The region of Southwest China covers the municipality of Chongqing and the provinces of Sichuan, Yunnan, and Guizhou in the People’s Republic of China. The topography of Southwest China differs considerably from that of the rest of southern China. To the east, the areas
surrounding low lying Middle and lower Yangzi River are composed of relatively large open expanses of land where the altitude rarely rises above 300 m. In prehistory, substantial portions of this area where covered in wetlands, such as the Jianghan plain. While eastern China is composed of a series of wide open plains, where the altitude rarely rises 100 meters above sea level, western China has a wide range of topographies. Both in Northern and Southern China, the western half of the People‘s Republic of China is characterized by a series of mountain ranges and by drastic changes in elevation compared with the low lying river valleys of the East. Southwest China is no exception to this rule and this region is comprised of several important physio-geographic zones. Yao (2010) has argued that unlike central China and Southeast Asia, the Yunnan Guizhou plateau lacks broad alluvial basins that both demarcate natural geographic regions and provide favorable conditions for reliable agriculture. Aside from the Sichuan Basin, she argues that only 4% of the landmass is considered arable and is concentrated in elevated lacustrine basins.
The Sichuan Basin forms the exception to this rule, and is the only very large low lying area in the whole of Western China. The Sichuan Basin was created as a by product of tectonic uplift of the Tibetan plateau (60 million years ago). This uplift created an inland sea that evaporated during the Middle Pliocene (ca. 3.4-1.6 Mya) leaving the Sichuan Basin separated from surrounding regions by mountain ranges on all sides (Li, et al. 2001). This basin can be divided into three main geographic zones: the Chengdu Plain in the northwest, the Central hills and Jialing river valley in the center and a series of valleys of vertical mountain chains in the East. The Chengdu Plain is the largest area of flat land in southern China and covers over 8000 square meters. The Chengdu Plain is covered in alluvium that is the result of 8 rivers deposing their gravel and sand onto this area (Sichuan Sheng Difang Zhibian Fuyuan Hui 1996).
The plain is surrounded by several mountain ranges. To the west, the basin is surrounded by the Hengduan mountain range that forms the first cordillera of mountains of the Qinghai Tibetan Plateau, here the altitude rapidly rises from 500 to 3000 masl. The north of the basin is surrounded by the Qinling mountain range that rises to 2000-3000 masl, separating the Sichuan lowlands from the Wei River valley. In the east, a southeastern extension of the Qinling range called the Daba Mountains divides Sichuan from the Han River valley of the Middle Yangzi region. The Wushan mountains and the Three Gorges (1000-3000 masl) flank the basin to the east, separating it from the low lying plains of the Middle Yangzi river valley (Flad and Chen 2013; Li, et al. 2001; Xiang, et al. 2007).
To the South and Southeast, the Yunnan-Guizhou plateau rises to heights of between 1000 masl in the south and east and 3700 masl in the Northwest, covering the provinces of Yunnan and
Guizhou. This area is criss-crossed by numerous mountain chains that create a wide variety of ecological habitats. These mountain chains run generally in a north-south orientation, perpendicular to the Tibetan plateau. In Western Yunnan Province, the southern continuation of the Hengduan mountain chain is a complex chain ranging in altitude from 1300-6000 meters. Aside from a few flat expanses of land surrounding large lakes like the Dianchi and Erhai, the majority of the Yunnan-Guizhou plateau is surrounded by high mountains traversed by deeply cut river valleys, creating a landscape that is highly orthographically defined. The foothills of the Yunnan Guizhou Plateau taper towards the south running into Northern Thailand, Laos, Myanmar and Vietnam, where they lower to an average height of 1000 masl. To the east, in Guizhou province these mountains reach an average height of 1200 masl, and karst processes have created ravines, sinkholes and small highly tapered mountains, leaving very little space with flat land. It is not surprising that in historic times, the lake valleys in the Yunnan-Guizhou plateau and the Sichuan Basin became the most densely populated regions in Southwest China (Lee 1982).
Southern China is affected by the East Asian monsoon. The East Asian monsoon creates a highly seasonal pattern of rainfall with over 60% of the annual precipitation occurring during the
summer months of June, July and August. The monsoon leads to different patterns of rainfall across China. In general, years of weak summer monsoons lead to more rain in the south and drought can ensue in the north. During years of stronger monsoons the pattern changes and the south receives less rain than in years of stronger monsoon. However, rain patterns across southern China are not even. Southeastern China receives on average more precipitation than
southwestern China as much of this rain falls on areas of higher altitude such as the Three Gorges or is lost in the shadow of the foothills of the Himalayas (Ding and Chan 2005).
