How wood varies in density among various kinds of trees 

(writing in progress)

A remarkable variation in Nature is the variation in wood density, from one kind of tree to another. Some trees have balsa-light wood while others have wood so heavy that, even when dry, it sinks in water.

Please see https://is.mendelu.cz/eknihovna/opory/zobraz_cast.pl?cast=19370.
 
Since all trees use their wood mainly as a skeletal material, designed to hold up the stems despite gravity, it is puzzling that wood should vary so greatly in density.

In this Post, I would like to establish some basic facts, in what can be a confusing topic.
 
Wood density can be confusing for various reasons, among which are the following.
 
Firstly, when the tree is alive its wood consists of both cell-wall (consisting of a combination of cellulose and lignin) and water, with some air-spaces. Because extremely loosely configured wood, such as that of palms, tends to be full of water in life, the living density of the stem may be great even in ‘soft-wooded’ trees.

If a palm, in a garden, falls on to a house, it can do great damage despite the ‘softness’ of its wood, simply because the stem is largely water and water is heavy. Just because an ebony has dense wood and a palm has the opposite of dense wood, does not mean that the weights of similar volumes of wood in ebony and palm are much different.

So, when we discuss wood densities we usually refer to the wood dried out to a large extent, as in the context of lumber/timber/woodwork.
 
Secondly, the density of wood can be measured fresh, air-dry or oven-dry. Because the air-dry measurement (in which there is still some water in the wood but relatively little water) is by far the most convenient to measure, it is air-dry density that one sees cited most frequently.
 
Thirdly, wood density varies from one part of the tree to another. For example, anyone who has ever sawn through eucalypts in his/her garden may realise that to saw through a mere sapling gives a different impression of wood density from sawing through the main bole.

This is perhaps analogous to the difference between sawing through cartilage and sawing through fully-formed bone, with the dense heartwood perhaps analogous to the extremely dense bone found in the jawbones (mandibles) of ungulates, which are do dense that even hyenas tend to spurn them as food.
 
Here I would like to present some basic information in terms of the oven-dry densities of the wood of various familiar genera of trees in the Northern Hemisphere, based on the website below. These can serve as a basis for comparison of Australian trees, which tend to have far denser wood in general.
 
Poplar (Populus) has extremely light wood, with an oven-dry density of only 0.43 tonnes/cubic metre.
 
The same is true for spruce (Picea).
 
Pinus and Tilia have somewhat denser wood with values around 0.47.
 
Larch (Larix), oak (Quercus), maple (Acer), beech (Fagus) and birch (Betula) have considerably denser wood than poplars, spruces and pines, with oven-dry wood densities of 0.60-0.65.
 
Robinia (which is common as a garden tree in Perth) has a far denser wood again, with a value of 0.76. Robinia has a dense wood comparable with that of eucalypts.
 
Now, the cell-wall material of wood has a density of about 1.5 tonnes/cubic metre. If one can imagine wood in which there are no air-cavities at all, i.e. a solid mass of wood analogous to a solid mass of plastic, then the density of the stuff would be about one-and-a-half-fold that of water.

Wood substance, per se, is heavy stuff indeed, and it is merely the presence of various proportions of air cavities in the structure of the wood that gives wood its variation in oven-dry density.
 
So, if one were hypothetically to expel all the air from the wood of a poplar (which would of course involve great shrinkage) you would wind up with a substance nearly four-fold denser than the real oven-dry wood. Another way of saying this is that, in oven-dry wood of poplar, the wood is nearly three-quarters air.

I do not have the figures for balsa wood. However, I would guess that in the case of balsa wood the oven-dry wood would be perhaps four-fifths air. Contrast this with dense wood such as that of Robinia or many eucalypts, in which the oven-dry wood would contain only perhaps half air.
 
In my forthcoming Posts about wood density, I will be citing air-dry densities, not oven-dry densities, so the values will be greater than those mentioned above. However, the framework I’ve just presented should serve as a good foundation for subsequent interpretations.
 Perhaps I can explain wood density as follows, to make it easy for anyone to see the biological interest in the topic.
 
