Charcoal and fossil wood from palaeosols, sediments and artificial structures indicating Late Holocene woodland decline in southern Tibet (China)

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Abstract

Charcoal and fossil wood taken from palaeosols, sediments and artificial structures were analysed in order to evaluate the regional pedoanthracological potential and to obtain information on Holocene environmental changes, particularly on possible past tree occurrences in southern Tibet. This research was initiated by the question to what extent this area is influenced by past human impact. Even recent evaluations have perceived the present treeless desertic environment of southern Tibet as natural, and the previous Holocene palaeoenvironmental changes detected were predominantly interpreted to be climate-determined. The material analysed – comprising a total of 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites (c. 3500–4700 m a.s.l.) – represents the largest systematically obtained data set of charcoal available from Tibet so far. 27 taxa were determined comprising trees, (dwarf-) shrubs and herbs as well as grasses. The predominant tree taxa were Juniperus, Hippophae, Salix and Betula. According to their present-day occurrence in the region, the genera Juniperus and Hippophae can be explicitly attributed to tree species. Further, less frequently detected tree taxa were Populus, Pinus, Quercus, Taxus and Pseudotsuga. Charcoal of Juniperus mainly occurred on southern exposures, whereas Betula was associated with northern exposures. In contrast, the (partly) phreatophytic taxa Hippophae and Salix showed no prevalent orientation. The distribution of radiocarbon ages on charcoal revealed a discontinuous record of burning events cumulating in the Late Holocene (c. 5700–0 cal BP). For southern Tibet, these results indicated a Late Holocene vegetation change from woodlands to the present desertic pastures. As agrarian economies in southern and south-eastern Tibet date back to c. 3700 and 5700 cal BP, respectively, and the present-day climate is suitable for tree growth up to c. 4600 m a.s.l., we concluded that the Late Holocene loss or thinning out of woodlands had been primarily caused by humans.

Introduction

Over the last decade several new studies have dealt with the questions what changes the vegetation cover of central and high Asia underwent during the Holocene and whether the present vegetation is primarily natural or was secondarily established following (past) human impact. Traces of a forest history, as represented by pollen, charcoal and fossil wood, indicate that larger areas of western China, Mongolia and of several Himalayan countries have undergone drastic vegetation changes in the last few millennia (e.g. Beug and Miehe, 1999, Ren, 2000, Shen, 2003, Schlütz and Zech, 2004, Byers, 2005, Feng et al., 2006, Huang et al., 2006, Miehe et al., 2007a, Zhao et al., 2007).

On the Tibetan Plateau, forming the largest alpine area in the world (c. 2.26 km2), treeless vegetation belts in the eastern sector between c. 3000 and 5000 m a.s.l. were often assumed to be natural. Studies dealing with the present-day vegetation and also those on the Late Holocene vegetation history have repeatedly claimed that the region's harsh climatic regime has been the main factor preventing the recent growth of trees and that human impact on vegetational changes has been limited to the recent past (e.g. Ni, 2000, Yu et al., 2001, Ren and Beug, 2002, Luo et al., 2004, Luo et al., 2005, Song et al., 2004). However, there are a growing number of studies which question this view. They point to the current distribution of zonal forest and woodland islands throughout the southern and north-eastern Tibetan Plateau, implying that viable and reproducing tree stands prove that current climatic conditions can support tree growth. Furthermore they point to the increasing palaeoecological evidence of larger forests still in the Mid- and at the beginning of the Late Holocene (e.g. Holzner and Kriechbaum, 1998, Frenzel, 2002, Bräuning, 2007, Kaiser et al., 2007, Mosbrugger et al., 2007, Ren, 2007, Schlütz et al., 2007, La Duo, 2008, Miehe et al., 2008b, Miehe et al., 2008a, Schlütz et al., 2009). Consequently, as climatic conditions at the peak of the Holocene desiccation and cooling period (e.g. He et al., 2004, Herzschuh, 2006, Zhao et al., 2007, Zhu et al., 2008) could support tree growth (the tree stands had been established long before the current anthropogenic warming commenced), they claim that climatic changes during the Late Holocene cannot have been the single driving force for a large-scale forest decline. Accordingly forest disappearance or thinning out might be explained by human impact comprising grazing of domestic animals, clearing for the establishment of arable land and settlements, and burning. Furthermore, Kaiser et al. (2006) and La Duo (2008) discuss a combination of climatic effects and anthropic factors, arguing that after an initial climate-driven opening of the vegetation cover by aridification and cooling, human impact increased this process. Nevertheless, as both the evidence for larger Holocene forests and that for early human impact are infrequent, widely-scattered and inadequately discussed so far, the dispute is not settled. With the present paper we want to contribute to this discussion by presenting the so far largest systematically obtained data set of charcoal available from the Plateau in order to provide new regional records of fossil woody taxa. Furthermore, we hope to stimulate additional high-resolution studies by showing the pedoanthracological potential of sites along an 800 km transect.

