Wood density provides new opportunities for reconstructing past temperature variability from southeastern Australian trees
Introduction
Tree-ring based reconstructions of climate have been critical for understanding past climate variability and placing recent climatic trends in a long-term context. However, for most of the Southern Hemisphere, including Australia, there are few tree-ring based climate reconstructions, which constrains our understanding of recent and potential future climatic changes. In Australia, the limited number of tree-ring based climate reconstructions is largely attributable to a lack of tree species that are suitable for dendrochronology. The dominant angiosperm genera do not generally produce visually distinct or strictly annual rings suitable for dendrochronological methods (Schweingruber, 1992), but recent progress points to the potential some of these genera may hold (Brookhouse and Brack, 2006, Brookhouse et al., 2008, Heinrich et al., 2009, Whinder et al., 2013).
In addition to this, the primary focus of Australian dendrochronology has been on tree-ring width (TRW) – the most commonly used tree-ring parameter in chronology development globally – which has failed to provide clear climatic signals in many of the species and sites examined to date. Recent successes in the reconstruction of past rainfall and/or drought indices have used TRW measurements from the mainland conifer Callitris columellaris (including the previously named C. intratropica, Farjon, 2005) in the north and west of the continent (Cullen and Grierson, 2009, D et al., 2008, O'Donnell et al., 2015). However, to date, the only temperature reconstruction derived from TRW is based on Huon Pine (Lagarostrobos franklinii) from one high-elevation site (Mount Read, 900 m.a.s.l.) in Tasmania (Cook et al., 1991, Cook et al., 1992, Cook et al., 2000). Globally, this is also one of the longest tree-ring based climate reconstructions (1600 B.C.E.–1991 C.E.). Despite considerable efforts over the past two decades, high-quality reconstructions of temperature based on low-elevation Huon Pine TRW (Buckley et al., 1997) and TRW of other long-lived conifer species i.e., Pencil Pine (Athrotaxis cupressoides, Allen et al., 2011) and Celery Top Pine (Phyllocladus aspleniifolius, Allen et al., 1999) in Tasmania have so far been unsuccessful.
Other physical wood properties (i.e., density, cell wall thickness (CWT), tracheid radial diameter (TRD), and microfibril angle (MFA); e.g., Drew et al., 2013) as well as chemical wood properties (i.e., stable isotope concentrations, e.g., Treydte et al., 2006, Brienen et al., 2012, and trace element concentrations, e.g., Poussart et al., 2006) are also known to record climatic information. In particular, strong temperature signals have been identified in the wood density of conifers across Europe (e.g., Briffa et al., 2002, Büntgen et al., 2010, Trouet, 2014) and North America (e.g., Briffa et al., 1992, D et al., 1992, Davi et al., 2003, Luckman and Wilson, 2005, Wilson et al., 2007). In many cases, wood density, particularly maximum latewood density (MXD), has been more strongly correlated with temperature and over a longer summer season than TRW (Briffa et al., 2002, Grudd, 2008, Tuovinen et al., 2009, Trouet et al., 2012). Consequently, MXD has been widely used in the Northern Hemisphere to build temperature-sensitive chronologies (e.g., Schweingruber and Briffa, 1996, Frank and Esper, 2005, Grudd, 2008) and to reconstruct summer temperatures over the last several centuries to millennia (e.g., Briffa et al., 1992, Luckman and Wilson, 2005). Recent work has also demonstrated the potential of various wood properties (e.g., CWT, TRD, MFA, and mean ring density) of several long-lived Tasmanian conifers as sources of past climatic information (Allen et al., 2012, Drew et al., 2013, Allen et al., 2013) and for reconstructing stream flow (Allen et al., 2015). Despite this potential, temperature reconstructions based on these wood properties have not yet been developed.
Here, we investigate the potential of several of these wood properties for reconstructing past temperatures in Australia. We focus on the native conifer, Athrotaxis cupressoides (Pencil Pine), which is endemic to high-elevation (700–1300 m.a.s.l.) areas of Tasmania (Farjon, 2005). In addition to TRW, we measured mean density, TRD, and CWT. Given the strength of climatic signals previously identified in these wood properties, we expect that chronologies based on wood properties, particularly mean density, will allow us to produce the first robust A. cupressoides-based temperature reconstruction in Tasmania.
Section snippets
Site description
We collected A. cupressoides samples at two high elevation sites (~ 1200 m.a.s.l.) in central Tasmania, Australia (41.742°S, 146.703°E; Fig. 1a). The first site is adjacent to Pine Lake. The other site is on a southwest-facing slope adjacent to Mickey Creek. These two sites are ca. 1.5–2 km apart on Tasmania’s Central Plateau. The Pine Lake-Mickey Creek (PLMC) site is approximately 100 km east of the Mt Read site (~ 900 m.a.s.l.) that was sampled to develop the only existing tree-ring based
Chronology and model development
Both the mean density and CWT chronologies were 655 years long (1355–2010 C.E.). CWT was calculated from mean density and the mean density and CWT time series were strongly correlated over the full period of overlap (r = 0.91; p < 0.001). Both the mean density and CWT chronologies showed high common signal strength over the length of the chronology (RBAR > 0.3; for mean density Fig. 3a–b; CWT data not shown). EPS values for the mean density chronology were higher than the 0.85 threshold from 1489 C.E.
Wood density as a proxy for temperature in southeastern Australia
Wood density contains a strong inter-annual climate signal, but failed to reproduce a long-term trend evident in the observed temperature. The absence of a long-term trend in the mean density chronology is largely attributable to a series of three years in the last decade (2005, 2006, 2007) when mean density values were much higher than average, albeit not extreme. However, if mean density is used as the sole predictor of temperature, the resulting reconstruction underestimates observed
Conclusions
Our findings highlight the potential to use the complementary climate signals in TRW and mean density to provide a more complete picture of inter-annual to centennial-scale variation in past temperatures in southeastern Australia. The relative weakness of the TRW signal alone meant, until now, it has not been possible to reconstruct temperatures from A. cupressoides. Wood density provides new opportunities for successful temperature reconstructions in southeastern Australia from tree species in
Acknowledgements
This research was funded by an Australian Research Council Discovery Project grant (DP120104320 to PJB). We are grateful to Michael Goddard for assistance in preparing core samples for analysis, Scott Nicholls for assistance in preparing and analysing samples, and the participants of the Dendroclimatology Masterclass as part of WorldDendro2014: Anders Brundin, Binod Dawadi, Nathan English, Maarit Kalela-Brundin, Robert Kennedy, Kathelyn Paredes, and Meritxell Ramirez-Olle. We are also grateful
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