Reconstructing Mallee fire history using Callitris verrucosa tree rings Report uri icon

abstract

  • The multi-year, tri-state Mallee Fire and Biodiversity (MFB) project aims to identify the properties of mallee eucalypt habitat mosaics, produced by fire, that enhance the persistence and status of a range of taxonomic groups. Large areas of mallee are described as ‘long-unburnt’ — that is, not burnt for over 38 years. The purpose of this project, Filling Fundamental Gaps in the Fire History of the Mallee, was to determine the ages of the various cohorts described as long-unburnt within the MFB study mosaics. Dendrochronology, the study of tree rings, can be used to determine tree age, annual growth rates, and the relationships of age and growth with environmental conditions. The native Australian conifer genus Callitris has known dendrochronological potential. Callitris verrucosa is fire sensitive and occurs within long-unburnt patches of mallee and MFB study mosaics. The ‘proof of method’ phase of this project (Zimmer et al. 2009) investigated the utility of C. verrucosa for dendrochronological analysis. Sampling stands of known age, we discovered that growth rings in C. verrucosa were anatomically distinct, and that ring counts from the majority of sampled trees were approximately equal to years since fire. Tree rings were successfully visually cross-dated (within tree), and at the mosaic-scale there was reasonable correlation among tree ring width series. We concluded that ring counts of cross-dated C. verrucosa could be used to determine the ages of long-unburnt cohorts within the MFB study mosaics. This final report is based on work that aimed to reconstruct the fire history of long-unburnt areas in the Murray-Sunset and Hattah–Kulkyne National Parks. Using C. verrucosa tree rings, we reconstructed the age structures of 12 C. verrucosa stands, including eight stands in long-unburnt areas. Despite our earlier findings, this study revealed that C. verrucosa was not an ideal candidate for fire history reconstruction. Mature C. verrucosa can survive fire, and recruitment can occur between fires. Nevertheless, stand age structures derived from tree rings may indicate ‘the period for which fires have been sufficiently patchy to allow survival of C. verrucosa trees’; up to 180 years in some areas. Sources of error in tree-ring-derived estimates include intra-annual ‘false’ tree rings and missing tree rings. At several sites no recruitment pulse was detected, despite previous research suggesting serotiny in C. verrucosa (e.g. Bradstock and Cohn 2002a). This may suggest that inter-fire recruitment is more important than first thought, or that the influence of low-severity fire (scorching but not killing trees, and releasing seed) is significant. However, our sample sizes were small and it is possible that the true age structure of the stand was not captured by our sampling. Other potential influences on C. verrucosa age structures are grazing pressure and drought. That C. verrucosa is not killed by all fires and recruits between fires, places a qualifier on our assumption that C. verrucosa would indicate the age of surrounding vegetation: this is only true in the case of severe fire, with successful recruitment in the immediate post-fire years. Currently, C. verrucosa tree-ring-derived age estimates may be best used in conjunction with other proxies, e.g. bomb pulse dating (Hua 2009) and studies based on increases in stem diameter (Clarke et al. 2010).

publication date

  • 2010