Searsia has dense wood, unusual for family Anacardiaceae, part 1
(writing in progress)
In the family Anacardiaceae, the genus Searsia (https://www.inaturalist.org/observations?place_id=any&taxon_id=72354&view=species) is common in southern Africa, usually in shrubby form.
Searsia has dense wood. The name ‘taaibos’ (Afrikaans for 'tough bush') reflects the combination of this density with a particular flexibility, that makes the stems hard to break.
It is well-known that the woody stems of Searsia are not brittle. However, it happens also to be true that, like so many ecologically similar shrubs in southern Africa such as Ebenaceae and Celastraceae, the wood is actually dense as well.
What many naturalists may not realise is that this is odd in Anacardiaceae. Few other genera in this family have dense wood.
Generally, anacardias, worldwide, have rather light wood. The most prominent member of this family in South Africa, namely Sclerocarya birrea (https://www.inaturalist.org/taxa/340245-Sclerocarya-birrea), shows this well.
Formerly, ‘Rhus’ included a diverse array of shrubs and small trees in Africa, Asia, and the Northern Hemisphere. This complex has now been broken up into Rhus proper, related Malosma, and a completely different genus called Searsia, which is restricted to Africa and southern Eurasia, as far east as southwestern China (https://ars.els-cdn.com/content/image/1-s2.0-S0254629916302605-gr1.jpg).
In addition to various other differences, this split between true Rhus on the one hand and Searsia on the other corresponds to a remarkable difference in wood density. Rhus and Malosma have soft wood, even for anacardias (ca 0.5). By contrast, Searsia has hard wood (probably >0.8, and in some cases >0.9). In terms of wood density, Searsia and true Rhus could hardly be more different.
I have previously pointed out the unique occurrence of dwarf ebonies in southern Africa (https://www.inaturalist.org/posts/61009-dwarf-ebonies-part-1-white-milkwood-as-symbolically-but-not-biogeographically-south-african# and see my article in July 2022 issue of http://plantlifesouthafrica.blogspot.com/).
Some spp. of Searsia might seem similar to dwarf ebonies in growth form and ecological niche. However, searsias do not qualify as dwarf ebonies, because arborescent anacardias, found in tropical forests and woodlands, do not share the wood densities of ebonies.
Shrubby searsias may in some sense be dwarfed, but they are not drawn from a family with ebony-like wood. Instead, what I m starting to see is that Searsia, so prominent/diverse under temperate climates in southern Africa, is an unusually dwarfed AND UNUSUALLY HARD-WOODED genus in its family.
The density of the wood of Searsia is all the more noteworthy because the wood remains dense in the fastest-growing species such as S. pendulina. These are fast-growing enough
- recover from frequent and repeated cutting by indigenous people for construction work and fish baskets etc., and
- to be successful street-trees when artificially planted in towns.
So, Searsia is not so much a ‘dwarf ebony’ as an ‘ebonised anacardia’.
My explanation for this would be that Searsia is adapted to withstand megaherbivory, mainly by the African savanna elephant (Loxodonta africana), but also by e.g. the hook-lipped rhino (Diceros bicornis).
I wrote in my diary, in 2012:
“A dominant tree in Malelane Mountain Bushveld is Combretum apiculatum. Although this species is dirt-common in Kruger Park, it has two noteworthy features. Firstly, it has extremely dense wood, being one of the few species of tree in Kruger Park to possess wood that, when dry, sinks in water. Secondly, many or most of the individual trees of this species, at least in Malelane Mountain Bushveld, have been pushed over by elephants, and have survived this but have 'grown upwards from their sides'. This has created a distinct (induced) life form of a tree with a horizontal trunk just above ground level, giving rise to vertical branches that reach about 5 m. Although the population density of trees of C. apiculatum may not have suffered from damage by elephants, it seems likely that the maximum or average height of the trees has suffered, simply because the trees generally do not grow upright, but on their sides instead.”
