 1000 years ago, and 2) some of DEF were replaced.

Keywords : Holocene, Plant silica, Vegetation changes, Thailand, Tropical seasonal forest

Plant Silica Fossils in the Soil from Sakaerat Environmental Research

Station, Northeast Thailand

Sei-ichi EGUCHI

, Naoki OKADA

**

, Somkid SIRIPATANADILOK

***

and Teera VEENIN

****

(Accepted November 16, 2013)

Department of Geography College of Humanities and Science, Nihon University: 3-25-40 Sakurajosui, Setagaya-ku, Tokyo, 156-8550 Japan ** Graduate School of Agriculture, Kyoto University, Kyoto, 606-8502

Japan

*** Depar tment of Forest Biology, Faculty of Forestr y, Kasetsar t University, Bangkok 10090, Thailand

**** Department of Forest Products, Faculty of Forestr y, Kasetsart 日本大学文理学部自然科学研究所研究紀要

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Figure 1 Topographic and vegetation map of Sakaerat Environmental Research Station

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Plant Silica Fossils in the Soil from Sakaerat Environmental Research Station, Northeast Thailand

est (DDF) and dry evergreen forest (DEF). In each site, soil layers up to 1m deep were described, and soil samples for silica analysis were collected from every 10 cm. Char-coal pieces in the soil layers were collected for radio car-bon dating, and plant leaves in the sites for identifying mother plants of silica.

Preparation methods of the samples are followed Egu-chi (1996). For each sample, the phytoliths were concen-trated as follows: addition of 30 % of H2O2 and heating, addition of 3N HCl and heating, ultrasonic treatment, and removal of fine particles. The eukitt was used for the mounting on preparats. The phytolith assemblages were counted under ×400 magnification, using a biological mi-croscope. Radio carbon dating of charcoal samples was conducted by Paleo Labo Co. Ltd.

3. Results 3.1 Stratigraphy

Following Dipterocarpaceae species produced silica in their leaves: Dipterocarpus costatus, D. intricatus, D.

obtu-sifolius, D. tuberculatus, Hopea ferrea, Shorea roxburghii,

and S. siamensis. Arundinaria pusilla, a typical bamboo species to DDF, also produced distinguishable silica, but

Shorea henryana, a representative member of DEF, and S. obtusa of DDF did not. From one pit in DDF (SD3),

carbon dating of charcoal pieces collected from 57 cm and 27 cm deep indicated 490±21 and 972±21 year BP,

re-spectively (Figure 2). In SD3, silica derived from current-growing Dipterocarpaceae and A. pusilla were found from the layers up to 40 cm from the surface. On the other hand, from the pit in DEF, charcoal included in a gravel layer and collected from 84 cm and 41 cm deep indicated 8732±34 and 937±22 year BP, respectively. Silica from Dipterocarpaceae were not found in the surface layer, but were found blow the layer up to 70 cm deep.

3.2 Phytolith Assemblages

Figure 3 is the plant silica diagram, showing the content (n/gram soil) of representative plant silica fossils collect-ed at SD3. There was a distinct difference in plant silica composition between lower (layers 6 – 5) and upper layers (layers 4 – 0). The former contained less motor-cell (<14000/gram soil) and short-cell (<1700/gram soil) of Bambusaceae, whereas the latter contained more motor-cell (23000/gram soil<) and short-motor-cell (2800/gram soil<). In addition, the latter contained two types of silica proba-bly derived from Shorea siamensis and S. roxburghii, whereas the former did not.

In SE1, a mixed deciduous forest site, layers 1 and 2 contained S. siamensis type of silica, although this species is not found in the site, and hence this type of silica is not observed in the top layer (Figure 4). Soil layers of SE2 and SE3 showed a similar trend as observed for SE1 in the composition of plant silica fossils (Figure 5); in all the

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Plant Silica Fossils in the Soil from Sakaerat Environmental Research Station, Northeast Thailand

cm from the surface at SE1 in the dry evergreen forest. This indicates that the site was once covered with DDF, and DDF was later replaced with DEF.

Present study shows that we can reconstruct vegetation changes in an area with simple species composition based on plant silica fossils derived from indicator species of the vegetation. Since charcoal produced by forest fire is easi-ly found in soil layers under the seasonaleasi-ly dry climate, it becomes useful materials to identify the age of the soil layers by 14C dating. Plant silica analysis is a useful meth-od to reconstruct vegetation changes in tropical seasonal forests. The detail of past vegetation in the area is expect-ed to be drawn on map after the application of the method to wider area.

5. Conclusion

From the results, we concluded 1) current DDF existed at least one thousand years ago, 2) some of DEF were once covered, and 3) plant silica analysis is useful in re-constructing past vegetation changes in tropical seasonal forests.

layers above bed rock, motor-cell (15000/gram soil<) and short-cell (1200/gram soil<) of Bambusaceae were abun-dant, and silica of S. siamensis type were observed. 4. Discussion

In the DDF site (SD3), the composition of plant silica fossils in the surface layers reflected the current vegeta-tion of the sites. The amount of plant silica decreased with the depth of soil layers, but the composition was not different. The observation indicates that current vegeta-tion has existed during the formavegeta-tion of the soil layers in-cluding plant silica derived from DDF elements such as S.

siamensis and S. roxburghii. The lower limit of the layers

is 40 – 50 cm deep from the soil surface, back to one thou-sand years ago based on 14C dating of the charcoal collect-ed in the soil layers. Therefore, we suggest that a deciduous dipterocarp forest including S. siamensis and S.

roxburghii with abundant Bambusaceae on the forest floor

has existed for more than one thousand years at the site in Sakaerat.

On the other hand, small amount of DDF elements was found in plant silica fossils collected from layers below 20

Eguchi, S., 1996. Laboratory analysis of phytolith. Kanto Plain 4, 25-28.

Eguchi, S., Okada, N., Siripatanadilok, S., Veenin, T., 2009. Opal phytolith fossils in the soil of tropical seasonal forest in Sakaerat, northeast Thailand. Proceedings of the FOR-TROPⅡ: Tropical Forestry Change in a Changing World 5, 149-156.

Kealhofer, L., 2002. Changing perceptions of risk: The develop-ment of agro-ecosystems in Southeast Asia. American An-thropologist 104, 178-194.

Kealhofer, L., Penny, D., 1998. A combined pollen and phytolith

References

record for fourteen thousand years of vegetation change in northeastern Thailand. Review of Palaeobotany and Pal-ynology 103, 83-93.

Penny, D., Kealhofer, L., 2005. Microfossil evidence of land-use intensification in north Thailand. Journal of Archaeologi-cal Science 32, 69-82.

White, J.C., Penny, D., Kealhofer, L., Maloney, B., 2004. Vegeta-tion changes from the late Pleistocene through the Holo-cene from three areas of archaeological significance in Thailand. Quaternary International 113, 111-132.

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Figure 1 Topographic and vegetation map of Sakaerat Environmental Research Station
Figure 3 is the plant silica diagram, showing the content  (n/gram soil) of representative plant silica fossils  collect-ed at SD3
Figure 4 Phytolith diagram of the study site SE1

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