University of Tasmania, Australia

What makes Callitris trees grow, and when? While this may seem like a simple, and perhaps a silly question, it is difficult to find an answer in the published literature. Yet a close relationship between the width of growth rings and climate variables underpins dendrochronology and dendroclimatology. To use the width of growth rings to reconstruct climate, we clearly need to understand how climatic, and other environmental variables, influence the width of growth rings.

Exploring the climatic determinants of Callitris growth rates, and the timing of growth, was the motivation for the ‘Four corners’ project. The project has involved establishing Callitris monitoring sites in each of four contrasting climatic settings:

1. Indian Island, near Darwin NT - warm, high rainfall, distinct winter drought;
2. Mt Baldy, near Atherton QLD - warm, high rainfall, less distinct winter drought;
3. Orford, near Hobart TAS - cool, weakly seasonal winter rainfall;
4. Garden Island, near Perth WA - classic Mediterranean climate - cool wet winter, warm dry summer.

Every three months for two years, we record the growth of 20 adult trees at each site. Growth is measured using band dendrometers permanently fitted to each tree. The dendrometers are simply spring loaded steel bands that wrap around the tree, and allow accurate estimates of changes in trunk diameter to be made. In addition, electronic dendrometers have also been fitted to five of the trees at each site, with diameter measurements recorded every hour. The electronic dendrometers are so accurate that the daily expansion and contraction of the trees, as water pressure inside the tree increases and decreases, can easily be seen. Finally, we also measure the isotopic composition of foliage (δ¹³C and δ¹⁸O), stem water (δ¹⁸O) and rainwater (δ¹⁸O). We will compare these measurements with the isotopic composition of the tree rings that form during the two years of the study.

While there are weather stations close to all our Callitris sites, we have installed temperature and relative humidity loggers on each tree, and a rainfall logger has been installed at each site. These data will provide a clear picture of the climatic conditions experienced by our trees. Additionally, every three months we measure the leaf water potential of the trees, which is a good indicator of water stress.

In sum, the coupled growth rate and climatic data we are collecting will allow us to build a picture of the patterns of Callitris growth in different climatic settings, and will indicate what climatic variables are most closely correlated with growth rate.