Late Holocene Vegetation Dynamic and Human Activities Reconstructed from Lake Records in Western Loess Plateau, China

Quaternary International 227 (2010) 38e45

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Late Holocene vegetation dynamic and human activities reconstructed from lake records in western Loess Plateau, China

Ke Zhang, Yan Zhao*, Aifeng Zhou, Huiling Sun

MOE Key Laboratory of Western China’s Environmental System, College of Earth and Environmental Sciences, Lanzhou University, 222 Tianshui Road South, Lanzhou, Gansu 730000, China article info abstract

Article history: A high-resolution pollen record from a mountain lake (2400 m a.s.l.) in the western Loess Plateau reveals Available online 11 June 2010 variations of vegetation and human activities during the last 3200 years. The pollen record indicates the vegetation changes from mixed deciduouseconiferous forest dominated by Betula, Quercus, and Abies at 3200e2200 cal BP, to forest steppe co-dominated by trees (Betula, Quercus) and herbs (mostly Artemisia) at 2200e1100 cal BP and then to steppe-like vegetation since 1100 cal BP. A distinct vegetation type shift occurred at ca.1100 cal BP, accompanied by the increase of anthropogenic indicators (cereal-type pollen, Humulus-type pollen and Pediastrum). Human deforestation might have contributed to this abrupt shift. Micro-charcoal abundance also shows a generally increasing trend during the last 1000 years, suggesting enhanced human activities. After 1100 cal BP, two deforestation phases (1100e750 cal BP and 350 cal BP-present) and one forest expansion phase (750e350 cal BP) are identiﬁed. These phases have good correlation with historical events, suggesting that increased population pressures, ﬂourished agriculture, and warfare are the main reason for these forest clearances. Anthropogenic activities appear to be the main controlling factor of the vegetation dynamics during the late Holocene, especially for the last 1100 years. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Quaternary environmental and climate shift based on pollen analysis have been carried out on the Loess Plateau (e.g., Sun et al., 1995, There has been a long-standing interest in the study of late- 2007; Li et al., 2003a; Feng et al., 2004, 2007; Tang and An, 2007; Quaternary landscape dynamics on the Chinese Loess Plateau (“the Wu et al., 2009). Most of these researches focused on the late- cradle of ancient Chinese civilization”), as this region is very Pleistocene and the early to mid-Holocene. Several of these studies sensitive to climate change (Li et al.,1988) and has witnessed strong show that the vegetation deteriorated since ca. 4000 cal BP (Zou human influence during the late Holocene (Liu, 1985). The Loess et al., 2009; Feng et al., 2006); however, these palynological Plateau is presently subject to desertification, soil erosion, and records do not have ideal age controls for understanding vegetation socio-economic stresses under the increased climate variability dynamics, as they are from loess profiles. Thus the vegetation (Wang et al., 2006). Understanding the interactions of past climate patterns on the Loess Plateau during the late Holocene, when change, human activity and environment response are of particular vegetation was probably greatly affected by human activities, importance to understand regional climate and response to global remain poorly understood. change (Dearing, 2006, 2008). This paper presents a high-resolution pollen record together As a core element of the environmental system, vegetation is the with charcoal, conifer stomata and Pediastrum data from a moun- key to the evaluation of past environmental evolution (Li et al., tain lake in the western Loess Plateau. The objectives were to 2003b). Palynological studies provide a useful tool for the recon- reconstruct the paleovegetation change during the late Holocene in struction of past vegetation in response to climatic variations and the study area and to investigate the human impacts on forest human activities. A number of investigations concerning the late- change by comparing with other regional paleoclimatic records from speleothems, archaeological records of human habitation and historical documents. This is the first high-resolution pollen record * Corresponding author. Fax: þ86 (0) 931 8912330. from lacustrine sediments to reveal the vegetation dynamics and E-mail address: [email protected] (Y. Zhao). human activities during late Holocene in the western Loess Plateau.

