Diversification of spinescence within the genus Rhamnus

Many members of the family Rhamnaceae (https://www.c82.net/twining/plants/?id=47) are indigenous to Australia. As far as I know, none is spinescent.

This is surprising, because

• Australia is, in some sense, the ‘world headquarters’ of foliar spinescence, and
• Rhamnaceae produce at least three types of spines on other continents.

The following excerpt emphasises the presence of foliar spinescence in Rhamnaceae, which is noteworthy given the lack of any foliar-spinescent spp. of this family native to Australia: https://bie.ala.org.au/species/http://id.biodiversity.org.au/node/apni/8761194

In this Post, I focus on the genus Rhamnus (https://www.inaturalist.org/observations?place_id=any&taxon_id=54812&view=species), in the Northern Hemisphere.

CATEGORIES OF SPINES

Foliar spines: borne on photosynthetic tissues (usually evergreen), these tissues being leaves, phyllodes, and (perhaps) cladodes.

Nodal spines: borne on non-photosynthetic stipules or short shoots (both of which, by definition, originate at nodes).

Caular spines: borne on non-photosynthetic stems, and located either along the stem (e.g. prickles in Rosa and Senegalia) or at its tip. The structure may be epidermal in origin, or it may retain a connection to the xylem and phloem of the stem.

RHAMNUS ILICIFOLIA AND ITS PARALLELS WITH GENUS  QUERCUS IN CHAPARRAL VEGETATION

Chaparral and macchia vegetation in mediterranean-type climates in the Northern Hemisphere contain foliar-spinescent, shrubby, evergreen spp. of the oak genus, Quercus (https://en.wikipedia.org/wiki/Oak).

The genus Rhamnus, which belongs to a different family (Rhamnaceae, as opposed to the oak family Fagaceae), shows evolutionary convergence with Quercus, in the same environment.

However, it is also noteworthy that the genus Rhamnus combines foliar spinescence with nodal spinescence in a way that – as far as I know – the genus Quercus does not.
 
Rhamnus, a smaller genus than Quercus, contains about 110 spp. These occur widely on several continents, and in both hemispheres. However, both Quercus and Rhamnus are absent from Australia as indigenous plants.
 
Rhamnus also differs from Quercus in generally being smaller plants (shrubs, not extending to tall trees as some oaks do), and in having fleshy fruits (as opposed to acorns).
 
Both Rhamnus and Quercus have some deciduous spp. and some evergreen spp. The foliar-spinescent spp. in both cases are evergreen.
 
In chaparral vegetation in California, Rhamnus ilicifolia (https://www.inaturalist.org/taxa/58308-Rhamnus-ilicifolia) and Rhamnus crocea (https://www.inaturalist.org/taxa/57243-Rhamnus-crocea) occur, with a similar type of foliage to that of coexisting shrubby evergreen spp. of Quercus. Both are, at least nominally, foliar-spinescent.

I do not know if there is a difference between ‘juvenile’ and ‘adult’ foliage in the foliar-spinescent spp. of Rhamnus. However, I suspect that this is so, based on the various photos I have seen on the web.
 
The foliar-spinescent spp. of Quercus are typical of chaparral vegetation in California. The same is true for Rhamnus ilicifolia and R. crocea.

A difference is that Rhamnus nowhere dominates the vegetation in the way Quercus does. While Quercus can be the dominant plant in stands of chaparral, Rhamnus is usually a subordinate/subsidiary plant.
 
There are at least three spp. of Rhamnus in the chaparral-like vegetation of the Mediterranean Basin in Eurasia and North Africa. However, as far as I know, none qualifies as foliar-spinescent.

Foliar-spinescent Quercus is typical of both macchia and chaparral, and the foliar-spinescent, evergreen, shrubby spp. of Quercus tend to coexist with various spp. of Rhamnus wherever they occur. Nevertheless, foliar-spinescent Rhamnus spp. are typical of chaparral, not of macchia.
 
Certain spp. of Rhamnus have nodal spinescence in the form of sharp twig-tips, these tips being non-green.

So the genus Rhamnus has some foliar-spinescent spp. and some nodal-spinescent spp. (the latter being the ones to which the common name ‘buckthorn’ applies).