It strikes me that this species of Combretum is emulating Searsia approach for managing proboscideans. So, I wonder: are the stems of Combretum as flexible as those of Searsia? A fact supporting this possibility is that many combretums are lianes, not free-standing plants.
I also note a possible parallel in Albizia (see my Post on Ndumo Game Reserve). Is Albizia petersiana, in some sense, a ‘searsia-like mimosa’?
Wood densities of Anacardiaceae
Most anacardias have rather soft wood of 0.5-0.8, with denser wood in Brazilian Schinopsis (ca 0.75) and moderately dense wood in Pistacia (0.8), the latter genus consisting of shrubs rather than trees. Searsia seems to have wood densities of 0.8-0.9 (few data as yet, which is understandable because most spp. are mere shrubs).
The following is a list of wood densities in genera of Anacardiaceae, worldwide, from the softest to the hardest wood.
Anacardium 0.38-0.5
Malosma 0.47-0.5
Rhus 0.45-0.53
Euroschinus 0.4-0.575
Schinus 0.5-0.56
Cotinus 0.54
Lannea 0.54-0.57
Spondias 0.46-0.64
Sclerocarya 0.56
Mangifera 0.52-0.68
Rhodosphaera 0.5-0.875
Myracrodruon 0.71
Ozoroa 0.67-0.80
Schinopsis 0.76
Pistacia 0.78-0.81
Searsia (lancea) 0.89-0.97
To date, I have found actual data on wood density for only one species of Searsia. However, qualitative descriptions of the wood of congeners suggest similar values for wood density.
The only possible exception I have read about is S. tenuinervis, but my attempts to substantiate this variation have failed, so I must assume that S. tenuinervis has similarly dense wood to S. lancea. Indeed, the roots of this species have such dense wood that they are used by indigenous San people to make arrowheads.
As readers can see from the above list, most anacardias have light to moderately light wood, <0.7 and in some cases <0.5.
For reference, the wood density of Populus is <0.5. My own experience with firewood of Populus suggests wood density of <0.5, and possibly as little as 0.4. Some anacardias have similarly light wood to poplars, and most have wood lighter than that of Robinia (which is not a particularly dense wood in the scheme of things, except perhaps to North Americans who talk of ‘hardwoods’ but in a different ballpark from e.g. Australians and South Africans).
Indeed, the stems of North American Rhus typhina are so soft that indigenous Americans used to hollow them out, for use as tubes and pipes.
Pistacia is the only anacardiaceous genus approaching Searsia in wood density.
This may perhaps make sense in light of the fact that Pistacia grows slowly (despite being partly cultivated, as opposed to wild).
Pistacia occurs naturally as far afield from the Middle East as Mexico. However, it also extends to East Africa, overlapping with Searsia in distribution. So, some similarity in wood density is not surprising. In growth-form and habitat, Pistacia is more comparable with Searsia than with any other genus of this family.
I have not found data for Heeria argentea (https://www.inaturalist.org/taxa/593611-Salix-mucronata). If Ozoroa is anything to go by, its wood density is probably about 0.7, considerably less than that of Searsia.
The bottom line:
It seems likely that Searsia is exceptional in its family for its dense wood, and its downplaying of chemical defence.
I suggest that the best way to interpret this exceptionally dense wood it in the light of the fact that, in this family, Searsia is the best example of a combination of
- shrubby growth-form, and
- distribution centred on the habitat of megaherbivorew.
Any anacardia growing in the habitat of megaherbivores might theoretically just have maintained the strategy of poisoning any animal that attempted to eat it (which is the pattern in the family generally). Therefore, the fact that Searsia has swopped toxins for physical/structural toughness/resilience indicates to me that it has ‘gone along’ with the sway of herbivory, and has emulated the tolerance of herbivory that is seen in various other woody plants in Africa.
http://link.springer.com/article/10.1007%2Fs001070050479#page-1
Schinus and Cotinus have wood densities of about 0.5, which means that they have soft woods.