The unique topographic configuration of Southwest China has led to a range of different climatic conditions across it. The Sichuan Basin for instance has a warmer winter climate than
surrounding areas and receives less precipitation (900-1300 mm) than in the mountains (1500- 1800 mm) (Sichuan Sheng Difang Zhibian Fuyuan Hui 1996). In the Chengdu Plain, however, precipitation is higher than in other parts of the Sichuan Basin and generally ranges between 1000-1300 mm per year, with some areas close to the mountains receiving as much as 1700 mm (Sichuan Sheng Difang Zhibian Fuyuan Hui 1996) (Flad and Chen 2013). In Yunnan, annual average rainfall in the region exceeds 1,000 millimeters on southwestern slopes at higher altitudes, while areas of the northwestern part of the region, in the rain shadow of the Tibetan Plateau, rarely receive more than 400 millimeters annually (Chang 1983). The wide range of physiographic and local weather conditions have led to this region being the largest centers of biodiversity in the temperate world. Even though the array of species in this region is not yet fully documented, vascular plant diversity is estimated at about 12,000 species. Roughly 40% of all species known to China grow in this region (Yang, et al. 2004).
Highland eastern Tibet is characterized by warm, wet summers and dry, frosty winters. Average high temperatures of 20 °C are reached as early as May, although nighttime temperatures are much lower due to the effect of altitude, reaching only a monthly average of 5°C). In June, July and August, the average high is remains at a monthly average of roughly 23° C, with lows between 9 and 10 degrees (Chang 1981, 1983).
4.2) Current Vegetation Patterns
The latitude of southwest China places it in what should be a subtropical vegetation zone, however the geophysical configuration of this area has led to the presence of a range of different ecosystems from permanent snow to temperatures perennially above 10° C. In the whole of China, natural vegetation types have been highly modified by modern agricultural practices, and it is sometimes difficult to reconstruct previous vegetation patterns. This is particularly the case in areas of warm and mild climates, which are well suited for agricultural practice. For the most part, only montane shrubland and alpine meadow occupy their real niche and remain untouched by modern agricultural activities.
4.2.1) The Vegetation of Sichuan
Compared to Yunnan, relatively little is known about the natural vegetation of the low lying areas of the Sichuan Basin. The Sichuan Basin has been intensively cultivated for at least the past 2000 years and retains little if any of its original flora. On current maps of vegetation, this entire area is marked as agricultural land (Wu 1980). A few areas in the Sichuan Basin support remnant patches of the climax forest vegetation. Lands dedicated to temples such as Dazushan and holy mountains like Mt. Emei can offer us clues to what the flora of the ancient Sichuan basin may have been. Unfortunately the primary vegetation on Mt. Emei starts at only 750 m, thus at an altitude that is higher than that of the rest of the Sichuan basin (Tang and Ohsawa 1997). This area corresponds to an evergreen broad leafed forest zone, with a tree canopy dominated by members of the Lauraceae family and Castanopsis. Paleobotanical data (see
discussion below) as well as an analysis of climate patterns tend to support the fact that the Sichuan Basin was covered by an Evergreen broad leafed forest (at least between 3600-3400 BP and 2600-900 BP) (Li 1980; Luo, et al. 2008).
An analysis of the vegetation in the higher altitudes of Mt Emei provides a picture of the
vegetation present in the southern mountain chains surrounding the Sichuan Basin. Compared to the mountains of the north and west, this area is marked by a warmer, more humid climate. Mixed evergreen and deciduous forest begins at higher altitudes in this region, between 1500- 2000 masl. Here the evergreen vegetation is dominated by Lithocarpus, Camellia,
Choroespondias, Prunus and Eurya (Tang and Ohsawa 1997). Between 2000-2500 masl, deciduous trees are replaced by conifers and Acer, Abies, Tsuga and Taxus dominate the assemblage along with Lithocarpus. Wild kiwi fruit (Actidinia sp.) and was also present in the sub-layer. A conifer-dominated forest begins at 2500 masl and reaches up till 3099 masl. Here the forest is dominated by Abies sp., although mountain ash (Sorbus sp.) that has an edible fruit was also present.
