Eucalyptus species with biomass potential for the British Isles

1. Seed sources

   • Most eucalyptus species are very variable in nature. Much of this variation is genetic, and this has great impact on growth. This presents opportunities, but also great uncertainties. Thus, the seed provenance of a particular species can influence cold tolerance, vigour and form of trees, often very markedly. Furthermore, growth in the wild is frequently a poor predictor of growth in planted stands. Thus, growth of trials and of specimen trees can only be interpreted with knowledge and understanding of the provenance of the seed used. Poor growth in past plantings is not necessarily an indicator of the potential of a species. Good growth may only be reproducible with plants from genetically-similar seed.

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2. Site preparation and subsequent management

   • This is a huge topic, much investigated elsewhere in the world. At this stage, detailed prescriptions for the British Isles are not possible. It is generally recognized that eucalypts are very susceptible to weed competition before they achieve a closed canopy. Therefore, site preparation and early establishment practices should focus on weed control. The practicalities of this are potentially costly, and compromises between achieving good early growth and the costs of weed control will probably need to be accepted. There are examples where very good early growth has been achieved without weed control, but these tend to be in high rainfall areas, notably in Ireland. See also the description of Eucalyptus nitens in Spain

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3. Planting

   • In the British Isles, eucalypts are normally planted in spring, ideally as soon as the risk of severe frost has passed. Planting stock is normally supplied as cell-grown plants. The plants can be unusually small, often only 15-20cm tall. This is the normal size of eucalyptus planting stock for forestry elsewhere in the world, although it is small compared to the forest nursery stock familiar in the UK. Such plants can establish very quickly, but this requires good weed control in the months following planting.
   • Because of limited and erratic demand at present, planting stock normally has to be grown to order.

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4. Wet sites, exposed sites and upland sites

   • The faster-growing eucalyptus species perform best on fertile, well-drained, lowland soils that are neutral or slightly acidic. Growth will be less good on other site types, and many species dislike very wet conditions. Several species (Eucalyptus dalrympleana, Eucalyptus macarthurii, Eucalyptus parvula) are said to perform well on alkaline soils, though this does not seem to have been tested critically. The species known to survive on difficult cold sites (e.g. some of the snow gums) tend to have very poor form and to grow slowly on such sites. However, some of the swamp gums and a few other species may have potential for biomass production on wetter lowland sites, and these deserve to be trialled on a small scale. Candidate species include Eucalyptus rodwayi, Eucalyptus aggregata, Eucalyptus camphora, and Eucalyptus stellulata. Certain other species appear better-suited for very exposed sites, including sites near coasts.

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5. Cold-hardiness

   • The cold-hardiness of eucalypts is a complex subject which is still not fully understood. In general, exposure to lower temperatures during autumn induces frost hardiness in the shoots. The degree of frost hardiness achievable varies greatly with species and seed provenance. Such hardiness is fairly well understood for the commoner species. However, unseasonal sudden frosts in autumn or spring can cause considerable damage to plants that are not fully hardened, and may kill young plants. Eucalypts growing in locations that are normally very mild in winter are also at risk of damage by sudden frosts.
   • There is one well-documented instance in Scotland of deeply-frozen soil killing young eucalypts. The symptoms suggested that the effect was essentially due to severe drought stress, because the roots were unable to take up water. Some damage to recent eucalyptus plantings on light soils in the UK may have been caused by unusually low soil temperatures.

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6. Protection from mammals

   • In general, eucalypts are unpalatable to browsing mammals, including rabbits and deer. The relative palatabilities of a range of species to deer have been documented in the USA¹. The notable exception to this unpalatabilty is Eucalyptus gunnii, which is browsed by many species. In the UK, species other than E. gunnii generally appear not to need protection from browsing, provided alternative forage is available for browsing mammals. In north-west Spain, Eucalyptus nitens plantations frequently are not given protection of any sort, despite high populations of deer (see Eucalyptus nitens in Spain).  The extreme unpalatability of Eucalyptus glaucescens has been confirmed in a planting in West Sussex in 2007.
   • Grey squirrels do not appear to damage eucalypts in Britain.

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7. Coppicing

   • Most eucalyptus species coppice freely, though there are exceptions. Coppicing offers the option of subsequent rotations, without re-planting costs. While this is superficially attractive, it may not be the most appropriate or cost-effective option in all situations. Additionally, it may not be appropriate for certain types of biomass fuel.

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The biomass potential of some eucalyptus species in the British Isles

The following species are ones that have been planted in biomass trials in the UK in recent years, or which have potential for biomass production based on evidence from plantings in the UK and elsewhere. Mention of a species does not mean an unqualified recommendation. Non-inclusion of a species does not mean that it may not be an appropriate one for certain locations. It would be prudent to plant several species at any new location, in order to be able to identify the most appropriate species for subsequent plantings.

Almost all the hardy eucalypts are vigorous, compared to other hardwoods in Britain. The comments on vigour of individual species refer to vigour relative to other eucalypts.

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• Eucalyptus gunnii

  • Established trees appear hardy throughout the British Isles. The species has moderate vigour and variable form. It can grow well on wet sites. Clonal selections grown for pulpwood in SW France have very good form, but yields are similar to those in the UK. It coppices freely. The species has a reputation for instability, though this has not been critically assessed. It is the most palatable species for browsing mammals. The wood appears to be of moderate basic density (approx 500 kg/m³) at age 15 years. Its logs have a reputation for being slow to dry.

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• Eucalyptus nitens

  • Exceptionally vigorous, potentially producing very large trees. Good form, though it tends to retain dead branches for some years. It has a reputation for being windfirm once established, though young trees can be unstable. Established trees are hardy to approx -12^oC. Coppicing ability depends on provenance and declines with age; 10 year stumps are unlikely to coppice well. Not normally browsed. The wood is of fairly low basic density for a hardwood, typically 470 kg/m³ at 10 years. Logs have a good reputation as fuelwood, and dry readily.

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• Eucalyptus dalrympleana

  • Vigorous species with excellent form; self-pruning. Many higher-elevation provenances likely to be more hardy than E. nitens. Stands in NE Spain were undamaged by frosts to -13.5^oC, and many trees withstood -17^oC without damage. Coppices well. Said to grow well on alkaline soils, as well as neutral and acid ones. Defined seed sources deserve further trialling.

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• Eucalyptus denticulata

  • Closely related to E. nitens, and formerly known as the Errinundra provenance of E. nitens. Slightly slower-growing and possibly slightly less hardy than true nitens, but reportedly has higher wood density than nitens at age 8 years². In contrast to nitens, it appears to coppice freely. Exceptionally windfirm on very thin mountain soils in Portugal. There is unlikely to be much provenance variation in this species.

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• Eucalyptus glaucescens

  • Related to E. gunnii, but more vigorous and highly unpalatable as browse. Most trees have very good form, and are self-pruning. Established trees probably hardy to -16^oC. Mostly planted for cut foliage and as a specimen in collections, but it deserves to be evaluated for its biomass potential. Appears to coppice well. There are some indications that provenances differ in vigour, form and hardiness.

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• Eucalyptus subcrenulata

  • Generally hardy and of good form. Provenances with higher vigour are likely to have lower cold-hardiness. Very unpalatable to deer. Coppices well. Not widely-planted but defined provenances deserve trialling.

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• Eucalyptus urnigera

  • Related to E. gunnii, but it has better form and is somewhat less palatable. There appears to be considerable variation in vigour and cold-hardiness between different seed provenances. Some trees planted in the UK in recent years appear to be natural hybrids of E. urnigera with E. gunnii, produced from seed collected in Scotland; this hybrid seed is also produced in New Zealand.