Contrasts in wood density among legumes and other trees in Kruger National Park

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

As I have shown in previous Posts, wood quality varies greatly among the trees in any flora.

For sake of simplicity, I have chosen just one initial reference to cite here, and the oldest possible one: Codd L E W (1951), Trees and shrubs of the Kruger National Park (https://www.abebooks.com/signed/Trees-Shrubs-Kruger-National-Park-Codd/22771974528/bd).

This reference is older than I am. However, it serves my purpose, because it reveals variation in the quality of wood that is so remarkable that it was measured when the botany of Kruger National Park was in its infancy.

I have collated Codd’s remarks about wood by categorising the quality into three categories: hard, medium, and soft. This is elementary, but it illustrates the main points I would like to make here.

The figures refer to specific gravity of the air-dry wood.

DENSE woods (>0.8):

Boscia albitrunca, Vachellia nilotica, Senegalia burkei, Senegalia nigrescens, Dichrostachys cinerea, Colophospermum mopane, Bolusanthus speciosus, Dalbergia melanoxylon, Entandrophragma caudatum, Spirostachys africanus, Pappea capensis, Berchemia discolor, Rhamnus zeyheri, Ziziphus mucronata, Dombeya rotundifolia, Ochna pulchra, Combretum apiculatum, Combretum imberbe, Combretum hereroense, Terminalia prunioides, Terminalia sericea, Sideroxylon inerme, Diospyros mespiliformis, Kigelia pinnata.

MEDIUM woods (0.6-0.8):

Vachellia tortilis, Vachellia sieberiana, Vachellia xanthophloea, Burkea africana, Cassia abbreviata, Peltophorum africanum, Philenoptera violacea, Garcinia livingstonei, Sygygium cordatum, Adina microcephala.

LIGHT woods (<0.6):

Ficus capensis, Faidherbia albida, Vachellia gerrardii, Vachellia robusta, Albizia tanganyicensis, Erythrina lysistemon, Pterocarpus rotundifolius, Kirkia acuminata, Kirkia wilmsii, Trichilia emetica, Euphorbia ingens (‘exceptionally light and tough’), Sclerocarya caffra, Lannea discolor, Adansonia digitata, Sterculia murex, Cussonia spicata, Rauvolfia caffra.

Codd does not mention wood quality for Commiphora (https://www.inaturalist.org/observations?place_id=6986&taxon_id=184094&view=species), probably because its wood is well-known to be so light as to be useless.

My commentary:

Wood quality is one of those aspects of trees and shrubs in Kruger National Park that is hard to observe directly, and almost as hard to infer from the visible features of the taxon in question. There is presumably rhyme and reason in the variation in quality of wood. However, a satisfactory explanation has yet to be thought out by ecologists.

It is tempting to assume that the faster a tree grows, the softer its wood. However, this is at best a partial explanation. It is easy to expect the baobab, effectively a giant succulent, to have soft wood; but euphorbias are more complicated.

Who, beholding a knobthorn (Senegalia nigrescens), marula (Sclerocarya birrea) and sycomore fig (Ficus sycomorus), could anticipate that the acacia has wood so much denser than that of the anacardia, and particularly the fig?

Who, familiar with the appearance of Trichilia emetica (https://www.inaturalist.org/taxa/595643-Trichilia-emetica) and Diospyros mespiliformis (https://www.inaturalist.org/taxa/340214-Diospyros-mespiliformis) as they live side by side on river banks in Kruger National Park, would have guessed that T. emetica has wood so inferior to that of D. mespiliformis?

One of the most interesting findings is that legumes vary so much in quality of wood, even within what was until recently considered a single genus.

To show the range of wood qualities:

Erythrina (https://www.inaturalist.org/observations?place_id=6986&taxon_id=82771&view=species) has exceptionally light wood, whereas Colophospermum mopane has exceptionally dense wood.

Within the Fabaceae, Erythrina contrasts with Dalbergia melanoxylon (https://www.inaturalist.org/taxa/191296-Dalbergia-melanoxylon).

Within the caesalpinioid legumes, Peltophorum africanum (https://www.inaturalist.org/taxa/138688-Peltophorum-africanum) has far lighter wood than that of C. mopane.

Within the mimosoid legumes, we have a remarkable variation among what all look to the untrained eye like mere ‘acacias’.

Albizia (https://www.inaturalist.org/observations?place_id=6986&taxon_id=47451&view=species) has soft wood, whereas spiny acacias tend to have hard wood. Dichrostachys (https://www.inaturalist.org/taxa/129706-Dichrostachys-cinerea), despite being a virtual weed, has hard wood, whereas the tall tree Faidherbia albida (https://www.inaturalist.org/taxa/343039-Faidherbia-albida) has soft wood. Vachellia robusta (https://www.inaturalist.org/taxa/559229-Vachellia-robusta) has far softer wood than Senegalia burkei.

