July 2, 2006

Approaches to Wood Quality: Eucalypts

Filed under: Eucalypts,Forestry Research,Hardwoods,Timber Growing — shem kerr @ 9:02 pm

In the early days of European exploration and settlement of Australia its timber was variously reported “too hard and ponderous for most uses” [James Cook]; “splits and warps” [Arthur Phillip]; “of little use for building either houses or boats” [George Thompson]; “when the gum bled out of the timber it became brittle and fell to pieces”, and ” the worst wood in any country or climate ever produced.” [J White]. Eucalypt wood was used pretty much just for rough slab huts, because the tools on hand weren’t really up to the job of working the hard wood. – The most significant Australian export of the 18th century was 1,8-cineole, or eucalyptus oil.

In the 20th century and on, one of the single largest uses of eucalypt wood in Australia been for pulp -even though the tools and knowledge to process the crop were there, the resourse was undervalued. The majority of eucalypts planted around the world have also been either for pulp or feulwood, ie quick grown high volume low monetary value crops.
In the early 1960s Australian defence scientists finally found a high value use for eucalypt wood in the synthesis of 3,4,5-trimethoxy-phenethylamine or mescaline. Later in that decade, and I hope that it wasn’t influenced by that work, Australian forestry scientists laid out the largest ever long term in-ground trial of Australian hardwoods. In 1998-9 the plug was pulled on the trial and the results were written up by Laurie Cookson in The In-ground Natural Durability of Australian Timbers. A significant finding of the trial was that termite damage only occured on sites where termites were present ( well, I think it’s significant) – and we could say the same about fungi. One could in hindsight say that someone had just spent 30 years learning stuff-all new over and above anecdotal evidence. It’s not quite the case – it was found that the rankings changed for some species depending on the site environment; and it’s this kind of thought provoking science that prods for finding better ways of finding out things. Since then accelerated testing methods have been worked out. What might have taken 30 year or more, can now be done in a matter of weeks by for example introducing bacteria and fungi or insects to the right environment in which to work on the wood. Non-destructive testing of strength of in-ground durability trials has also been developed. And the biology of the building and its site is given more consideration in design specification.

In recent years there has been heightened interest in naturally durable timber due to concerns about the toxicity of timber treatment chemical effects on health and the environment. Eucalypts have been a target for promotion because some have both durability and fast growth. Potential problems arise when enthusiastism obscures fact; when promoters start pushing a species outside of its natural range, or beyond its physiological capabilities;and/or in short rotation crops such as for posts and poles – harvested in as little as 6 to 8 years. Add to these problems the little bit of stretching that’s going on here – with at least one New Zealand promoter labelling a class 3 durable eucalypt ( not to be used in contact with the ground) as class 2 (marginal in ground use). I’m not sure wether we have another snake oil industry here, until I can tell you so.


Probably a good starting point is to look at where, how, and with whom the tree lives when it’s at home, that is: environment, physiology, and ecology. While eucalypts grow over large areas of Australia, there are several hundred ‘species, each has evolved to compete well in a certain sometimes niche environment – as to climate, soil, and neighbours. Some neighbours are bothersome, and either need to be deterred or outgrown. Some eucalypts just grow fast; others take the strategy of chemical deterence, which may takes extra energy and resources; some try a mixed strategy. There may be point when a young tree needs more nutrients than the soil can provide – and transfers elements from the senescing? sapwood further out and up in the tree where it’s needed to power fast growth. This leaves the core heart both wearker and less resistant to rot than heartwood subsequently created around this core. Some growers think that the core has rotted because of age – this is only part of the story – as core wood further up the trunk only a few years younger is of a different nature.

CoreRotInEucalyptusLog CloseupofCoreSmallEndofEucalyptLog

One could put forward the possibility that provision of the missing nutrients would fix this situation. On the other hand it may just encourage faster growth further up and out and create an even larger core. At the present time there is no way of knowing if a short rotation post crop is a viable proposition without chemical treatment. If the sapwood rots off, then the post is lose in the ground. One could peel the sap off before use. Australian research suggests that retention and protection of the sapwood makes a post stronger, as sapwood contributes over half its strength.

Twelve years is probably the lower limit for growing a durable post – and that’s all it’ll probably be: just one post height of duable wood. Perhaps by 15 years the trunk diametre will be large enough to cut into quarter rounds with the sap and core-heart removed; and you might get 3 post height’s worth of durability out of the tree. But why not go the extra 5 to 10 years or even futher and have the option of a higher value sawn or cut timber log? Then take your lower value post material from the short lengths between the branches on the upper logs – and get 2 crops. The more lengthy crop has a higher proportion of its mass useable as timber ( or posts ); has a higher percentage as stronger more durable heartwood.

Sometime soon I’m going to need to do some economic analyses on different

regimes and species. This post could also do with some editing, but I’d prefer to get it out first. There are a whole lot of references here under the page: Special Needs List, Solid timber, Eucalypts.



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