Elsevier

Forest Ecology and Management

Volume 260, Issue 12, 15 December 2010, Pages 2079-2087
Forest Ecology and Management

The potential for gene flow from exotic eucalypt plantations into Australia's rare native eucalypts

https://doi.org/10.1016/j.foreco.2010.08.049Get rights and content

Abstract

Hybridisation through pollen dispersal from exotic plants is increasingly recognised as a threat to the genetic integrity of native plant populations. Its genetic impact can be greater in rare taxa, due to their vulnerability to pollen swamping by more abundant congeners. We assessed the likelihood of pollen dispersal from exotic eucalypt plantations into all of Australia's rare native eucalypts, and conducted a case study of Eucalyptus perriniana, which is rare in Tasmania. The Australia-wide study involved spatial analyses of the locations for each rare species superimposed on distributions of eucalypt plantations, which were combined with known taxonomically based reproductive barriers. 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. These eight proximal taxa are considered priorities for monitoring. In the most extreme case, 30% of point locations originating from herbarium records and field surveys for Eucalyptus conglomerata were within 1 km of exotic plantations. In the case study, E. perriniana revealed considerable reproductive compatibility with adjacent recently established Eucalyptus nitens plantations. However, F1 hybridisation between these species was limited, with 0.2% of the 18,625 seedlings grown from 100 single-tree open-pollinated seedlots being hybrids. For now, the probability of exotic gene flow into E. perriniana appears to be low, however this probability is likely to increase as more E. nitens flowers in the surrounding landscape. These studies suggest that understanding the breeding system and biology of these populations may reveal surprising resistance to such exotic hybridisation as well as identifying high risk situations to focus conservation management.

Research highlights

▶ The potential for genetic contamination of Australia's rare native eucalypts from hybridisation with non-local eucalypt plantations was assessed. ▶ 70% of the 74 nationally listed rare eucalypt taxa were at negligible risk. ▶ Rare taxa most at risk of hybridisation with plantation species were identified. ▶ A case study aimed at testing the risk assessment and adaptive management strategies for a rare taxa in a high risk landscape was undertaken. ▶ Hybrids between the rare and plantation species comprised 0.2% of the open-pollinated seed collected from the rare species, but this rate is expected to increase as flowering increases in the plantation.

Introduction

Hybridisation between species or between naturally allopatric populations is now recognised as being an important contributor to the evolution (Abbott et al., 2003, Ellstrand and Schierenbeck, 2000) and invasion (Ellstrand et al., 2010, Gaskin and Schaal, 2002, Vila et al., 2000) of many exotic plant species. Movement of pollen and its potential genetic and ecological consequences became a public concern following the deployment of genetically modified (GM) crops, as escapes of transgenes into non-transgenic populations were reported (Stewart et al., 2003). These studies highlighted the importance of considering pollen movement and its impacts more generally (Ellstrand, 1992, Ellstrand and Elam, 1993). Gene flow from non-GM crops or exotic species into native populations through hybridisation and introgression has been recognised as posing a risk to the genetic integrity and survival of “pure” species populations (Ellstrand, 1992, Rhymer and Simberloff, 1996, Wolf et al., 2001). These risks have the potential to be higher in the case of rare species and small populations, due to their vulnerability to reproductive swamping (i.e. source/sink effects, Ellstrand and Elam, 1993, Field et al., 2009, Levin et al., 1996, Wolf et al., 2001) and sensitivity to compounding adverse environmental conditions (Rhymer, 2008).

Eucalypts represent a central component of the Australian biota, being the dominant tree in the majority of non-arid woodland and forest communities (Williams and Woinarski, 1997). There are nearly 900 eucalypt taxa (CPBR, 2006), which are endemic to Australia and its surrounding islands. Within these, 74 taxa are currently listed as endangered or vulnerable at the national level in Australia and require special attention. A notable characteristic of eucalypts is their propensity for interspecific hybridisation (Potts and Wiltshire, 1997). Under natural circumstances eucalypts are often observed to hybridise (Griffin et al., 1988, Potts and Wiltshire, 1997), however, the degree to which this occurs is limited by pre-mating barriers such as spatial isolation and flowering asynchrony (Keatley et al., 2004, Potts and Wiltshire, 1997), and post-mating crossing-incompatibilities. The latter prevents successful hybridisation between the three genera of eucalypts (Angophora, Corymbia, and Eucalyptus) and also between the major subgenera within Eucalyptus (Griffin et al., 1988, Potts et al., 2003).