Acacias cannot be said, at least in Africa, to have a typical density of wood: it depends on the genus and on the species.

When it comes to the symbiosis between acacias (in the loose sense) and large mammals, the patterns are also puzzling.

Senegalia spp., Vachellia spp., Faidherbia sp., and Dichrostachys sp. all support large herbivores in various ways, using generosity rather than deterrence. Yet, their qualities of wood vary greatly. The last three genera all produce ‘carob’ pods, offering food to herbivores in return for dispersal and sowing of their seeds. And it has been suggested that at least one species of Senegalia is pollinated by the giraffe.

And yet these plants vary greatly in quality of wood, with Dichrostachys having exceptionally dense wood despite never growing to tree form, while Vachellia tortilis (https://www.inaturalist.org/taxa/489563-Vachellia-tortilis), so well-adapted to damage by the African savanna elephant (Loxodonta africana), has medium-density wood.

There are few data for acacias in Dale and Greenway, but note the difference between Vachellia sieberiana at only 0.7 and V. lahai at 1.25.

Combretaceae (Combretum and Terminalia) all seem to exceed unity in this parameter, which means that, even when air-dried, their wood will sink in water.

Entandrophragma (https://www.inaturalist.org/observations?place_id=any&taxon_id=183572&view=species) is the genus to which the tallest of all African trees belongs. It is noteworthy that the values for wood density given by the two sources above are so inconsistent..

Again, note the difference between Trichilia (<0.6) and Diospyros mespiliformis (>0.8).

Spirostachys africana forms a handy benchmark for the mind because its wood density is unity – just at the threshold of sinking in water in the air-dry condition.

Balanites maughamii is a tough-looking tree, with structural defences against the African savanna elephant. However, its wood has medium density, no greater than that of a savanna protea, Faurea saligna (which occurs near Numbi Gate in Kruger National Park).

The incongruous-looking Salix along the Sabie River, which shoots spring foliage as if living in the Northern Hemisphere, has a fairly typical wood density for a willow, of about 0.55. However, note that at least some spp. of albizias have wood similarly light despite looking like non-spinescent acacias.

What emerges are the following handy benchmarks:

Think of marula (Sclerocarya) as a neat 0.5, and tamboeti (Spirostachys) as a neat 1.0, in air-dry specific gravity of wood.

The other taxa can be considered relative to these standards, and they certainly do vary greatly, in ways that remain to be fully explained.

Publicado el julio 31, 2022 10:59 MAÑANA por

milewski

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https://www.krugerpark.co.za/Kruger_Park_Facts-travel/explore-kruger-vegetation-trees.html

Publicado por milewski hace casi 2 años

But does density actually mean anything?
Are the important parameters not conductance:
.* the ratio of vessel empty tubes (and critically their size) to structural support;
.* the amount of medullary rays capable of lateral water transport (do they do any secondary modification if they contain live cells?);
.* the shearing strength at branches;
.* the weight of canopy supported by the stems;
.* the strategies for preventing xylem collapse, embolism and recovering vessel use under water stress.

Mention is made of elephants: but what about longhorn beetles, termites and fungi - wood needs to accommodate these as well.
There is no inherent reason why fast growing plants cannot change their wood density in older stems.
Also, is only the outer rings of xylem used (it is for water transport)?: it is hard to believe that the inner wood is purely discarded by trees - surely it serves at least a function (structural support is also mainly the outermost layers of wood).
There will also be a dichotomy in function between wood itself and bark: e.g. fire or termite protection is more likely to be by bark

Basically wood anatomy is still a black box. For instance, I tried to find out the possible adaptive significance of Protea wood - the "Silky Oak" diagnostics caused by the high ratio of medullary rays that requires the vessels to weave sinuously through the wood (seen in external view). It is probably this that makes the wood porous and useless for buckets. But what it means for and to the plants in terms of water conductance and structural support is impossible to evaluate.

With wood having so many disparate selection pressures, it is hardly likely that any explanations based largely on wood density will be forthcoming.
((PS: is it my imagination or are Grey Squirrels far heavier (i.e. denser) than other animals (mammals: obviously birds dont count) of the same size? And might this apply also to other animals that might regularly fall out of trees?))

Publicado por tonyrebelo hace casi 2 años

@tonyrebelo @wynand_uys

Many thanks for your comments.

Perhaps I can begin to answer two of the points you raised, viz. what is the main significance of wood density, and how much of the wood is functional other than for sheer support.

The main reason why the density of wood is ecologically meaningful is that dense wood seems to represent a remarkable waste of photosynthate.

If a tree can support itself by means of light wood (which is largely hydrated in the living specimen, despite almost all the wood being dead = metabolically defunct), then any extra density would seem to mean wasted energy.

