Dendrochronological Studies and the Determination of Tree Ages in the Australian Tropics

Abstract

In temperate regions the ages of trees can be measured by counting the annual rings revealed on an increment core. The central principle of dendro- chronology - crossdating - simply ensures greater accuracy. In the tropics and sub-tropics growth periodicity may not be clearly limited to a particular season, so that it is necessary first to ascertain whether any anatomical structures delimit annual periods. This problem is discussed with particular reference to the Austra- lian tropics. The climate and forest vegetation of tropical Australia are described. Studies on Eucalyptus and Callitris in open woodlands in the monsoon climates of northern Australia indicate that trunk diameter growth is confined to the wet season. The resulting growth bands, although predominantly annual, are difficult to count, but the trees are not long-lived. However, trees 1000 years old, and possibly considerably older, do occur in the tropical rain forests of eastern Australia. A variety of techniques for measuring age is discussed and it is con- cluded that in the case of Araucaria, and probably some other conifers, the growth bands are approximately annual. Discrepancies between radiocarbon dates and ages derived by other techniques do occur however, and indicate a need for further research. Dendrochronological studies on Pisonia grandis are described in detail to illustrate how ring-width measurements and correlation with climatic factors can sometimes be used to verify the annual nature of the growth rings.