Gene flow from introduced Eucalyptus plantations into native eucalypt species

There has been a massive expansion of Eucalyptus plantations in Australia in the last decade. As most eucalypt species are native to Australia, this has raised concerns about genetic pollution of native species gene pools. To assess the risk of genetic pollution, five key aspects of gene flow from E. nitens plantations in Tasmania were studied. Firstly, the pattern of pollen dispersal from plantations into native E. ovata forests was examined at three sites. The frequency of F1 hybrids among 119 000 progeny, grown from open-pollinated seed collected from E. ovata trees at varying distances from E. nitens plantations, was determined. A leptokurtic pattern of pollen flow was evident, with the mean level of hybridisation at the plantation boundaries (0-100 m) being 14%, dropping to 1% by 200-300 m and continuing at this level to the extent of the studied areas (max. 1600 m). Secondly, the first exotic E. ovata x nitens F1 hybrids actually established in the wild were discovered and verified using morphological and allozyme analyses. Thirdly, field trials were established to assess the fitness of the exotic E. ovata x nitens F1 hybrids in the wild. The hybrids displayed poorer early-age performance relative to their pure E. ovata half-sibs. However, many were still alive and growing successfully two years after planting. Fourthly, surveys of flowering time were conducted on native Symphyomyrtus species, which were potentially at risk of gene flow from plantations. This was combined with a spatial analysis of the proximity of eucalypt plantations to each native species, and used to produce an overall assessment of the potential for exotic hybridisation and gene flow. These results indicated that at least eight of the 16 Symphyomyrtus species in Tasmania appear to have significant potential for pollination by E. nitens, and will need further study. Finally, the crossability of E. nitens pollen on native female Symphyomyrtus species was assessed using artificial pollination techniques. A number of species did not produce hybrids with E. nitens, indicating that post-mating barriers may be acting to prevent hybridisation. Overall results suggest that the main risk of genetic pollution in Tasmania is limited to a few species. Of those species, the ability of first and later generation hybrids to survive to reproductive maturity and backcross with native populations, allowing the introgression of exotic genes, is yet to be assessed.