The role of extreme rain events in driving tree growth across a continental-scale climatic range in Australia

Rainfall regimes in many parts of the world have become increasingly dominated by fewer, but more extreme, rainfall events. Understanding how tree growth responds to changes in the frequency and intensity of rain events is critical to predicting how climate change will impact on forests and woodlands in the future. In this study, we used five tree-ring records of the native Australian conifer Callitris columellaris that span a large (> 20 degree) latitudinal and climatic gradient from the mesic (tropical) north to the xeric (semi-arid) south of Australia to investigate how inter-annual and spatial variation in the delivery of rainfall (the intensity and frequency of rain events) influences tree growth. In semi-arid biomes (~300-400 mm rainfall annually), tree growth is most strongly related to the amount of rainfall from heavy (> 75th percentile) rain days or the number of extreme (> 90th percentile) rain days, regardless of differences in the seasonal distribution and average intensity of rainfall among sites. Our findings also indicate that there is likely a minimum threshold amount of daily rainfall (~5 mm) that is required to stimulate tree growth in the semi-arid zone. In contrast, in the tropics (> 800 mm annual rainfall), inter-annual variation in growth is best explained by total growing season rainfall or the number of rain days > ~5 mm (~50th percentile of rain days) rather than extreme rainfall. Our findings indicate that not all rain events are important for driving tree growth, which has important implications for interpreting climatic signals in tree rings. Our findings also indicate that projected increases in the intensity of extreme rain events are likely to have contrasting impacts on tree growth across biomes, with greater and positive impacts on growth in semi-arid biomes and potentially negative impacts on growth in tropical biomes of Australia.