To what extent do foliar-spinescent acacias coexist with foliar-spinescent grasses in central Australia?

@arthur_chapman @abedggood @mattbarrett @iancastle @jeremygilmore @alan_dandie

Australia is the continent on which foliar spinescence is best-developed (http://ianfrasertalkingnaturally.blogspot.com/2020/06/spinifex-prickly-heart-of-australia.html).

One of the largest-scale manifestations of this is the dominance of vast areas by 'porcupine grasses' (Triodia, https://en.wikipedia.org/wiki/Triodia_(plant)).

Hummock grassland (https://www.anbg.gov.au/photo/vegetation/hummock-grasslands.html and https://www.awe.gov.au/sites/default/files/documents/mvg20-nvis-hummock-grasslands.pdf), dominated by Triodia, occurs over much of the central Australian area studied by Latz (1995, https://books.google.com.au/books/about/Bushfires_Bushtucker.html?id=xlRmPgAACAAJ&redir_esc=y and https://books.google.com.au/books/about/Bushfires_and_Bushtucker.html?id=CH-uugEACAAJ&redir_esc=y).

In this vegetation, the dominant plant is hummock grass (https://inaturalist.ala.org.au/taxa/132351-Triodia), but there are many shrubs scattered here and there. These include at least 18 spp. of Acacia that are particularly associated with Triodia, while being common enough to warrant consideration.

Some of these acacias are foliar-spinescent. Their phyllodes are terete, with ‘pungent’ tips (https://www.wordnik.com/words/pungent#:~:text=from%20The%20Century%20Dictionary.&text=Specifically%20%E2%80%94%20In%20botany%2C%20terminating%20gradually,sharp%20points%3B%20stinging%3B%20acrid.). They are thus convergent with Triodia in the extreme adaptation of the form of their foliage.
 
I asked the following questions for this central Australian study area:

  • of the 19 spp. of Acacia particularly associated with Triodia, which ones are foliar-spinescent? and
  • of all the Acacia spp., are the spp. with foliage most similar to that of Triodia the most strongly associated with Triodia?

According to my analysis, the answers are:

  • only a few spp. are foliar-spinescent, and
  • no, the species of Acacia most convergent in foliage form with Triodia does not coexist with Triodia; rather it tends to occur in mutual exclusion with Triodia.

This means that, in this central Australian study area, Acacia in hummock grassland is typically not foliar-spinescent, despite the foliar-spinescence of the dominant grasses.

However, the picture is complicated by the fact that many or most of the Acacia spp. occupy some sort of successional niche in hummock grassland, post-fire. In the case of those spp. most congruent with Triodia in the successional cycle, there are indeed a few foliar-spinescent spp. of Acacia.

Hummock grassland is unusual, for semi-desert vegetation, in burning intensely, and depending on wildfire for its regeneration.

When hummock grassland burns, ash is deposited. This then provides nutrients for a flush of ‘fireweeds’, which are usually soft-leafed plants living only a few years while the more slow-growing Triodia begins to recover.
 
Some spp. of Acacia qualify as ‘fireweeds’. This is true despite the hardness of their wood, which is one of the most important causes of tyre punctures on tracks through vegetation ‘normally’ dominated by Triodia.
 
In this study area, the ‘woody fireweed’ spp. of Acacia are

None of these spp. is foliar-spinescent.

Furthermore, none of them grows strictly in temporal association with the hummocks of this vegetation type. This is because they tend to senesce before Triodia achieves dominance of the vegetation. Their role is successional. Once they die, they remain only as buried, durable seeds, waiting for potentially decades before the next intense fire.
 
It may be a surprise that ‘fireweeds’ include such hard-wooded shrubs. However, there is no surprise that these spp. of Acacia are non-spinescent.
 
Then there is a category of spp. of Acacia which also occupy a successional, germinative role, but live longer and do not depend on the most intense fires for their success. It would be misleading to call these fireweeds, because they can live for several decades (albeit not as long as the plants of Triodia, which tend to continue their growth radially, in the form of rings with expanding bare centres),
 
These germinative, fire-promoted spp. of Acacia in the study area are

Again, none of these is foliar-spinescent. They can be found coexisting with Triodia (the acacias on the wane while the hummock grass is still on the rise). However, but they effectively form a non-spinescent upper stratum (up to a few metres high) over the grasses.
 
Finally, there are spp. of Acacia here which tend to regenerate vegetatively after fire. The above-ground stems tend to die in fires, but there is re-sprouting from the base. Some of these spp. are clonal, and sucker to reproduce vegetatively. The spp. are

(Another sp., namely A. minyura, https://en.wikipedia.org/wiki/Acacia_minyura, has a niche that is best characterised as similar to that of mulga (A. aneura, https://en.wikipedia.org/wiki/Acacia_aneura).)
 
Among these spp., there are two with spinescent phyllodes, namely

Both spp. are shrubs about 2 m high.

This means that A. inaequilatera (https://en.wikipedia.org/wiki/Acacia_inaequilatera) is among the most intensely spinescent spp. in its genus. And yes, it does indeed coexist with Triodia, at more or less the stage of the successional cycle when hummock grass dominates the area. Acacia inaequilatera has corky bark protective against fire.

The additional defence in the case of A. maitlandii (https://en.wikipedia.org/wiki/Acacia_maitlandii) is a certain amount of resin (on the twigs rather than the phyllodes).
  
The following shows Acacia inaequilatera over Triodia https://apps.lucidcentral.org/wattle/text/entities/acacia_inaequilatera.htm. The following shows the sclerophyllous, ‘pungent’-tipped phyllodes of A. inaequilatera: https://en.wikipedia.org/wiki/Acacia_inaequilatera#/media/File:Acacia_inaequilatera_foliage.jpg.

This is an example of an intensely spinescent wattle growing together with spinescent hummock grass. However, such correspondence in spinescence between the hummock grasses and the tall shrubs growing with them is more the exception than the rule in central Australia.

The species of Acacia in the study area most strongly convergent in foliage form with Triodia is A. tetragonopylla (https://www.inaturalist.org/taxa/519206-Acacia-tetragonophylla). The phyllodes of this species are terete and ‘pungent’.

Acacia tetragonophylla

  • relies on germination to regenerate, but also grows slowly, and
  • is extremely drought-resistant, but poorly-adapted to fire.

The bottom line is:
The habitat of A. tetragonophylla does not correspond to that of Triodia. Instead, this shrub occurs in woodlands and on hills, where Triodia is scarce or absent.

Publicado por milewski milewski, 24 de junio de 2022

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Peter Latz (https://en.wikipedia.org/wiki/Peter_Latz_(botanist)) studied a vast semi-arid area in central Australia, mainly in the Northern Territory but extending into adjacent Western Australia and South Australia. His focus was ethnobotanical but his book is a good source of more general biological information.

Publicado por milewski hace alrededor de 2 meses (Marca)

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