Adaptive extremes in hummock grasses

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Readers, before reading this Post, please behold the following scene. The first photo is at ground level, and the second is from above: https://www.istockphoto.com/photo/australian-desert-gm594921940-101985513?phrase=red%20soil%20australia and https://www.alamy.com/stock-photo-spinifex-carpet-aerial-view-showing-patterns-of-spinifex-grass-red-122699733.html.

Many species of plants, worldwide, are foliar-spinescent.

However, some of these are particularly noteworthy, for their extreme combinations of adaptive features including foliar spinescence.

There are more than 100 spp. of Triodia (hummock grasses, https://en.wikipedia.org/wiki/Triodia_%28plant%29 and https://www.alamy.com/dunes-with-spinifex-grass-strzelecki-desert-south-australia-image9074071.html?imageid=9EAAA7EB-D58F-40D8-8C3B-60873AD52ECC&p=29927&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0 and https://www.alamy.com/stock-photo-desert-in-australia-with-spinifex-grass-australia-76155378.html?imageid=955B40B6-5C7C-4CE2-8A63-614C807E6B84&p=851480&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0).

All members of this genus are restricted to Australia.

Triodia dominates the vegetation over vast areas (https://www.researchgate.net/figure/Occurrence-of-Triodia-dominated-grasslands-in-Australia-26-source-Anon-2007-In_fig1_262603054 and https://www.agriculture.gov.au/sites/default/files/documents/mvg20-nvis-hummock-grasslands.pdf).
 
A LAST REFUGE FOR A NEARLY-EXTINCT BIRD

Triodia longiceps (https://www.inaturalist.org/taxa/502769-Triodia-longiceps and https://keys.lucidcentral.org/keys/v3/triodia/key/pilbara_triodia_app/Media/Html/entities/triodia_longiceps.htm and https://keys.lucidcentral.org/keys/v3/AusGrass/key/AusGrass/Media/Html/TRIODIA/TRILONC.HTML and https://alicespringsdesertpark.com.au/connect-with-nature/plants/plants/bull-spinifex and https://www.agric.wa.gov.au/rangelands/hard-spinifexes-triodia-spp-pilbara-western-australia and https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2910969) is an example of a foliar-spinescent plant with an extreme adaptive syndrome.
 
Within Triodia there is considerable variation in the degree of spinescence:

• ‘soft spinifexes’ are only weakly foliar-spinescent, often relying more on chemical defence (via resin) than physical defence, whereas
• ‘hard spinifexes’ are strongly foliar-spinescent (and usually not resinous). However, they vary considerably in the length of the leaves, and the shape and size of the hummock.

Triodia longiceps, although a noteworthy species, has currently only four observations in iNaturalist.

The species is actually widespread in semi-arid Australia, all the way from the Pilbara (https://en.wikipedia.org/wiki/Pilbara) in Western Australia to nearly Townsville in Queensland (https://en.wikipedia.org/wiki/Townsville). The distribution is mainly tropical. This is one of the more widespread of the spp. of Triodia.

In T. longiceps, the hummocks

• are particularly large, reaching 1.5 m high (https://www.inaturalist.org/observations/44462381 and https://www.inaturalist.org/observations/34621181), leading to the name ‘giant grey spinifex’,
• do not form rings as some congeners do, and
• present a wall of spines, impenetrable to e.g. the human hand.

Furthermore, the leaves

• are relatively elongated for Triodia, being 10-25 cm long, yet
• are extremely spinescent, i.e. they manage to be rigid and ‘pungent’ (https://www.dictionary.com/browse/pungent), despite being extremely slender.

There are few plants, worldwide, that can match this degree of ‘pungency’ relative to the ratio of length/width of the leaf. This species seems particularly sclerophyllous, even among members of its genus.

So, not only are the leaves extremely spinescent, but the spinescence is configured to be particularly impenetrable for any approaching herbivore.

Furthermore, the dull hue seen in several photos suggests that T. longiceps may perhaps be diallagous (see below). 

The night parrot (Pezoporus occidentalis, https://www.inaturalist.org/taxa/73180-Pezoporus-occidentalis) has recently been rediscovered in arid Australia. See https://www.bushheritage.org.au/species/night-parrot?gclid=EAIaIQobChMIq43PhqrF4wIVzIRwCh2SBw-VEAAYASAAEgJXa_D_BwE and https://www.dpaw.wa.gov.au/plants-and-animals/threatened-species-and-communities/threatened-animals/night-parrot.

As it turns out, part of the reason why this bird has eluded ornithologists for decades is that it has a habit of sheltering within the hummocks of T. longiceps by day.

If not for this species of hummock grass, this extremely endangered parrot might well be extinct, owing to depredations by feral Felis catus (https://en.wikipedia.org/wiki/Feral_cat).
  
 PHOTOSYNTHETIC RESURRECTION IN COMBINATION WITH FOLIAR SPINESCENCE
 
Most forms of grass become dry and yellowish/greyish in dry seasons, and grow new leaves once rains fall. This makes them technically deciduous. However, there are a few grasses in which the same, old leaf surfaces re-green, i.e. 'resurrect' repeatedly.

