The northeastern margin of QTP encompasses the eastern part of Qinghai Province, including the areas along the Huangshui River and upper reaches of the Yellow River from Tongde County to Xunhua County, as well as the Gonghe Basin (Figure 1). This subregion is an important transitional zone between the QTP and Loess Plateau, marking the boundary between the East Asia summer monsoon and the northwest arid region. It features mostly north-south-oriented, high mountains, deep and broad valleys, well-developed river terraces, and elevations ranging between 1,650 and 5,206 m above sea level. The region is climatically sensitive and ecologically vulnerable. It is classified as having a temperate-arid and semi-arid continental climate, with mean annual precipitation between 252 and 535 mm, and mean annual temperature between 3 and 8°C (Jia et al., 2019). The dominant biome in this area is temperate grassland with distinctive vertical zonality. This area is typical of the agropastoral areas where river terraces are densely cultivated and it has one of the highest rates of agricultural land use on the QTP. In spite of the fact that this region accounts for only 5.18% of the province’s total area, the population exceeds 70% of the province’s total. Cropland area accounts for 73% of that found in the province (Zhao, 2016).

The Dianziping profile was collected from Gaomiao basin, Ledu County, Qinghai Province, which is characterized as having a temperate-plateau, semi-arid climate. The basin sits at an elevation of 1,870 m, has a mean temperature of 7.6°C and a mean precipitation of 333.7 mm annually. The Huangshui River runs through the basin from west to east. In the river valley, flat terrain and intact terraces are common. At present, cropland is widely distributed on terraces and slopes, and most of the plants observed in the valley are cultivated species such as Triticum aestivum, Vicia faba, Pisum sativum, Juglans regia, Zanthoxylum bungeanum, Populus, Salix, and others. In the foothills, temperate grasslands are dominant, with Stipa bungeana occurring as the dominating species; clustered Stipa glareosa and artemisia gmelinii are commonly observed. In the high mountains, shrub species such as Potentilla fruticosa, Salix oritrepha, Rhododendron lapponicum, and a few Picea crassifolia, Picea wilsonii are abundant. Mountain regions above 3,200 m are mainly Kobresia dominated alpine grassland.

The scope of this study extends from the Neolithic Age to the historical time period (5.3–0.11 cal ka BP). This time frame coincides with the occurrence of the Majiayao, Qijia, Xindian, and the Kayue cultures. The Majiayao culture, renowned for its excellency in pottery, was centered in the Hehuang Valley and mainly practiced farming. Based on radiocarbon dating, the Majiayao culture can be categorized into three different subsets, the Majiayao type (5.3–4.5 cal ka BP), Banshan type (4.5–4.3 cal ka BP), and Machang type (4.3–4.0 cal ka BP). Around 4.2 cal ka BP, the Qijia culture (4.2–3.6 cal ka BP) replaced the Majiayao culture in the region, representing the onset of the Bronze Age. During the mid-late Bronze Age, people of the Kayue culture (3.6–2.7 cal ka BP), who practiced both farming and grazing, and the Xindian culture, which practiced farming, occupied the area. While the Kayue culture was mainly distributed in the western part of the study area, the Xindian Culture (3.6–2.6 cal ka BP) was found in the east (Xie, 2002; Chen et al., 2015).

Prehistoric population sizes are estimated mainly by settlement archaeology, grave analysis, bio-archaeology, relics study, and other analytical methods (Fang, 2007). In this study settlement archaeology and grave analysis were used for population estimation. Calculation of the prehistoric population was based on both the sizes of prehistoric sites (settlement archaeology) and the number of graves excavated at each site (grave analysis). In light of the fact that there were no record of prehistoric population and former studies of the specific human population, we used the data attained from officially excavated sites. Uncertainty around the population estimates are the result of inaccuracies in grave numbers due to both natural, and human disturbances such as grave robbery and village construction. These disturbances have resulted in an inaccurate number of recorded graves and excavated human bones. Consequently, the population estimates for this study are likely low, but provides a reasonable approximation of the population and general population growth trends, if used consistently across all sites.

