A stable oxygen, but not carbon, isotope chronology of Callitris columellaris reflects recent climate change in north-western Australia

We investigated the dendroclimatic potential of stable carbon (delta C-13) and oxygen (d(18)O) abundances in tree rings of Callitris columellaris F. Muell. Tree-ring chronologies were constructed from the central Pilbara, north-western Australia and span 1919-1999. Variation in d18O was more strongly related to climate than d13C; ecological and physiological factors may have dampened the climate signal in the d13C chronology. Tree-ring d18O was most strongly correlated with relative humidity ( RH) and rainfall (r=-0.36 and -0.39) of the wettest months of the summer period, January and February. The correlation with RH reflects its effect on evaporative enrichment of leaf water. However, tree-ring delta O-18 may also partly reflect the variability in O-18 signatures of rainfall, which are influenced by the amount of rainfall and atmospheric humidity. From the d18O chronology, we inferred that from 1919 to 1955 summers were relatively dry and warm, but since 1955, summers in the Pilbara region have become increasingly cooler and more humid. Since 1980, conditions have been the wettest and coolest of the last 80 years. These inferred changes in climate correspond to a measured increase in rainfall since 1980 in north-western Australia associated with a greater intensity of tropical cyclones. We conclude that d18O abundances in tree rings of C. columellaris have significant potential for reconstructing the climate of semi-arid Australia, a region for which observational climate records are sparse.