2. Regional setting Berberis circumserrata, and Salix cathayana. The steppe and grass- land is dominated by Pteridium aquilinum, Stipa baicalensis, Festuca Tianchi Lake (351505100e351505500N, 1061802800e1061802800E, ovina, and Artemisia. Between 2100 and 2800 m, the vegetation 2430 m a.s.l.) is a small (2 km2) freshwater lake (maximum depth of is characterized by sub-alpine trees and shrubs. The deciduous 8.2 m) on the Liupan Mountains, situated at 30 km east of Zhuanglang trees are dominated by Betula utilis and P. davidiana, with few County, Gansu Province, northwestern China (Fig. 1). The lake is conifer trees (Pinus armandii and Juniperus formosana). Shrubs are mainly fed by rainfall and underground water. Climate in this region is the dominant vegetation, mainly consisting of Cotoneaster acutifo- mainly controlled by the Asian monsoon with a warm, wet summer lius, Hippophae rhamnoides, Rosa acicularis, Rosa sertata and Salix and cold, dry winter. Mean annual precipitation is 677 mmwith peaks caprea. The herbaceous taxa mainly include Carex rochebruni, occurring during JuneeSeptember, mean annual temperature is Beckmannia syzigachne and Poa pratensis. At present, dwarf-shrub 8.2 C, and the minimum temperature of the coldest months of the and steppe are the dominant vegetation on the mountain slopes year is 26 C (January) while the maximum temperature of the surrounding Tianchi Lake, mainly consisting of R. omeiensis, warmest months of the year is 30 C (July), based on the nearest Berberis amurensis, H. rhamnoides, Lonicera spp., Betula spp., Rubus meteorological Liupan Mountain station (at 2845 m a.s.l.). spp., Poa annua, Artemisia argyi, Ageratum spp., Inula japonica, Natural vegetation of this region is temperate forest steppe. The Taraxacum ofﬁcinale, and Leontopodium nanum. Vegetation is modern vegetation displays stronger secondary features, but it still different on the north and south slopes: the northern slope is shows a clear zonal distribution along the altitudinal gradient characterized by dwarf-shrub, and the southern slope is charac- (Wang, 1988). Between 800 and 1400 m, the vegetation is domi- terized by steppe and some human-planted Pinus trees. Fig. 1 nated by human-planted trees, as well as natural shrubs and some clearly shows this disturbed landscape, with combination of scattered trees. The grass mainly includes Artemisia, Cyperaceae, grassy steppe and shrubs near the lake. and some cultivated crops. Between 1400 and 2100 m the vegetation is characterized by forest steppe. Betula platyphylla, Betula 3. Materials and methods albo-sinensis and Quercus liaotungersis are the common dominant deciduous species, together with Tilia paucicostata, Populus Ten surface pollen samples were collected from the sites rep- davidiana and Fraxinus chinensis. Shrubs mainly consist of Salix resenting different vegetation types near the lake and 3 surface lake cheilophia, Ostyopsis davidiana, Corylus heterophylla, Rosa omeiensis, samples from the Tianchi lake, in order to investigate the

