Population status of eucalypt trees on the River Murray floodplain, South Australia

For populations to persist, recruitment must keep pace with mortality. In variable environments, opportunities for reproduction occur patchily in time and space, and favourable conditions must occur sufficiently often to allow growth to maturity (hence 'recruitment'). The risks of local extinctions may be increased by anthropogenic factors. This scenario is illustrated by two tree species, river red gum (Eucalyptus camaldulensis) and black box (E. largiflorens), on the floodplain of the River Murray, South Australia. Fixed area plots were established at Banrock Station, where large, mature trees are common, although red gum outnumber black box by about four to one. Trunk diameter was measured as a surrogate for tree age. The smallest diameter (0-10 cm) black box are nearly as common as seedlings of that species, whereas the smallest diameter red gum (0-10 cm) are 10 times more abundant than seedlings. Small trees of both species occur in localized clumps, and the respective size-class distributions exhibit series of peaks and falls, suggesting episodic recruitment and opportunistic survival. Population viability calculations suggest that more than 100% of existing saplings need to survive to maintain the local black box population (i.e. there are too few saplings). The red gum population apparently requires a smaller proportion of survivors, but the calculations may be biased by the clumped distribution of the saplings. Based on population structure and viability estimates, black box at this site appear to lack sufficient regeneration to compensate for adult mortality while red gum appear to have a much better balance. The methods established here may be useful for assessment of stands of these species in other areas. Copyright (c) 2005 John Wiley F Sons, Ltd.