The climate becomes colder and drier to the northwest of the mountain ranges. In Jiuzhaigou, vegetation characteristic of much colder climates dominates (Winkler 1998). Between 2000- 2700 meters, a montane mixed forest belt of deciduous trees such as different species of maple (Acer spp.), oak (Quercus aliena), conifer trees such as (Pinus armandii, P. tabulaeriformis, Picea spp., Tsuga spp.) as well as Juniper exists (Winkler 1998). At an altitude of 2700-3200 m an alto-montane bamboo cloud forest exists, where higher rainfall leads to the presence of
montane rhododendron forest, where bamboo is replaced by rhododendron. A sub-alpine forest zone dominates the higher zones (3500-3800 masl) and here the numbers of juniper increase (Winkler 1998).
4.2.2) The vegetation of the Yunnan-Guizhou Plateau
The geophysical characteristics of the Yunnan-Guizhou plateau have led to a wide range of ecotypes being present in this region. The lowest point in Yunnan province is 76.4 m above sea level on its southeastern edge, while the highest point is over 6000 m at Kagebo peak. The
topographic slope from northwest to southeast has created a large temperature gradient across the plateau (Li and Walker 1986). Tropical evergreen forest occupies only the most southern part of the province west of the Ailaoshan. Its maximum altitude of occurrence ranges from 100m in the east to 1500 m in the southwest. Humid tropical rainforest occurs only in Southeast Yunnan in Hekou and Jinping counties below altitudes of 400m. The flora of both of these areas is most like Southeast Asia.
Annual mean temperatures range between 22-26 °C and the mean of the coldest month is 18 °C. This area experiences large amounts of rainfall, measuring over 2000mm per year. A seasonal tropical evergreen forest is found in southern Yunnan in basins below 100m, however, this type of forest experiences cooler and drier winters. Here the annual mean is slightly cooler ranging between 20-25 °C (Li and Walker 1986).
One of the most widespread types of forest in Yunnan is the subtropical evergreen broad leafed forest. In central Yunnan, this forest grows between 1000-2000 meters above sea level. Moving
southward this forest can be found at altitudes as high as 2800 meters above sea level. These forests are dominated by different types of oak trees (such as Cyclobanalopsis and Lithocarpus) as well as by members of the chinkapin family (Castanopsis sp.), all of which contain edible fruit. When anthropogenically disturbed, this forest is replaced by the Yunnan pine tree (Pinus
yunnanensis), which also produces edible pine nuts. The large numbers of edible nut species present in this area provides some suggestions as to the kinds of foods that may have been
consumed by the foragers that once inhabited this region. This forest grows in areas with a strong seasonality and whose annual mean ranges between 13-18 C. Areas with this forest rarely see temperatures below 0 and the mean of the coldest month ranges between 3-8 ° C.
Subtropical evergreen sclephyllous forest is found on the slopes of the Jinsha river valley. Winters in this area are dry and cold and experience snow. In the montane forests the annual mean is below 10° C, whereas in the lower reaches of the valleys, this warms to 15-18° C, and only milder frosts are experienced. Rainfall in this region is low and only ranges between 700- 900 mm. Subtropical deciduous broadleafed forest covers small areas of Yunnan and is composed primarily of different species of oak tree.
Two types of secondary forest are present throughout much of Yunnan. These are largely due to human disturbance, making it difficult to estimate what kind of vegetation occupied this area in the past. Secondary forest regions comprise subtropical conifer forest dominated by Pinus yunnanensis ranging from 600-3500 masl. Another type of pine tree, Pinus armandi is confined to areas of higher altitude (2000-3000 masl). Subtropical evergreen shrubland also occupies a large area of Yunnan.
A montane conifer forest is only found in the northwest of the province, where it grows above 2700 m in altitude. P. densata is an important type of pine in this forest and is often accompanied by Abies, Betula and Larix sp. Tsuga and Larix dominated forests are other important high altitude forest types. Montane shrubland covers areas of over 4000 masl and is often dotted with areas covered by alpine meadow, where Kobresia meadow is dotted by wild strawberries,
Potentilla sp. A savannah ecotype occupies the low lying basins of some of the rivers below 1200 masl, where annual rainfall is low such as the Yuan River basin, the Nu and the Lancang. In upland Guizhou province, this vegetation pattern follows similar altitudinal trends.
