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- Materials and methods
- Natural vegetation in the geological past
Loess, a wind transported accumulation derived from arid inland areas, covers an area of ~440 000 km2 on the Loess Plateau in north-central China (Liu 1985). The main body of the Chinese Loess Plateau (CLP) is found in the middle reaches of the Yellow River (Fig. 1) and is characterized by an arid and semi-arid climate. Destruction of vegetation cover on the CLP, as a result of long-term human activities, has resulted in severe soil erosion. About 1·64 billion tonnes of sediment are transported into the Yellow River each year (Liu 1985), raising the riverbed downstream and thereby causing frequent devastating floods. To tackle this issue, China has invested hundreds of billions of Yuan in numerous afforestation campaigns since the end of 1950s (Wang et al. 2007; Chen, Shao & Li 2008b).
Figure 1. Map showing the study sites (solid circles) and the distribution of loess in the middle reaches of the Yellow River (adapted from Liu 1964).
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Afforestation has led to a considerable increase in forest resources and a decrease in dust-storm activity in some areas (Shen et al. 2003; Cao 2011). However, it has also caused environmental degradation in the arid and semi-arid regions (Cao 2008; Cao et al. 2011a,b), including low survival rate of trees (Guo et al. 2001; Wang et al. 2007), increased soil erosion (Normile 2007; Wang et al. 2010), exacerbated water shortages (Cao, Chen & Yu 2009) and deep soil desiccation (Chen, Shao & Li 2008a; Wang, Liu & Liu 2009).
To improve the tree survival rate, some local governments have adopted extreme approaches, including the digging of parallel trenches to collect slope run-off (Cao, Chen & Liu 2007); the introduction of exotic, fast-growing species (Cao, Wang & Chen 2010) and the removal of herbaceous vegetation under trees to reduce competition for moisture (Cao, Chen & Yu 2009); and installing expensive irrigation systems (Shen et al. 2003). For example, 660 million Chinese Yuan has been invested since 1999 in the North and South Mountains Greening Project in Lanzhou, the largest city in the upper reaches of the Yellow River (Fig. 1). Water from the Yellow River has been used to maintain an artificial forest on mountains along both sides of the valley, involving a network of ~3000 km of pipes. In contrast, herbs are only sparsely present on the adjacent, nonirrigated slopes and hills, which have a distinctly different landscape.
A major reason for the failure of afforestation is the improper selection of plant types (Cao 2011). Therefore, two questions must be asked: (i) What kind of trees or herbs should be planted under current climatic conditions and to withstand global warming? and (ii) How can sustainable alternative incomes be provided for farmers in the Loess Plateau? Knowledge of the native Loess Plateau vegetation is central to these questions, because naturally grown native plants are fully suited to the local climate and soil (Sun et al. 1998; Li 2001). However, the natural vegetation on the Loess Plateau has been destroyed by human activities, such as clearance, cultivation and grazing, over a long period (Liu et al. 1996).
The Earth's climate has varied naturally between glacial and interglacial states, driven by periodic variations in the Earth's orbit (Hays, Imbrie & Shackleton 1976; Ding et al. 2002; Lisiecki & Raymo 2005), and global ecosystems have continually adjusted to accommodate the changing climate, showing cyclic shifts between alternative states (Whitlock & Bartlein 1997; Yu et al. 2000; Shi et al. 2003; Jiang & Ding 2005; Ni et al. 2010). Warm periods in the geological past are widely used as analogues for current and future warming. For example, the global change community and the Intergovernmental Panel on Climate Change (IPCC) have taken the Holocene Optimum (~9–5 ka), Marine Isotope Stage (MIS) 5e (~130–116 ka) and MIS 11 (~420–360 ka) as analogues for evaluating the effect of different warming scenarios on various ecosystems (e.g. Kukla et al. 1997; Loutre & Berger 2003; Jansen et al. 2007; Kopp et al. 2009; Tzedakis et al. 2012).
The climate in northern China was cold and dry during glacial periods, and warm and humid during interglacials (Ding et al. 2002; Yang & Ding 2010). Geological records have shown that present climatic conditions are similar to those in the early Holocene (Xiao et al. 2004; Jiang et al. 2006). Although the present temperature is ~2 °C lower than the temperature in the Holocene Optimum (Shi et al. 2003), if the current warming rate continues, it will rise to the Holocene Optimum temperature by AD 2100. Therefore, the vegetation present in the early–mid-Holocene is a good reference for potential natural vegetation on the Loess Plateau and will provide valuable insights into the restoration of vegetation for policy makers.
To date, native vegetation on the Loess Plateau, as derived from various records, remains controversial, and assertions have been made that it was predominantly forest (Shi 1981, 1991; An, Feng & Tang 2003; Tang & An 2007; Shang & Li 2010), forest-steppe (Zhu 1983, 1994; Liu et al. 1996) or steppe (Sun et al. 1997; Xie et al. 2002; Li, Zhou & Dodson 2003; Jiang & Ding 2005). At present, the climatic conditions of the Loess Plateau vary significantly between regions, with ~250 mm mean annual rainfall and a mean annual temperature of ~8 °C in the north-west and ~650 mm mean annual rainfall and a mean annual temperature of ~14 °C in the south-east. Therefore, observations at a single site or a few geological samples may not represent the whole Loess Plateau vegetation picture that well, and systematic paleovegetation studies are urgently required before a practical vegetation restoration strategy can be implemented. In this study, we present pollen records from six sites across the plateau, with the objective of identifying the natural vegetation types since the Last Glacial Maximum (LGM) and to suggest suitable plant types for vegetation restoration under global warming conditions.
