Eucalyptus macarthurii / Eucalipto macarthurii / Eucalipto de Macarthur / Eucalipto peludo / Camden Woollybutt / Paddy's River Box / Cold Hardy Eucalyptus / Eucaliptos resistentes a las heladas
Eucalyptus macarthurii . Camden Woollybutt Highland Forests in Northwestern Iberia
Eucalipto de Macarthur. Bosques cultivados colinos de Eucalipto Lanudo de Camden en el Noroeste Ibérico
Following the Tasmanian Blue Gum (Eucalyptus globulus ssp. globulus) in our journey from sea level to 1700 meters elevation we walked in a transect across the foggy Eucalyptus belt in the Atlantic and Cantabrian coasts of Northwestern Iberia to reach a first stop in the inland plains and low hills of the Galician Highlands. Here E. globulus, affected by adverse winters, no longer prevails. A series of cold hardy Eucalyptus species starts to become an important element of rural landscape after recurring species trials for the last 50 years. Among these, several Tasmanian Oak species and several Eucalyptus species from higher elevations in the ranges of South Eastern Australia and the Tasmanian highlands. The Shining Gum tree is one of these. But today we stop at small forests of another species, Eucalyptus macarthurii.

The Eucalyptus of Sir William Macarthur can reach up to 40 meters height when grown at forestry plantings on good sites, or remain a shorter tree of wider crown and retaining basal branches when grown as isolated specimen (20-30 m). Its juvenile leaves are ovate-lanceolate, sessile and opposite on round branchlets. Adult leaves are alternate, long and falcate, flexible, concolorous on both sides and very fragrant. Its bark can be quite variable depending on the season and tree age, from almost black due to water absorption to light grey when dry. It is fibrous, woolly, and normally persistant up to three quarters height, fitting its vernacular name Camden Woollybutt. Tree tops show witish soft bark and some patches of grey, while some bark strings can hang up on main upper branches. Flowers and capsules are small and in sevens.

Also known as Paddy's River Box, it was introduced to experimental arboreta in the Iberian Peninsula by the 1950's and from here jumped to small trials and small commercial plantings from the 1970's on. Its quite good cold hardiness (resisting down to -10ºC and more than 80 frost events per year) and its average growth rates, made of it one of the alternative species to E. globulus for inland areas or beyond 500 meters above sea level before the widespread planting of E. nitens. It has medium potential as a resource for cellulosic pulp altough its pulping properties are inferior to E. globulus and E. nitens and this makes it little favoured by local pulp mills, configured to process E. globulus only. Its timber can range from pale to reddish and could find alternative applications for board production, or due to its natural colour for solid wood if clearwood from big diameter logs grown at longer rotations and subject to special silviculture was produced. It could also be a candidate species for biomass production targeted to green energy generation. In the Iberian Northwest this species currently reaches annual increments in the range of 15 to 25 cubic meters of wood per hectare and year.

Its natural habitat is quite restricted to the Blue Mountains and Southern Highland Tablelands of New South Wales (Australia) and is recognised as a vulnerable species today. Here it stands 20 to 40 frost events per year, minimum temperatures down to -7ºC and ocassional snowfalls, also receiving 700 to 1000 mm of rainfall per year.

Widely cultivated in South Africa, where it ranks among the cold hardiest species, and also in New Zealand, where other species have been favoured over this one along time. Tested for forestry in Northern and Southern Spain and in Portugal too, it seems a quite plastic species for both acidic and alkaline soils, growing better on non sandy but somewhat deep and clayish ones. Also tested in many other temperate climates in the world. Studies conducted in the 1980's by Pezeshki and the University of Washington (USA) at Lee Experimental Forest (Snohomish County) proved its cold hardiness reaches its limits around -14ºC, but can coppice after that.



Photo 1 (left): +20 year old specimen tree of Eucalyptus macarthurii in a small yard grove at 550 meters height in the Highlands of Galicia (Northwestern Spain). They have reached 25 meters height and diameters up to 80 cm and resisted -10ºC well.