Imagine that a tree grows and then dies. Its wood dries out, and this produces a log.
 
Now, the chemical nature of wood, as a combination of cellulose and lignin plus some mineral matter such as calcium and silica, is such that if this log contained no air spaces at all the density of the log would be 1.5 tonnes per cubic metre.

The chemical substance of wood, which is a kind of complex polymer analogous to a biological plastic, is so dense that it is 1.5-fold heavier than water.
 
Wood is heavy stuff, and the only reason why real logs are actually much lighter (in the air-dry condition) than portrayed above is that in real life all logs, without exception, contain lots of air in the tiny cavities among the fibres of cellulose glued together with lignin.

Wood is naturally ‘spongy’ in the sense that its fibrous structure is automatically grown with much space among the fibres, and as the dead wood dries out the space loses the water filling the wood during life, producing in a dry log a structure of heavy material lightened up with an aerial component.
 
A log of 1.5 tonnes per cubic metre is so much denser than water that, even if there is much air in the porous spaces of the air-dry log, the log could still sink in water. It is only when the air-dry wood contains very much air that it actually floats in water.

Most species of trees do have air-dry logs that float in water, because most species of trees do indeed have wood so full of air cavities that the density of the actual woody substance is ‘outweighed’ by the buoyancy of the numerous air cavities.
 
In the case of the densest woods, such as tuart (Eucalyptus gomphocephala) or sugar gum (Eucalyptus cladocalyx), the air content of the air-dry wood is limited enough that the air-dry wood still sinks in water. The values for air-dry wood density in such cases is >1 tonne per cubic metre of wood. There is simply not enough air cavities in the fibrous structure of the wood of these dense-wooded eucalypts to buoy them up in water.
 
However, most taxa of trees, including many eucalypts, have enough air cavities in the wood that the oven-dry logs would float in water. And the most porous forms of wood, such as poplar and balsa, have so much air that the wood floats high in the water, a bit like cork.
 
Although all eucalypts have dense wood relative to Virgilia, a hint of analogous variation can be discerned within eucalypts, exemplified by the main two species in southwestern Australia, jarrah (Eucalyptus marginata) and karri (Eucalyptus diversicolor). These two species belong in the same genus even after the splitting of the old Eucalyptus into several genera. So they are closely related, and indeed their timbers can easily be confused based on appearance.
 
Jarrah dominates forests with a heathy understorey on extremely nutrient-poor soils, whereas karri replaces jarrah on somewhat richer soils (although still poor in e.g. Zn) under copious rainfall, where the forest is even taller. There is a difference in the life history strategies w.r.t. fire: jarrah tends to survive fire and resprout epicormically, whereas karri tends to die in the fire and regenerate germinatively, not vegetatively as in jarrah. So it makes sense that karri would have the more perishable wood: after all the new generation of saplings depends on recycling of the nutrients tied up in the wood of its parents’ generation.
 
It is not that jarrah is a particularly long-lived tree for a species that can reach 40m high; it lives only <500 years. However, jarrah does tend to survive most of the fires it experiences, whereas karri tends to die in the first fire it experiences (partly owing to the frequency of mild fires in the jarrah understorey).
 
True enough, the timbers of these two species conform to predictions. Jarrah is a termite-resistant wood, whereas karri wood is notorious for attracting termites. It’s hard to say whether karri wood is eaten by termites merely because it lacks chemical defences against termites, or because it actually contains something attractive to termites. However, the point I’d like to make here is that karri wood is certainly regarded by foresters and timber workers as more perishable than jarrah wood DESPITE BEING DENSER. Karri (which is not a softwood by any definition) is rapidly consumed by termites, presumably in concert with fungi. Karri wood almost seems to serve as ‘bait’ for termites whereas jarrah is so durable that, after being used for decades as sleepers or whatever, it can in some cases be redeployed for furniture.
 