At least eight eucalypt species and a few artificial hybrids are being deployed in industrial hardwood plantations in Australia (National Forest Inventory, 2006). This plantation estate has expanded dramatically over the last two decades, and now covers nearly 1 million ha in Australia (950,000 ha, Gavran and Parsons, 2009). Its expansion is likely to continue due to increasing public pressure to reduce native forest harvesting, reduce the Australian trade deficit in forest products and to develop carbon off-set industries (National Forest Inventory, 2006). Due to the often weak barriers to hybridisation within eucalypts, there is concern over the potential for pollen-mediated gene flow from plantations into native eucalypt populations (Kanowski et al., 2005, Potts et al., 2003, Salt et al., 2005, Strauss, 2001, Wardell-Johnson et al., 1997). This concern is based on the fact that genetic material used for stocking plantations is typically established well out of its native range, as locally exotic species, hybrids, provenances or genotypes (Barbour et al., 2008a, Potts et al., 2003).

Considerable work has been conducted to assess the potential for exotic gene flow from the major temperate plantation eucalypts in Australia, Eucalyptus globulus and Eucalyptus nitens. This work has demonstrated that F1 hybridisation involving plantations and their adjacent native species can occur (Barbour et al., 2008b, Barbour et al., 2003, Barbour et al., 2005a, Barbour et al., 2005b, Barbour et al., 2006b, Barbour et al., 2002) and that F1 hybrid seedlings are establishing in the wild at some locations (Barbour et al., 2008b, Barbour et al., 2003). In addition, work in the subtropical and tropical regions of Australia has identified a potential for such gene flow (Barbour et al., 2008a, Kanowski et al., 2005). An important aspect of assessing the off-site risks from planting Eucalyptus that has not been addressed is the likelihood of exotic gene flow to the rare eucalypt species of Australia.

The aim of this study was to assess the likelihood of pollen-mediated gene flow from eucalypt plantations into all nationally listed endangered or vulnerable (herein referred to as rare) eucalypt species of Australia to better focus resources to the protection of species most at risk. This assessment was conducted through the integration of spatial analyses of the proximity of plantations to each native species, and the knowledge of the reproductive barriers that exist amongst them. Following this, a detailed case study was conducted into the likelihood of exotic gene flow into a rare Tasmanian species, Eucalyptus perriniana, from surrounding E. nitens plantations, by assessing levels of F1 hybridisation between the two species and their reproductive biology.

Section snippets

Spatial assessment

All 74 eucalypt species or subspecies classified as endangered or vulnerable in 2009 by the Australian Government (Department of the Environment, Water, Heritage and the Arts, www.deh.gov.au/cgi-bin/sprat/public/sprat.pl) were assessed for their spatial distribution relative to eucalypt plantations. Three species on the list, Eucalyptus aquatica, Eucalyptus olivacea and “Eucalyptus sp. Howes Swamp Creek” were not included in the analyses as they were no longer regarded as legitimate taxa and

Spatial assessment of plantations across Australia

Of the 74 nationally listed rare eucalypt taxa of Australia, 22 were found to have point locations (representing an individual or population) within 10 km of a eucalypt plantation of the same genus (Table 1). Looking specifically at each genus, no point locations for the four rare Corymbia species were found within this distance of Corymbia plantations. No known Angophora species are established as plantations, so neither of the two rare Angophora were found to be at risk. All 22 rare eucalypt

Discussion

Despite the broad overlap of the geographic distribution of both Australia's rare eucalypt taxa, and the current eucalypt plantation estate (Fig. 1), only 22 of the 74 endangered or vulnerable taxa were found within 10 km of a plantation. The four species predicted to have highest likelihood of exotic gene flow are E. alligatrix subsp. limaensis, E. conglomerata, E. kabiana, and E. strzeleckii; either because they had more than 10% of their distribution within 1 km of plantations, or due to

Acknowledgements

The authors wish to thank the CRC for Forestry for financial support, Gunns Ltd. and the Department of Primary Industries and Water (Threatened Species Unit) for permission to conduct seed collections and field observations, and Scott Nichols, Anthony Mann, Jnthony Bloomfield and Matthew Lacombe for their assistance. We also thank the National Plantation Inventory for providing the GIS shape files for the distribution of eucalypt plantations, and the following agencies for the point location

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