In general, data on vegetation history can be obtained using several methods/plant remains (e.g. pollen, macro remains, charcoal) and archives (e.g. lake sediments, peat, palaeosols). Evidence and interpretation of past environmental changes substantially depends on how well the archive used represents the area. Palynology usually gains insights into (sub-) regional vegetation patterns (e.g. Berglund and Ralska-Jasiewiczowa, 1986, Seppä, 2007, Sugita, 2007). However, because of possible long-distance pollen transport, it remains uncertain whether certain taxa really did grow on the respective sites. In contrast, macroscopic charcoal and fossil wood reliably reflect the local growth of woody taxa (e.g. Schoch, 1986, Lynch et al., 2004, Carcaillet, 2007) except for some potentially problematic archives (e.g. fluvial-lacustrine sediments, artificial structures). In comparison to palynological research, which is comparatively well-established on the Plateau, charcoal analysis (anthracology) and fossil wood analysis (xylology) have been rarely performed so far. The charcoal and fossil wood presented in this study were primarily derived from palaeosols and sediments during research on the Late Quaternary geomorphic evolution of the Lhasa area in southern Tibet (Kaiser et al., 2006, Kaiser et al., 2009, Kaiser et al., in press-a), and in the context of biogeographical research on the distribution of juniper forests in south-eastern Tibet (Miehe et al., 2008a). They were thus not obtained in a strict anthracological framework. Nevertheless, as the whole material comprised 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites covering a biogeographical transect from the coniferous forests in the east, via the present border of the closed forest area, to forest outposts/relics in the west (Fig. 1, Appendix 1), a palaeoecological appraisal is very much worthwhile.

The overall question to be addressed by this paper is: What can charcoal and fossil wood primarily found in palaeosols and sediments tell us about the Holocene environmental history of southern and south-eastern Tibet? More specifically we wanted to i) describe taxa composition, topographic pattern and dating of (woody) plant fossil assemblages; ii) interpret and discuss the results with respect to the regional environmental history, including fire history, by means of further biogeographical–ecological, palaeobotanical, geoscientific and archaeological evidence; and iii) evaluate the regional pedoanthracological potential.

Section snippets

Study areas

We had two main study areas, comprising the Lhasa area in southern Tibet and the deep river gorges of south-eastern Tibet, which as currently classified in the Atlas of Tibet Plateau (1990) belong to two biogeographical zones: open dwarf shrubland pastures and coniferous forests, respectively (Fig. 1, Appendix 1). In the following, mainly information on climate and vegetation/land-use will be given. Further aspects, such as details on geology, geomorphology and pedology, can be taken from e.g.

General remarks and sampling

In general, (pedo-) anthracological studies have already been performed at several (sub-) alpine sites throughout the world, mostly comprising areally high-resolution palaeoecological case studies (e.g. Tessier et al., 1993, Carcaillet and Thinon, 1996, Tinner et al., 1996, Carcaillet and Brun, 2000, Carnelli et al., 2004, Di Pasquale et al., 2008). In contrast, our data have so far been collected mainly for geomorphical purposes and characterise palaeosols and sediments. Thus we commonly

Origin and taphonomic aspects of the samples

Most samples derived from palaeosols (n = 34). Further samples originated from fluvial-lacustrine (n = 6), aeolian (n = 6) and colluvial sediments (n = 5). Four samples (RET 1, RET 8, LSW 3, GHU 1) were taken from artificial structures (burial mounds, field terraces), partly in combination with palaeosols. The sampling depth varied from 20 to 1000 cm (mean = 281 cm), whereas the thickness of the layers sampled ranged from 2 to 200 cm (mean = 24 cm). In six cases different levels per profile were sampled (

Assignation of growth forms

As the Juniperus species cannot be distinguished from each other on the basis of their wood anatomy, so far the assignment of charcoal and fossil wood must consider both shrubby and treelike growth forms. Juniper trees (e.g. J. tibetica; Fig. 3B) are found up to an altitude of c. 4900 m a.s.l., whereas juniper shrubs (Juniperus pingii v. wilsonii) form thickets (‘krummholz’) in the treeline ecotone between c. 4200 and 5300 m a.s.l. (Farjon, 2005, Miehe et al., 2007b, Miehe et al., 2008a).

Conclusions

  • (1)

    In southern and south-eastern Tibet, charcoal frequently occurs in palaeosols, various sediments and artificial structures forming an important palaeoecological proxy. In particular dry-site palaeosols regularly contain strongly dispersed and fragmented macroscopic charcoal. Moreover, wet-site palaeosols bear well-preserved fossil wood.

  • (2)

    Most taxa determined represent (dwarf-) shrubs, followed by trees and herbs/grasses. The genera Hippophae and Juniperus statistically prevail, representing tree

Acknowledgements

We are grateful to Gen Dun, Tibet-Museum Lhasa, and Shargan Wangdwei, Tibetan Authority of Cultural Relics Lhasa, whose knowledge in regional archaeology enabled us to sample some profiles at archaeological sites. The fieldwork was partly carried out under the auspices of the Lhasa-Marburg University partnership; special thanks go to our colleagues La Duo and Tsering Dorgeh for their untiring support. We also thank Christiane Enderle, Marburg University, for preparation of some figures, James

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