Pistacia has denser wood, at about 0.8, which is similar to that of some acacias.
Cotinus coggygria, with wood density 0.54
https://books.google.com.au/books?id=7PvFBAAAQBAJ&pg=PA101&lpg=PA101&dq=cotinus+wood+density&source=bl&ots=opPrTTQMy_&sig=dq_GnSGD6Qho287GkPLHfWwlSJg&hl=en&sa=X&ved=0CDgQ6AEwA2oVChMI3MK7kdKZxwIVIWOmCh1EAAnf#v=onepage&q=cotinus%20wood%20density&f=false
Pistacia lentiscus, with wood density 0.78
Pistacia terebinthus, with wood density 0.81
Schinus molle, with wood density 0.50
Schinus terebinthifolia, with wood density 0.56
In the following study of wood density in the caatinga of eastern Brazil, the two anacards are Schinopsis and Myracrodruon. Both seem to have fairly dense wood, perhaps greater than the average expected for this family. My readings of the axis are that Schinopsis has wood density about 0.76 and Myracrodruon about 0.71.
Jatropha and Commiphora have remarkably soft wood at about 0.3. None of the study species has particularly dense wood, Mimosa being the densest at about 0.83. In a semi-arid environment similar to the caatinga in Africa or Australia, many or most trees would have dense wood (>0.75), although of course soft-wooded Commiphora occurs also in Africa.
http://www.researchgate.net/profile/Andre_Lima14/publication/256941095_Phenology_and_wood_density_of_plants_growing_in_the_semi-arid_region_of_northeastern_Brazil/links/0c9605329dead4b11b000000.pdf
Another example of soft wood in Anacardiaceae is Spondias mombin:
Spondias mombin
http://worldagroforestry.org/treesnmarkets/wood/data.php?id=1#
Euroschinus falcata
http://www.dpi.nsw.gov.au/data/assets/pdf_file/0003/390054/NSW-Rainforest-Trees-Park-IX-Families-Euphorbiaceae,-Anacardiaceae,-Corynocarpaceae,-Celastraceae,-Siphonodontaceae,-Icacinaceae.pdf
Rhodosphaera rhodanthema
http://www.dpi.nsw.gov.au/data/assets/pdf_file/0003/390054/NSW-Rainforest-Trees-Park-IX-Families-Euphorbiaceae,-Anacardiaceae,-Corynocarpaceae,-Celastraceae,-Siphonodontaceae,-Icacinaceae.pdf
Mangifera indica
http://www.wood-database.com/lumber-identification/hardwoods/mango/
Sclerocarya birrea
http://www.worldagroforestry.org/treedb2/AFTPDFS/Sclerocarya_birrea_ssp._caffra.pdf
Anacardiaceae: SEARSIA:
Searsia lancea
http://database.prota.org/PROTAhtml/Searsia%20lancea_En.htm
Searsia chirindensis
http://www.plantzafrica.com/plantqrs/searsiachirind.htm
Searsia pendulina
http://www.plantzafrica.com/plantqrs/searsiapend.htm
to be continued in https://www.inaturalist.org/journal/milewski/68675-searsia-has-dense-wood-unusual-for-family-anacardiaceae-part-2-comparison-with-californian-shrubs-including-rhus-and-malosma#...
(writing in progress)
Comments
@wynand_uys
Density of wood of Searsia pendulina:
If we accept that all searsias have dense wood (probably >0.8 and in some cases >0.9), then it should come as no surprise that the wood seems to make good 'coals' for barbecuing (https://www.4x4community.co.za/forum/showthread.php/228536-Best-type-of-wood-for-braaing/page5).
However, I mention Searsia pendulina in particular, to emphasise that this wood quality seems to apply even to the fastest-growing of searsias.
Searsia pendulina (https://www.inaturalist.org/taxa/593906-Searsia-pendulina) has become popular as a street-tree in South Africa, because it grows so rapidly (up to 1.5 m per year). This might lead us to predict that it is somewhat like Virgilia (https://www.inaturalist.org/observations?place_id=any&taxon_id=379588&view=species), in growing fast, dying young, and having light wood.
