Foliar-spinescent Acacia maitlandii, an associate of foliar-spinescent hummock grasses

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Foliar-spinescent grasses in the genus Triodia (https://en.wikipedia.org/wiki/Triodia_(plant)) dominate the vegetation over large areas in semi-arid Australia. Various spp. of Acacia are associated with Triodia (see https://www.inaturalist.org/journal/milewski/67576-to-what-extent-do-foliar-spinescent-acacias-coexist-with-foliar-spinescent-grasses-in-central-australia).
 
Essentially, Acacia can occupy three different types of niche in vegetation dominated by Triodia, as follows.

• Acacia can be somewhat like a woody version of a ‘fireweed’, growing rapidly after intense fire combusts the hummocks of Triodia, and then dying after a few years as the Triodia once again usurps its space. My impression that these short-lived spp. of Acacia tend to be neither foliar-spinescent nor particularly flammable. They are essentially successional to Triodia in the fire-cycle. The seeds tend to be hard, capable of remaining dormant for decades.
• Acacia can take over with the senescence of the hummocks of Triodia in the absence of fire. Although Triodia is ‘designed to burn’, it can happen that a given stand somehow evades being burnt for several decades. In such cases the hummock grass tends to lose vigour, and shrubs or low trees of Acacia tend to increase at the expense of the grass, simultaneously reducing the flammability of the whole vegetation. I suspect that most spp. of Acacia in this category are not foliar-spinescent.
• Acacia can co-occur with the hummocks of Triodia as a minor component of the mature vegetation. The species in focus in this Post, namely Acacia maitlandii (https://www.inaturalist.org/taxa/926523-Acacia-maitlandii and http://www.flora.sa.gov.au/cgi-bin/speciesfacts_display.cgi?form=speciesfacts&name=Acacia_maitlandii), is an example. This species happens to be foliar-spinescent, paralleling Triodia in this way.

Acacia maitlandii (https://en.wikipedia.org/wiki/Acacia_maitlandii and https://florabase.dpaw.wa.gov.au/browse/profile/3434) is

• widespread in semi-arid Australia from the Pilbara in the west to Queensland in the east (https://en.wikipedia.org/wiki/Acacia_maitlandii#/media/File:Acacia_maitlandiiDistMap573.png),
• a shrub of only about 2 m height, which means that it does not rise high above the hummocks of Triodia (https://bie.ala.org.au/species/https://id.biodiversity.org.au/taxon/apni/51285931), and
• always sparsely distributed, never dense or dominant.

Acacia maitlandii accompanies Triodia merely as a sparse, low ‘upper stratum’. It has a lignotuber/roostock of sorts, which means that it can regenerate vegetatively after wildfire, provided that the combustion is not intense. The seeds of A. maitlandii are relatively soft, distinguishing it from the many 'hardseeded' species of Acacia.
 
Judging from the form of its elaiosome, A. maitlandii is dispersed and sown by ants, which is consistent with a semi-pyrophilic niche (http://worldwidewattle.com/speciesgallery/maitlandii.php).

There are many spp. of Acacia with spinescent phyllodes. However, A. maitandii seems to be among the most strongly spinescent spp. in the genus. Its phyllodes vary in shape but are always spinescent (https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Acacia~maitlandii and http://worldwidewattle.com/speciesgallery/maitlandii.php).
 
An odd feature of A. maitlandii is that it has resinous twigs, presenting another possible parallel with Triodia (various spp. of which are also resinous). I presume that one of the functions of the resin in A. maitlandii is to deter herbivores, but I do not know if – as in the case of Triodia – it also promotes flammability.

I know of few foliar-spinescent plants, other than spp. of Triodia, which combine resinousness with foliar-spinescence. Therefore, this combination in A. maitlandii deserves further investigation.
 
The following shows the ‘pungent’ tips of the sclerophyllous phyllodes of Acacia maitlandii: https://en.wikipedia.org/wiki/Acacia_maitlandii#/media/File:Acacia_maitlandii_flowers_and_foliagee.jpg and https://www.inaturalist.org/observations/91523066 and https://www.inaturalist.org/observations/121559495 and http://worldwidewattle.com/speciesgallery/descriptions/pilbara/html/maitlandii.htm.
 
The following show that the shape of the phyllodes varies within A. maitlandii: http://worldwidewattle.com/speciesgallery/images/maitlandii.jpg and https://apps.lucidcentral.org/wattle/text/entities/acacia_maitlandii.htm. 
  
The following show that the crown of A. maitlandii tends to be rather sparse: https://en.wikipedia.org/wiki/Acacia_maitlandii#/media/File:Acacia_maitlandii.jpg and https://www.inaturalist.org/observations/9158107.
  
The following shows the coexistence of A. maitlandii with Triodia: http://www.flora.sa.gov.au/cgi-bin/speciesfacts_display.cgi?form=speciesfacts&name=Acacia_maitlandii.

Publicado el junio 25, 2022 02:46 MAÑANA por milewski