The most distinct change in vegetation cover brought by anthropogenic activity is the conversion of natural to cultivated lands. The total area of the cropland can be used to estimate the extent of human disturbance to natural vegetation. Elevation, slope aspect, and gradient, as well as soil types, are considered as limiting factors for cultivation (Luo et al., 2015; Han et al., 2020). During the prehistoric time periods, the distance to the river was also considered a limiting factor for cultivation due to the lack of vehicles for transportation and undeveloped irrigation technology. Generally speaking, the suitable land area available for cultivation was limited by one social factor; population, and four natural factors; elevation, slope, water source, and soil type.

A 3.5 m profile with intervals of 4 and 6 cm (280–311 cm) was collected from Dianziping (36°26′28″N, 102°34′51″E, 1,938 m.a.s.l.), Gaomiao basin for the purpose of dating and pollen analysis. Fifty samples were prepared according to standardized protocols and the experiment was carried out using standard procedures (Li et al., 1995). A known number of Lycopodium spores (27,637 ± 563 grain/slice) were added to each sample for calculating the values of pollen and charcoal concentration. Carbonate and silicate were removed with 10% hydrochloride and 40% hydrofluoric acid, respectively, and washed through 10 μm sieves. Five charcoal samples from the stratum were collected at depths of 108, 173, 194, 205, and 241 cm for AMS dating. These charcoal samples were sent to Quaternary Dating Laboratory for AMS14C dating in Peking University. Ages were calibrated using IntCal13 atmospheric curve (Reimer et al., 2013), using the program OxCal v4.2.4 (Bronk Ramsey, 2015).

Table 3 illustrates the population estimations of the different cultures. The Neolithic residential population size increased from 40 people in the early stages of the Majiayao culture to approximately 70 people during the Machang type. The residential population dropped to 50 people during the Qijia cultural period, but recovered during the Xindian cultural period and augmented to a similar rate of the Machang type. This indicates that, in spite of low population, the Xindian sites were more densely populated. The total population of the Majiayao type reached 9,900 people but fell by nearly half during the Banshan period. During the Machang type, the population increased dramatically and reached the peak of the Neolithic Age with a total population of 39,200 people in the northeast part of the QTP. Numbers decreased significantly during the Qijia cultural period and reached 22,790 people. The study area was occupied by both the Xindian culture and the Kayue culture during the mid-late Bronze Age when the total population reached approximately 61,330 people. Around 2.0 cal ka BP, during the Han dynasty, Zhao Chongguo, an official in Hehuang Valley, reported in his letter to the emperor, that there were about fifty thousand Qiangs (refers to the Kayue people) in the area. This supports the estimated results of the Kayue population.

The statistical result of the reconstructed cultivated land showed that during the Majiayao culture, the cultivated land area grew from approximately 150–460 km² during the Machang type. However, this was decreased during the Banshan type, falling to approximately 25 km² (Table 4). During the Qijia Culture, it decreased slightly to 330 km² and then, was followed by an increase during the mid-late Bronze Age. During this period, cultivated land area expanded to approximately 1,080 km² in the west. In the eastern region of the study area, the cultivated land area was 80 km². The total cultivated land area in the northeast margin of the QTP reached 1,160 km².

Figure 2 illustrates the temporal and spatial distribution of cultivated land throughout the prehistoric time period. Rivers played a central role in the distribution patterns of the cultivated land area. During the early Majiayao culture, the cultivated lands were sparsely distributed along the two major rivers: the Huangshui and the Yellow Rivers. During the late Neolithic Age, the cultivated land area expanded along the Huangshui River and the east edge of QTP appeared to be heavily cultivated. In the mid-late Bronze Age, the cultivated land area expanded westward, to the source region of the Yellow River and its tributaries, and northward to the areas along the Beichuan River, a main tributary of the Huangshui River.