Fig. 1. Location and settings. A. Location of the Loess Plateau (yellow area). Black dots show other palaeoclimate sites discussed in the text and the rectangle represents our study region. B. Tianchi Lake (black dot) on the Liupan Mountains. White dots show other sites mentioned in the text. C. Photo of Tianchi Lake. Black dot shows the coring location of the core GSA07-1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). 40 K. Zhang et al. / Quaternary International 227 (2010) 38e45 relationships between pollen assemblages and modern vegetation Table 1 in the study area. Samples S1eS3 were from surface lake sediments AMS radiocarbon dates from Tianchi Lake (the upper 6 m of core GSA07-1), southwest Loess Plateau, China. along a transect from the west to the east. Samples S4eS9 were collected from the meadow and steppe at the mountain (along the Lab Depth Material d13C 14C date Error Calibrated age & s transect from west to east) near the lake at different elevations. number (cm) dated ( VPDB) (yr BP) ( yr) (cal yr BP-2 range) Sample S10 and S11 were from dwarf-shrubs the southern side of A01e163 162 Tree leaves 29.0 680 30 634e680 the lake. Sample S12 and S13 were from forest patches dominated e e e A01 184 183 Tree leaves 26.9 855 35 688 801 by Larix and Butula, respectively. The surface samples (S4 S13) A01e223 221 Tree leaves 21.0 1080 35 932e1056 were taken from moss cushions and surface soils. A01e262 260 Tree leaves 11.6 1255 30 1122e1277 The 11-m-long sediment core (GSA07-1) was taken from the A02e021 302 Tree leaves 18.7 1440 45 1288e1406 deepest part of Tianchi lake (8.2 m) using a UWITEC piston corer in A02e097 382 Tree leaves 21.0 1775 30 1610e1745 A02e148 436 Tree leaves 24.3 2060 30 1948e2118 September 2007. This study focuses on the upper 6 m of the core A02e198 489 Tree leaves 23.1 2355 30 2333e2464 GSA07-1 in order to make a high-resolution reconstruction of the A02e225 517 Tree leaves 12.7 2585 40 2695e2774 vegetation change over the late Holocene, which went through A03e043 600 Tree leaves 16.6 2895 45 2912e3165 large changes in vegetation based on the coarse resolution pollen analysis results from the entire core (Zhao et al., 2010). The chronology was based on 10 AMS 14C dates on terrestrial occurred in samples S2 and S3. Samples S4eS9 were from steppe plant macrofossils (tree leaves) at the AMS Dating Laboratory in and meadows consisted of Poaceae, Artemisia and Cyperaceae Beijing University. Radiocarbon ages were calibrated to calendar- (based on field investigation). The pollen assemblages of samples year ages using CALIB5.01 (Reimer et al., 2004). The age-depth S4eS9 are characterized by Betula (average 17%), Artemisa (average model was based on 2nd polynomial curve. All dates discussed in 20%), Elaeagnaceae (average 10%) and Quercus (average 10%). S8 has this paper are in calendar years before present. a significant high value of Cyperaceae (47%), probably originating A total number of 154 fossil pollen samples were taken for pollen from Cyperaceae plants at the western lake shore. Samples S10 and analysis with a temporal resolution of about 20e30 years. Pollen S11 were from dwarf-shrubs consisted of H. rhamnoides, R. acicu- preparation followed the modified acetolysis procedure (Fægri and laris and some herbaceous plants (such as Poaceae, Polygonum). Iversen, 1989), including HCl, NaOH, HF, and acetolysis treatment, Sample S10 has a very high pollen percentage of Elaeagnaceae and fine sieving to remove clay-sized particles. A known quantity of (38%) with some Betula, Artemisa and Poaceae, whereas the pollen Lycopodium spores was added to each sample, to calculate absolute assemblage from S11 is characterized by Betula (22%), Artemisia pollen concentration, micro-charcoal concentration, Pediastrum (25%) with some Elaeagnaceae, Salix and Roacea. Sample S12 has concentration and stomata concentration (Maher, 1981). The slides a high value of Larix (13%) comparing with other samples, and it were analyzed using a Nikon microscope at a magnification of also shows a high value of Betula, Artemisia and Quercus. S13 has the 400, with 1000 magnification used to examine detailed struc- highest value of Betula (27%) and Artemisia (35%) of the thirteen ture when necessary. Taxonomic identification followed Wang samples. The results suggest that the modern pollen assemblages (1995) aided by modern reference collections. The cereal-type could fairly reflect the vegetation type that grows around the pollen was distinguished from other Poaceae pollen on the basis of Tianchi Lake region, and the pollen assemblages can be used to size of grain, pore and annulus. About 300e500 terrestrial pollen interpret vegetation change in the study area. grains were counted for each sample level, and pollen percentages were calculated based on the sum of terrestrial pollen. The defini- 4.3. Fossil pollen assemblages tion of the boundaries of pollen assemblage zones was assisted by stratigraphically constrained cluster analysis using CONISS, avail- A total of 48 taxa were identified in 154 samples from the upper able in the computer program TGView 2.0.2. Micro-charcoal parti- 600 cm of the core GSA07-1 (Fig. 3). The pollen assemblages were cles were counted on pollen slides following Tinner and Hu (2003) divided into three zones based on stratigraphically constrained and Finsinger and Tinner (2005). Micro-charcoal concentration is cluster analysis (CONISS). calculated based on Lycopodium spikes. Conifer stomata were identified following Trautmann (1953), Wan et al. (2007) and some 4.3.1. Zone GSA-1 (3200e1100 cal BP; 615e235 cm) modern reference collections. This zone is characterized by a gradually decreasing trend of tree pollen. It can be divided into two subzones: zone GSA-1a 4. Results (3200e 2200 cal BP; 615e435 cm) and zone GSA-1b (2200e1100 cal BP; 435e235 cm), mainly based on relative changes in abundance 4.1. Lithology and chronology of Abies, Picea, Betula, and Artemisia pollen types.