4.2.3) The Vegetation of Tibetan plateau:
Between the Chengdu Plain and the highlands of the Tibetan Plateau a wide range of different vegetation and climatic zones are present. The type of vegetation is directly related to altitudinal changes (Xu, et al. 2006). Several factors change the range of the extent of vegetation on the Tibetan plateau. Due to the heating of the mass of the Tibetan plateau, its effective heat is greater than on mountains of the same latitude and altitude (Chang 1981). Higher levels of solar
radiation also contribute to it having a higher temperature. For these reasons, the altitudinal limits of vegetation types are much higher on the plateau than on mountains in Yunnan or Western Sichuan, and both the snow line and tree line are between 900-1500 masl higher than in other areas. The mountains surrounding the plateau form a rain shadow that leads to an arid to semi arid climate on the plateau itself (Chang 1981). The majority of the plateau is dominated by low and high shrub tundra in the eastern half of the plateau and by graminoid and forb tundra in the western part of the plateau (Ni and Herzschuh 2011). In the southeast this is interspersed with
an evergreen taiga montane forest. Vegetation growing in warmer conditions is only found in Bhutan, where a deciduous montane forest grows.
Figure 4.1 Growing degree days using a circular model on a 10 °C base for Southwest China.
An analysis of accumulative growing degree days shows a marked trend in temperature between eastern and western south China (Figure 4.1). Although much further south in latitude, the altitudinal effect of the Three Gorges, Western Sichuan highlands and the Yunnan-Guizhou plateau mean that temperatures are much lower here than in areas of similar latitude in southeastern China. Except for the southern tip of Yunnan, this region shows temperatures similar to that of latitudes that are much further north in northern China.
4.3) Ancient Climate
The present climate of southwest China does not reflect past conditions. It is therefore necessary to consider the effect that climate change has had on past agricultural production. A number of macroregional trends separate the climate of the earlier Holocene from that of today. Bond (1997) has proposed that climate fluctuations occurred in the North Atlantic approximately every
1470+-500 years throughout the Holocene (Table 4.1).
Table 4.1 “Bond events” recorded on a global scale.
No Date ka Description
0 0.5 ka Little Ice Age
1 1.4 ka Migration Period 2 2.8 ka 1st millennium BC drought 3 4.2 ka 4.2 kiloyear event, maybe triggering collapse of Akkadian empire 4 5.9 ka 5.9 kiloyear event 5 8.2 ka 8.2 kiloyear event
6 9.4 ka Cold event in China as
well as Norway
7 10.3 ka
8 11.1 ka Transition from the
younger Dryas to the Boreal
Over recent years, a growing body of evidence has allowed us to better reconstruct changes in climatic conditions during the Holocene in China. See Table 4.2 for a summary of this data in Southwest China. One common trend discussed in all papers is the presence of a global climatic optimum during which temperatures were warmer and where higher precipitation values were
debated, however dates for its beginning range from 9000-7500 BP, and it is hypothesized to end between 5000-2500 BP (Shi, et al. 1993). In particular, between 8700-8500 BP, the temperature in western China appears to have increased by 4-5 ° C (He, et al. 2004; Shi, et al. 1993).
However, the exact start and end dates of this transition to warmer temperatures are unclear (He et al. 2004).
During this period, Shi et al. (1993) argue that two periods of time represent substantial warming events: between 8500-8400 BP and 3000-2900 BP. Prior to the earlier warm event, a marked cold event appears to have occurred c. 8900- 8700 BP and another cooling event appears to have occurred c. 3000- 2900 BP. Another important trend involves the arrival of maximum monsoon strength during the early-middle Holocene (6000-4800 cal. BC). According to An et al. (2000) maximum precipitation in Northern China took place between 8000 and 5000 BC, however in the middle and lower reaches of the Yangzi River Valley this occurred between 5000 and 3000 BC.
Another major event is known as the ―Holocene Event 3‖ or the ―4.2 kiloyear BP aridification event‖ during which cooler and drier temperatures prevailed (Arz 2006). This event has been implicated in the cultural collapse around the world (Davis and Thompson 2006; deMenocal 2001; Gibbons 1993; Staubwasser, et al. 2003; Wu and Liu 2004). In addition to this event, there appear to have been several periods of time during which weak summer monsoons affected northern Asia. These include arid events around 5000-3000 BC, 1000 BC and around 1000 AD (Shi, et al. 1993).
However, using data from one region to apply it to another may not present a picture that is representative of past climate trends (He, et al. 2004). Several problems exist in interpreting different paleoclimatic records. The first of these issues is that large uncertainties exist in the