Sporopollen has the advantages of being a very tiny unit, being abundant and having a wide range of distribution. It reflects changes in vegetation very directly and also is sensitive to and reflects changes in the environment. Therefore, sporopollen has been applied broadly to reconstruct prehistoric environments. The perfect study site for collecting sporopollen is a small closed lake without a drainage outlet. Karst areas are the best places to study the relationships between climate and the environment, and an understanding of the evolution of past environments is needed before they can be adequately protected. Baixian Lake, a small closed lake located in a karst area in Guizhou is selected for this study.
Through an analysis of pollen assemblages of the sediment profile in Baixian Lake, we discuss the vegetation and climate change in the past 5.5 ka using organic ¹⁴C dating. The records of the pollen assemblages reveal four stages to the vegetative evolution and climate changes in the Baixian Lake region. From 5500-4500 aB.P., various types of vegetation were relatively rich, with Pinus, Quercus, Cyclobalanopsis, Lithocarpus/Castanopsis/Castanea and other woody plants more frequent than other species. Plants and grass species preferring mesic habitats comprised as high as 30% of the flora on average while the percentage of herbs of non-mesic habitats as well as drought or cold tolerant herbs were relatively low, averaging 2% of the flora. Ferns also comprised a relatively low percentage of the flora, less than 10%. These findings agree with climatic records based on cave related data, which implies that this period experienced a warm and humid climate during that time frame. From 4500-2750 aB.P., drought-resistant herbs and ferns began to appear more frequently, which implied a transition occurred as the climate changed. Trees of cool hygrophilous habitats appeared while some woody plants with seasonal resistance to drought decreased in frequency compared with the previous period. Xerophytic herbs such as Artemisia and ferns such as Lepisorus began to appear. From 2750-1500 aB.P., changes in woody plant species occurred. Tropical and subtropical woody species increased during this period, although small fluctuations in their populations occurred in the second half of this period with these species comprising up to 7% of the flora. Hygrophilous herbal or moist herbal content declined to 15%. The content of pollen from aquatic herbs in the Cyperaceae declined to 20000 grains/g and continued a downward trend in the next period. Hygrophilous herb species declined and drought tolerant herbaceous species increased in frequency. Cold-or drought-resistant herbs increased to 8% of the pollen flora; fern spore content in the sediment band from this time period was high, averaging 30%; sporopollen content in the Polypodiaceae and Lepisorus increased; this all indicated the climate tended to be cool and dry. Since 1500 aB.P., woody plants and moisture-loving herbs continued to decline, while hygrophilous herbs and coniferous trees almost disappeared, and plants and grasses of mesic habitats made up about 15% of sporopollen samples. Gramineae pollen content increased and Artemisia pollen content increased to 100000 grains/g, reaching a maximum at 400000 grains/g. Hygrophilous Cyperaceae content declined to almost nothing. Herbs and ferns continued to increase, while cold or drought tolerant herb pollen content increase and fluctuated, the highest point reaching 12%. Fern spores in the samples continue to increase in number, in this period, aridification obviously intensified, reached 35%. The vegetation tended to be dominated by the plants adapted to conditions with rocky desertification. The pollen assemblages in Baixian Lake showed that temperature and precipitation gradually decreased, while the vegetation obviously degraded in the last 5000 aB.P.. This study also reveals that the climatic events (e.g., the continuous dry climate) can result in rocky karst desertification, and supports the idea that the environment in the karst region is innately fragile. Therefore, scientific assessment of aridification and the associated affect on the environment in the Baixian Lake region is essential in addressing climate change, and future research in this field is needed.