Is spinescence in cycads associated with dry climates?

@dewet @fahrenheit_66 @kucycads @jayhorn @tonyrebelo @ludwig_muller @jeremygilmore @botaneek @troos

Most naturalists might agree that there seems to be a general association between dry climates and spinescence in plants (https://en.wikipedia.org/wiki/Deccan_thorn_scrub_forests and https://en.wikipedia.org/wiki/Northwestern_thorn_scrub_forests and https://en.wikipedia.org/wiki/Madagascar_spiny_forests and https://en.wikipedia.org/wiki/Sonoran_Desert and https://www.anbg.gov.au/photo/vegetation/hummock-grasslands.html and https://en.wikipedia.org/wiki/Caatinga).
 
After perusing most of the nearly 300 spp. of cycads described in detail by D L Jones (2002, https://www.amazon.com/Cycads-World-Ancient-Plants-Landscape/dp/1588340430v and https://www.penguinrandomhouse.com/books/206597/cycads-of-the-world-by-david-l-jones/), I can make a preliminary test of this notion for cycads.
 
My test is a simple one: I went through all the species to identify those living in semi-arid climates, defined as < 500 mm/year of mean annual rainfall. I also noted the general configurations of spinescence for all species in each of the genera of cycads.
 
The results are as follows.
 
Firstly, the numbers of spp. in each genus occurring in semi-arid climates are as follows:
 
Bowenia: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=129812&view=species
 
Ceratozamia: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=129813&view=species
 
Chigua: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=67326&view=species
 
Cycas: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=129782&view=species

Dioon: D. sonorense
https://www.inaturalist.org/observations?place_id=any&taxon_id=129814&view=species

Encephalartos: E. hirsutus, E. concinnus, E. lehmannii, E. longifolius, E. horridus (this latter sp. also extending into climates with > 500 mm/year mean rainfall)
https://www.inaturalist.org/observations?place_id=any&taxon_id=129815&view=species
 
Lepidozamia: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=129816&view=species

Macrozamia: M. macdonnellii, M. sp. ‘Eneabba’, M. moorei
https://www.inaturalist.org/observations?place_id=any&taxon_id=129824&view=species

Microcycas: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=136621&view=species

Stangeria: nil
https://www.inaturalist.org/taxa/135513-Stangeria-eriopus
 
Zamia: nil
https://www.inaturalist.org/observations?place_id=any&taxon_id=67326&view=species

In summary, there are 9 spp. of cycads, worldwide, that live in semi-arid climates, representing three genera.
 
By the way, a biogeographical anomaly that emerges clearly: South America is unusual in that no cycad penetrates its semi-arid regions.
 
Dioon sonorense (https://www.inaturalist.org/taxa/136079-Dioon-sonorense) is hardly spinescent. Its petiole lacks prickles, and its pinnae are not ‘pungent’ (https://www.finedictionary.com/pungent) although the pinnal margins have three ‘small spines’. This species is certainly not more spinescent than expected for its genus.
 
Encephalartos hirsutus (https://www.inaturalist.org/taxa/136105-Encephalartos-hirsutus) has ‘very pungent’ tips to the pinnae, but lacks marginal spines on the pinnae. The proximal pinnae are not reduced to spines, although I assume that they are spinescent like all the other pinnae.
 
Encephalartos concinnus (https://en.wikipedia.org/wiki/Encephalartos_concinnus#) has relatively complex spinescence for its genus:

• the tips of the pinnae are ‘pungent’,
• the pinnal margins each have 1-4 spines (which I assume also to be effectively ‘pungent’),
• the proximal pinnae are reduced to spine-like structures, and
• these structures are multilobed, i.e. each possesses several points.

Encephalartos lehmannii (https://www.inaturalist.org/taxa/135957-Encephalartos-lehmannii) has ‘very pungent’ tips to the pinnae (which are particularly sclerophyllous), the margins of the pinnae lacking spines except in juveniles (which have a ‘single basal spine’ on each pinna), and the most proximal pinnae spine-like (which I take to mean as pungent as the other pinnae but lacking the pinnal blade).
 
Encephalartos longifolius (https://www.inaturalist.org/taxa/135958-Encephalartos-longifolius) has ‘pungent’ tips to the pinnae, but these are instead blunt and rounded in one district. The pinnal margins usually lack spines, but 1-3 spines sometimes occur on the lower margin near the base of the pinna. The proximal pinnae are reduced in size but not modified in shape to be ‘spine-like’.
 
Encephalartos horridus (https://www.inaturalist.org/taxa/136109-Encephalartos-horridus) has

• pinnae with ‘pungent’ tips,
• lower margins with 1-3 prominent pungent-tipped lobes to 4 cm long which are twisted out of the plane of the pinnae and form an impenetrable barrier, and
• proximal pinnae which are reduced in size and lacking lobes, but not usually spine-like.

The leaves of this species form an entangled crown, i.e. dense clusters of overlapping foliage. Encephalartos horridus is described as having ‘rigid, extremely spiny leaves’.
 
Macrozamia macdonnellii (https://www.inaturalist.org/taxa/135846-Macrozamia-macdonnellii) has ‘thick-textured’, rigid pinnae with ‘pungent’ tips. The proximal pinnae are gradually reduced to pungent spine-like structures.
 
Macrozamia sp. ‘Eneabba’ seems hardly leaf-spinescent. The pinnae are ‘leathery’ with tips that are not described as ‘pungent’ but merely ‘tapered to a long, drawn-out apex’. The proximal pinnae are reduced in size, the most proximal pair ‘sometimes spine-like’.
 
Macrozamia moorei (https://www.inaturalist.org/taxa/135849-Macrozamia-moorei) has pinnae ‘tapered to a pungent apex’, the proximal pinnae being ‘gradually reduced to a series of rigid spine-like structures’.
 
Please note that none of the above spp. possesses a spinescent petiole or rachis (https://www.dictionary.com/browse/rachis#:~:text=rachis-,rhachis,part%20that%20carries%20the%20barbs).
 
Discussion:

• From these results we can see that the sole semi-arid cycad in the Americas is hardly spinescent. This undermines the notion of particular spinescence in dry climates.
• In the case of Encephalartos, there is plenty of spinescence in the semi-arid spp. However, there is not necessarily more than expected for this genus, which is a particularly spinescent genus of cycads.
• In the case of Macrozamia, once again the hypothesised trend is hardly supported. The three semi-arid spp. vary in spinescence, one of them being possibly non-spinescent.

My conclusion:

The idea that spinescence is particularly associated with plants of semi-arid conditions is not supported by cycads. In these plants, several genera do penetrate semi-deserts. However, the spp. concerned are not, in general, more spinescent than their mesic relatives.

Posted on Ιούλιος 03, 2022 0750 ΜΜ by milewski