Dwarf ebonies, part 1: white milkwood as symbolically but not biogeographically South African

This Post is the first in a series about the peculiar occurrence of dwarf ebonies (Sideroxylon, Euclea and Diospyros) in South Africa.

Although the wood of the white milkwood (Sideroxylon inerme, https://en.wikipedia.org/wiki/Sideroxylon_inerme and https://www.kariega.co.za/blog/in-bad-odour) is pale, it can be considered a form of ebony because the family Sapotaceae is closely related to the family Ebenaceae (https://en.wikipedia.org/wiki/Ericales#/media/File:EricalesRose2018.png).

And although the white milkwood can reach 15 metres high, its ability to persist for centuries as a knee-high shrub (e.g. see https://www.inaturalist.org/observations/83814910) on the windswept coast near Cape Point (https://en.wikipedia.org/wiki/Cape_Point) and Cape Agulhas (https://en.wikipedia.org/wiki/Cape_Agulhas) means that in some sense it can be described as a dwarf ebony.

South Africans have a special symbolic regard for the white milkwood (https://www.inaturalist.org/taxa/362175-Sideroxylon-inerme).

The reasons are that this species is:

  • the only indigenous tree persisting to the southernmost tip of Africa,
  • capable of producing an 'arborescent thicket' in windswept littoral vegetation that is otherwise merely shrubby,
  • remarkably hardy in association with dense wood, which allows it to survive urbanisation (http://tropical.theferns.info/viewtropical.php?id=Sideroxylon+inerme)
  • non-flammable in a region generally subject to wildfires, and
  • proclaimed an historical monument where its durability as a natural landmark has been utilised by various explorers over the last half-millennium (e.g. https://en.wikipedia.org/wiki/Treaty_Tree).

However, the local appropriation of the white milkwood is at odds with its actual affinities on a global basis. Whether geographically or ecologically, Sideroxylon hardly belongs in South Africa.

This genus originated in central America (https://www.researchgate.net/publication/266396476_Revisiting_the_biogeography_of_Sideroxylon_Sapotaceae_and_an_evaluation_of_the_taxonomic_status_of_Argania_and_Spiniluma) and is today associated mainly with North America rather than Africa.

Even in the African region, the genus - and indeed the white milkwood itself - is associated as much with tropical islands in the Indian Ocean as with the temperate zone in South Africa (https://www.jstor.org/stable/2417064 and https://www.inaturalist.org/observations/36099669).

The occurrence of Sideroxylon in South Africa is an outlier in the sense that this genus is far more speciose in the southern United States of America and the Caribbean.

For example, there are 11 indigenous species of Sideroxylon in Florida alone (e.g. https://www.inaturalist.org/taxa/168950-Sideroxylon-lycioides), the natural companions of which include Pinus (https://en.wikipedia.org/wiki/Sideroxylon_tenax and https://www.fdacs.gov/content/download/82393/file/CIRCULAR_Buckthorns_SIderoxylon.pdf).

The North American species are tardily deciduous (e.g. https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=m830), and differ ecologically in this way from the white milkwood, which is fully evergreen.

Unlike the white milkwood (https://prota4u.org/database/protav8.asp?g=pe&p=Sideroxylon+inerme+L.), the North American species of Sideroxylon do not have particularly dense wood (https://www.fpl.fs.fed.us/documnts/fplrp/fplrp325.pdf). The specific gravities of the dry wood, measured in kilograms per cubic meter, are more than 1.0 in the former vs only about 0.75 in the latter.

Particularly surprising to South African naturalists may be the fact that most or all of the North American species have a capacity for spinescence (e.g. https://www.inaturalist.org/photos/7045078 and https://www.inaturalist.org/observations/36812876 and https://www.inaturalist.org/observations/24496215). This contrasts categorically with the white milkwood, which is completely devoid of spines despite occurring in floras with so many spinescent species of plants that it would be hard to list them all.

The contrast is best exemplified by Sideroxylon tenax, which grows as a shrub on coastal dunes in Florida. This species parallels the white milkwood in habitat and the size of the plants, but looks so different that no South African would find it familiar (https://www.thesurvivalgardener.com/an-unknown-eleagnus/ and https://www.eattheweeds.com/tag/sideroxylon-tenax/ and https://plants.jstor.org/compilation/Sideroxylon.tenax).

Instead, any South African naturalist first encountering Sideroxylon in North America (e.g. https://www.inaturalist.org/observations/3521873) might be reminded of Gymnosporia (Celastraceae). Gymnosporia buxifolia (https://en.wikipedia.org/wiki/Gymnosporia_buxifolia) naturally occurs side-by-side with the white milkwood at the southern tip of South Africa (e.g. https://www.inaturalist.org/observations/30029230), making the situation all the more confusing.

The southerly extension in Africa is to some degree paralleled in South America, where Sideroxylon obtusifolium reaches the coast of Uruguay at similar latitudes to the southwestern Cape of South Africa (e.g. https://www.inaturalist.org/observations/69115540 and https://www.inaturalist.org/taxa/273914-Sideroxylon-obtusifolium).

However, as far as I know the South American species does not grow in stunted form, and thus does not qualify as a dwarf ebony (Uruguayan iNaturalists please correct me if I am wrong). Nor does it extend into a mediterranean-type (winter-rainfall) climate. The growth-form of S. obtusifolium (https://www.inaturalist.org/observations/61197783) remains in line with Mexican species rather than converging with the white milkwood.

