Ecology of the pappusgrasses Schmidtia and Enneapogon in southern Africa and Australia

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Mokala National Park (https://en.wikipedia.org/wiki/Mokala_National_Park) in South Africa is ecologically interesting because it occurs at the junction of three major vegetation types, viz. treeless grassland, savanna, and low shrubland. In other words, this small national park straddles the Highveld (https://en.wikipedia.org/wiki/Highveld), the Kalahari (https://en.wikipedia.org/wiki/Kalahari_Desert), and the Karoo (https://en.wikipedia.org/wiki/Karoo).

A particularly abundant grass in Mokala National Park, particularly on sandy substrates, is Schmidtia pappophoroides (https://www.inaturalist.org/taxa/336805-Schmidtia-pappophoroides).

See https://www.inaturalist.org/observations/103383472 and https://www.inaturalist.org/observations/72990067 and https://www.inaturalist.org/observations/103383477.

This is puzzling, because S. pappophoroides is not particularly typical of any of the three biomes referred to above.

How, then, can this abundance be interpreted in ecological terms?

Schmidtia is a pappusgrass (https://www.inaturalist.org/journal/milewski/67844-introducing-pappusgrasses). As such, it is related to Enneapogon - which occurs in Mokala National Park but also has a cosmopolitan distribution (https://en.wikipedia.org/wiki/Enneapogon).

We can best find clues to the ecology of pappusgrasses in Australia, because Enneapogon

• is fully indigenous to Australia, and diverse here in number of species, and
• has been particularly well-studied on this continent.

Beadle (1981, https://books.google.com.au/books/about/The_Vegetation_of_Australia.html?id=_5rwAAAAMAAJ&redir_esc=y and https://catalogue.nla.gov.au/Record/1562329) gives information on Enneapogon in Australia, on pp. 540-541. This suggests that this genus can best be thought of as successional, and suited to times/places where only limited biomass can be supported.
 
Enneapogon occurs in ‘ephemeral grasslands’ on sandy soils in Australia. The vegetation is always woodlands or shrublands, not treeless grasslands. The grasses are called ephemeral because, even if they are technically perennials, they tend to be killed by drought after only a few years.
 
Here are further facts that I gleaned from Beadle (1981):
 
In dry parts of tropical Australia, woodlands of Eucalyptus terminalis and E. argillacea occur on slopes, with mean rainfall 300-600mm per year. Here, the grasses restricted to annuals, leaving much bare ground. The soils are not particularly sandy. Enneapogon is patchily common in these woodlands, the spp. being E. pallidus, E. avenaceus, and E. polyphyllus. Sporobolus is always co-dominant. Other genera of grasses occurring in these communities are Tragus, Chloris, Dactyloctenium, Aristida, and Brachyachne.
 
Over large areas of the arid zone of Australia (formerly dominated by Acacia or Atriplex/Maireana), the vegetation has been degraded by pastoralism. As a result Enneapogon avenaceus has become common, functioning effectively as an annual. It now commonly covers large areas as monospecific stands, only 20 cm high. Beadle shows this in a photo, Fig. 20.19, which is obviously a form of treeless grassland, although not fully natural. “Even if the tussocks of Enneapogon do not grow during the second year, the dead tussocks remain and, in these, either new plants of E. avenaceus or another annual species become established.”
 
Furthermore, Beadle (1981) informs us that:

• In the mulga lands, Enneapogon (e.g. summer-growing E. avenaceus) are restricted to hills with rock exposures. Sandy patches of mulga have hummock grasses (Triodia, https://en.wikipedia.org/wiki/Triodia_(plant)) instead.
• Enneapogon avenaceus also occurs in eucalypt woodlands in the arid zone, on loams, where it is just one of various grasses (Aristida, Danthonia, Eragrostis, Neurachne, Paspalidium, Stipa).
• Enneapogon avenaceus also penetrates areas characterised by the saltbush Atriplex vesicaria on gilgaied clay soil, although it is not dominant here. Ditto for saline/sodic sands bearing the shrub Maireana pyramidata (which vaguely resembles 'ganna' in southern Africa, https://en.wikipedia.org/wiki/Caroxylon_aphyllum).
• On some claypans, the dominant grass is the tall Eragrostis australasica, with E. avenaceus forming a peripheral zone, in this case mildly sodic rather than saline.
• Right across the arid zone of Australia, one finds narrow belts of clay, bearing the grass Eragrostis xerophila. Sometimes Enneapogon planifolius is one of the associated grasses, along with Chloris, Dicanthium, Eragrostis (other spp.), Panicum, Sporobolus and Triraphis.