- Top of page
- Materials and methods
- Natural vegetation in the geological past
Our study sites represent a range of geomorphological units and are characterized by different climatic conditions (Table 1), but they all have very thick loess deposits (60–300 m). Chinese loess is mainly composed of loosely cemented silt (Liu 1985; Yang & Ding 2008), which allows rainwater to infiltrate quickly (Chen, Shao & Li 2008b; Yang et al. 2012). In areas with thick loess, therefore, the water in the surface soil is insufficient to maintain forests. Only in areas of thin loess underlain by bedrocks (e.g. deep gullies, incipient floodplains and low river terraces), where the underground water table is relatively high, can trees and shrubs grow under the appropriate conditions.
Our pollen results all show that herbs, rather than trees or shrubs, were dominant on the Loess Plateau in both the cold–dry period and the warm–humid period, which is consistent with pollen data from many other loess sections (Sun et al. 1997; Li, Zhou & Dodson 2003; Jiang & Ding 2005; Tang & An 2007; Shang & Li 2010). It should be noted that the pollen records from several sections in river valleys indicate that trees were abundant in the Holocene (An, Feng & Tang 2003; Wu et al. 2009; Shang & Li 2010). However, these sections are all located in the riparian zones of first-order tributaries of the Yellow River. In the semi-arid Loess Plateau, these riparian zones cover a very limited area.
Most areas in the Loess Plateau are covered by thick loess (>20 m) (Liu 1985); therefore, our records, together with pollen data from many other sites (Sun et al. 1997; Li, Zhou & Dodson 2003; Jiang & Ding 2005; Tang & An 2007; Shang & Li 2010), may well be representative of the entire loess area. Because of this, it appears that priority should be given to planting herbs rather than trees or shrubs in current and future greening programmes.
Most plants selected for past and present afforestation programmes have been trees or shrubs tolerant to arid conditions, including Robinia pseudoacacia, Caragana intermedia, Amorpha fruticosa, Pinus tabuliformis, Populus davidiana, Ulmus pumila and Hippophae rhamnoides (Shi & Yang 2002; Cao 2008). However, these species are rare in the natural vegetation record, except for a few species of Pinus and Ulmus. Our results show that the native vegetation species on the CLP in the past were mainly from the Poaceae and Asteraceae families, which are well adapted to arid and semi-arid climates and to various topographic and geomorphological units in the loess area. For example, Agropyron cristatum, a perennial species from the Poaceae family, is very tolerant of drought, cold and grazing and produces high herbage yields in early spring (a season when feed is in short supply). It also has a fibrous root system that stabilizes disturbed soil. Members of the Poaceae and Asteraceae families are, therefore, likely to be ideal candidate species for ongoing greening programmes. Choosing the most appropriate species is an urgent task for ecologists.
At present, more than 70 million farmers live on the CLP. Any ecological restoration must therefore be combined with income generation for local farmers, to promote sustainable development. According to the early–mid-Holocene pollen records, we suggest Juglans, Corylus and Selaginella sinensis as candidate species for the ongoing greening programmes in areas south-east of Heshui and Fuxian (Fig. 1). Juglans (walnut) and Corylus (hazelnut) seeds are important in the food industry. In a recent study (Ottaggio et al. 2008), taxanes, including 10-deacetylbaccatin III, baccatin III, paclitaxel C, and 7-epipaclitaxel, were found in Corylus shells and leaves. Paclitaxel is a popular and expensive anticancer drug, so Corylus can be used as a medicinal source material. Selaginella sinensis, a native species of fern, is a traditional Chinese medicine for the treatment of hepatitis, cholecystitis, eczema and burns. In addition, the Asteraceae family, which flourishes on the CLP in both cold and warm conditions, is an important source of medicines (Dharmananda 2012). For example, the most effective antimalarial drug artemisinin is derived from Artemisia annua (a member of the Asteraceae family) (Covello 2008). The Loess Plateau, therefore, has tremendous potential to be a source of medicinal plants, and this prospect deserves serious consideration in the greening programme.
In summary, as reviewed by Cao (2011), excessive reliance on afforestation has caused significant negative environmental impacts in northern China. According to our pollen records, priority should be given to herbs (especially the Poaceae and Asteraceae families), rather than trees or shrubs, in current and future greening programmes. Only in the deep valleys, with very thin loess and the riparian zones of large rivers, should some trees and shrubs be considered as candidate species. To combine ecological restoration with economic growth, Juglans, Corylus and some medicinal plants (e.g. Selaginella sinensis) should also serve as useful candidate species for the south-eastern Loess Plateau.