Vegetational And Environmental Changes Since15Ka BP Recorded By Lake Lugu In The Southwest Monsoon Domain

Lake Lugu, located at the southeastern Tibetan Plateau (27°40′-27°44′N，100°46′-100°55′E), is the second largest deep plateau-lake in Yunnan-GuizhouPlateau. A880cm long core (at27°40′49″N，100°48′E) taken from44.5m water depthof the lake shows that its sediments are rich in organic matters with leaf remains butwithout distinguishable physical features. The age-depth model of the Lake Lugu coreis established from16AMS14C dates. This paper presents pollen assemblages of itsupper474cm, together with the measurement of the TOC and CaCO3to reconstructpast vegetation and environment changes during the last15000years. Among the70samples analyzed, a total of81pollen and spore taxa were identified. Trees were thedominant pollen in the entire profile and the average content of trees was92.1%, andthe maximum value was as high as97.3%. On average, herb accounts for7.9%onaverage. One of the typical features from the pollen diagram identified from thesediments from Lake Lugu is that the content of Pinus spp. is high all through theprofile, which averagely can reach about38.5%-74.6%. The samples from the top3cm contents too much water than sediments, therefore, it’s difficult to sample andthey were not considered here. Based on the pollen percentages of the major taxa andCONISS results, the pollen record from the Lake Lugu was divided into six mainzones:(1) From15000a BP to12300a BP, pine forests together with alpineconiferous and broadleaved mixed forests occupied the drainage of Lake Lugu,indicating a cool and humid conditions.(2) It was followed by a warm and dry periodmarked with a decline of pine forests and an increase of deciduous broadleavedforests from12300a BP to10300a BP.(3) During the Early and Middle Holocene(10300-7500a BP and7500-3800a BP), high coverage vegetation was dominated bypine forests, together with a few alpine coniferous forests consisting of hemlock, firand spruce, indicating warmer and wetter conditions than those in15000-12300a BP.However, more evergreen oaks occurred in the Middle Holocene, suggesting thatclimatic conditions were not as humid as in the Early Holocene.(4) Pollen spectrafrom3800a BP to3300a BP showed a marked decline of pine forests and a distinctexpansion of sclerophyllous evergreen oak forests, reflecting a500-year cool and dryclimate event.(5) From3300a BP to the present, pine forests recovered from itsshrink of3800-3300a BP, but did not reach the Early and Middle Holocene level;evergreen broadleaved Cyclobalanopsis forests and coniferous hemlock-fir-spruceforests frequently occurred in the lake drainage. Past vegetation indicated thattemperature and humidity in the Late Holocene were lower than those in the Earlyand Middle Holocene. Lake environmental changes revealed by TOC and CaCO3were consistent with climatic changes reflected by pollen spectra. The TOC contents vary between3.78%-13.80%(with average of8.79%). Low lake levels weregenerally parallel with cool and dry climatic conditions. TIC content is one of theproxies for water levels. Based on this assumption, it is believed that high lake levelsappeared between13600a B.P. and3800a BP, during that period the water levelfluctuated with a periodicity of600-1230a. Overall, pollen, TOC, and CaCO3recordsfrom Lake Lugu revealed a trend of temperature rise-high temperature-temperaturedrop, superimposed with a series of cool and dry events.