These are neither truly evergreen nor true resurrection plants (https://en.wikipedia.org/wiki/Resurrection_plant). They are more precisely described as 'diallagous' (George, 2002).
  
George’s study of diallagy in Western Australia can be found in full here: https://www.rswa.org.au/publications/Journal/85(1)/george85(1).pdf .
 
George (2002) found that various families, genera, and spp. of graminoids show diallagy. At least one species of Triodia shows this kind of physiological resurrection after drought.

Triodia is evergreen but drought-adapted. Therefore, it is unsurprising that diallagy has evolved within this genus.

And indeed, George (2002) observed diallagy in Triodia danthonioides (https://ausgrass2.myspecies.info/content/triodia-danthonioides).
However, this species is not typical of its genus, because

• it occurs in relatively well-watered habitat, in a limited area,
• it is one of the few spp. of Triodia that grows in kwongan (https://en.wikipedia.org/wiki/Kwongan), and
• it lacks the hummock growth-form.

So, does diallagy occur in any of the more typical members of the genus?

George (2002) did not investigate the tropical parts of Western Australia, where Triodia wiseana (https://www.flickr.com/photos/jantrac/9290241811/ and https://www.flickr.com/photos/jantrac/9290257709/ and https://www.inaturalist.org/taxa/1239787-Triodia-wiseana and https://www.inaturalist.org/observations/98136882 and https://florabase.dpaw.wa.gov.au/browse/profile/704 and https://keys.lucidcentral.org/keys/v3/AusGrass/key/AusGrass/Media/Html/TRIODIA/TRIWIS.HTML and https://ausgrass2.myspecies.info/content/triodia-wiseana and https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2919470), and other possibly diallagous spp. of Triodia, occur.
 
Based on the published observation of diallagy in T. danthonioides, I examined various photos of T. wiseana.

These do indeed show non-green hummocks. Triodia wiseana is a ‘hard spinifex’ with sclerophyllous, long-lived leaves (as opposed to the short-lived, perishable leaves typical of non-sclerophyllous grasses).

Therefore, T. wiseana seems to be a prime candidate for diallagy in a foliar-spinescent grass. This combination makes physiological sense, but, as far as I know, has not previously been documented in any plant species.
 
Based on photos, I suspect that another diallagous species of hummock grass is Triodia intermedia (https://www.inaturalist.org/observations/105772131 and https://www.agric.wa.gov.au/rangelands/hard-spinifex-triodia-intermedia-western-australian-rangelands).

Even the common and familiar Triodia basedowii (https://www.inaturalist.org/taxa/1030928-Triodia-basedowii and https://www.inaturalist.org/observations/91501414 and https://www.anbg.gov.au/photo/apii/id/dig/38089) may possibly be diallagous.
 
Diallagy, as a form of resurrection in plants, is not as easily recognisable as that of true resurrection plants - the leaves of which are non-sclerophyllous, yet able to remain non-green and dormant, sometimes for years on end.

Nonetheless, I suspect that Triodia, in sclerophyllous form, extends to physiological resurrection in at least a few widespread and ecologically important spp., that actually dominate vegetation over considerable areas. If so, they add up to the most extensive incidence of 'resurrection' in vegetation, anywhere on Earth.
  
The following shows what may be Triodia wiseana or Triodia intermedia in the Kimberley region of tropical Western Australia, in its normal dull-green state: https://www.alamy.com/stock-photo-australian-spinifex-grass-landscape-pilbara-western-australia-171465440.html?imageid=4C1F3ED4-2FBC-4593-A823-463F04CE4FD0&p=150170&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0.
  
The following show Triodia wiseana in its normal dull-green state: https://www.flickr.com/photos/jantrac/9290257709 and https://www.flickr.com/photos/21331264@N04/9179771015.
 
In the following (spp. yet to be identified), the lack of any green hue in the leaves need not indicate that the leaves are dead. Instead, these leaves may be able to respond to rain by re-greening and resuming photosynthesis.

https://www.alamy.com/stock-photo-colourful-australian-outback-landscape-with-golden-spinifex-grass-89271367.html?imageid=3166B8D2-5458-480C-A0D4-6C99A72AA0C6&p=195722&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0

https://www.alamy.com/spinifex-grass-palm-valley-northern-territory-australia-image242279808.html?imageid=D18546AE-33A9-440A-B527-721FA4661C0D&p=794157&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0

scroll in https://www.fishing-victoria.com/viewtopic.php?t=23375

The following shows what may be the start of the re-greening process in Triodia: https://www.alamy.com/stock-photo-spinifex-grass-in-the-purnululu-national-park-bungle-bungle-australia-34637508.html?imageid=ED33ADFC-9ADA-4A14-A955-3657E2956536&p=104079&pn=1&searchId=ff4ea337355047d5533cf5450cec7d1e&searchtype=0.

Publicado el julio 24, 2022 07:18 MAÑANA por milewski