The upper 600 cm of the GSA07-1 core is well laminated, except 4.3.2. Zone GSA-1a (3200e2200 cal BP; 615e435 cm) for four short intervals of brownish clay at 513e567 cm, This zone is overwhelmingly dominated by tree pollen (up to 375e387 cm, 238e309 cm and the uppermost 8 cm. All 10 AMS 90%), mainly including Betula (20e40%), Quercus (15%) and Abies 14C dates are in good order and the chronology indicates that the 6- (20e50%). Herb pollen is less abundant with the maximum value of m-deep core extends back to the last 3200 years (Table 1). The 30% dominated by Artemisia and Rosaceae. Abies pollen increases sediment accumulation rates based on the age-depth model are signiﬁcantly at ca.2850 cal BP, and then it gradually decreases to relatively consistent, averaging 1.85 mm/y. 10%. Betula pollen reaches a lower value of 15% at ca.2850 cal BP, and then gradually recovers to its former level (35%). The pollen 4.2. Modern pollen assemblages concentration varies signiﬁcantly with a very low value (averagely 14 000 grains/g) between ca. 2820 and 2700 cal BP. Pollen assemblages from samples S1 and S3 show that pollen assemblages are dominated by Betula (average 20%), Artemisia 4.3.3. Zone GSA-1b (2200e1100 cal BP; 435e235 cm) (average 45%) and Myriophyllum (average 30%), together with some This zone is characterized by a high percentage of tree pollen Quercus, Elaeagnaceae and Cyperaceae (Fig. 2). Cereal-type pollen (65%), but it shows a gradually decreasing trend. Betula and Quercus K. Zhang et al. / Quaternary International 227 (2010) 38e45 41

Fig. 2. Percentage pollen diagram from regional modern samples in the study region. Only major taxa shown. are the predominant deciduous tree taxa with detectable amounts concentrations fluctuate between 31124 and 622 965 grains/g with of conifer pollen (10%). The percentage of herbs and shrub pollen a relatively low mean value (e279 970 grains/g). show a slight increasing trend compared with zone GSA-1a. The pollen concentrations fluctuate significantly between 2800 grains/ 4.3.6. Zone GSA-2b (750e350 cal BP; 167e92 cm) g and 1 355 300 grains/g during this period. This zone is characterized by a sudden increase of tree pollen (including Betula and Quercus) at the expense of the herb and 4.3.4. Zone GSA-2 (1100e350 cal BP; 235e92 cm) shrub component. The tree pollen percentages increase to 45%, The pollen assemblages are characterized by a significant while the herb and shrub pollen percentages decrease to 40% and decrease of tree pollen and increase of herbaceous pollen. This zone 15%, respectively. The conifer pollen shows a slightly increase can be divided into two subzones: zone GSA-2a (1100e750 cal BP; before they almost disappeared. The Elaeagnaceae and Humulus- 235e167 cm) and zone GSA-2b (750e350 cal BP; 167e92 cm), type pollen are all considerably lower than those in the zone mainly based on relative changes in abundance of Betula, Quercus, GSA-2a. The pollen concentration remains at a high value and Artemisia pollen types and tree pollen percentage. (e536 360 grains/g).

