@jeremygilmore Comments please?
One of the obvious biotic differences between southern Africa and Australia is in the incidence of Myrtaceae (https://en.wikipedia.org/wiki/Myrtaceae).
Myrtaceous trees and shrubs cover much of Australia across a wide range of climates and soils. By contrast, they tend to be few and far-between - and therefore easily overlooked - in southern Africa.
A main ecological difference between these continents is the extreme nutrient-poverty of Australia, together with a limited incidence of herbivores and an extreme incidence of wildfires.
This raises the question:
How do the indigenous Myrtaceae of southern Africa relate to poor soils, herbivores, and wildfire?
The following notes may begin to shed some light on this topic, focussing on syzygiums (https://en.wikipedia.org/wiki/Syzygium), a genus indigenous to both continents.
EASTERN SOUTH AFRICA (see https://upload.wikimedia.org/wikipedia/commons/a/ad/St_Lucia_Estuary_OSM.png)
The relevant myrtaceous tree species here are Syzygium cordatum (https://www.inaturalist.org/taxa/338719-Syzygium-cordatum) and Syzygium guineense (https://www.inaturalist.org/observations?taxon_id=132507). These have succulent fruits and are dispersed and sown by frugivores. In this way they differ from most fire-adapted Myrtaceae in Australia, as well as most woody plants adapted to poorly-drained ground worldwide.
Syzygium cordatum occurs in stunted form in the area of Lake Sibhayi (https://en.wikipedia.org/wiki/Lake_Sibhayi). This a nutrient-poor part of Amatongaland (https://en.wikipedia.org/wiki/Amatongaland and https://www.researchgate.net/figure/The-Mkuze-Wetland-System-showing-the-extent-of-the-floodplain-and-the-location-of_fig1_7139313).
According to Tinley K L (1976, The ecology of Tongaland), a main habitat of S. cordatum in this area is the level just above that of the lake edges, where the upper soil is not waterlogged although the subsoil probably is. In such situations S. cordatum is the only tree, growing with the grasses and sedges Urelytrum squarrosum, Bulbostylis contexta, Monocymbium ceresiiforme, Eragrostis sp., Imperata cylindrica, Mariscus sp., Andropogon sp., Ischaemum arcuatum, Eragrostis capensis and E. chapelieri, Perotis patens, and Dactyloctenium geminatum.
Another main habitat of S. cordatum is ‘woodland’ (which I take to be savanna). One variant is ‘umdoni veld’, in which S. cordatum shares the tree stratum with many other taxa, e.g. Strychnos spinosa and S. madagascariensis, Dichrostachys cinerea, Albizia adianthifolia, Mundulea sericea, Hyphaene natalensis, Brachylaena discolor, Sclerocarya caffra, Ziziphus mucronata, Trichilia emetica, Apodytes dimidiata, Ficus stuhlmannii, and Acacia nilotica ssp. kraussiana. “In the eastern section [of the Lake Sibhayi area] Syzygium cordatum is the dominant woodland species changing to Terminalia sericea in the west, where the latter is dominant. The central part of Sibayi has an overlap of both types of vegetation.”
Tinley notes that “This country is commonly used by hippos for grazing purposes”. Although nobody expects S. cordatum to be eaten by the hippopotamus (https://www.inaturalist.org/taxa/42149-Hippopotamus-amphibius), the habitat is surely affected by this megaherbivore in Amatongaland. This defies resemblance to Australia, where megaherbivores are absent.
Swamp forest (see https://en.wikipedia.org/wiki/Forests_of_KwaZulu-Natal) occurs in the Sibhayi area as in other parts of Amatongaland, where the water runs rather than remaining stagnant. The tallest trees are Ficus hippopotami, Macaranga capensis, Voacanga thouarsii, Morella serrata, and Halleria lucida, and the main plant near ground level is the scandent fern Stenochlaena tenuifolia. Syzygium cordatum occurs at the edge of this poorly-drained forest, not within it.
Syzygium cordatum also occurs at the edges of two other types of forest, namely coastal forest and dune forest, in the Sibhayi area (see https://en.wikipedia.org/wiki/Forests_of_KwaZulu-Natal; bushpig, bushbuck, cane rat, red duiker, and suni occur in both types, but the nyala is absent from dune forest.)
