Hybrid Eucalyptus Identification: 4 main considerations1) Close Cousins or not: Taxonomic Distance
- Eucalyptus crenulata Blakely & de Beuzevill. [1939]: Subtribe Eucalyptinae, Genus Eucalyptus, Subgenus Symphyomyrtus, Section Maidenaria, Series Neglectae, Subseries Crenulatosae
- Eucalyptus nitens (H. Deane & Maiden) Maiden [1913]: Subtribe Eucalyptinae, Genus Eucalyptus, Subgenus Symphyomyrtus, Section Maidenaria, Series Globulares, Subseries Nitentosae
- Hybrid category: Intra-sectional (Maidenaria), Inter-serial (Neglectae x Globulares)
- Hybridism rank: Common. There are many known examples of natural and man-made Eucalyptus hybrids using species of two different Series within Section Maidenaria.
2) Bee-Made or Man-Made? Checking Habitat Overlap
Eucalyptus crenulata is an endemic species present in just two tiny wild populations in South Central Victoria. Here it survives as part of plant communities where E. ovata, E. viminalis, E. camphora and E. radiata are the only eucalypts sharing its habitat.
E. nitens has a more widespread natural distribution in the highlands of Central Victoria and further North along the Great Dividing Range into New South Wales. Both species share a somewhat close geographic distribution in Central Victoria but their habitats do not overlap and there is clear reproductive isolation between both gene pools. This means natural hybrids between E. crenulata and E. nitens are not possible in the wild. Hybrid strains can only originate from cross-pollination between compatible parent trees of closely related species planted in some other location.
3) Measure the Beast: Comparative Botanical Diagnosis
One of the most useful parameters to aid Eucalyptus species identification is a detailed study of their reproductive organs: flower buds, flowers and seed capsules. Their numbers, shapes and sizes are key to assist taxonomic study and accurate diagnosis.
Fig. 3: Morphologic assessment of purported hybrid Eucalyptus crenulata x nitens. (Click image to enlarge)
For the case of this probable hybrid strain we have the following morphologic clues:- Lightly crenulate glaucous juvenile leaves (E. crenulata trait, see Fig. 1)
- Verrucose non aristate glaucous juvenile stems (E. crenulata trait, see Fig. 1)
- Distinctively beaked flower buds (E. crenulata trait, see Fig. 1)
- Barrel shaped non globose glossy green flower buds (non E. crenulata trait, should be glaucous)
- Flower buds and seed capsules in groups of 7 or lower numbers (non E. crenulata trait, should be in groups of 11 or more)
- Early transition from sessile orbicular juvenile leaves to opposite stalked falcate leaves before reproductive maturity (non E. crenulata trait, should not show transition to adult foliage)
4) Ask the Parents: Ancestry of the Cultivated Strain
Are you wondering why the "Get to know as much as possible about seed provenance before planting an Eucalyptus" mantra is a good one? Because when a case like this arises, morphology is not enough!
Going further, seed provenance is not enough. Traceability to the original parent trees for at least a couple of generations might be needed. Detective work.
For this case we are lucky enough as to trace back the seed originating this curious specimen to E. crenulata trees planted ex situ from its natural habitat. In this particular location at least another Eucalyptus species grows which could provide all those traits observed as non E. crenulata. And that species is E. nitens (see Fig. 2).
Why Eucalyptus Botany is seldom an easy task
Discussed above, a strange looking eucalypt for which we have followed early and adult growth until reproductive maturity, which allowed observation of apparent intermediate characters between a clear E. crenulata ancestry (verrucose juvenile stems, crenulate juvenile leaf margins, beaked flower buds) and another eucalypt species bearing flower buds and seed capsules in groups of 7, having orbicular juvenile leaves, falcate adult leaves and barrel shaped capsules.
Detective work yielded nice clues of the other parent in this mix being E. nitens, which, as far as we know, would mean a first detailed record of the hybrid strain E. crenulata x nitens, making Barclay's Hybrid an unique specimen and a nice contribution to genecological studies in Genus Eucalyptus.
