Ecological notes on Syzygium (Myrtaceae) in southern Africa, part 2
...continued from https://www.inaturalist.org/journal/milewski/66507-ecological-notes-on-syzygium-myrtaceae-in-southern-africa-part-1#
In summary so far:
Although Tinley (1976) 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.
Mkuze area:
We now move farther south again, but still on the coastal plain, in the vicinity of the Mkuze River in Amatongaland.
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).
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.
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/subspeciated 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, Kwazulu-Natal, South Africa),
- 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, with no known taxonomic correlation. In S. guineense it is expressed mainly as a proliferation of subspecies - the full allopatry among which remains unclear to me.
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. Do syzygiums similarly vary intraspecifically and/or hybridise interspecifically in Australia? And does this genus penetrate wildfire-prone vegetation (savanna) on that continent?
Comments
Vegetation at Mkuze in southernmost Tongaland, with particular reference to Syzygium cordatum, based on Tinley K L (1976) The ecology of Tongaland (based on fieldwork done in the ‘fifties).
Syzygium guineense is listed by Tinley as occurring in a restricted habitat at Mkuze. This is farther south than I thought it extends. The habitat here is swamp forest, where S. guineense occurs together with Ficus hippopotami, Schefflera umbellifera, Erythrina prob. caffra, Macaranga capensis, Rauvolfia caffra, Stenochlaena tenuifolia, Veacanga thouarsii, Myrica serrata, and Cyperus sp. The congener Syzygium cordatum occurs at the margin of the same swamp forest, together with Cussonia sphaerocephala, Dialium schlechteri, Setaria chevalieri, Kraussia floribunda, Albizia adianthifolia, and Phoenix reclinata. Syzygium cordatum extends to other vegetation types in the Mkuze area, with a surprising range of plant heights.
In treeless grassland and Hyphaene palm vegetation, on white sand, Syzygium cordatum occurs as a dwarf, as does Parinari curatellifolia. Although the latter is only scattered through the grassland, S. cordatum occurs in pure stands six inches high (up to a foot high). This is a bizarre growth form of S. cordatum (effectively a thicket ca 10 cm high!).
In savannas and woodlands adjacent to the grassland, S. cordatum grows as a small tree (ca 5m high) together with various other tree species. Here, S. cordatum is a minor component but qualifies as a tree.
Woodland in places is dominated by S. cordatum. “Umdoni woodland patches occur in the palm belt west of the Mbazwane river. It is usual for these patches to be pure stands of Syzygium cordatum, forming an open park-like vegetation. The only vegetation [Ken means plants] noted under the trees is grass, with some small herbs and perhaps several small Phoenix reclinata palms. The largest stands of Syzygium cordatum occur between the Bigibigi and Masenga streams where they grow along the edges of the seasonal, grass-covered marshes common between these streams. Trees are between 15’
and 25’ in height [i.e. ca 5-8m]. Some of the larger trees probably reach 30’.”
So, S. cordatum at Mkuze ranges from a dwarf only 6 inches high in treeless vegetation through to a tree 9 m high. The species here also varies from dominating the vegetation in which it occurs (which happens patchily over the full range of its plant height) to occurring as just one small tree species among many other species of trees. Tinley does not mention the drainage, but I imagine that, wherever S. cordatum occurs, there is some degree of waterlogging, at least in the subsoil.
Effects of the hippo (and fishes) on Syzygium guineense:
Syzygium guineense in Ndumo and elsewhere in Amatongaland (as far south as Mkuze) grows only in habitats affected by the hippopotamus. This has been overlooked in the past because there is no grazing in this kind of forest.
However, the hippopotamus often rests in this kind of ‘riparian forest’ during the winter, when the ground is dry.
It may be surprising that the hippo sometimes rests in such dense vegetation (where it is shaded by the forest canopy) by day. However, it does so in preference to returning to the cold and leech-infested water, where, in this season, the mature males also make a policy of killing any immature males they encounter).
On this basis, I presume that some manuring takes place in this situation. The habitat of S. guineense here is potentially affected by the hippopotamus as well as the African bush elephant and the African savanna buffalo (even if only by trampling).
I also now realise that, in summer, this type of forest (i.e. riparian forest containing S. guineense) tends to be shallowly inundated in the rainy season, making it analogous with the habitats of ‘flooded gum’ (e.g. Eucalyptus rudis), rather than those of eucalypts in general. Since the water is overflow from perennial streams/rivers such as the Pongolo and Mkuze, a possible implication is that, during the rainy season, there will be fishes swimming around among the syzygiums.
