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Eucalypt Hybridisation

How do we make informed decisions on whether to plant non-indigenous native eucalypts without running the risk of polluting the indigenous eucalypt gene pool?

First of all, many of the eucalypts hybridise or change naturally. 

Prehistoric relatives were genetically the same, but as the environment changed (soil, drainage, rainfall, wind, temperature, humidity, etc) over millennia, many trees died out, unable to cope with the changes, while others, now in isolation, were able to hang on and change with their surroundings.  The reason Australia has over 800 different eucalypt species is due to this fact.

One study shows certain eucalypt species in east facing protected valleys in the Grampians having more genetic diversity than the same species just over the ridge in west facing exposed hillsides(1).  As these two populations of trees evolve, their physical attributes change as the trees adapt to the local environment.  Over many generations each population no longer resembles their mates over the ridge.  If given enough time they will become a new species though they are still closely related to the other trees over the ridge.  Like cousins they share not too distant relatives. Think of a chihuahua and a great dane.

All eucalypts are related, however, species say, from WA are much more distantly related to our Otway species than the Grampians scenario above.   The isolation has been so great and for so long that they are not even closely related.  Like a domestic cat and a lion.

So we know that a chihuahua and a great dane are related closely enough to breed with one another, however, they don’t share certain physical attributes to make that very easy!  Also, these breeds evolved in isolation from each other.  It’s not until humans enter the scene and start moving these dog breeds across the globe that the potential of the breeds to cross is even possible, however unlikely.

The eucalypts were first classified in 1934 by Blakely (2) and have been reclassified many times since then as new knowledge comes to hand.  Classification puts the various species in sub groups depending on their relations with each other.  For example, the classification listing for manna gum is:

1. Family: Myrtaceae
2. Genus: Eucalyptus
3. Subgenus: Symphyomyrtus
4. Section: Maidenaria
5. Series: Viminales
6. Species: viminalis

In most good Eucalypt reference books, the classification listings will be shown for each species.  The more closely related two species are, the closer they will be listed in the classification.  The relation is based on physical attributes like bark type, reproductive parts, leaf shape and many more.  For example:

*not indigenous to Otway region

On face value, the first two and last two species are closely related to each other.  Most of us in the Otways also know that messmate and mountain ash have hybridised sometime in the past producing the “otway messmate” (Eucalyptus obliqua x regnans).  Many hybrids between close relatives often exhibit reduced vigour and reproductive output compared with the parents (3), however, the otway messmate has thrived producing a high value commercial timber as well as being able to reproduce and maintain a viable population.  It also appears to have no detrimental environmental effect on local ecosystems. Botanists believe one day this hybrid will be classified as a species (4). 

The yellow and brown stringybarks are also very closely related but only one grows in the Otways.  If a farmer planted a few hundred yellow stringybarks on her/his Otway property could this result in “sowing the seeds” of a potential hybrid somewhere down the road?  I’m afraid the answer is not an easy yes or no! 

Here are some general points about introducing exotic eucalypts into another ecosystem. (referenced from 5)

1. Generally, small and/or isolated remnant reserves may be more susceptible to “reproductive swamping” from large plantations but only if they are close relatives.  Isolated paddock trees and remnants are also extremely important as “gene banks” where genetic material may be needed for future environmental rehabilitation projects.
2. Eucalypts are well known for weak reproductive barriers.  In one study, over 55% of 528 species examined were involved in at least one natural hybrid event.  However, even with close relatives (between species, series and sections), hybrids rarely show vigour let alone produce genetically viable offspring.  This doesn’t mean they don’t have an influence on gene flow into surrounding ecosystems.
3. Studies show hybrids support more herbivorous insects, pathogenic fungi and even greater susceptibility to browsing by brushtail possums, than their parents.
4. Distance of pollen flow from introduced trees into the indigenous population depends on the species involved, what carries the pollen, and physical barriers between the introduced and indigenous trees.
5. Annual and seasonal flowering overlap between the species may coincide or it may not.
6. Most blue gum and shining gum pulp plantations produce few flowers due to the shading of a high stocking rate, not to mention harvesting of trees at a young age before they reach a reproductive age. (6)
7. Physical barriers in reproductive systems between species strengthen as the taxonomic distance between the two species increases.

Like I said, there is no easy answer.  Suffice to say that every caution should be taken when considering planting exotics, and even non local provenances on your property.   Here are two worthwhile reference books that explain eucalypt classification:

1. Costerman L, (1981), Trees and Shrubs of Southeastern Australia, Weldon
2. Nicolle D, (2006), Eucalypts of Victoria and Tasmania, Bloomings

References

1. Pollock L, et al, (2013), Chloroplast DNA diversity associated with protected slopes and valleys for hybridizing Eucalyptus species on isolated ranges in south-eastern Australia, Journal of Biogeography
2. Blakely WF, (1934), A Key to the Eucalypts, Forestry and Timber Bureau
3. Potts B, et al, (2001), Genetic Pollution From Farm Forestry Using Eucalypt Species and Hybrids, RIRDC Joint Venture Agroforestry Program
4. Broadhurst L, (2013) Improving the Past, Present and Future of Australian Restoration- the Role of Genetics, “Sowing Seeds of Success” Glenelg Hopkins CMA Public Forum
5. Potts B, et al, (2003), Genetic Pollution of Native Eucalypt Gene Pools- Identifying the Risks, Australian Journal of Botany
6. Barbour R, (2008), Assessing the risk of pollen-mediated gene flow from exotic Eucalyptus globulus plantations into native eucalypt populations of Australia, Biological Conservation

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