Charcoal particles provided reliable evidence for fire. Small charcoal particles could be transported across long distances by wind, especially in dry areas (Huang et al., 2006). These small particles provided evidence for regional fires. Alternatively, large charcoal particles tended to travel only short distances, and serve as a good indicator for local fires (Patterson et al., 1987; Clark, 1988; Miao et al., 2017). Charcoal particles exceeding 100 μm are considered too massive to be suspended and transported over long distances by wind (Clark, 1988). Dianziping site charcoal analyses reveal a concentration of charcoal particles <50, 50–100, and >100 μm at 200–225, 200–250, and 200–210 cm respectively. These findings indicate the occurrence of regional fire events at a period represented at 200–250 cm, and local fire events at 200–210 cm, suggesting increased anthropogenic activity around 3.8–3.9 cal ka BP. The regional fire events appear to have occurred within the cultural layer, while evidence is absent from other layers. It is, thus reasonable to equate the high concentration of charcoal particles during this period as an indication of human activity. In addition, a sharp increase of Poaceae pollen was also observed in this section, which may further suggest that human intervention to the natural vegetation had occurred.

In the early Majiayao culture, the population density was low, and a relatively small area of land was cultivated. These areas were sparsely distributed along the Yellow and Huangshui Rivers. At this point, the anthropogenic impacts on natural vegetation was small. However, in the late Majiayao culture, the population increased dramatically, and agriculture became the primary means of sustenance (Xie, 2002). Notably, the population had increased three to four-folds and reached 39,200 during the Machang period. Concomitantly, the area of cultivated land increased exponentially and reached a peak of 465 km². Although the cultivated land had decreased during the Banshan Type, the cultivation intensity, attained by dividing the cropland area with the duration of that culture and area of that particular culture (Zhuo et al., 2013), had steadily increased since the beginning of the Majiayao Culture. This trend indicates that, despite the decreasing population, the settlement areas were still densely populated, and surrounding areas were intensely cultivated, even during the Banshan period. Disturbance to the neighboring area continued by expanded areas of cultivation (Figure 4).

During the mid-Holocene climatic optimum, coniferous and broadleaf forests grew abundantly in northeast QTP (Zhou et al., 2011). However, shifts in arboreal pollen were recorded around the Changning region (Dong et al., 2012). Around 5.1 cal ka BP, tree pollen such as Picea, Ulmus, and Betula accounted for ±33% of the total, while plant taxa associated with human activity such as Poaceae, Asteraceae, Brassicaceae, Apiaceae, Fabaceae, and Rosaceae (Li Y. Y. et al., 2008) only accounted for ±13%. Around 5.0 cal ka BP, tree pollen decreased and at around 4.2 cal ka BP, reached a low point with approximately ±10% in relative abundance. In contrast, synanthropic species significantly increased during that period, and reached a peak of ±43% of the total pollen assemblage, and in which, Poaceae accounted for ±41% (Figure 5). The decrease in tree pollen was associated with an increase in human-indicator species, especially in Poaceae during the optimum climate condition of mid-Holocene. This was also highly correlated with the population growth and expansion of cropland, as well as the high intensity of cultivation, suggesting human-induced deforestation and land cover change (Figure 4). Anthropogenic disturbance to the original vegetation was evident. The farmers of the Neolithic Age actively deforested neighboring areas to create cropland and to meet the needs of an increasing population. In short, anthropogenic impacts on the environment were initially apparent at approximately 4.3–4.0 cal ka BP on this part of the plateau.

In addition, the firing of pottery and utilization of wood resources also likely accelerated deforestation. Pottery-making technology during the Majiayao culture was highly developed, and this culture was renowned for its exquisite pottery. According to previous research, a temperature of 800°C is required for the firing of pottery (Papachristodoulou et al., 2006). Wood might have been the most attainable source for fuel at that time and a substantial number of trees utilized for pottery making. Additionally, among the excavated tombs of the Majiayao culture, wooden coffins (coffin made of wood) accounted for 84% of the burials (Qinghai Province Cultural Relics Management Branch Archaeological team, Chinese Academy of Social Sciences, Institute of Archaeology, 1984). To sum up, it appears that tree resources were abundant during the early Majiayao culture. Cropland areas may have been previously forested, but, trees were definitely logged for various of wood usage. It is thus reasonable to conclude that large areas of forests were brought down during this period.