4.3.5. Zone GSA-2a (1100e750 cal BP; 235e167 cm) 4.3.7. Zone GSA-3 (350 cal BP-present, 92e0 cm) The pollen assemblages are characterized by a sudden appear- The tree pollen decreases sharply to ca.15% at the beginning of ance of cereal-type pollen and also by a signiﬁcant decreasing trend this zone, then stays at a relatively lower value (ca.10e25%) until of tree pollen, mainly resulting from the reduction of Betula the last decade. Artemisia (up to 60%) and Poaceae (up to 20%) and Quercus. The conifer pollen (Abies, Piceae and Pinus) did not dominate the herb component. It is also quite noticeable that change so much in this zone. The herb pollen (mainly Artemisia) cereal-type pollen is at a considerably high value compared to increased from 30% to 80%. The Elaeagnaceae and Humulus-type those from zone GSA-2b. The Humulus-type and Elaeagnaceae pollen also show a considerable increase in this zone. The pollen pollen also exhibits a slight increase. The pollen concentration is

Fig. 3. Percentage pollen diagram from core GSA07-1 at Tianchi Lake, Gansu, China. Only major taxa shown. 42 K. Zhang et al. / Quaternary International 227 (2010) 38e45 characterized by a high ﬂuctuation between 7171 grains/g and 795 885 grains/g with a generally decreasing trend.

4.4. Micro-charcoal, stomata and Pediastrum

Micro-charcoal shows a relatively steady and low concentration (averagely 19 800 particles/g) in the pollen zone GSA-1 (3200e1100 cal BP). It increases in pollen zone GSA-2 (1100e350 cal BP), and it reaches the highest level (up to 125 300 particles/g) in the pollen zone GSA-3 (350 cal BP-present). The Abies and Pinus stomata concentrations are high in the pollen zone GSA-1a (3200e2200 cal BP), low or absent in the pollen zone GSA-1b (2200e1100 cal BP) and GSA-2a (1100e750 cal BP). Both of the Abies and Pinus stomata show relatively high concentration in the pollen zone GSA-2b (750e350 cal BP), after which the Abies stomata almost disappeared while the Pinus stomata gradually decreased. The ﬁrst distinct increase of Pediastrum occurred at ca.1200 cal BP and it reached the highest value from ca.500 cal BP to the present.