But...
"At this subpopulation, the mature Buxton Gums are thought to be hybrids between E. crenulata and E. ovata (Adams & Simmons 2000), and the characteristics of individual trees may not be entirely consistent with the species description in the botanical literature (Blakely & de Beuzeville 1939). Hybridisation between related Eucalyptus species is common and widely documented (Pryor & Johnson 1971, 1981; Griffin et al. 1988). Undoubtedly the exchange of genetic material facilitated by hybridisation has been important in the evolution of this large genus (Potts & Wiltshire 1997). (...) Most of the wild ‘Buxton Gums’ at this site are now thought to be introgressed hybrids between E. ovata and E. crenulata (Simmons & Parsons 1976; Adams & Simmons 2000). A limited number of ‘pure’ E. crenulata were recorded from the site in the 1970s (Simmons & Parsons 1976) and early 1980s (Albrecht 1983). However, pure breeding E. crenulata are thought to be no longer present (Adams & Simmons 2000). Genetic analysis of the population is required to confirm this."
White M.; Murphy A. H. & Downe J. (2006)
No matter what, an unique plant!
The previous excerpt points to more detective work needed. Suddendly, Ian's Hybrid has become one of three potential very rare Eucalyptus:
- A first record of E. crenulata x nitens
- A first record of E. ovata x crenulata in North America
- A first record of (E. ovata x crenulata) x nitens
So maybe the thing to do now is a Spaniard annoying some Australian about an Eucalyptus growing in the United States. Aren't these amusing International Plants?
Ian Barclay (BSc Hort. WSU) has been growing rare plants since last century and scattering them all over Washington State (USA) and elsewhere. Among other things he has trialled over 130 species of Eucalyptus trees in the Pacific Northwest as an endeavour to obtain a range of forms and provenances of horticultural merit and to obtain select trees for superior performance and cold-hardiness, which is the most important effort in Eucalyptoculture for Northwestern America since the days of Stan Gessel.
References
- White M.; Murphy A. H. & Downe J. (2006) National Recovery Plan for the Buxton Gum Eucalyptus crenulata. Department of Sustainability and Environment, Victoria.
- Adams, R. & Simmons, D. (2000) Survival, health and recruitment of Eucalyptus crenulata (Buxton Gum) - Base line data and monitoring guidelines. Unpublished report to the Upper Goulburn Field Naturalists and the Department of Natural Resources and Environment, Funded under the Threatened Species Network Community Grant Scheme. NRE, Alexandra.
- Blakely, W.F. & de Beuzeville, W.A.W. (1939) Contributions from the New South Wales National Herbarium 1: p. 37.
- Pryor, L.D. & Johnson, L.A.S. (1971) A classification of the Eucalypts. Australian National University Press, Canberra.
- Pryor, L.D. and Johnson, L.A.S. (1981) Eucalyptus, the universal Australian. In Ecological Biogeography of Australia. (Ed A. Keast). Dr W Junk, The Hague.
- Griffin, A.R.; Burgess, I.P. & Wolf, L. (1988) Patterns of natural and manipulated hybridisation in the genus Eucalyptus L’Herit.–a review. Australian Journal of Botany 36: 44–61.
- Potts. B.M. & Wiltshire, R.J.E. (1997) Eucalypt genetics and genecology. In Eucalypt Ecology. Eds: J.E. Williams and J.C.Z. Woinarsky. Cambridge University Press, Cambridge.
- Simmons, D. & Parsons, R. F. (1976) Analysis of a hybrid swarm involving Eucalyptus crenulata and E. ovata using leaf oils and morphology. Biochemical Systematics and Ecology 4: 97–101.
- Albrecht, D.E. (1983) Land use case study-Eucalyptus crenulata, Yering. Report to Burnley Horticultural College, Victoria (unpubl.).
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