A 22 570-year Record of Vegetational and Climatic Change from Wenhai Lake in the Hengduan Mountains Biodiversity Hotspot, Yunnan, Southwest China

The Hengduan Mountains, with their strong altitu-dinal vegetation zonation, form a biodiversity hotspot which offers the potential for comparison between sites in order to understand how this zonation arose and how it has responded to climate change and human impacts through time. This paper presents a 22 570-year pollen record of vegetational and climatic change based on a core 320 cm in depth collected from Wenhai Lake on Jade Dragon Snow Mountain, one of the highest peaks in the Hengduan Mountains region of Yun-nan, Southwest China. From 22 570 to 21 140 cal yr BP, the vegetation was dominated by broad-leaved forest (comprising mainly Quercus, Betula and Castanopsis), accompanied by needle-leaved forest (mainly Pinus and Abies), indicating a rather cold and dry climate relative to the present followed by cold and wet conditions. In the period between 21 140 and 19 350 cal yr BP, the vegetation was still dominated by broad-leaved forest and needle-leaved forest as before but with a notable increase in Betula pollen and a sharp decrease in Quercus pollen, implying a relatively cold and dry climate with several fluctuations in humidity. The period 19 350 to 17 930 cal yr BP was a transition stage from broad-leaved forest to needle-leaved forest, with a dramatic decrease in Quercus pollen and a maximum reading for Abies pollen, reflecting the coldest and driest climate since 22 570 cal yr BP. The expansion in needle-leaved forest dominated by Pinus and Abies (22 570–17 930 cal yr BP) along with an increase of Betula might correspond to the Last Glacial Maximum (LGM; the start of the LGM perhaps occurred prior to the basal age of the core). Between 17 930 and 9250 cal yr BP, needle-leaved forest declined and broad-leaved forest began to increase at first, suggesting increases in temperature and humidity, while towards the end of the period, needle-leaved forest expanded and broad-leaved forest shrank, indicating a colder and drier climate, possibly corresponding to the Younger Dryas. From 9250 cal yr BP to the present, the vegetation has been dominated by needle-leaved forest (comprising mainly Pinus, Abies and Tsuga), interspersed with broad-leaved Quercus and Betula, reflecting a significant decline in humidity from the early to late Holocene. During this period, human activity likely increased in this region, with impacts on the vegetation such as a distinct decrease in Pinus and Quercus pollen and an increase in Polygonaceae pollen in the …

Abstract.The Hengduan Mountains, with their strong altitudinal vegetation zonation, form a biodiversity hotspot which offers the potential for comparison between sites in order to understand how this zonation arose and how it has responded to climate change and human impacts through time.This paper presents a 22 570-year pollen record of vegetational and climatic change based on a core 320 cm in depth collected from Wenhai Lake on Jade Dragon Snow Mountain, one of the highest peaks in the Hengduan Mountains region of Yunnan, Southwest China.From 22 570 to 21 140 cal yr BP, the vegetation was dominated by broad-leaved forest (comprising mainly Quercus, Betula and Castanopsis), accompanied by needle-leaved forest (mainly Pinus and Abies), indicating a rather cold and dry climate relative to the present followed by cold and wet conditions.In the period between 21 140 and 19 350 cal yr BP, the vegetation was still dominated by broad-leaved forest and needle-leaved forest as before but with a notable increase in Betula pollen and a sharp decrease in Quercus pollen, implying a relatively cold and dry climate with several fluctuations in humidity.The period 19 350 to 17 930 cal yr BP was a transition stage from broad-leaved forest to needle-leaved forest, with a dramatic decrease in Quercus pollen and a maximum reading for Abies pollen, reflecting the coldest and driest climate since 22 570 cal yr BP.The expansion in needle-leaved forest dominated by Pinus and Abies (22 570-17 930 cal yr BP) along with an increase of Betula might correspond to the Last Glacial Maximum (LGM; the start of the LGM perhaps occurred prior to the basal age of the core).Between 17 930 and 9250 cal yr BP, needle-leaved forest declined and broad-leaved forest began to increase at first, suggesting increases in temperature and humidity, while towards the end of the period, needle-leaved forest expanded and broad-leaved forest shrank, indicating a colder and drier climate, possibly corresponding to the Younger Dryas.From 9250 cal yr BP to the present, the vegetation has been dominated by needle-leaved forest (comprising mainly Pinus, Abies and Tsuga), interspersed with broadleaved Quercus and Betula, reflecting a significant decline in humidity from the early to late Holocene.During this period, human activity likely increased in this region, with impacts on the vegetation such as a distinct decrease in Pinus and Quercus pollen and an increase in Polygonaceae pollen in the upper 30 cm of the core.The marked decline in Quercus pollen compared with the early stage of this period, in particular, in the Wenhai core can be correlated with that observed in the Haligu core (situated about 2 km away) between 2400 cal yr BP and the present.

