Climate Change And Fire Episodes In The Past1700Years Documented By The Sediments In The Tropical Xishuangbanna, Southwestern China

The tropical rainforests throughout the world are experiencing area declining due to the global warming since the late Holocene, because they are sensitive to the precipitation change that affects plant productivity and survival. The frequent and severe droughts and fires are causing habitat fragmentation. More research into droughts in the tropical forests has drawn more attention on the forest-climate relationships, rain-forest deforestation and habitat fragmentation. In the tropical Xishuangbanna, primary rain forest is well-preserved. The climate history of this region, including drought episodes, has been reconstructed in part using pollen and phytoliths from surrounding areas. Gu et al.(2008) focused their research on climate change in the tropical Xishuangbanna using phytolith and charcoal records. The current research aims to reconstruct the local paleovegetation, palaeoclimate, and fire history of Xishuangbanna, using phytolith, and charcoal records from alluvial sediments in the well-preserved rain forest area. Phytolith is an important and reliable tool for reconstructing local vegetation and climate. Grass phytoliths, in particular, offer a promising way to differentiate grasses at the subfamily level and to infer subtle changes related to the palaeoenvironmental conditions. Phytolith records here are used to reconstruct vegetation change, while charcoal particles are recorded as paleofires indicators. This study aims to reconstruct the vegetation history in response to climate change, and fire occurrences over the past1700years in the tropical Xishuangbanna rain-forest area.We aim to assess the impact of climate change on the local vegetation and to establish the correlations between drought events and forest fires which might respond to the fluctuation of Indian Ocean Monsoon. The objectives are (1) to identify diversity of phytolith morphotypes, to enrich the database of phytolith in Southwestern China;(2); to reconstruct the local paleovegetation by phytolith assemblages;(3) to understand the interactions between climate change and vegetation successions, showing which vegetation type corresponds to a specific climate;(4) to reconstruct fires history in the tropical Xishuangbanna using charcoal particles;(5) to reveal relationships between fire episodes and droughts, especially with El Nino events occurring;(6) to predict how global warming affects rainforest in the future.The use of multiproxies such as fossil phytoliths, and fossil charcoal records in this work is the first case in our study area. Our analysis demonstrated that phytoliths and charcoal particles are reliable proxies for the reconstruction of paleoenvironments in the tropical forests.Phytoliths are silica casts in the plant cells or spaces between cells. Detailed descriptions for anatomical terms are divided into seven groups:short cells, long cells, bulliform cells, hair cells, pteridophyte type, broad-leaved type and gymnosperm type. The first four phytolith types are originating from Poaceae epidermis. Long cells are mainly composed of elongates (smooth and echinate). Short cells include rondel, saddle, cross, trapeziform (sinuate and polylobate), and bilobate. Bulliform cells consist of parallelepipedal bulliform and cuneiform bulliform cells. Usually, short cells can be classified as the Pooideae (rondel and trapeziform), Panicoideae (bilobate, cross, and cylindrical polylobate), Chloridoideae (short/square saddle), Bambusoideae (oblong concave saddle) based on the previous research.In the current study, two major phytoliths indices were calculated so as to show various aspects of microclimate and vegetation compositions in the tropical Xishuangbanna. The various indices have been formulated to indicate phytoliths information in different parts of the world. These indices are fundamental in showing characteristic micro-environments of a particular locality and have a wide application to a broad range of environments including the Aridity index (Iph) and climatic index (Ic).On the basis of phytolith identification and indices, different zones of phytolith assemblages have been discriminated and interpreted in terms of climate and major vegetation changes.Based on the research of top soils, fan-shaped, dumb-bell shaped as well as saddle-shaped phytolith mainly appears in the warm and humid climate, hence belong to warm type, while pointed shaped, and Pooideae types mainly takes shape in cold climate, hence belong to the cold type, therefore, the ratio of the phytolith content between the warm and cold type can be used to calculate the index of warmth (Iw), by which the process of climatic change can be revealed (Wang Weiming,2003). Meanwhile, according to the soil moisture content that the plants need, the C4type from the grass family can be divided into the Chloridoideae (with the square saddle-shaped phytolith) which belongs to the arid type and the Panicoideae (with the dumb-bell shaped phytolith) which is the opposite of the arid type. So the ratio got from the equation that Iph (the index of aridity)=Chloridoideae/(Chloridoideae+Panicoideae) can be used to indicate the degree of aridity. Further more, by studying the contrast of the content of the herbaceous and wood plants and comparing to the modern plant of the research area and the information of phytolith from the top soils, we can reconstruct the floral landscape of one area, reappear the paleoclimatic changes, and find the reliable measurement of microbotanic fossils which are of the climatic meaning (such as the drought).According to the phytolith statistics and Iw, four zones of phytolith assemblages can be discriminated and interpreted in terms of major vegetation constituents and climate change at ML3section. From the top to the bottom, there are four assemblage zones have been discriminated for their ecoclimatic and paleovegetation reconstruction at ML3section.The statistics of the density as well as the quantitative study of the charcoal grains that is of the size more less than50μm, more less than125μm and more than125μm show that there were three phases with dramatic changes in the rainforest ML3and totally19fire events have taken place. For1700years, the climate has been warm and humid in general in the rainforest of Bubang (BB section). Based on the Ic and Iph, four climatic phases have been discriminated. The rainforest in Bubang of Xishuangbanna also experienced three phases with dramatic change, including38fires in total.Phytolith records and high resolution of charcoal particles carried out in the Holocene sediments from tropical rainforest in Xishuangbanna are employed to reconstruct the succession history of paleovegetation over the past1700years.33phytolith morphotypes have been discriminated and divided in seven major groups according their shape and their ecological significance for reconstructing vegetation history. Microcharcoals, macrocharcoals and magnetic susceptibility were used to assess fires occurrences. Depth distribution pattern of climate indices (temperature and aridity indices) indicates that the research area has undergone4phases of climate changes as following:(1)1700-1200cal. a BP, alternation of warm-humid and warm-arid, temperature declining in response to the Bond1Event and weak Indian Ocean Summer Monsoon;(2)1200-700cal. a BP, alternation of hot-humid and hot-arid, temperature rising in response to the Medieval Warmth Period and strong Indian Ocean Summer Monsoon;(3)700-150cal. a BP, warm-arid, temperature declines in response to the Little Ice Age and weaker Indian Ocean Summer Monsoon;(4)150cal. a BP-present, changing from hot-arid to hot-humid, temperature rising up in response to the Modern Warmth. Fire indicators such as micro/macro-charcoals, burnt phytoliths, and highly-weathered phytoliths are exploited to reconstruct the fire episodes occurred in the past1700years. Tropical rainforest in Xishuangbanna has undergone3phases of ecological changes as following:(1) early stage has limited local small fires in response to the strong Indian Ocean Summer Monsoon;(2) intermediate stage has higher fire frequencies and scopes in response to the weak Indian Ocean Summer Monsoon;(3) late stage has higher fire frequencies and scopes in response to the weaker Indian Ocean Summer Monsoon. Research on interactions between climate change, fire history, and vegetation are significant to understand the fire occurrence and ecological response in tropical rain forest. Our studying might provide innovative evidence for the evolution of Indian Ocean Monsoon in the late Holocene.