Growth and survival of two north Australian relictual tree species, Allosyncarpia ternata (Myrtaceae) and Callitris intratropica (Cupressaceae)

Allosyncarpia ternata (an angiosperm) and Callitris intratropica (a gymnosperm) are two fire-sensitive tree species of the Australian monsoonal tropics. Studies using historical aerial photography have revealed recent expansion of A. ternata rainforests. There has simultaneously been a widespread collapse of C. intratropica populations in northern Australian savannas, presumably because of cessation of traditional Aboriginal landscape burning. To explain the demography behind these contrasting trends, stand structure, survival, and growth of the two species were recorded over a 16-year period at the boundary of a rainforest patch and also in adjacent savanna, in Kakadu National Park. Ages of the largest trees of each species, estimated by using a Bayesian analysis of tree-diameter increments, were approximately 433 years for A. ternata and 235 years for C. intratropica on the rainforest boundary, and 417 years for C. intratropica in the adjacent savanna. Densities of juveniles (seedlings and re-sprouts < 0.5 m high) were 325-6,000 times higher for A. ternata than for C. intratropica. Life-table calculations indicated there was sufficient recruitment of A. ternata, but not C. intratropica, to overcome observed mortality rates and maintain a stable population. This is almost certainly because A. ternata re-sprouts prolifically after fire whereas C. intratropica is an obligate seeder. These results highlight the critical need for careful fire management to maintain populations of a characteristic Australian gymnosperm over much of its range.