Introduction
The Hengduan Mountains are located in the north of the Mountains of Southwest China biodiversity hotspot, the most biologically diverse temperate ecosystem in the world (Conservation International, 2010), sandwiched between the Honghe Basin to the east and the Qinghai-Xizang Plateau to the west.They comprise five main ridge systems char-Y.F. Yao et al.: A 22 570-year record of vegetational and climatic change acterized by vertical vegetation zonation and separated by four deep drainage systems, created during the Himalayan orogeny beginning in the Tertiary and continuing into the Quaternary (Myers et al., 2000;Ying, 2001).The floristic diversity of the region is particularly high: the Hengduan Mountains are situated in Yunnan Province, which, despite covering just 4 % of China's land area, contains ca. 15 000 species of higher plants, almost 50 % of the country's total.
The marked altitudinal zonation of vegetation in the Hengduan Mountains offers the potential to compare between sites in order to build up an understanding of how this zonation arose and how it responds, through time, to climate change and human impacts.Jade Dragon Snow Mountain (rising to 5596 m a.s.l.) is one of the highest peaks in the Hengduan Mountains region, and is particularly appropriate for the study of past and present diversity using palynological data because it supports a number of natural wetlands and lakes containing abundant, well-preserved palynomorphs, at a range of altitudes.Thus, sampling of core sediments from different sites has the potential to generate pollen data relative to both time and altitude (at a given locality), which will ultimately enable us to estimate changes in both floristic composition and diversity over time and their response to climatic change.
During the past decade, pollen analysis has been employed extensively for understanding Quaternary vegetation and climate history in China (e.g.Xu et al., 2002;Xiao et al., 2004;Zhao et al., 2007;Li et al., 2011;An et al., 2013;Cao et al., 2013;Jiang et al., 2013).However, few such studies have been conducted in the Hengduan Mountains (Jiang et al., 1998;Shen et al., 2006;Jones et al., 2012;Song et al., 2012;Cook et al., 2013;Xiao et al., 2014).Previously, we have investigated changing climate and vegetation over the past 9300 years based on pollen analyses of a core 400 cm in depth from a wetland site at Haligu (3277 m) on Jade Dragon Snow Mountain (Song et al., 2012).This paper presents a 22 570-year record of vegetational and climatic change from Wenhai Lake, also on Jade Dragon Snow Mountain.We aim to use pollen data to develop insights into changing floristic diversity and to draw inferences about past climate and anthropogenic influences in the region during the late Quaternary.

Study area
Wenhai Lake (26 • 58 59 N, 100 • 09 54 E), an enclosed icescour lake at an altitude of 3080 m, is located at the southern end of Jade Dragon Snow Mountain in Yulong County of Lijiang City, northwestern Yunnan, Southwest China (Fig. 1).It forms part of Lashihai Swamp Natural Reserve and is approximately 23 km northwest of Lijiang.The lake has a surface area of about 0.16 km 2 , with a maximum water depth of ca. 4 m.It is hydrologically recharged by rainfall and glacial meltwater from the surrounded mountains, without river wa-ter inputting into the lake.It is a seasonal lake, receiving abundant rainfall in the rainy season (May to October) and less rainfall in the dry season (November to April).The outflow is dammed, with a dam constructed in 2012.Therefore, the source of lake sediment is relatively simple and stable.This area is a key region linking the Qinghai-Xizang Plateau with the Yungui Plateau, and also is a boundary region between the Hengduan Mountains area of northwestern Yunnan and the plateau area of eastern Yunnan.More than a thousand years ago, Wenhai was an important stop on the ancient "Tea-Horse Road", a route for trading tea and horses between inland agricultural and remote nomadic regions (Luo, 2003).Yulong County is home to several ethnic minorities, with the Naxi being most numerous.
The study area is strongly influenced by the southwest monsoon coming from the Indian Ocean.Thus the summers are warm and humid and the winters cool and dry.The mean annual temperature (MAT) and mean annual precipitation (MAP) measured at Lijiang (situated below the study site at about 2200 m) are 12.8 • C and 935 mm, respectively.About 90 % of the annual precipitation falls in summer, between June and October.The warmest month is July, with a mean temperature of 17.9 • C, and the coldest month is January, with a mean temperature of 5.9 • C (Feng et al., 2006).
The regional vegetation and climate of the Jade Dragon Snow Mountain area are strongly related to elevation gradients.At increasing elevations on the mountain slopes, MAT shows a decreasing trend, while MAP displays a reverse trend.For example, MAT and MAP are 12.6 • C and 772 mm, respectively, at 2393 m; MAT decreases to 5.4 • C and MAP increases to 1600 mm at 3200 m; and MAT further decreases to −3.3 to −4.7 • C and MAP increases to more than 2400 mm at the snow line (4800 m; He et al., 2000a, b).Four main vegetation zones can be recognized: zone 1, semi-humid evergreen broad-leaved forest-pine forest (about 2400-3000 m); zone 2, needle-and broad-leaved mixed forest-sclerophyllous evergreen broad-leaved forest (about 3000-3300 m); zone 3, cold-temperate needle-leaved forest (about 3300-4200 m); and zone 4, alpine heath scrub and meadow (above 4200 m; Wu et al., 2006).From our personal observations, the present vegetation around the Wenhai Lake catchment is dominated by oaks (Quercus pannosa Hand.-Mazz) and pines, primarily Pinus yunnanensis, with P. armandii Franch.at slightly lower elevations, and smaller numbers of Tsuga dumosa (D.Don) Eichler and P. densata Mast.are also present.Here, the most abundant shrubs are ericaceous, including rhododendron species, especially Rhododendron mucronatum (Blume) G. Don, R. racemosum Franch., R. yunnanense Franch.and R. delaveyi Franch., together with Vaccinium bracteatum Thunb.and Pieris formosa (Wallich) D. Don.Herbaceous taxa are diverse, with some of the most speciose genera being Anemone, Gentiana, Primula and Roscoea.This area is now heavily influenced by human activities, such as felling of timber and grazing, so some patches are barren of vegetation.

