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Assessing the risk of gene flow into Australia’s rare native eucalypts

Brad Potts
University of Tasmania

Figure 1.  The distribution of hardwood and softwood plantations in Australia (from Gavran and Parsons 2008)

Australia’s hardwood plantation estate is 0.99 million hectares and is virtually all eucalypts (Figure 1; Gavran and Parsons 2010).  By comparison, the area of native eucalypt forest is estimated to be 116.4 million hectares ( Bureau of Rural Sciences 2010, p. 11). In many cases, plantation eucalypt species have been planted outside their natural range in areas where they could hybridise with native eucalypts from the same subgenus with which they do not co-occur naturally (see related article in Biobuzz 12; view related publications).  Such hybridisation may or may not impact on the gene pool of the proximal native species, depending upon many factors including: the strength of the reproductive barriers; hybrid fitness; geographic proximity; and relative sizes of the source and sink populations (Potts et al. 2003).  All else being equal, it is the rare populations and species of Eucalyptus which are likely to be most at risk from gene-flow from large-scale plantings of non-local eucalypts due to their greater vulnerability to pollen swamping.  

To focus management and research priorities, we assessed the likelihood of pollen-mediated gene flow from eucalypt plantations into all eucalypt species and subspecies in Australia that, as of 2009, were listed as nationally endangered or vulnerable.  This assessment was published recently in Forest Ecology and Management (Barbour et al. 2010).

In this study, we integrated spatial analyses of the proximity of plantations to each rare native species with the knowledge of the reproductive barriers that are likely to exist between the plantation species and the rare species.  Point location data for the rare taxa was provided by State agencies.  The distribution of eucalypt plantations throughout Australia, current to 2008, were provided as GIS shape files from the National Forest Inventory. These shape files delineated the taxa used in plantations at the genus level only.  Depending upon climatic zone, these eucalypt plantations are typically established using E. globulus (subgenus Symphyomyrtus, 64% of the total area), E. nitens (Symphyomyrtus, 20%), E. pilularis (subgenus Eucalyptus) and E. grandis (Symphyomyrtus) (combined at 4%), E. dunnii (Symphyomyrtus, 3%),  Corymbia (species and hybrids from section Politaria, 2%) or other minor taxa such as E. cladocalyx (Symphyomyrtus), E. occidentalis (Symphyomyrtus) and E. cloeziana (subgenus Idiogenes) (6% of the total) (Parsons et al. 2006 ).

Of the 74 nationally listed rare eucalypt taxa, 22 had locations within 10 km of plantations of the same genus, and eight were within 1 km. The eight taxa identified as priorities for ground-truthing, and monitoring if the risk is validated, are E. alligatrix  subsp. limaensis, E. conglomerata, E. glaucina, E. johnsoniana (click here for more information on E. johnsoniana), E. kabiana, E. morrisbyi, E. paludicola (click here for more information on E. paludicola) and E. strzeleckii.  In the most extreme case, 30% of point locations originating from herbarium records and field surveys for E. conglomerata were within 1 km of exotic plantations.  Two taxa, E. alligatrix subsp. limaensis and E. kabiana, had 100% of their distribution within 10 km of plantations and another two, E. conglomerata and E. strzeleckii, had more than 10% of their distribution within 1 km of plantations.

In Tasmania, the preparation of Forest Practices Plans includes the assessment of off-site gene flow risk (see Forest Practices Authority technical report).  A case study aimed at testing the risk assessment and adaptive management strategies for rare taxa in a risk landscape is currently in progress and involves a Tasmanian population of the locally rare E. perriniana (see related article in Biobuzz 12).

Barbour RC, Wise SL, McKinnon, GE, Vaillancourt RE, Williamson, GJ and Potts BM (2010) The potential for gene flow from exotic eucalypt plantations into Australia’s rare native eucalypts. Forest Ecology and Management 260: 2079–2087. (read)

Gavran M, Parsons M (2008) National Plantation Inventory 2008 Update, National Forest Inventory. (Bureau of Rural Sciences: Canberra).

Gavran M, Parsons M (2010) Australia’s Plantations 2010 Inventory Update, National Forest Inventory.  (Bureau of Rural Sciences: Canberra) ( read)

Parsons M, Gavran M, Davidson J (2006) National Forest Inventory. (Bureau of Rural Sciences: Canberra)

Potts BM, Barbour RC, Hingston AB, Vaillancourt RE (2003) Genetic pollution of native eucalypt gene pools - identifying the risks.  Australian Journal of Botany 51: 1-25. (read)

Biobuzz issue thirteen, December 2011