5. Discussion

5.1. General vegetation history over the last 3200 years

The fossil pollen assemblages suggest that vegetation was dominated by a mixed deciduouseconiferous forest in this region during 3200e2200 cal BP. The broadleaved trees mainly consist of Betula and Quercus. The high percentage of Abies pollen and occurrence of the Abies stomata indicate that Abies may have been present locally or near the lake. At 2200e1100 cal BP, the pollen assemblages indicate less dense forest cover compared with the previous interval. The vegetation has transformed from mixed Fig. 4. Comparison of tree pollen percentage from Tianchi Lake and regional summer deciduouseconiferous forest to forest steppe. The herbaceous monsoon records from speleothem oxygen isotopes over the last 3200 years. A. Tree component increase clearly at the expense of trees. The broad- pollen percentage from Tianchi Lake. B. d18O record from Dongge Cave (Wang et al., d18 d18 leaved trees mainly consist of Betula and Quercus, together with 2005). C. O record from Heshang Cave (Hu et al., 2008). D. O record from Wan- xiang Cave (Zhang et al., 2008). some Salix and Juglans trees. The existence of Pinus and Abies stomata suggest that there were still few Abies and Pinus trees scattered around this region. vegetation transition took place from ca.1100 cal BP, when the trees During the period of 1100e750 cal BP, the pollen assemblages began to decrease significantly. The distinct vegetation type shift indicate an open landscape characterized with steppe dominated from forest to forest steppe could have been caused by human by Artemisia and Poaceae. The broadleaved trees declined sharply activities rather than climate change, as paleoclimatic records during this period. The first appearance of cereal-type pollen (Wang et al., 2005; Hu et al., 2008; Zhang et al., 2008) in this region suggests some cultivated crops in this region. The period of do not show substantial climate change at this time (Fig. 4). 750e350 cal BP is characterized by the forest regeneration that The first appearance of cereal-type pollen suggests the upland includes re-expansion of Betula and Quercus in the region. Although expansion of cereal cultivation most probably associated with there are low pollen percentages of Abies and Pinus, the existence of strong deforestation at ca.1100 cal BP. The Humulus-type pollen also their stomata suggest that Abies and Pinus trees may still have been increased dramatically during this period, which may indicate scattered near the lake. substantial opening-up of woodland by human clearance (Fig. 5). After ca.350 cal BP, herbs and shrubs increased again and cereal- Pediastrum has been commonly used as an indicator of nutrient type and other Poaceae types reach the highest level. Abies trees status in temperate freshwater lakes (Del Zamaloa and Tell, 2005). almost disappeared during this period. The increase of tree pollen The sudden increase of Pediastrum at ca.1100 cal BP may suggest the together with the reduction of the Artemisia during the recent eutrophication of the lake as a result of increasing human activities. decades possibly indicates a slightly denser vegetation cover in this Farmers probably fertilized the soil using manure on the mountain region. slopes for cultivation, meanwhile, the excretion of human and domestic animals brought into the lake by the rain could also lead 5.2. Human-induced vegetation type shift at 1100 cal BP to nutrient increase due to the lake’s small size. The micro-charcoal also shows a great increase, suggesting intense human-induced fire The gradually decreasing trend of tree trees since 3200 years at activities. All of this evidence supports the hypothesis that human Tianchi Lake generally agrees with changes in monsoon intensity activities could have contributed to the sharp vegetation type shift inferred from the d18O isotope record (Fig. 4), suggesting sensitive at 1100 cal BP. vegetation response to weakening monsoon and moisture decrease In the Loess Plateau, cultivated land per person decreased from in this region. This indicates that climate might be the main 2.81 to 1.8 ha during ca.1350e1200 cal BP due to the dramatically controller of the vegetation change at this time. Forest vegetation increased population (Wang et al., 2006). Xu (2003) compiled an shows a moderate decline since 2200 cal BP. The most significant approximate estimation of the location of the boundary between K. Zhang et al. / Quaternary International 227 (2010) 38e45 43