Ken Tinley, in his 1976 report Ecology of Tongaland, says the following about the hippopotamus in the Mkuze area sensu lato (note inconsistent use of 'marsh' and 'swamp').
“Lying-up haunts of the individual herds occur in a variety of situations and vegetation types. Those herds occurring in the central...flood plain [of the Mkuze area sensu lato] have lying-up spots close to the water’s edge, in either riparian forest (of the Mkuze River) or patches of coastal forest, as well as in dense woodland which has a closed understorey. During summer months when riparian and swamp forest areas have water over their understorey, the lying-up haunts in the wooded and coastal type forests become more popular. In the central flood plain hippo lie-up during the day in suitable shallows in clumps, as in the Pongolo Pans. In the swamp areas the animals tramp down large stands of papyrus on to patches of thick mud, so that the water is a suitable depth. Lying up spots were noted mainly along the eastern shore of Mosi Pan, the western wooded areas of both Tshanetshe and Mpempe Pans (on the south of the river), and in a patch of coastal forest on the west bank of Enhlankha Pan...During the winter period the flood plain itself in the central section appears to be the main source of grazing. Large sections, such as the Neshe Pans, are completely covered over by iMbuku watergrass [Echinochloa pyramidalis, implying a floating meadow]. Hippo in Mosi and Oyengu Pans graze out some way into the country on either side of the pans, which supports a dense ground cover of grass in woodland. Hippos frequenting the swamp [probably = marsh of the papyrus type] country appear to walk out considerable distances over the open lala palm and grassveld, in search of suitable grass patches. Favourable grazing is found along the edges of the many marshes which occur in the lala palm and grass areas. This grazing is done in terrain similar to most parts of the country surrounding Lake Sibayi [farther north in Tongaland]. The grass country on either side of the swamp [probably = marsh] area is not dense, but forms an open cover, growing thicker near patches of bush and shrubs. Large fires are numerous in the entire flood plain area, but have their greatest effects in the open grass country. I can only presume at this stage that a fair amount of nocturnal and local movement takes place when the country is burnt out.”
My commentary:
This implies that the belt of low savanna of Syzygium cordatum adjacent to Oyengu Pan is grazed by the hippopotamus, and that the park-like physiognomy of this vegetation is partly owing to the effects of this megaherbivore. Tinley does not state this, but I infer it. This may help to explain why S. cordatum is not more eucalypt-like than it is: it is affected by a megaherbivore.
It seems that, as on the Pongolo floodplain, the hippopotamus in the Mkuze area rests partly away from the water, out in the dense vegetation (undercover of with trees) on well-drained ground. I infer that in winter this includes rest by day, whereas in summer it refers only to rest by night (after the night’s grazing but before dawn).
Overall, I infer that all stands of S. cordatum throughout Amatongaland are naturally part of the habitat spectrum of the hippopotamus. So, one cannot consider the ecology of S. cordatum without invoking this megaherbivore. In a sense, ‘waterberry is a hippo myrtle’. More particularly, grazing and trampling by the hippopotamus may help to explain how this fleshy-fruited myrtle manages to attain tree size despite the effects of fire in a landscape that is, overall, nutrient-poor. Put another way: eastern Amatongaland is nutrient-poor and sandy enough to be suitable for a fire-tolerant myrtle, but trampled and grazed by megaherbivores enough to keep this myrtle from converging much with eucalypts. The link is that both S. cordatum and the hippo are, in a loose sense, inhabitants of marshes. We have an interesting tension between an Australia-like ecosystem (sand, nutrient-poverty, fire, unpalatability) and an Africa-like ecosystem (rich alluvia, fire-suppression, heavy grazing, manuring, trampling) that plays out in a kind of complex mosaic with seasonal variation.
This is because a) eastern Amatongaland is a nutrient-poor plain dotted with marshes of various size, b) the marshes are deep enough and nutrient-accumulating enough (by alluvial processes) to support the hippopotamus, and c) the hippopotamus must range far from one marsh to another over the course of the year.
Somewhere in this complexity of environmental factors has arisen a niche for an African myrtle, living beyond the rainforest habitat to which most African and American myrtles are restricted.
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