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Hybrid vigour key to assessing risk of gene flow from exotic plantations

Matthew Larcombe
School of Plant Science
University of Tasmania

Figure 1. Clare Brooker walks among large numbers of E. ovata seedlings that were becoming established a year after a wild fire in 2005.  It was in this area that the 80 hybrid pairs were located.

My FWPA and CRCF-funded PhD project involves investigating the risk and management of exotic gene flow from eucalypt plantations to neighbouring native eucalypts, in particular from blue gum (Eucalyptus globulus) plantations across the southern Australian mainland. Hybridisation (between the plantation blue gum and native forest species) is a prerequisite for gene flow to occur and previous issues of BioBuzz have outlined how my project is investigating: blue gum pollen movement in the landscape (read article in BioBuzz 11); the species that are likely to hybridise with blue gums (read article in BioBuzz 12); and whether or not hybrids are establishing along the plantation native forest boundary (link to article in BioBuzz 13). Although investigating or anticipating the occurrence of hybridisation has been the focus of these previous articles, hybridisation is only the first step in the gene flow pathway. For gene flow to actually occur the hybrids between plantation and native species must reach maturity, flower and cross (hybridise) back to the native species.  This process needs to occur repeatedly over multiple generations for exotic genes from the plantation species to become permanently incorporated into the gene pool of the native species (i.e. for exotic gene flow and introgression to occur). Therefore, the relative success or “fitness” of hybrid seedlings is the next logical step in the gene flow pathway that should be investigated.

In 2006, Robert Barbour identified a site in southern Victoria where exotic E. globulus x ovata hybrids were establishing in native forest adjacent to a plantation. Both the native forest and the plantation had been burnt in a wildfire a year earlier, triggering a large recruitment event. Barbour established a monitoring site where he paired each of 80 hybrids he identified with a pure E. ovata seedling establishing from the same recruitment event (Figure 1). Initial measurements indicated that the hybrids seemed to be more susceptible to browsing insects, animals and leaf fungi (Barbour et al., 2008).

Figure 2. The "Barbour wild hybrid site" in April 2006 (left) and December 2010 (right).  In less than five years, the previously open vegetation reverted to scrub, making relocating the hybrid and pure species pairs very difficult.

In December 2010 I revisited Barbour's hybrid site to try to relocate the ‘hybrid-pure pairs’ and assess the survival rate of the hybrid seedlings compared to pure species seedlings. The first thing that struck me about the site was that it looked totally different from Barbour’s photos (Figure 2). Over the four years post-fire, the site had been completely engulfed by scrub: relocating the pairs was going to be difficult!  After four days of intensive scrub-bashing I managed to relocate 33 of the 80 pairs. Even with this reduced sample set the survival results were striking.  Of the potential 66 saplings, 27 were still alive; 22 were pure E. ovata and only five were E. globulus x ovata hybrids. These results were consistent with glass-house and field trials that showed reduced hybrid fitness and survival in E. nitens x ovata hybrids (Barbour et al., 2006); they are the first record of increased mortality in plantation x native eucalypt hybrids in the wild.  The increased hybrid mortality is likely to act as a barrier to gene flow because reduced hybrid fitness is likely to result in mortality before the tree reaches maturity, preventing the hybrid genes from entering the native species gene pool.

Figure 3. Left: an unusually vigorous and healthy E. globulus x ovata hybrid. This plant was significantly taller and in better condition than the surrounding pure E. ovata. Right: the more typical hybrid condition.

Although the trend in the results (above) is for reduced fitness in the exotic hybrids, it should be noted that while searching for the hybrid-pure pairs in the scrub, several other hybrid saplings were located (plants that were not included in the study of Barbour et al. (2008) but were most likely from the same recruitment event).  Some of these saplings showed increased vigour in comparison to surrounding pure E. ovata (Figure 3). It is, therefore, possible that a small number of hybrid individuals with increased vigour could reach maturity and cross back to the native species, leading to gene flow in the long term. Alternatively, it is possible that a reduction in adult fitness or some level of hybrid incompatibility could prevent this second generation hybridisation.  It is currently unclear what impact a small number of “fit hybrids” might have in the long term, but further investigation and monitoring of their adult fitness will be required.


Barbour, R.C., Potts, B.M. & Vaillancourt, R.E. (2006) Gene flow between introduced and native Eucalyptus species: Early-age selection limits invasive capacity of exotic E. ovata x nitens F1 hybrids. Forest Ecology and Management, 228, 206-214. [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]

Biobuzz issue fourteen, May 2011