Coring and sampling
A sediment core 320 cm in depth was obtained from Wenhai Lake in January 2005 using a Russian corer, which consists of a 40 cm long steel chamber (diameter 10 cm) and 1 m long steel rods.Coring was done in 40 cm overlapping steps (0-40, 40-80, 80-120 cm, etc.).To avoid contamination, the chamber was cleaned carefully before starting each new round of coring.The core was labelled in the field, wrapped in plastic foil and placed in halved PVC tubes.A detailed lithological description of the core is presented in Fig. 2.

Radiocarbon dating
Two samples from the core, at 155 and 320 cm in depth, were taken for accelerator mass spectrometry (AMS) radiocarbon dating, which was performed at the Scottish Universities Environmental Research Centre (SUERC) in Glasgow, Scotland, UK.The 14 C ages are quoted in conventional years before present (before AD 1950).Bulk samples from the core were used because fragments of plant material suitable for analysis were not present.Age calibration was set up using the calibration curve from Reimer et al. (2004) by means of the calibration program OxCal v3.10 (Bronk, 2005).Date ranges are cited in calibrated years AD/BC at 95 % probabil-ity, with end points rounded to the nearest 10 years (Mook, 1986;Foster et al., 2008).

Pollen analysis
Six surface soil samples near the core were collected for comparison with the preserved pollen assemblage.Thirtytwo samples were taken from the core itself, at 10 cm intervals, for pollen analysis.Thirty grams of each sample was processed by means of heavy liquid separation (Moore et al., 1991;Li and Du, 1999) followed by acetolysis (Erdtman, 1960).Pollen grains and spores were identified using modern pollen slides, palynological literature and monographs (IBCAS, 1976;IBSCIBCAS, 1982;Wang et al., 1995).All samples yielded abundant, well-preserved palynomorphs.Pollen samples were examined using a Leica DM 2500 light microscope at a magnification of 400 ×, and at least 300 pollen grains and spores were counted in each sample.Pollen grains and spores were divided into four categories: trees and shrubs, herbs, pteridophytes, and aquatic taxa.Pollen data were expressed as percentages and graphed using Tilia.Graph, and pollen zones were determined with CONISS in the Tilia program (Grimm, 1997).

Chronology
Two

Surface samples
Fifty palynomorphs were identified from the six surface soil samples collected in close proximity to the core, including 29 families and 7 genera of angiosperms, 3 genera of gymnosperms, 9 families and 1 genus of pteridophytes, and 1 genus of alga (see the Supplement).The pollen assemblage is dominated by trees and shrubs, at percentages ranging from 79.5 to 97.0 % of the total pollen and spores.Pinus pollen (62.3-87.1 %) dominates in all six surface samples, followed by Abies (3.3-10.7 %), Quercus (0-5.5 %) and Ericaceae (0-4.