However, as far as I know, no species of Rhamnus has both types of spines. This is partly because the foliar spines are associated with evergreenness, while the nodal spines in this same genus are associated with winter-deciduousness. I.e. there are two different ecological syndromes within the genus Rhamnus.
 
The following contain useful illustrations and distribution maps:mhttps://calscape.org/Rhamnus-ilicifolia-(Hollyleaf-Redberry)?srchcr=sc567cfe95b4f92 and https://www.calflora.org/cgi-bin/species_query.cgi?where-calrecnum=7076 and https://calscape.org/loc-california/Rhamnus%20crocea().
  
The following show the foliar spinescence of Rhamnus ilicifolia in chaparral vegetation in California: https://en.wikipedia.org/wiki/Rhamnus_ilicifolia#/media/File:Rhamnus_ilicifolia.jpg and https://www.calflora.org/cgi-bin/viewphoto.cgi?arg=/app/up/mg/59/mg17840-0.jpg.

NODAL AND CAULAR SPINESCENCE IN GENUS RHAMNUS
 
Above, I showed foliar spinescence in the genus Rhamnus, and I mentioned that the same genus also contains spp. with different categories of spinescence.
 
Here I introduce the widespread, common, rather weedy Rhamnus cathartica (https://www.inaturalist.org/taxa/54811-Rhamnus-cathartica and https://www.waterwereld.nu/wegedoorneng.html and https://open.alberta.ca/dataset/a4bf2d0a-6433-417f-a62e-64ec5304f4d2/resource/717fffdd-5320-428f-a4da-b772d77ec444/download/ard-weeds-common-buckthorn.pdf and https://www.friendsofthewildflowergarden.org/pages/plants/buckthorncommon.html).

This exemplifies the main forms of spinescence other than foliar spinescence in this genus.

Rhamnus cathartica is indigenous to Europe, North Africa, and western Asia. It is widely invasive in North America. Its natural habitat is cold in winter.
 
In R. cathartica, there are definite spines (https://www.flickr.com/photos/38984611@N03/3601890128 and https://www.botanickafotogalerie.cz/fotogalerie.php?lng=en&latName=Rhamnus%2520cathartica&czName=&title=Rhamnus%2520cathartica%2520%7C%2520&showPhoto_variant=photo_description&show_sp_descr=true&spec_syntax=species&sortby=lat), with a single ontogenetic nature. However, these are somewhat complicated to categorise functionally because of a kind of heteroblasty (https://en.wikipedia.org/wiki/Heteroblasty_(botany)).
 
The spines are essentially sharp, non-green twig-tips.

Each spine originates at the penultimate node of a given branchlet. (The node is marked by leaves or buds.) However, it is a shoot, not a stipule. The end of the spine-forming shoot presumably corresponds to the next node, but is reduced to a ‘pungent’ structure (https://www.merriam-webster.com/dictionary/pungent).
 
As the branches of R. cathartica grow, they eventually lose all leaves, as they are overtopped and outgrown by the crown. However, instead of dying and falling off, the spine-tipped branchlets grow into more or less permanent branches, becoming stouter than they were originally but retaining the sharp tip.

These long, sturdy spines then protect the main branches, presumably from large herbivores.
 
The spines at first project just beyond leaf-bearing nodes, and later come to be deployed in a part of the plant well below the crown. Therefore, I would call these spines initially nodal spines, and later caular spines.

It is hard to find any photos showing how the rigid, pointed side-branches start as foliage-bearing stems.

Even when the spines have leaf-bearing nodes proximally, they are rather poorly described as nodal spines. Hence I suggest that the best way to categorise the spinescence in R. cathartica is as caular spinescence, with the possibility of initial nodal spinescence as part of the ontogeny of the spines.

First the spines defend the leaf-bearing nodes from being stripped by large herbivores. Later, they defend the major branches from being pulled off and broken.

It is easy to envisage the nodal-spinescent function playing out with the extant fauna (e.g. deer). However, it is hard to envisage the caular-spinescent function, without invoking particularly large herbivores, such as proboscideans.
 
Rhamnus cathartica is winter-deciduous. This fits the relative defencelessness of the leaves, and the defensiveness of the branchlets and branches. There is little advantage in defending the leaves themselves, because they are naturally short-lived, and are due to be discarded at the end of a single growing season, anyway.
 
