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Measuring hybrid establishment along plantation boundaries

Matthew Larcombe
PhD candidate
School of Plant Science
University of Tasmania

At a Tasmanian boundary survey site near Parramatta Creek, Matthew Larcombe (right) shows Gunns Ltd. Forest Practices Officer, Brodey Frost (left), the distinguishing characteristics of E. globulus x E. ovata hybrids that were grown from open pollinated seed collected from native E. ovata growing beside a Victorian E. globulus plantation.

Blue gum (Eucalyptus globulus) is being used as a plantation species in temperate regions of Australia, both within the natural range of the species (e.g., Victoria and Tasmania) and outside its native range (e.g., in Western Australia, South Australia and parts of Tasmania and Victoria).  I am investigating the risk of gene flow from locally exotic blue gum plantations to neighbouring native forest species.

Hybridisation is a prerequisite for gene flow to occur between species and, as such, the risk of hybridisation and barriers to hybridisation have been the main focus of research attention to date. Barbour et al. (2008) showed that although the E. globulus plantation estate is large, just over half of the 300 plantations that they surveyed had no cross-compatible species adjacent to them, making gene flow from these plantations unlikely. However, it has also been shown that where cross-compatible species do exist, hybridisation can occur.  Hybrids have been found in open-pollinated seed collected from native species beside plantations, and established hybrid seedlings have been observed near plantations in Tasmania and Victoria. But what are the frequency, success and relative importance of  hybrid seedling establishment relative to plantation-wildling and pure native species seedling establishment?  I am monitoring hybrid establishment around plantations in order to produce a risk assessment of pollen-mediated gene flow from blue gum plantations to other native species in Australia.  

It has been shown that the proportion of exotic hybrids in open-pollinated seed collected from native species growing adjacent to shining gum (E. nitens) plantations is greatest within 200 m of the plantation boundary (Barbour et al., 2005).  This is consistent with the estimates of pollen dispersal studies of E. globulus (Mimura et al., 2009), making the plantation-native-forest interface the front-line of any gene flow event. In order to determine the actual frequency of hybrids establishing in the wild (a vital first step in the exotic gene flow pathway), I am surveying plantation-native forest boundaries across south-eastern Australia.

A plantation-native forest boundary in Gippsland, Victoria, that was burnt in the 2009 Black Saturday bush fires. 

The survey program targets harvest-age E. globulus plantations that border native stands of E. ovata (or closely related species within the series Foveolatae) because these species are known to hybridise and the morphology of hybrids is well documented and easily distinguished in the field (Figure 1). Surveys involve visually screening all seedlings that are growing along an identified boundary and (i) classifying them as "pure native", "pure plantation" (i.e. wildlings) or "hybrid" phenotype; and (ii) allocating each to a size class to give an indication of recruitment cohorts (i.e., which year the seed was produced).  At some sites radiata pine (Pinus radiata) seedlings are also establishing in native forest. These are also being recorded to give an indication of the relative dispersal capabilities of Australia’s two most important plantation species (E. globulus and P. radiata are the most widely planted and most economically important plantation species in Australia). As eucalypts tend to recruit most effectively following disturbance events, some surveys will target areas in Victoria that were burnt in the 2009 Black Saturday bush fires (Photo 2) and these will be compared to unburnt areas in Tasmania, Victoria and South Australia.

Although the survey program has only just started, early results from three plantations representing 3.1 km of plantation boundary in Tasmania and Gippsland, indicate very low levels of hybrid establishment in comparison to native and plantation species seedlings, with just two of 455 eucalypt seedlings (0.4%) being identified as putative hybrids involving exotic E. globulus. The native cross-compatible species seedlings (n = 281) were the most common along the boundaries so far, with more-or-less equal proportions of E. globulus (n = 163) and Pinus radiata (n = 172) wildlings. It will be interesting to see if this pattern is maintained as the sample size increases.

Barbour, R.C., Potts, B.M. & Vaillancourt, R.E. (2005) Pollen dispersal from exotic eucalypt plantations. Conservation Genetics, 6, 253-257. [read]
Barbour, R.C., Otahal, Y., Vaillancourt, R.E. & Potts, B.M. (2008) Assessing the risk of pollen-mediated gene flow from exotic Eucalyptus globulus plantations into native eucalypt populations of Australia. Biological Conservation, 141, 896-907. [read]
Mimura M, Barbour RC, Potts BM, Vaillancourt RE and Watanabe KN (2009) Comparison of contemporary mating patterns in continuous and fragmented Eucalyptus globulus native forests. Molecular Ecology 18, 4180–4192. [read]

Biobuzz issue thirteen, December 2010