Population dynamics of an arid zone mistletoe (Amyema preissii, Loranthaceae) and its host Acacia victoriae (Mimosaceae)

The principal host of the mistletoe, Amyema preissii, near Alice Springs is the short-lived, fast-growing tree, Acacia victoriae. In order to describe the dynamics of their interaction, the fate, size and phenology of mistletoes were monitored in natural host stands, infection experiments were conducted to determine establishment success and growth rates of seedlings, hosts were repeatedly disinfected in a 14.6-ha paddock surrounded by infected source trees, and mistletoe reinvasion of disinfected trees was monitored. In the unmanipulated population of A. victoriae, 46–54% of trees (> 2 m in height) were infected (5.8–6.7 mistletoes per infected tree). Establishment percentage of mistletoe seeds deployed on host branches varied between 11–24% after 12–23 months, and most naturally-dispersed seedlings established on branches ≤ 20 mm in diameter. Estimates of natural mistletoe dispersal to uninfected trees varied between 2 and 12 seeds per infected tree over periods of 8–23 months. Examination of young mistletoes showed that 24% of 122 clumps consisted of more than one mistletoe, with 27 occurrences of double infection and two occurrences of triple infection at the same point on the host branch. Mistletoes grew rapidly: in one sown cohort, plants achieved canopy diameters of 152–170 cm within 18–23 months, and half were flowering 15 months after germination. In unmanipulated populations, the largest mistletoes had a maximum canopy diameter of 250 cm and a maximum host branch diameter proximal to the haustorium of 100 mm, but modal sizes of these measures were 80–120 cm and < 10 mm, respectively. Turnover in the host population in the disinfection paddock was rapid: 60% of trees died between 1990 and 1995, mortality being balanced by recruitment. In unmanipulated stands of host trees, larger trees were more likely to be infected and supported larger numbers of mistletoes per tree than small trees. Once mistletoes had been removed from trees in the disinfection paddock, larger trees were still disproportionately infected by reinvading mistletoes and trees that had been infected previously were more likely to be reinfected. In order to fully describe the spatial and temporal dynamics of this host–mistletoe combination, further studies are needed of mistletoe persistence and mortality through time and annual seed production as a function of mistletoe size.