Pollen diagram zonation and description
Pollen analysis of the core samples shows a high degree of taxonomic diversity.The palynoflora comprises 83 paly-nomorphs, which can be identified to 45 families and 13 genera of angiosperms, 1 family and 7 genera of gymnosperms, 12 families and 3 genera of pteridophytes, and 2 genera of algae (see the Supplement).Some of the selected palynomorphs extracted from the core are illustrated in the Supplement.
A greater diversity of palynomorphs was recovered from the core samples than from the surface samples.However, many of the taxa found in the core but missing from surface samples are not present in the upper part of the core and are no longer present in the immediate area and thus do not contribute to the local pollen rain.Examples include Cedrus, Dacrydium and Taxodium amongst the gymnosperms and the angiosperm taxa Actinidiaceae, Anacardiaceae, Carpinus, Clethraceae, Flacourtiaceae, Icacinaceae, Juglandaceae, Liquidambar, Myrsinaceae, Palmae and Tilia.Some of these taxa have a subtropical distribution, and their closest occurrence to the study site is at much lower elevation near the Jinsha River or considerably further south in Yunnan.Other taxa such as Araceae, Araliaceae, Campanulaceae, Caprifoliaceae, Caryophyllaceae and Umbelliferae are present in the immediate area but are entomophilous plants with relatively lower pollen production which might be expected to be under-represented in the local pollen rain.
A cluster analysis performed using Tilia (with CONISS) divided the pollen diagram into five distinct zones (Fig. 2).Brief descriptions of each zone are as follows.
The percentage of tree and shrub pollen in pollen zone 3 (79.7-92.2%) is more similar than the previous zones to that of the surface samples (79.5-97 %).As in the surface samples, Pinus pollen dominates in pollen zone 3, followed by Abies and Quercus.The percentages of herb pollen (pollen zone 3: 5.2-13.7 %; surface samples: 1.8-4.1 %) and aquatics (pollen zone 3: 0-5.8 %; surface samples: 0-3 %) in pollen zone 3 are higher than in the surface samples.However, a comparatively lower percentage of pteridophyte spores is recorded in pollen zone 3 (0.7-7.1 %) than in the surface samples (0.3-15.3 %).

Climatic implications of the principal palynomorphs from Wenhai Lake
The palynoflora found in the Wenhai core includes a large number of potential climate indicators.For example, Pinus is currently distributed below 3200 m elevation in Southwest China and is commonly found in slightly warm and moderately dry habitats.Pinus yunnanensis Franch., P. densata Mast.and P. armandi Franch.are the dominant species on the mountains of northwestern Yunnan (KIBCAS, 1986).
Tsuga is a cold-tolerant and hygrophilous conifer, requiring a MAT of 8.4 to 10.5 • C and a MAP of about 1000 mm for favourable growth in Yunnan (WGYV, 1987).One species, Tsuga dumosa (D.Don) Eichler, and one variety, T. chinensis (Franch.)E. Pritz.var.forrestii (Downie) Silba, are recorded in northwestern Yunnan (KIBCAS, 1986;Wang et al., 2007).Abies is strongly associated with cold and dry habitats with a MAT of 2-8  , 1987).From our personal observations, some small Quercus trees are present up to about 3800 m.Artemisia is mainly distributed in temperate areas of mid-to high latitudes of the Northern Hemisphere, usually in arid or semi-arid environments (Valles and McArthur, 2001).The genus Artemisia is considered an indicator of steppe climate (Erdtman, 1952) and moderate precipitation (El- Moslimany, 1990).There are 54 species and 8 varieties growing in Yunnan (KIBCAS, 2003a).Cyperaceae is a cosmopolitan family with ca.5000 species and 104 genera.Many species of this family commonly grow in wetlands and surrounding areas, adapted to open and sunny conditions.About 26 genera and 272 species occur in Yunnan.The high frequency of Cyperaceae pollen may indicate humid conditions (KIBCAS, 2003b;Sun et al. 2003).

