Gene flow between introduced and native Eucalyptus species: Flowering asynchrony as a barrier to F1 hybridisation between exotic E. nitens and native Tasmanian Symphyomyrtus species

Eucalyptus nitens has recently been introduced to the island of Tasmania for use in commercial plantation forestry. The current area of the E. nitens plantation estate now stands at ca. 117,000 ha. E. nitens is potentially cross compatible with 16 of the island’s 29 native eucalypt species. Interspecific flowering asynchrony was assessed as it is a potential barrier to pollen-mediated gene flow from E. nitens plantations in Tasmania. Flowering was assessed across 41 field sites containing these compatible native eucalypt species and/or E. nitens. Two years of field data from these sites, combined with previously published flowering data, showed that flowering phenology varied considerably with species, altitude and season. In addition, assessments of a field trial indicated that significant genetic variation at the provenance and family levels exists within E. nitens. This genetic variation was limited, however, and it is argued that the marked delay in flowering time with altitude observed in plantations of this exotic is mainly due to environmental effects such as the seasonal heat sum. Assessments of flowering synchrony between E. nitens and each native species found variation due to season and species. Of the 16 potentially cross-compatible species, six appear at low risk of pollen-mediated gene flow from E. nitens due to low levels of flowering synchrony (proportion of flowering period synchronous with E. nitens 0.18). Of the remaining species, E. archeri (0.50), E. ovata (0.58) and E. rodwayi (0.55) displayed the highest levels of flowering synchrony with E. nitens. The results demonstrate the importance of gaining greater empirical knowledge of factors affecting pollen-mediated gene flow from exotic species to precisely identify species and populations at greatest risk of exotic gene flow. When combined with studies of other potential reproductive barriers, such as spatial isolation, crossing incompatibilities and reduced hybrid fitness, further reductions in the number of native Tasmanian eucalypt species found to be at significant risk of exotic gene flow from E. nitens, are expected