Information on wood density in the mimosoid legume shrub, Dichrostachys cinerea

@tonyrebelo @jeremygilmore @wynand_uys @troos @botaneek @graham_g @richardgill @jan-hendrik @ludwig_muller

An article by Fernandez et al. (2015) presents values for the wood density of Dichrostachys cinerea (https://en.wikipedia.org/wiki/Dichrostachys_cinerea).

https://www.sciencedirect.com/science/article/abs/pii/S0961953415300763
https://www.researchgate.net/publication/281310025_Sickle_bush_Dichrostachys_cinerea_L_field_performance_and_physical-chemical_property_assessment_for_energy_purposes
https://pubag.nal.usda.gov/catalog/5362099
https://dial.uclouvain.be/pr/boreal/object/boreal:164968

These were determined on an OVEN-DRY basis, instead of the usual air-dry basis.

Oven-dry values somewhat underestimate the air-dry values. However, the difference should not be much more than 10%, because air-dry wood contains only about 10% water.
 
Stems with diameter <2 cm have oven-dry wood density 820 kg per cubic metre, whereas stems with diameter >2 cm have oven-dry wood density 840 kg per cubic metre.
 
These values are more or less in line with the information I gave in a previous Post.

I have not calculated the likely air-dry values. However, they sound like about 900-940 kg per cubic metre, the former value probably lacking any heartwood, and the latter value including a narrow core of heartwood. (Fernandez et al. 2015 do not mention the question of heartwood at all.)
 
The paper compares wood density for D. cinerea (including bark) with those for various other taxa of trees (including bark). Again, all values are oven-dry.

The comparison is worthwhile, despite being somewhat complicated to relate to data elsewhere.

Values are: Dichrostachys 780, Populus 360, Salix 600, Eucalyptus 540, Allocasuarina 650, Leucaena 430, and Paulownia 270 (all values include bark).
 
My commentary is as follows.
 
Eucalyptus has a pattern, which I noticed many years ago when cutting down saplings in my garden, of having soft wood in the saplings despite the hard wood in the fully-formed mature stems.

This explains why the wood density is only 540 in Eucalyptus compared with 600 in Salix.

This pattern of ‘juvenile softness’ is not so marked in the case of Allocasuarina (https://en.wikipedia.org/wiki/Allocasuarina).

Populus and Paulownia (https://en.wikipedia.org/wiki/Paulownia) are extremely light-wooded trees, growing extremely rapidly, and useful as plantations. This is because the wood, although not dense (wood density air-dry of only about 300-400 kg per cubic metre), is suitable for various objects, and can produced quickly.

On the basis that it is an angiosperm, not a gymnosperm, Paulownia has a reputation as ‘the fastest-growing hardwood’. This genus originates in China, and is grown for its light but durable wood. This is used for boat-building and making surfboards.

The wood density of Dichrostachys is 2.9-fold greater than that of Paulownia.

Salix, although related to Populus, has somewhat denser wood, which is actually denser than that of Sclerocarya and Lannea.

Leucaena, although a legume and thus related to Dichrostachys in that way, has wood far less dense than that of Dichrostachys. In terms of air-dry wood the respective values would be about 470 for Leucaena vs 860 for Dichrostachys (bear in mind that in both cases these values include bark).
 
The bottom line seems to be that Dichrostachys does indeed have dense wood for such a fast-growing, weedy plant, whether compared with poplars and willows, eucalypts and casuarinas, or other legumes.

Indeed Dichrostachys seems to achieve, in mere sticks with diameters of a few cm, the kinds of densities achieved as mature boles in plantation eucalypts.

Of course, many naturalists in South Africa value sticks of Dichrostachys as fuel for barbecues. However, this paper helps to quantify why these sticks make such good 'coals' for roasting meat.
 
Putting the bottom line into an even more meaningful generalisation:

Dichrostachys as a ‘woody weed’ (e.g. in Kruger National Park) has a SHRUBBY growth-form instead of a tree form. However, its great ‘woodiness’ makes up for the thinness of its stems, w.r.t. 'biomass' (which is actually partly necromass) and carbon-content.

Dichrostachys is

  • a rather spindly plant,
  • always multistemmed = shrubby (in an inverted cone shape, rather than having a bole), and
  • limited in height (usually only a few metres, despite its potential as a species of growing to about 7 m high),

However, the amount of wood contained in this plant is surprising.

So, any measurements of stem diameters are only part of quantifying this formidable woodiness, and would not do the plant justice without including a factor for wood density.

Posted on August 03, 2022 04:17 AM by milewski milewski

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