Early Holocene vegetation dynamics of Lake Barrine Basin Northeast Queensland Australia




Chen, Yinshuo

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This thesis presents the results of an investigation of vegetation dynamics and the related environmental changes at a crater lake, Lake Barrine, near the modern rainforest sclerophyll woodland boundary in tropical Australia during the early Holocene (ca 10000 - 3500 B.P.), using fine resolution pollen analysis techniques such as small interval radiocarbon dating, "absolute" pollen frequency, and various statistical techniques (e.g. sequence splitting, time series analysis, model fitting, etc.). A bathymetric survey was carried out on Lake Barrine in 1983 and several sediment cores were taken from various water depths to reveal the general stratigraphy of the lake sediment. Three cores were used in the pollen analytical study. The sedimentation rate during the period concerned has been estimated to be constant at 0.6 mm per year from 22 radiocarbon dates of the three cores. The within-sample interval is 17 years and between-sample interval 34 years in the master core. Thirty-three selected pollen taxa and charcoal particles were counted and a sequence-splitting treatment was applied to each individual taxon. The numbers of significant splits at each sample level were used to zone the pollen diagrams. The results show that the major vegetation change from sclerophyll woodland to rainforest took place about 6800 B.P. During the sclerophyll woodland period, charcoal influx rate was very high, indicating that fire was an important environmental factor. Time series analysis was used to analyze fire frequencies and the response of various sclerophyll taxa to the fire. The results show that before 9300 B.P. the fire frequency was high {less than 50-year intervals)after which it dropped to a 230-year interval probably as the result of an increase in rainfall. Some rainforest trees invaded the area during that period, but the fire prevented the further development of forest. By 6800 years B.P, perhaps due to further increase in rainfall or perhaps purely by chance, fire was absent from the area for a sufficient period to allow rainforest trees to expand and to replace the sclerophyll woodland as the dominant vegetation type. During early rainforest development different rainforest tree taxa expanded at different times. The processes of population expansion of several taxa have been successfully fitted to one of two population growth models, namely, the exponential model and the logistic model. Doubling time for each taxon was calculated and varied from less than 100 years to more than 300 years. The secondary trees had shortest doubling time, while the gymnosperm canopy trees had the longest. By comparison with modern analogues, it is suggested that the early rainforest experienced three major climatic periods: wet-cool (6800 - 6100 B.P.); wet-warm (6100 - 5000 B.P.); and dry-warm (after 5000 B.P.). The results of the investigation show that fine resolution pollen analysis can not only reveal general changes in vegetation and environment, but also provide ecologically important information on vegetation dynamics.






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