Casuarina ecology: factors limiting cone production in the drooping sheoak, Allocasuarina verticillata

Abstract

Allocasuarina verticillata is an important species for biodiversity conservation on Kangaroo Island (South Australia) because it is the primary food source for the endangered glossy black-cockatoo, Calyptorhynchus lathami halmaturinus. Two potentially limiting factors, pollen and soil nutrients, were studied in the context of A. verticillata as foraging habitat for glossy black-cockatoos. Cone production was not limited by the amount of pollen available to female plants. The soils on which A. verticillata occurs on Kangaroo Island were low in nutrients. Available N, P and K were significantly increased via the application of slow-release fertiliser and the added nutrients resulted in a corresponding increase in the productivity of A. verticillata. The additional nutrients increased the number of cones produced on female branches, branch growth and potentially therefore, tree size. Since cone profitability appears to increase with tree size, the additional growth may also result in greater cone profitability in the long term. Adding slow-release fertiliser to small female A. verticillata trees and revegetation on sites with higher concentrations of soil nutrients would benefit the cockatoos. This is because other studies have shown that the cockatoos increase their foraging efficiency by cropping cones from large trees with greater cone profitability and branches with high densities of cones.