The Application of Dendroclimatology to Reconstruct Long-Term Rainfall Records for Subtropical Southeast Queensland, Australia

Abstract

In the tropical and subtropical regions of eastern Australia multidecadal periods of floods and droughts have major economic and environmental consequences. Due to the short duration of instrumental rainfall records in this region the temporal pattern of these extreme events is poorly understood. Subtropical Southeast Queensland (SEQ) is one area that frequently experiences floods and droughts and is lacking in both instrumental rainfall records and sources of proxy data. There are old growth forest stands found in National Parks where dendroclimatology, the reconstruction of climate using tree-rings, can be applied. Tree rings have been widely utilized in temperate environments to provide annually-resolved centennial-scale climate information. However, in tropical and subtropical regions dendroclimatology has been underutilized, as many species in these regions are difficult to analyse. These species have short life-spans, poorly preserved timber, and are believed to exhibit numerous ring anomalies making dating of ring series difficult. Due to this, few species have been analysed for relationships between tree growth and climate. Recent reviews of both tropical and Australian dendrochronology have suggested that if a multi-technique approach is applied more species could be found suitable for use in reconstructing climate. This thesis first reviews the dendroclimatological history of Australia evaluating 36 studies across the continent that examined tree and shrub species for growth-climate relationships. This review showed that all four climate zones of Australia; temperate, arid, tropical, and subtropical, contained species that had the potential to provide high-quality, long-term climate reconstructions in areas under represented by instrumental data. Only four climate reconstructions have been developed in Australia. In all of these studies a combination of traditional ring-width measurements and modern analysis techniques allowed for the reconstructions to be developed. Several species located in SEQ had been assessed for growth-climate relationships and were found to grow as a result of rainfall conditions, with trees in the Araucariaceae family demonstrating the most potential. These trees are longer lived than most tropical/subtropical species, are known to put on annual rings, and grow as a result of environmental conditions which led them to be targeted in this thesis. Secondly, to test the spatial patterns of SEQ rainfall the instrumental data network was evaluated with Pearson correlation analysis undertaken on 140 rainfall stations active during 1908 to 2007. Stations are clustered into groups that correlate at r = 0.80, 0.85, and 0.90 levels. The pattern produced indicates that rainfall across SEQ is not uniform with correlated groups being separated by the Great Dividing Range with both coast-inland and north-south separations. To determine the effect this spatial variability has on the spatial applicability of rainfall reconstruction, the 140-year Toona ciliata tree-ring width record developed by Heinrich et al. (2009) is compared with the different rainfall groups and subgroups observed across SEQ. The rainfall reconstruction is found to best represent the spatial subgroups within which it is contained rather than representing the entire regional rainfall network. This suggests that several sites within SEQ need to be targeted to develop rainfall reconstructions for the region. Consequently three spatial distributed sites are targeted within SEQ with cores collected from Araucaria cunninghamii trees in D’Aguilar and Lamington National Parks and Araucaria bidwillii trees in Bunya Mountains National Park. These Araucariaceae cores are assessed for ring anomalies though the application of visual ring dating with false, faint, locally absent, and pinching rings found to be present in both species. However, through the use of bomb-pulse radiocarbon dating of A. cunninghamii samples anomalous rings are identified and annual growth patterns determined. Dendrometers are installed at the Lamington and Bunya Mountains sites to determine the exact growth-climate relationships for these species in SEQ. The amount of growth experienced by these trees is driven by annual rainfall while minimum temperatures are shown to be influencing the start and conclusion of the growth seasons. These results suggest that annual rainfall can be reconstructed from Araucariaceae trees when dating is verified using radiocarbon dating to account for ring anomalies. Two statistically significant rainfall reconstructions developed from A. cunninghamii trees are then presented. The first is a 69-year rainfall reconstruction for Brisbane developed from the D’Aguilar site. The trees at this location are all found to exhibit faint, indeterminate ring boundaries making them unsuitable for traditional visual ring-dating. A new method is presented using x-radiographs and density patterns scanned using an Itrax core scanner to build this reconstruction. Thirty-nine 12mm cores from twenty trees have their ring boundaries identified on the Itrax produced images based on features visible in the radiographs and the density patterns. Bombpulse radiocarbon dating is performed to verify the chronology. Climate response function analysis demonstrates that growth in driven by Austral annual (June-May) rainfall. The second reconstruction is a 164-year record developed from eighteen trees located in Lamington National Park. A field sampling strategy is applied where multiple cores are collected from the upslope, downslope, and across-slope sides of the study trees to help eliminate anomalous ring issues identified as being prevalent in these species. This chronology is then developed using traditional visual ring identification complemented by bomb-pulse radiocarbon dating to verify and correct the ring counts. Climate response function analysis indicates that Austral annual (June-May) rainfall is driving growth in these trees. Comparisons to both a Brisbane rainfall station (located about 100km from the Lamington site) and a station local to Lamington are undertaken with the rainfall signal at the local site more closely related to tree growth. Drought conditions are well represented by this chronology. The El Niño Southern Oscillation is found to be driving rainfall for the Lamington area but only since the 1940s.