Vegetation and climate history at Wenhai
Based on the climatic preferences of the major taxa recovered from the Wenhai core, the palynological record reveals a detailed history of shifting vegetation and climate change in this region during the past 22 570 years (Fig. 3).From 9250 cal yr BP to the present day (pollen zone 5), Pinus underwent a process of decrease at the depth of 50-80 cm (from 38.1 to 14 %), increase from 30 to 50 cm (from 14 to 56.8 %; this shows a trend consistent with the Haligu core between 4000 and 2400 cal yr BP), and decrease again at 0-30 cm (from 56.8 to 14.5 %), while Tsuga displays an opposite trend.Abies shows a distinct increase during this period.Quercus increased at the beginning of the stage, then decreased after that.This pollen assemblage indicates that needle-leaved forest (comprising mainly Pinus, Abies and Tsuga) dominated the areas surrounding Wenhai Lake during the Holocene, interspersed with broad-leaved elements (mainly Quercus and Betula), reflecting a significant decline in humidity from the early to late Holocene, which might be related to a strong Asian summer monsoon over Southwest China during the early Holocene and a reduced monsoon in-tensity in the mid-to late Holocene (Dykoski et al., 2005;Kramer et al., 2010).

Palynological signals of human activity detected in the Wenhai core
Based on pollen and other evidence, human influences on postglacial vegetation have been inferred in Europe and North America (e.g.Brugam, 1978;Hirons and Edwards, 1986;Smith and Cloutman, 1988;Russell et al., 1993;Parker et al., 2002), as well as in China (e.g.An et al., 2002;He et al., 2002;Xu et al., 2002;Song et al., 2012).The present authors have previously published another palynological investigation within the Hengduan Mountains (Song et al., 2012).
The altitude of the previous study site, Haligu, is 3277 m, where there is no current human settlement.The present study area, about 2 km far from Haligu, is close to a Naxi ethnic minority settlement at Wenhai village.Moreover, Wenhai was an important stop on the ancient "Tea-Horse Road" (Luo, 2003), making it likely that a wide variety of cultivated plants were introduced into the agricultural system around the lake, as evidenced by an ethnobotanical survey of traditional edible plants (including 45 cultivated plants) used by the Naxi people in Wenhai village (Zhang et al., 2013).
In the Wenhai core, several observations may be interpreted in terms of increasing anthropogenic impact in the region.First, Pinus and Quercus pollen decreased distinctly at the depth of 0-30 cm, which is probably linked with the fact that local people felled the trees for house construction or fuel wood.We detected that Quercus pollen decreased steadily in the Haligu core during the period from 2400 cal yr BP to the present day and observed heavy present-day coppicing of Quercus for firewood, resulting in much reduced pollen production.The comparison of both cores may substantiate the existing human impacts in the region.Second, the occurrence of abundant Labiatae pollen also indicates increasing human activity.From our personal observation, the Naxi people in Wenhai village currently cultivate several Labiatae species, including Perilla frutescens (L.) Britton and Mentha spp., as edible herbs and for medical utilization.Third, the increase in Polygonaceae pollen (likely to be Fagopyrum) could be an important indicator of human activity in the region, as the Naxi people continue to plant buckwheat as an important crop today.Although we have no absolute way to confirm these signals correlated with increased human settlement in the region, we believe a further ongoing study of two soil pit profiles with high-resolution dating and sampling from the village of Wenhai will give us even more information about human activity on Jade Dragon Snow Mountain.
The Supplement related to this article is available online at doi:10.5194/bg-12-1525-2015-supplement.

Figure 1 .
Figure 1.(a) The location of Wenhai Lake in northwestern Yunnan, China, (b) the position of core and surface soil samples, (c) the location of Wenhai Lake in the Hengduan Mountains, (d) the location of Wenhai Lake on Jade Dragon Snow Mountain, and (e) an enlarged photograph of Wenhai Lake (c, d and e are taken from Google Earth).

Figure 3 .
Figure 3. Schematic diagrams showing vegetation succession over the past 22 570 years at Wenhai.
AMS radiocarbon dates, 14 075 ± 40 yr BP (17 150-16 350 cal yr BP) at a depth of 155 cm and 19 075 ± 50 yr BP (22 760-22 380 cal yr BP) at a depth of 320 cm, give a relatively reliable basis for deciphering the vegetation and climate history in and surrounding Wenhai Lake.The lithology of the entire core is mainly characterized by clay with the exception of a peat deposit at depth of 45-55 cm.Given continuity and stability of sedimentation during the past 22 570 years, an age-depth curve in cal yr BP, reflecting the sedimentation pattern, can be constructed for the core (see the Supplement).Although the age-depth model is obtained based on two dates, it roughly shows the sedimentation rates are ca.0.1 and 0.28 mm yr −1 for the depths of 0-155 and 155-320 cm, respectively.Ages of other depths are deduced by assuming that the sedimentation rate is constant for the lower and upper sections of the core, i.e. 9250 cal yr BP at 80 cm, 17 930 cal yr BP at 190 cm, 19 350 cal yr BP at 230 cm, and 21 140 cal yr BP at 280 cm depth, which are determined as the timing of the changes of the pollen zones.