Given this broadened view, will South African naturalists find their appreciation of one of the botanical symbols of this country to be boosted, or deflated?

to be continued in https://www.inaturalist.org/posts/61027-dwarf-ebonies-part-2-euclea-tomentosa-as-a-substitute-for-the-ericas-missing-from-the-cape-flora#...

Posted on January 07, 2022 01:46 AM by milewski milewski

Comments

Interesting. We have a milkwood forest at Cape Point.
https://www.inaturalist.org/observations/102162727

Posted by dianastuder about 2 years ago

Thanks Antoni! I look forward to subsequent parts of this story, please do tag me in them!

Posted by adriaan_grobler about 2 years ago

@adriaan_grobler Hi Adriaan, You are most welcome and I will be sure to tag you as requested, with regards from Antoni.

Posted by milewski about 2 years ago

I am particularly interested in this as all three genera occur very frequently at Archaeological habitation sites, and have many uses which are recorded ethnobotanically.

Posted by yvettevanwijk1941 about 2 years ago

@yvettevanwijk1941 Hi Yvette, Many thanks for this comment, with regards from Antoni.

Posted by milewski about 2 years ago
Posted by milewski about 2 years ago

Some of the coastal Milkwoods in exposed sites such as at Pinnacle Point and Blombos, dorm quite extensive prostrate mats, probably pruned by the salty wind.

Posted by yvettevanwijk1941 about 2 years ago

@yvettevanwijk1941 True, and this is a main reason why I have included Sideroxylon inerme in the category of dwarf ebonies. As far as I know, the stands to which you refer are clonal as opposed to continually regenerating from seed. This means that an entire mat, covering many square meters, can be a single, centuries-old individual plant which shifts somewhat back and forth according to deposition/erosion of beach sand. The amount of foliage borne by the individual may exceed that of a large tree, but it is all located less than one meter above-ground. This is the context of my use of the term 'dwarf' for S. inerme, and what I find remarkable is that on no other continent at similar latitudes does any other member of the clade Sapotaceae-Ebenaceae take this form.

Posted by milewski about 2 years ago

Super interesting, thank you.

Posted by jeremygilmore about 2 years ago

Just another observation, I have found quite a few Diospyros dichrophylla in the Little Karoo which have quite long sharp "thorns", which I think are the remnants of small branchlets on the main trunk. I have not seen this mentioned anywhere?

Posted by yvettevanwijk1941 about 2 years ago

@yvettevanwijk1941

SPINESCENCE IN DIOSPYROS

Many thanks for pointing this out (https://www.inaturalist.org/observations/65420348). Actually, it is mentioned and illustrated in the Wikipedia account of Diospyros dicrophylla: https://upload.wikimedia.org/wikipedia/commons/4/48/Diospyros_dichrophylla05.jpg.

These structures, which are not really spines and might perhaps be better-described as struts, might function as an anti-elephant defence, discouraging gross damage to the bole or main stems. So it is interesting that Loxodonta africana in the Knysna area seems never to have been recorded foraging on, or breaking, any species of Diospyros.

I looked through the hundreds of observations of D. dichrophylla in iNaturalist and did find one similar illustration of this kind of 'spinescence': https://www.inaturalist.org/observations/76152352.

The second-last photo in https://www.inaturalist.org/observations/77415018 suggests that a similar quasi-spinescence may occur in Diospyros simii, despite the tendency of that species to be a scrambler rather than an upright shrub/small tree.

In the case of Diospyros lycioides, the only relevant photos show quasi-spines on branches that are still relatively small: https://www.inaturalist.org/observations/11222942 and https://www.inaturalist.org/observations/13433486. This seems to make D. lycioides the species, at least within the southern African flora of Ebenaceae, that is the closest to conventional spinescence.

I suspect that the very name 'lycioides' was based on these quasi-spines.

The reluctance of Diospyros to become spinescent is illustrated by Diospyros ramulosa, which lacks spines even when subject to extreme folivory by Procavia capensis (https://www.inaturalist.org/observations/11064860).

Beyond southern Africa:

Diospyros montana of India is described as spinescent: http://tropical.theferns.info/viewtropical.php?id=Diospyros+montana. This is borne out by two observations in iNaturalist: https://www.inaturalist.org/observations/32000999 and https://www.inaturalist.org/observations/1366870.

Diospyros rhombifolia is also described as somewhat spinescent (https://treesandshrubsonline.org/articles/diospyros/diospyros-rhombifolia/), but I have yet to see illustrations.

So, as it turns out, quasi-spinescence seems fairly common in Diospyros, across a wide distribution from southernmost South Africa to India.

Posted by milewski about 2 years ago

Thanks milewski, in our area (Wilderness and Knysna in the southern Cape, Diospyros dichrophylla is regarded by some elders in the modern KhoiSan communities as sacred or not to be cut or burnt, it is also used for magical purposes by Xhosa Sangomas. Maybe the elephants in the Knysna forest kniow a thing or two!?

Posted by yvettevanwijk1941 about 2 years ago

Thanks for posting this Antoni, mega interesting! Are you ever in the Southern Cape Garden Route area? It would be great to ask you in person to look at the vegetation around archaeological habitation sites and get your take on the possible history and reasons for them having so many of the Ebonies etc.

Posted by yvettevanwijk1941 over 1 year ago

@yvettevanwijk1941 Hi Yvette, Many thanks for your kind invitation. I will keep you up to date on any travel plans, and it would be wonderful to join you in your field sites, from Antoni

Posted by milewski over 1 year ago

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