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#...