Syzygium cordatum is thus absent from vegetation that is free of wildfires (coastal and dune forest as well as swamp forest), or subjected to permanent inundation at the surface (swamp forest). On the other hand, it also seems to be excluded from considerable areas of grassland owing to wildfire and/or good drainage. Where the intermediate conditions suit it, it can be the most prominent species of tree in savanna.
In summary so far:
Although Tinley does not put it together this way, I infer that S. cordatum is widespread in the Sibhayi area of Amatongaland wherever there is a combination of
- nutrient-poor soils,
- enough grass to carry wildfire, and
- some degree of waterlogging in the subsoil.
However, the hippopotamus is likely to be naturally present wherever S. cordatum is common in Amatongaland. This 'megaherbivory' tends to undermine any analogy with Australia.
Farther south, on the coastal plain near the Mkuze River:
On the western edge of Oyengo Pan is a small belt of trees of S. cordatum. On the coastal side is a plain of nutrient-poor whitish sand, characterised by 'lala palm' (Hyphaene) scattered in grassland. (In the same area, ‘in places small patches of bush occur on large ant-hills’). In this zone, S. cordatum occurs extensively along the edges of treeless marshes covered in graminoid vegetation. The trees of S. cordatum are 8-9 m high, and the only plants growing under them are grasses.
I infer that S. cordatum forms savanna here, withstanding wildfires without becoming stunted. However, S. cordatum also grows in the largely treeless grassland (characterised by Hyphaene and Phoenix) as a dwarf, as does Parinari curatellifolia (https://en.wikipedia.org/wiki/Parinari_curatellifolia).
The tallest vegetation (reaching 18 m high) in this coastal belt of the Mkuze area is swamp forest, occurring along the main drainage lines. Neither S. cordatum nor Syzygium guineense is mentioned as present.
Moving to the northwest, closer to the Lebombo hills on the western side of the Mkuze area, we find drainage lines such as the Ophanzi, where riparian forest (not to be confused with swamp forest) reaches 18 m high and includes S. guineense. Similar riparian forest, dominated by Ficus sycamorus, occurs along the Mkuze River itself.
It is apparent that both S. cordatum and S. guineense are associated with partly wet conditions. Riparian forest of Ficus sycamorus tends to be inundated in summer owing to overflow of rivers, but in winter the soil is well-drained. In the case of S. cordatum, the typical situation seems to be the edges of marshes, where there is permanent waterlogging of the subsoil. Although S. cordatum may be dwarfed by frequent wildfires, there does not seem to be any obvious relationship between the height of S. cordatum and either drainage or wildfire.
My conclusion so far:
In southeastern Amatongaland, S. cordatum forms a short (about 5 m high) savanna, in which it can be the main tree. This savanna seems to be fire-prone, implying that even trees of S. cordatum can withstand frequent wildfires under certain regimes. This fire-tolerance is unique for myrtaceous trees in southern Africa, and seems somewhat convergent with confamilials (not necessarily congeners) in Australia.
Syzygium cordatum seems to be well-adapted to savannas at the edges of marshes in Amatongaland, which are too nutrient-poor for other trees, too poorly-drained for tall trees in general, and subject to wildfire (albeit not to the exclusion of 'megaherbivory').
A complication is the ability of S. cordatum (in parallel with Parinari but with greater abundance) to adopt a dwarfed form in situations where all trees are excluded by whatever factor it is that excludes trees in the pale sandy belt of Amatongaland. Also deserving more thought is the parallel between this S. cordatum and two genera of palms (Hyphaene and Phoenix), which are likewise capable of growing into fairly large trees but are usually found in stunted form in Amatongaland.
MALAWI AND ZAMBIA
Syzygiums are prominent plants in the Malawi-Zambia area, contributing to catenas in miombo (https://en.wikipedia.org/wiki/Miombo) and locally dominant in a wide variety of growth-forms. The nutrient-poverty with which syzygiums are associated, and the extreme plasticity of forms, are noteworthy in any comparison of Myrtaceae in southern Africa and Australia.
Burtt Davy et al. (1958), on page 15, provides information about Syzygium mumbwaense (https://plants.jstor.org/compilation/Syzygium.mumbwaense; White (1962) seems to regard this as Syzygium guineense ssp. huillense, which is confusing) in miombo in Malawi. I infer that
- its habitat, being sandy and nutrient-poor, resembles Australia,
- other nutrient-poor floristic elements such as Uapaca, Protea, and Burkea, co-occur, and
- the syzygium adopts a multi-stemmed, mallee-like form where there is some accumulation of laterite (https://en.wikipedia.org/wiki/Laterite), a substrate type widespread in Australia.