Overall, the spinescence of R. cathartica is consistent not so much with the defence of the leaves as with the defence of the gross infrastructure of the plant. This, therefore, seems to be a case of the persistence of a defence formerly adaptive to now-extinct megafauna.
 
The following provide useful information on Rhamnus cathartica: https://en.wikipedia.org/wiki/Rhamnus_cathartica and https://www.michigannatureguy.com/blog/2013/07/23/common-buckthorn-michigan/ and https://mnfi.anr.msu.edu/invasive-species/CommonBuckthornBCP.pdf.

The following (http://www.michigannatureguy.com/blog/wp-content/uploads/2013/07/Rhamnus-cathartica-thorns-inner-bark.jpg) shows the spines of Rhamnus cathartica at a stage of growth when the spine-tipped branches have already lost their ability to produce leaves.

Note that the spines are, right from the start, non-photosynthetic. This rules out their being categorised as foliar spines. What we see conforms with the category of caular spines:
 
The following (https://www.flickr.com/photos/38984611@N03/3601890128) shows that these caular spines persist as the branch continues to thicken into a main stem of the shrub, in R. cathartica. Note the radial orientation of the spines, such that this main stem is defended from various directions of attack by a large animal.
 
The following shows the spines retained, below the crown, on stems that no longer produce foliage (https://extension.umn.edu/sites/extension.umn.edu/files/commonbuckthorn-150.jpg).
 
DISCUSSION
  
In the genus Rhamnus, most spp. have no form of spine at all.

However, in the spinescent forms of Rhamnus, there is – as is generally true in plants across the board – a tendency to mutual exclusivity between foliar spines on one hand, and nodal/caular spines on the other hand.
 
This mutual exclusivity in categories of spinescence can occur between closely-related genera, or it can occur within a given genus. In the case of Rhamnus, it occurs within the genus: some spp. are foliar-spinescent, others nodal/caular spinescent

The two forms of defence tend to be incongruous with each other. This is because the former is associated with evergreenness and sclerophylly, while the latter is associated with deciduousness and orthophylly.
 
Rhamnus illustrates these patterns and principles. There is one species, namely Rhamnus crocea, that does combine the two categories of spinescence in marginal form. I.e. it illustrates a kind of compromise in which the species has not quite ‘decided’ on one category of spinescence over the other, and so has both categories in a weak form. In this sense it is ‘the exception that proves the rule’.
 
In chaparral in California, R. ilicifolia attains clear foliar-spinescence (although I get the impression that, even in this species, the leaves sometimes lack the spines, perhaps in ‘adult’ foliage).

Rhamnus crocea is closely related to R. ilicifolia, and also lives in the same vegetation type (as a smaller shrub).

However, it tones down the foliar spinescence while toning up the rigid, barrier-like configuration of branching that appears in caular/nodal-spinescent congeners, such as Eurasian R. cathartica and R. lycioides (https://www.dreamstime.com/rhamnus-lycioides-plant-rhamnus-lycioides-plant-garden-image116765426).
 
The result, in the case of R. crocea, is a bit like ‘falling between two stools’. This is because:

• its small leaves are variable, and even at their spiniest are not particularly good examples of spinescent leaves; and
• its stems have a clear ability to bar stripping of the foliage by large herbivorous mouths, but lack the sharp points necessary for qualification as a caular-spinescent or nodal-spinescent plant.

Overall, in terms of comparison with Australian plants:
Rhamnus crocea resembles an intermediate between a nodal/caular spinescent Coprosma (https://www.inaturalist.org/observations?place_id=any&taxon_id=70254&view=species), and a foliar-spinescent erica, two plant forms which occur for example in Tasmania. I cannot think of a real plant in Australia that looks particularly like R. crocea. However, the elements are there.
 
http://southwestdesertflora.com/WebsiteFolders/All_Species/Rhamnaceae/Rhamnus%20crocea,%20Redberry%20Buckthorn.html
 
https://www.calflora.org/cgi-bin/species_query.cgi?where-calrecnum=7075
 
https://en.wikipedia.org/wiki/Rhamnus_crocea#/media/File:Rhamnus_crocea_range_map.jpg
 
http://ucjeps.berkeley.edu/eflora/eflora_display.php?tid=41069

Publicado el julio 24, 2022 07:42 MAÑANA por milewski