The 'wilderness effect' and the decline of Callitris intratropica on the Arnhem Land Plateau, northern Australia

Abstract

An aerial survey along a transect from eastern side of the Arnhem Land Plateau where Aboriginal people still lead a semi-traditional lifestyle, to the unoccupied western side of the Plateau, revealed systematic differences in the proportion of living and dead Callitris intratropica trees. Multiple regression analysis showed that the highest proportion of dead C. intratropica stems occurred on unoccupied, level terrain dominated by open Eucalyptus forests, with a minor or complete absence of Allosyncarpia ternata closed-canopy forests. A detailed study of one population of C. intratropica in western Arnhem Land adjacent to a small patch of A. ternata forest, known as Round Jungle, showed that the population had a unimodal size-class distribution, reflecting a low density of stems less than 10 cm in diameter at breast height (dbh). A computer simulation model was developed on the basis of estimates of annual fecundity, mortality and growth rates derived from observations of the stand. Sensitivity analyses suggested that a well-stocked stand could be transformed to one similar to that observed at Round Jungle after 50 years, if annual mortality rate of the immature stems (i.e. <12 cm dbh) was greater than 85%. Under these conditions, the stand would become extinct after 325 years. Variation in estimates of mature-stem (>12 cm dbh) mortality and fecundity had much less effect on the predictions of the model than the rate of mortality of the smallest size class. The model suggests that C. intratropica populations can rapidly fluctuate in response to changes in fire regime, while extinction is a gradual process and is consequently unlikely if some seedlings can escape burning, for instance by establishing in fire-protected microsites. This conclusion is consistent with the observed greater mortality of C. intratropica on sand sheets that have little topographic variability at the micro- or mesoscale, compared with other habitat types in areas that are currently unoccupied by Aboriginal people. Our study shows that predicting the fate of individual populations will require careful consideration of local factors such as the presence of micro-topographically safe sites for seedling establishment, as well as the surrounding pattern of vegetation and landforms that mediate the impact of fire on C. intratropica. However, we suggest that rather than refining details of the adjustment of C. intratropica in response to changed fire regimes associated with European colonisation, subsequent research should focus on the effect and significance of these changes for other organisms.