Qijia culture represents an early period of the Bronze Age. During this period, the overall climate of the QTP gradually shifted from the mid-Holocene’s warm and wet to drier and colder climate (Liu, 2016; Wei H. et al., 2020). This transition continued through the entire Qijia period on the QTP and was synchronous with increasing climatic instability and extreme weather events (Wang et al., 2021). According to Liu (2016), the Genggahai region received relatively low amounts of precipitation between 4.1 and 3.1 cal ka BP. The excavation of the Lajia site confirmed that the area was experiencing frequent mountain torrents, indicating an increase in severe rainstorms (Chui et al., 2019; Wang et al., 2021). Driven by the overall cooling, drying climate, and the increase of extreme weather events, the human population in the area declined slightly, and the production mode of the agricultural settlers also shifted during this period. Farmers appeared to have adopted grazing as an alternative means of sustenance (Xie, 2002; Chen T. et al., 2019). Evidence suggests that people were, at least partially, engaged in pastoralism. Burial objects unearthed during the Qijia culture shifted from pigs to sheep, implying the onset of pastoralism on the northeast margin of QTP (Liu, 2014). Consequently, the cultivation intensity demonstrated a dramatic fell during the Qijia culture (Figure 4). The area of cultivated land decreased compared to that of the Machang Type.

The Dianziping pollen analysis indicates that during the Qijia culture, pine and spruce forests, as well as deciduous broadleaf trees (birch and maple), had persisted in the area. Tree pollen accounted for ±70% and shrubs accounted for ±26.8% of the total pollen assemblage (Figure 3). Trees, especially coniferous taxa, appeared to have been abundant in the area while broadleaf trees were scarce. Therefore, climate change had caused vegetation succession in the high-altitude areas. However, forests in the lower elevation (∼1,870 m) areas in the Hehuang Valley persisted indicating that these areas were still warm and wet enough for the growth of these tree species. Since cultivated land area was also concentrated in the eastern edge of Huangshui Valley. It seems that this part of the QTP was intensely cultivated despite its low human population numbers, which could be drawn from the relatively high intensity of cultivation (Figure 4). Meanwhile, analysis of the charcoal in Dianziping indicated that the incidence of regional fire was frequent between 3.9 and 3.8 cal ka BP (Figure 3). These fires were unlikely to be started naturally as lightning events are typically followed by thunderstorms and heavy precipitation on the QTP. As such, these findings further demonstrated anthropogenic disturbance during this period.

Based on series of archaeological excavations, there are approximately 366 graves of the Qijia culture, among which 288 (approximately 80%) graves used wooden coffins. Over half of these were made from a single cut of wood with diameters of approximately 50 cm. Although both the population and cultivated land area in the northeast margin of the QTP decreased during the Qijia culture, forest destruction in the region continued. Despite climate change being a major driving force for the reduction of deciduous broadleaf forests, human destruction to the forests, especially to the disappearance of coniferous forests on the river terraces in the valleys, are undeniable. In conclusion, it appears that during the Qijia culture, large scale ecological shifts were mainly driven by climatic changes. The changes obviously caused forest decline in high altitude areas and ecological succession in lower areas (Kramer et al., 2009a; Kramer et al., 2009b). On the other hand, intensive cultivation and lumber utilization in settlement areas accelerated the pace of deforestation at lower elevations.

During the mid-late Bronze Age, the northeast QTP was occupied by people of the Xindian and Kayue cultures. The relatively cold and dry climate, dictated the people to undertake some changes concerning their means of sustenance (Jia, 2012; Dong et al., 2013; Chen et al., 2015; Ren et al., 2018). One such change to the farming practice was a shift from the monoculture of millet to developed multiple crops. Barley, as cold-resistant crop, became staple in the harsh highlands (Ma et al., 2016; Zhang and Dong, 2017). Sheep, as cold-tolerant livestock, were introduced into this region in the early Bronze Age via cross-mainland cultural exchange, and grazing livestock was broadly adopted (Dodson et al., 2013; Dong, 2018). In addition, the number of pigs associated with funeral sties began to decrease and were replaced by sheep, horses, and cattle (Liu, 2014), which implies the growing importance of grazing in the Kayue culture. The development of pastoralism allowed farming activities to expand from low altitude to high altitude areas despite the dry and cold climate, and to settle permanently in areas with elevations above three thousand meters (Chen et al., 2015). Consequently, Kayue culture developed quickly in the higher altitude areas, practicing both grazing and farming. The Xindian culture was mainly distributed at lower altitude areas in the east of the study area, and was sustained by traditional farming. The shifts in production structure boosted the population growth and the development of agriculture. The total population in the northeast margin of QTP reached approximately 61,330 people, and the cropland area amounted to over 1,160 km² (Table 4). These croplands were mainly distributed along the Yellow River, and its major tributaries, expanding into the upper reaches of the Yellow River. As figure four illustrates, the adoption of grazing reduced cultivation intensity but the exponential population growth enlarged the overall impacted area. The population growth and the subsequent cropland expansion significantly resulted in human-induced environmental destruction in the natural areas, especially at high altitudes. Furthermore, increasing charcoal concentration in Gonghe Basin during the Kayue culture was accompanied by increasing human imprints and disturbances to the high-altitude areas (Miao et al., 2017).