The air-dry wood density of jarrah is 0.8, whereas the corresponding figure for karri is 0.9. Counterintuitively, karri is the denser wood. At 0.9 karri is a dense wood by any standards, a remarkable fact given that this tree can grow to >80m high and grows at the typical plantation-eucalypt rate of about 1m per year, decade after decade.
 
[As a side-note: I see that the ‘green’ wood densities of jarrah and karri are respectively 1.1 and 1.2. This means that the combination of water and wood in the living trunks does not merely add up to a density of 1, as I’ve speculated in emails to you in the past on the topic of variation among trees in wood density. These eucalypts have dry woods approaching the wood densities needed to sink in water, and in addition they contain enough water, when the tree is alive, that the ‘green’ trunks (most of the wood is of course dead, but it’s still moist) would certainly sink in water! That’s real food for thought because it means that there’s not simply a range of proportions, among trees, in which wood substitutes for water, e.g. palms and eucalypts both having ‘green’ density of 1 but the palms containing mainly water and the eucalypts containing mainly wood. Instead, eucalypts actually ‘overshoot’ in their wood composition, so that the living tree has a trunk that sinks in water. Since another eucalypt found in the same area of southwestern Australia,  tuart (Eucalyptus gomphocephala), has air-dry wood density of 1.03 and ‘green’ wood density of 1.25, you can see that eucalypts actually ‘overpack’ their trunks with lignin, being truly ‘superhard’ woods in that sense.]
 
The bottom line is that karri, despite being a truly hard wood, is susceptible enough to destruction by termites that it is poorly regarded as a timber, and too often just reduced to woodchips. This palatability to the degraders of wood is surprising from the point of view of a) its hardness and b) the termite-resistance of many, many other ‘eucalypts’ sensu lato. However, it is unsurprising from the point of view of a) the need for the dead trees to rot rapidly to make way for the new cohort after fire (most trunks are not combusted although the trees are killed) and b) the well-known phenomenon in which other eucalypts, also with hard wood, are hollowed out in the living state by termites such as Coptotermes (which encourage fungal attack).
 
Although I haven’t read of Virgilia being attacked by termites (of which there are few species in its habitat), it’s certainly attacked by a remarkable guild of other wood-consuming insects and fungi. But then again, Virgilia wood is remarkably lacking in density; there are no figures available because nobody would dream of treating Virgilia as timber, but my guess is air-dry density of about 0.3. So, when one corrects for density differences, one could almost take the view that karri, and other pyrophilic eucalypts of the same ecological strategy, is at least as inviting of consumption/degradation/rotting by insects and fungi as Virgilia is?
 
But the question remains: why does karri make its wood so dense in the first place; why does it ‘waste’ so much energy on wood? Why do eucalypts and their relatives tend to be ‘superhard’?
 
And I can think of another wonderful question arising from all this: if one cuts a section of Virgilia wood from the living tree, i.e. a length of ‘green’ wood, what is its green density as determined by placing the cut length of wood in water. Is the wood extremely buoyant, which would indicate that whatever water present in the soft wood still leaves much empty space, i.e. a ‘green’ wood density of <0.6? Or is there so much water in the ‘green’ wood of Virgilia that the living trunk, albeit not ‘hyperdense’ as in the case of eucalypts, approaches the same density as water (i.e. a ‘green’ density value approaching 1, and consisting of say 3 parts mass of wood and 6 parts mass of water?
 
If Virgilia wood in the living state also balsa-like, or is it just that most of the ‘green’ wood is water and when the wood dries out there’s just a flimsy fibrous structure remaining?
  
The questions that have preoccupied me for decades now are:

• why would taxa of trees vary so greatly in their wood density?
• what is the biological value of dense wood as opposed to non-dense wood, and what are the ecological patterns in the distribution of wood densities in the vegetation on Earth?
• what use is wood density to trees, once the basic structural requirements of wood as the material of stems are met?
• what are the adaptive advantages in particular of dense wood, which is energetically more expensive than non-dense wood?
• why do all trees on Earth just have the least expensive wood consistent with the requirement that their stems hold up against gravity and wind? 

(writing in progress)

Publicado el julio 30, 2022 10:15 TARDE por milewski