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

An aerial survey along a transect from eastern side of the Arnhem Land Plat eau 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 r egression analysis showed that the highest proportion of dead C. intratropi ca 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 we stern Arnhem Land adjacent to a small patch of A. ternata forest, known as Round Jungle, showed that the population had a unimodal size-class distribu tion, reflecting a low density of stems less than 10 cm in diameter at brea st height (dbh). A computer simulation model was developed on the basis of estimates of annual fecundity, mortality and growth rates derived from obse rvations of the stand. Sensitivity analyses suggested that a well-stocked s tand could be transformed to one similar to that observed at Round Jungle a fter 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 e xtinct after 325 years. Variation in estimates of mature-stem (>12 cm dbh) mortality and fecundity had much less effect on the predictions of the mode l than the rate of mortality of the smallest size class. The model suggests that C. intratropica populations can rapidly fluctuate in response to chan ges in fire regime, while extinction is a gradual process and is consequent ly unlikely if some seedlings can escape burning, for instance by establish ing in fire-protected microsites. This conclusion is consistent with the ob served greater mortality of C. intratropica on sand sheets that have little topographic variability at the micro- or mesoscale, compared with other ha bitat types in areas that are currently unoccupied by Aboriginal people. Ou r study shows that predicting the fate of individual populations will requi re careful consideration of local factors such as the presence of micro- to pographically safe sites for seedling establishment, as well as the surroun ding 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 assoc iated with European colonisation, subsequent research should focus on the e ffect and significance of these changes for other organisms.