On the contrary, the wood is dense (see under 'uses' in http://pza.sanbi.org/searsia-pendulina).
Searsia pendulina may perhaps be short-lived (van Wyk describes it as such, but I would like to see the evidence). However, it certainly does not have light wood, and my guess would be that the density is >0.8.
Searsia pendulina coexists with, and ecologically resembles, Salix mucronata (https://www.inaturalist.org/taxa/593611-Salix-mucronata). However, the two genera differ greatly in wood density.
A combination of rapid growth with dense wood is best-known in mimosoid legumes such as Vachellia karroo, which normally lives for only about 25 years, yet has dense wood (>0.8 and probably up to 0.89). Dichrostachys cinerea is a woody weed, and yet it has notoriously dense wood (0.9?). Senegalia mellifera can be described along similar lines.
Searsia pendulina is ecologically similar to V. karroo (at least in its more restricted original habitat of alluvia along the Gariep and Vaal Rivers). In both cases we have fast-growing woody plants that manage to make dense wood, despite their continual replacement of leaves, rather ‘weedy’ character, and niches as successional plants after disturbance.
Bottom line:
As in acacias and other mimosoids (including short-lived spp.), even the fastest-growing searsias seem to have dense wood. This would not seem remarkable in the southern African context, were it not for the fact that, overall, the family to which Searsia belongs – unlike mimosoid legumes – generally has fairly light wood.
Density of Searsia wood not owing to aridity:
There is an idea out there that plants in dry climates tend to have dense wood, so that the xylem vessels can resist cavitation in drought.
This seems disproven by an extremely well-known anacardia, viz Schinus molle (=S. areira). The native habitat of S. molle is the Peruvian Andean desert, i.e. it is naturally a plant of arid zones (helping to explain why it does so well at Samara and elsewhere in the Karoo).
Schinus molle does seem ‘drought-adapted’. Yet is has soft wood (about 0.5 if memory serves). In a sense the indigenous counterparts within the same family in southern Africa are the various karees (e.g. Searsia lancea), but these are quite different in wood density (>0.8).
So, the climatic aspect of the environments seems to explain little about wood density in anacardias?
A note from personal experience on density of wood of Eucalyptus gomphocephala:
Eucalyptus gomphocephala (https://www.inaturalist.org/taxa/162753-Eucalyptus-gomphocephala), indigenous to the limestone-based coastal dunes in southwestern Australia, has extraordinarily dense wood.
So, when I saw that municipal workers had lopped some dead branches from a local specimen, I carried a few pieces of the branches home to examine them. The sections had diameter ca 5-10 cm, and were already air-dry (having been dead on the tree for some time).
What I noticed is that the branch wood does not feel as heavy as I expected. This is approximate, of course, but my rough guess would be a density of about 0.75, perhaps similar to oak or other typical hardwoods of the Northern Hemisphere. I was expecting something more like 0.9.
My interpretation:
I can only imagine that the branches (diam. ca 5-10 cm) have wood that is less dense than that of the main trunk. I cannot yet offer an explanation of how the wood in a given stem becomes denser as the stem thickens, other than to suggest that the heartwood is considerably denser than the sapwood, and that it is virtually only sapwood that occurs in branches <10 cm diam.
Naturalists in the Western Cape of South Africa are familiar with the various spp. of Searsia, so typical of this region. However, how many realise that the same genus also occurs under a mediterranean-type climate in the Maghreb of North Africa, plus the Levant?
https://www.inaturalist.org/taxa/874132-Searsia-tripartita
https://www.inaturalist.org/taxa/919676-Searsia-pentaphylla
Under a different climate, there is also a species restricted to the island of Socotra, off the Horn of Africa: https://www.inaturalist.org/taxa/1376573-Searsia-thyrsiflora.
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