Fig. 5. Climate-human-environment interactions over the last 2000 years. A. Tree pollen percentage from Tianchi Lake. B. Cereal-type pollen percentage and Humulus-type pollen percentage from Tianchi Lake. C. Micro-charcoal concentration from Tianchi Lake; D. Temporal variation in sedimentation rate in the lower Yellow River (dark cyan line)(Xu, 2003) and north limit of agriculturaleanimal husbandry boundary (dark yellow line). The west-east-trending Weihe River and the Yellow River below Tongguan is used as a baseline for the distance (south to north) calculation (Xu, 2003). E. The population of the Loess Plateau during historical periods (Chen, 1990). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). the agriculture and animal husbandry areas based on the historical 5.3. Deforestation phases over the last 1100 years literature on the Loess Plateau (Zhu, 1999). The major shift of the agricultureeanimal husbandry boundary to the north at ca.1000 cal The first substantial tree reduction occurred at ca.1100e750 cal BP suggests the expansion of the agricultural area and enhanced BP, and this period was coincident with the TangeSong dynasty. human-induced vegetation deforestation (Xu, 2003). Historical The first appearance of cereal-type pollen and large increase of the documents also show substantial deforestation during the Tang Humulus-type pollen during this period (Fig. 5) suggest the dynasties (618e907 AD ¼ 1332e1043 cal BP) (Wang, 1994; Shi increased human activities in the upland region. Tang dynasty is et al., 1981). Bray (1984) indicates that terracing in the central a period of great prosperity in politics, economics and culture with Loess Plateau is believed to have been introduced well over 1100 very dense population and a flourishing agriculture. More upland years ago. Geomorphological and geochemical records suggest the regions were exploited for cultivation, together with the greatly increased sedimentation rate of the Yellow River at ca.1000 cal BP increased demand for wood as a building material and for fuel all as a consequence of intensive human activities though with slightly led to large-scale destruction of forest (Zhu, 1991). Historical different timing (Saito et al., 2001; Xu et al., 1998). All of these documents recorded that significant areas of natural forest in the studies suggest that significant human disturbance on the vegeta- central and western Loess Plateau were logged and transported to tion and landscape occurred at ca.1100 cal BP on the Loess Plateau, nearby areas, which were the most civilized and populated in supporting the pollen and charcoal records from Tianchi Lake. China at that time (Shi, 2001). After the Tang dynasty, the western 44 K. Zhang et al. / Quaternary International 227 (2010) 38e45

Loess Plateau experienced a long period of political disturbance (3) The results reveal several distinct deforestation phases which due to border conflicts between Song dynasty and nomads (Xixia, show good correlation with historical human activities. The Jin and Liao). As a natural barrier, the Liupan Mountain region has first significant deforestation occurred at 1100e750 cal BP, witnessed a large amount of warfare and conflict. Historical correlating with the late TangeSong dynasty, a period with documentation states that there was a wide-spread deforestation dense population, flourishing agriculture as well as plenty of by the different armies at this time (Shi, 2001; Zhu, 1999). Armies border conflict. Forest expansion during 700e350 cal BP from each side gathered in this region and built numerous barriers corresponds to the Yuan and the early Ming dynasty, with and shelters. Moreover, soldiers were encouraged to open more sharply decreasing population and many abandoned agricul- land and grow corps during periods of peace (Shi, 2001). In ture land. The final substantial forest clearance occurred at ca. summary, increased anthropogenic activities in both the later Tang 350 cal BP. Vegetation history inferred from pollen record dynasty and Song dynasty should have contributed to the exten- generally agrees with the deforestation records based on the sive deforestation during 1100e750 cal BP. historical documents in the Loess Plateau. The timing of forest recovery between 750 and 350 cal BP is coincident with the territorial expansion of the entire Yuan Acknowledgments dynasty and part of the Ming dynasty. This period was coincident with a weak monsoon, inferred from the speleothem records We would like to thank Furong Li for field assistance and Virginia (Fig. 4). Furthermore, the rainfall intensity reconstruction based Panizzo for linguistic corrections. This project was supported by the on historical documents in Longxi area (ca.150 km from Tianchi National Science Foundation of China (NSFC Grant# 40771212 and Lake) also indicates a relatively dry climate during this period Grant# 40971056), NSFC Innovation Team Project (#40721067) and (Tan et al., 2008). Therefore, the forest recovery may mainly MOE New Century Talent Program (NCET-07-0402). result from decreased human activities rather than the climate variations. The Humulus-type pollen from core GSA07-1 References decreased during this period, suggesting less human clearance (Fig. 5). The Yuan dynasty was ruled by the Mongolian people, Bray, F., 1984. Science and civilization in China, 6, Part II: Agriculture. Cambridge who used to be nomadic. They abandoned many farm lands or University Press, Cambridge. Chen, S.B. (Ed.), 1990. Issue on the Population of the Loess Plateau of China. Science turned the farmland into pasture (Shi, 2001). 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