A quite common species in the tablelands and low hills of Central Galicia, it is normally cultivated in small patches both on forestlands and abandoned agricultural lands. Typical silviculture applied to the species is minimal, starting with quite dense stocking per hectare and relying on coppice as main regeneration strategy after harvest. However, lack of proper management on its very vigorous coppice regrowth (as shoot selection to obtain one new trunk per stump) often gives these small forests a messy jungle-like outlook. Sadly, this will also affect negatively the number of future useful logs having a minimal diameter for industrial processing, which can compromise financial viability of this crop in the long term. Properly planned plantings done at lower stocking per hectare and using an additional non coppicer interplanted foster species (non coppicing eucalypts, fast growing conifers or fast growing broadleaved trees) could yield interesting multiproducts. If the woollybutts are also pruned early and the foster species thinned at mid rotation (10 to 15 years), interesting fat logs could be grown in medium rotations (20 to 30 years).


Photo 2 (top left): Dense small forestry plantation resulting from unmanaged coppice of E. macarthurii after a third harvest cycle done on an initially highly sotcked planting (1600 trees/ha) at 500 m elevation. Vigorous coppice assures low casualty rates in remaining stumps, but requires an early shoot selection to produce sizeable new trunks. Lower stocking per hectare should have been preffered at initial planting time.

Photo 3 (bottom left): Natural regeneration from seed can easily happen with E. macarthurii if circumstances favour abundance of light and water, proper temperatures and absence of competence for light in the first months. The photo depicts new shoots on a coppiced E. globulus stump (left) and two feral seedlings of E. macarthurii (right and foreground) resulting from natural regeneration in a mildly frost prone area at 450 m elevation.

Photo 4 (right): Proper planting of E. macarthurii for small diameter timber production at 600 m elevation. Forestry stocking favours competence for light among the trees and they discard their lower branches as photosynthetic efficiency declines. Early pruning operations are needed on yet alive branches if any clearwood is to be produced, but this would be unnecessary for targets as pure woody biomass production for green energy generation. The depicted plantation is not a mixed species one, so thining would require control of new shoots to avoid unnecessary competence for those trees to be harvested at the end of cycle.

A prolific flowering species, Eucalyptus macarthurii quickly releases seed if timber harvest is done on mature trees loaded with ripe capsules in the wrong season of the year and if non woody biomass is left after log extraction on bare soil without finely and timely crushing and mixing it with soil as an organic fertiliser for a further timber crop. If natural regeneration from seed happened and the new grove was left unmanaged a very thick closed forest would develop, which could force early thining and control of stumps with a suitable herbicide to avoid further regrowth if the target is getting any useful log of reasonable diameter. This is not always cost effective and hence a potential problem. However, this same apparently negative scheme could be greatly positive if the species was used for phytorremediation projects as a green filter, being planted or sown directly on well prepared soils to form thick protective barriers. This way the interlocked root systems could retain dangerous waters (urban sewage, mining effluent leakages, heavy metal polluted waters) for a reasonable long time avoiding clean water contamination, and these fast growing trees could store toxic substances in biomass for a noticeable amount of time.

An avid Eucalyptus pioneer in Spain, Forestry Engineer Manuel Martínez Bolaños wrote in the 1950's an interesting piece on E. macarthurii: "Market values for this perfumary oil are four times higher than E. citriodora and twelve times higher than E. globulus" . No doubt, essential oil markets have evolved quite a bit since then and harvest, distillation and purification costs of this extra product from this species make it a viable commercial initiative just in appropriate circumstances.

However, it is true this species is distinctive from others in what regards to essential oils. Distinct from 1,8-cineole (typical oil component of E. globulus or E. polybractea and also known as eucalyptol) and citronellal (typical of C. citriodora and also known as citral), E. macarthurii yields geraniol (both geranyl acetate up to 75% also and free geraniol) in amounts of crude oil in the order of 0.5 to 1.3% of fresh harvested weight. A very variable trait, select strains could yield more.

Not only its leaves are aromatic. Geraniol is also present in E. macarthurii inner bark, which makes timber harvest for this species quite attractive, as debarking floods the air with an intense fragrance similar to jasmin. This feat, be it at timber harvest time or when inspecting standing trees, can sometimes aid to more accurately identify this species, or, within this species, high oil yielding specimens that could be screened for further laboratory tests on its essential oil richness.


Photo 5 (left): Juvenile leaves of E. macarthurii, a species rich in geraniol oil


© 2007 Gustavo Iglesias Trabado / GIT Forestry Consulting - Consultoría y Servicios de Ingeniería Agroforestal