Many trees, with dense wood, seem to treat their boles as 'dumps' for energy. Note that there is no physiological mechanism for retrieving energy once it is allocated to wood. The energy can only be released by decay or combustion, in which case the tree that produced the energy has forfeited it anyway.

So, dense wood is a 'one-way sink' of remarkably large amounts of biologically fixed energy.

This raises the obvious questions of a) why make that photosynthate in the first place, and/or b) why not allocate that photosynthate to more useful tissues, where the investment can bring a return?

This suggests that trees with dense wood are receiving some benefit from the density. But what could that benefit be?

Turning now to the question of how much of the sapwood is functional for drawing water upwards:

The bole of a tree can be divided into metabolising tissues (cambium and phloem), dead tissues (bark and heartwood), and non-metabolising tissues that remain conductive of water (outer sapwood). The latter has lost its organelles and mitochondria, so it is technically necromass, not biomass,

This means that possibly 90% of the mass of a standing, photosynthesising tree is necromass, not biomass.

What tends to be overlooked is that the functional sapwood tends to be only a narrow outer layer of sapwood (see https://www.canstockphoto.com/sap-on-cut-tree-6395462.html and https://www.istockphoto.com/photo/freshly-cut-tree-trunk-showing-lines-of-sap-gm153824009-20109937).

What this means is that much/most of the mass of the bole tends to consist of a peculiar kind of material: hydrated, intact, undegraded xylem that is both non-metabolic (i.e. dead) and non-conductive. Its sole function seems to be support.

So it is actually this stuff, which does not seem to have received a satisfactory name but which I could call 'non-conductive sapwood', that poses the main puzzle in terms of the variation in wood density among species.

Why would a combretum make this non-conductive sapwood several-fold more expensive of photosynthetic energy than a fig does?

Is photosynthetic energy really a cost? If you have the leaves, then carbon may well be a waste product. Yes, you need to phytosynthesize, but there may be many other factors in play: having enough leaves may well be served by making many more for defoliation by insects and frost, and leaves critical for spring growth may be superfluous in summer, but still functional. So it may be that some plants produce a surplus of carbon, seasonally or temporarily, that might as well be used for something, rather than just shutting down the leaves.
Especially where carbon is needed to sequester nitrogen or minerals from the soil, the rate limit may well be the amount of protein that can be made, in an "energy flooded" system. So carbon may well be a "waste product" that plants can use to increase competitive ability, or survivabilty. Protein is needed for seeds, but flowers and fruit can be loaded with free carbon to be bigger, or more, or better. So some plants can afford to spend 30% of their carbon resources purely on flowering: it is not a cost - just the system in idle as critical limiting elements are mined.

An analogous system, would be labour for vines. they (in sufficient numbers) are needed for pruning in winter, and harvesting grapes in autumn, but they need to be maintained all year: they might as well be used for something, rather than just left idle (or discarded, and regrown: but even then, after "harvesting" they are there - why discard them before their maintenance exceeds resources?).

Of course, everything needs to be taken in context. An excess of carbon might be used for wood in Combretum and fruit in Figs.

I agree that, under certain conditions, photosynthate can be so cheap that it is, to all intents and purposes, 'dumped' by the plant. Eucalypts exemplify this, because they photosynthesise so rapidly, relative to their rate of acquisition of nutrients, that they manage to combine rapid growth with dense wood.

(Bear in mind that this is not as cheap as it may at first seem, because eucalypts demand transpirational water at a rate unsustainable in many environments.)

However, if we accept this principle of 'photosynthetic affluence', then the question shifts to: why/how is a combretum more energy-affluent, in the same environment, than a fig?

And I think you may already have hinted at an answer:

The life history strategy of the fig is that it has chosen to allocate 'surplus' energy not to wood but instead to fruit-pulp. This makes fig trees 'fountains of life' for guilds of frugivores, in a way not emulated by combretums - or indeed by most fleshy-fruited trees.

So, now we have a working hypothesis. Whenever we encounter a tree with wood denser than 0.8, we might well ask 'why is this foliage 'hyperphotosynthetic'? (an answer might be because it taps water particularly rapidly, relative to nutrients).

By the same token, when we encounter a tree with wood lighter than 0.5, we might well ask 'what is this plant allocating unusually large amounts of energy to?' (an answer might be that the taxon concerned is not just a fleshy-fruiter, but a 'hyperfruiter').

So here are some thoughts to dwell on.

Next time you throw a heavy piece of eucalypt wood on the barbecue, think of all the extra water it took to subsidise that 'dumping' of energy. And next time you hoist a dead log of fig overhead like styrofoam, think of all the birds and monkeys that metabolised the energy not dumped inside that porous wood.

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