Wood was used as raw material for construction, production tools, and fuel for the fire (Wang et al., 2014). Generally, charcoal remains from the burning of dead wood were rich in mycelium (Marguerie and Hunot, 2009; Vidal-Matutano et al., 2017). In the charcoal analysis from the Kayue and the Xindian cultural sites, mycelium was rarely encountered (Liu, 2019). Based on the absence of mycelium, it can be inferred that the increasing population during the mid-late Bronze Age could have resulted in massive logging for fuel. In the Liuwan site, six graves of the Xindian culture were excavated, but none used wooden coffins (Qinghai Province Cultural Relics Management Branch Archaeological team, Chinese Academy of Social Sciences, Institute of Archaeology, 1984). The excavation of Hetao Zhuang grave, located along a tributary of Huangshui River in Milagou, revealed 342 graves, 102 of which used wood coffins (Qinghai Province Cultural Relics Archaeological Institute, 2004). In Guide Shan Pingtai site, along the Yellow River, 90 graves of the Kayue Culture were excavated, of which, 32 graves contained wooden coffins made of woods with diameter of only 10 cm. This suggests that forests along the Huangshui River were mostly exhausted. Trees were quite limited even along the upper reaches of Yellow River where only small trees and shrubs remained. Moreover, Liu (2019) states that Pinus was easily shaped, yet hard to corrode, making it a favorable choice for use by prehistoric humans. However, the number of broadleaf trees used by prehistoric Kayue people was significantly higher than that of coniferous species. Approximately two-thirds of the wood came from broadleaf trees. This demonstrates that the availability of conifers such as Pinus has been greatly reduced, but the climate still allowed the growth of deciduous broadleaf trees. In addition, the pollen assemblage from Fengtai site in Huzhu County disclosed a variety of Poaceae taxa that no longer existed in the area. This is a testament to the rich floral biodiversity in the region during the Kayue culture and that climate allowed their abundance (Jin et al., 2006). In sum, despite having a climate that was optimal for supporting the diversity of flora and growth of deciduous broadleaf trees during that time, the forest resources were being depleted. This is supported by both archaeological evidence of wood utilization and by the pollen assemblage from Dalianhai, which showed a decreasing trend of Pinus and Picea and steady increase of Poaceae (Cheng et al., 2013) (Figure 6).

Ecological studies conducted on the grasslands of the QTP show that grazing by domesticated livestock caused an increase of Stellera chamaejasme across the landscape (Huang et al., 2017; Wei H. et al., 2020; Wei H.-C. et al., 2020). This species is toxic and was not consumed by livestock. Grazing of other species accelerates the growth of Stellera, enabling it to eventually become the dominant species on the grassland (Liu et al., 2004). The increasing abundance of Stellera chamaejasme, therefore, is an indicator of overgrazing and grassland degradation (Miehe et al., 2014). The pollen assemblage from Genggahai illustrates a rapid proliferation of Stellera chamaejasme around 3.6–3.0 cal ka BP in the surrounding areas. This indicates intensive grazing activity resulting in the degradation of previously Artemisia dominated steppe (Huang et al., 2017). This is consistent with the results of a sediment core analysis in Lake Muge, where the occurrence of frequent anthropogenic fire events showed expansion of cropland and pasture. The resulting environmental impacts of these activities induced great alterations of the natural vegetation (Sun et al., 2016).

