Genetics of eucalypts: traps and opportunities

Eucalypts are amongst the most planted hardwoods in the world (Doughty 2000). They are native to Australia and islands to its north (Ladiges et al. 2003) where they occupy diverse ecological habitats ranging from sea level to the alpine tree line, from high rainfall to semi-arid zones, and from the tropics to latitudes as high as 43° south (Williams and Woinarski 1997). In habit they vary from shrubs and multi-stemmed mallees, to giant trees (Hickey et al. 2000; Nicolle 2006), and include the tallest flowering plants on earth (Eucalyptus regnans – 99.6 metres, http://www.gianttrees.com.au/). They are generally sclerophyllous, and adapted to low nutrient soils and fire (Ashton 2000; Eldridge et al. 1993; Florence 1996). In the broad-sense, eucalypts encompass species of the genera Eucalyptus L'Hérit., Corymbia Hill and Johnson and Angophora Cav. (Ladiges 1997; Appendix 1). A key feature of the majority of Eucalyptus (sensu stricta) and Corymbia (bloodwoods) is the fusion of either the petals and/or sepals to form an operculum from which the eucalypts derive their name (Eldridge et al. 1993; Ladiges 1997). The latest formal taxonomic revision of eucalypts (Brooker 2000) recognizes just over 700 species that belong to 13 main evolutionary lineages (subgenera/genera; Appendix 1), and EUCLID (Euclid 2006 - an important electronic resource for practitioners) lists 894 eucalypt taxa. The major subgenera exhibit different ecological and reproductive characteristics (Florence 1996; Ladiges 1997; Harwood 2011 this volume) and closely related species are usually ecologically differentiated (Florence 1996; Williams and Woinarski 1997).

Most eucalypt species belong to the subgenus Symphyomyrtus, and it is mainly species from three sections of this subgenus that are used in plantation forestry world-wide (Appendix 1; see Hardwood this volume). This is certainly the case in Australia where there has been a major expansion of the eucalypt plantation estate in the last two decades. This eucalypt plantation estate reached 0.92 million ha in 2010 and is approaching the area of softwoods (Gavran and Parsons 2011). Most Australian eucalypt plantations occur in temperate regions and the estate is dominated by Eucalyptus globulus (58.4%; i.e. 538,000 ha) and E. nitens (25.5%, i.e. 235,000 ha) (Gavran and Parsons 2011). There are breeding and deployment programs in Australia and overseas for both Eucalyptus globulus (Potts et al. 2004) and E. nitens (Hamilton et al. 2008). These species are mainly grown for pulpwood.

However there is increasing interest in producing solid wood products (e.g. sawn timber, veneer, composites) from these plantations (Nolan et al. 2005; Beadle et al. 2008; Wood et al. 2009; Washusen 2011; Welsford and Henson 2011). At least 7% of the broadleaf plantations are managed for this purpose (Gavran and Parsons 2011), which includes Forestry Tasmania‟s solid wood estate of approximately 19,655 ha of E. nitens and 5,462 ha E. globulus (Wood et al. 2009). We here overview some of the key genetic issues associated with the breeding and deployment of these industrial plantation species and recent research aimed at understanding the genetic opportunities for growing these species for solid wood products.