What emerges is that Enneapogon

• occurs widely, here and there, in semi-arid Australia,
• tolerates sodicity and even perhaps salinity, and ranges from sand to clay and stony shallow soils,
• is always naturally associated with shrubs or trees, but is capable of achieving a rather unnatural commonness on degraded surfaces where the woody plants have largely died off, and
• does not grow with hummock grasses, being mutually exclusive with them.

The edaphic and climatic distinctions between hummock grasses (which are fire-prone) and Enneapogon are rather subtle within Australia.

Enneapogon obviously prefers somewhat nutrient-richer soil than that typically associated with hummock grasses, with a corresponding difference in soil texture. However, the main point about Enneapogon is that it is essentially a grass of somewhat degraded sites, i.e. it is only competitive where larger plants have been removed.

It would be simplistic to describe Enneapogon as an annual. Instead, it is best thought of as a small, short-lived, insubstantial grass which cannot compete with more substantial plants. It is one of various smallish tussocks forming an ‘understorey’ to woody vegetation (typically Acacia but also halophytic amaranths) in semi-arid Australia.

It is only where the competitors have been removed that Enneapogon can approach dominance, and this is not strictly natural.
 
This may help us to understand the abundance of a related pappusgrass, in South Africa. The implication is that part of the reason for the dominance of Schmidtia pappophoroides in Mokala National Park is disturbance - originally by livestock, and now by dense populations of wild grazers.

Australia is the continent of wildfire. Fire is unlikely in mulga (https://en.wikipedia.org/wiki/Mulga_(habitat)) and saltbush vegetation, except on sandy patches suited to Triodia = hummock grasses. Although many grasses in Australia benefit from fire, nothing in Beadle’s description hints at any role of fire w.r.t. Enneapogon.

Beadle does not state this as such, but I infer that fire-prone vegetation is mutually exclusive with Enneapogon in Australia, as I think it is on other continents. I see this as more important than any distinction in substrate-preference between hummock grasses on the one hand and Enneapogon on the other.

The main aspects that I have learned about Enneapogon are as follows:

• These are small, annual-like grasses, superficially looking as if they might spring up after fire, in a kind of successional stage as the main fire-prone plants recover. However, I doubt this.
• As effectively an 'annual', Enneapogon is not a post-fire successional grass. Instead it characterises categories of disturbance other than fire, i.e. usually overgrazing.
• Some of the features of Enneapogon are somewhat specialised for herbivory (in a positive sense, i.e. supportive to grazers instead of being inimical). This is because it manages to regenerate from seed within the previous small tussock. That is to say, it regenerates in its own infrastructure, and thus achieves some of the continuity and reliability of a lawn grass, without having to produce the same mat-form and rhizomes as a lawn - which would be impractical in such a dry climate and where woody competition is so strong.

Enneapogon, Schmidtia, and other pappusgrasses are not fire-weeds, not lawn-formers, and not unpalatable ‘scab-plants’ that keep herbivores off until cover has been restored. Instead, they are adapted to a regime of herbivory where the following conditions apply:

• no grass can be really vigorous because the dominant plants are woody plants, and
• there is enough grazing to ensure that the biomass of grass is kept sparse.

In conclusion for now:

The inference is that it is the herbivores in Mokala National Park that are responsible for keeping pappusgrasses dominant. Were the herbivores to be removed or drastically reduced, I suspect that S. pappophoroides would become scarce. I am unsure what would replace it in savanna of Vachellia erioloba (https://en.wikipedia.org/wiki/Vachellia_erioloba) on Kalahari sand. Perhaps a coarse species of Stipagrostis?

Posted on Ιούλιος 02, 2022 0915 ΜΜ by milewski