In all, during this period, both the human populations and the cultivated land areas had increased significantly due to a progression in environmental adaptability. The Kayue people had moved up to the middle and high-altitude areas and practiced grazing as a primary means of livelihood, and crop cultivation as alternative sustenance. As a result, the pristine forests in these areas were largely destroyed, and the grassland cover was disturbed by both cultivation and grazing activities. It appears that anthropogenic impacts had been the major driving force for land deterioration and forest decline during this period.

The analysis of Genggahai and Dalianhai pollen assemblages show that since 1.6 cal ka BP, herbaceous pollen had increased, while tree pollen decreased (Cheng et al., 2013; Liu, 2016). In Genggahai region, the growth of herbaceous pollen, such as Stellera, Artemisia, and Chenopodiaceae was evident, and degradation of the grassland was apparent. The soil in the region was arid and low in organic matter, and the region was successively transformed into a desert steppe (Liu, 2016). In the Dalianhai area, at 1.4 cal ka BP, the humidity gradually increased, and the grassland began to expand. However, forested areas were not restored due to the slow succession rate of forest re-generation itself and the extensive deforestation that previously occurred in the area (Cheng et al., 2013). Soil water retention capacity was reduced, signs of soil erosion in the surrounding area were apparent, and grassland degradation was evident with the increasing growth of Stellera. It appears that anthropogenic activity, including over-grazing, was a driving force for land degradation.

The three main expansion periods of cultivation during the historical period in China occurred during the Han Dynasty, the Song Dynasty and mid-Qing Dynasty (Fang et al., 2019). In approximately 2.2 cal ka BP, the Han Dynasty colonized the areas along the Huangshui River and implemented “military cultivation” and “immigration garrison” policies to supply food to the military in the area (Gong, 2012). To ensure the implementation of these policies, over ten thousand soldiers were staged in the region to cultivate the barren land in Ledu (along the Huangshui River). Advanced iron plow techniques were introduced, and agriculture became highly developed (Gu, 2007). Postal stations and bridges were also constructed in the area during this period. According to the Legend of Zhao Chongguo and Xin Qingji, during the construction, over six thousand trees were logged from the adjacent mountains and transported along the Huangshui River. This depleted the forests in the Huangshui Valley, leaving trees mainly on the adjacent mountains. In the Tang Dynasty “military cultivation” and expansion of cropland in the Hehuang Valley reached approximately 321 km² (Wu et al., 2017). During the Song Dynasty, according to Qing Tang Record, only a few clusters of pine trees were observed along the Huangshui River. The total cropland area in Hehuang Valley was approximately 305 km², congruent with Tang Dynasty (Guo et al., 2021). By the Ming Dynasty, the population had increased rapidly in the area (Cui et al., 1999; Zhang et al., 2016). In the late Ming Dynasty, 1650AD, the cropland area in the Hehuang Valley reached approximately 433 km² (Zhao, 2016). The structure of forest vegetation in the valley had changed significantly, with poplar and birch replacing the original pine and spruce forests (Qinghai Local Chronicle Compilation Committee, 1993). By the Qing Dynasty, dry farming had been widely developed and the cultivated land gradually expanded into the hills and mountains (Wang et al., 2000). In 1747AD, Yang Yingju recorded in Annals of Xining Fu, that most of the mountains along the middle reaches of the Huangshui River were barren lands (Yang and Wang, 1747). Timber for construction of Xining Fu had to be transported from distant mountains. By the late Qing Dynasty, in 1845AD, the cropland in Hehuang Valley reached approximately 1,505 km².

In conclusion, croplands were abandoned and re-cultivated frequently and expanded continually; forests and shrubs were heavily harvested. Cropland expansion increasingly pushed the carrying capacity of the land toward the edge of limitation. Large areas of barren hillsides were formed in Hehuang area. Soil erosion and landslides occurred more frequently following the deterioration of the natural vegetation. There were no major climate fluctuations during the historical period suggesting the destruction of vegetation cover was overtly induced by anthropogenic factors. Currently, natural forest in the area is patchy and scattered across the hills and slopes found at elevations ranging from 2,600 to 3,400 m (Zhou et al., 1986). This further demonstrates that the destruction of forests and the deterioration of grassland during the historical period were primarily due to anthropogenic forces.