In the area of Lilongwe (https://en.wikipedia.org/wiki/Lilongwe), the catena features miombo vegetation. A ‘lateritic’ layer, locally called ‘lubwe’, is exposed in places by erosion. “The lubwe always defines the lower limit of heavy leaching of the soil and is a zone of seepage...The lubwe band is characterised by the small tree Syzygium mumbwaense...This band may occur below the Uapaca zone where a drift of quartz overlies the basic complex.”
The typical catena hereabouts, from uppermost in the landforms to lowermost, is as follows:
- on quartz sand on the hilltops, Uapaca kirkiana - Brachystegia floribunda
- next, one step lower, Brachystegia longifolia – Faurea speciosa, a zone in which Protea abyssinica occurs as scattered plants
- next, Brachystegia boehmii – Isoberlinia globiflora on reddish sandy soils, where Burkea africana may also occur
- next, a zone characterised by the dwarf form Syzygium guineense ssp. huillense
- next, Terminalia on sand, and
- at the bottom, treeless grassland on clay in the 'dambo' (https://en.wikipedia.org/wiki/Dambo).
According to the same source, some areas of riparian forest in Malawi are co-dominated, on granitic soils, by S. cordata, S. guineense and Bridelia. In ‘swamps’ throughout the ‘low montane’ areas of Malawi, syzygiums occur to some extent.
An unidentified syzygium occurs with the palm Raphia, adjacent to streams in swampy land at altitudes of 1067 m in southern Malawi. This apparently survives despite long grass and frequent wildfires in this vegetation type.
Moving across the border to Zambia:
According to White (1962), syzygiums occur as follows.
Syzygium cordatum can grow to as high as 22 m and even occasionally has buttresses at the base. It is widespread in Zambia and dominant or co-dominant in swamp forest and fringing forest. This suggests that the vegetation dominated by S. cordatum in Amatongaland, South Africa, described above, represents a southerly attenuation of a typical vegetation type in the miombo biome.
Syzygium guineense is widespread and polymorphic in Zambia, as follows.
- ssp. afromontanum (https://www.inaturalist.org/taxa/1280025-Syzygium-afromontanum), a tree as high as 22m, occurs in the uplands,
- ssp. barotsense (https://www.inaturalist.org/observations?place_id=any&taxon_id=601314) is riparian in Barotseland and also along the tributaries and Zambesi itself above Victoria Falls, where it dominates vegetation, sometimes with several stems from a common base (this ssp. possibly occurs also in Ndumo Game Reserve, South Africa; it is often partly submerged by flooding; the fruit floats and is probably partly dispersed by water),
- ssp. huillense (https://www.inaturalist.org/observations?taxon_id=338658), a suffrutex only 30 cm high or a shrub up to 1m high, suppressed by frost and fire, is widespread in western Zambia apart from Barotseland, at the edges of sandy dambos, and
- ssp. macrocarpum (https://www.zimbabweflora.co.zw/speciesdata/species.php?species_id=142390 and https://plants.jstor.org/stable/10.5555/al.ap.visual.jspcrandu00000862), a small, semi-deciduous tree up to 11 m high, occurs in woodland at the edges of dambos in a zone above that occupied by ssp. huillense, in low, open miombo and on rocky hills.
Hybrids between S. cordatum and S. guineense are common in Zambia.
Apart from the syzygiums already mentioned, Zambia also has S. owariense (https://www.inaturalist.org/observations/106142654), an evergreen tree up to 22m high (sometimes slightly fluted at the base), similar to S. guineense ssp. macrocarpum. Syzygium owariense occurs in the northern province and parts of central province of Zambia, and is dominant or co-dominant in swamp forest and fringing forest.
OVERALL DISCUSSION
The relationship of syzygiums in southern Africa to wildfire and herbivory seems inconsistent and needs further elucidation. However, what is clear is the polymorphism/raciality/plasticity of growth form in both of the widespread species.
In S. cordatum this is expressed mainly as a remarkable variation from stunted to arborescent. In S. guineense it is expressed mainly as a proliferation of subspecies.
Either way, what seems to emerge is that just a few species achieve the equivalent of many species, in terms of filling various ecological niches. Doe syzygiums in Australia also vary so greatly intraspecifically?
_female.jpg){kind=link}
