Caesalps on southern continents, part 1

Africa, South America and Australia are all southern continents ( and, but they differ biogeographically.

Africa broadly straddles the equator and has always been connected to Eurasia. South America was formerly isolated, but is now connected via North America. Australia remains an isolated southern landmass, an island continent.

Africa has a balance of wet and dry environments, South America has extensive wetness, and Australia has extensive drought (

The comparative biogeography of various families of animals, plants, fungi, and algae on the southern continents is of obvious interest. Here I investigate the caesalps (including both and, a major category of mainly woody and mainly tropical plants capable of dominating vegetation - but with seemingly unpredictable ecological patterns.

I realise that, among the legumes, the pea (Fabaceae) and mimosa (Mimosaceae) families are the clearly-defined ones and 'Caesalpiniaceae' refers to what is really a heterogeneous assemblage of 'other legumes' ( and and However, I think we can learn something interesting about biogeography by categorising the non-pea and non-mimosa legumes as caesalps.

With this caveat in mind, caesalps are surprisingly different in their incidence among our three continents. By incidence I mean the size of the plants and the proportion they occupy in the vegetation.

In a series of four Posts, I will sketch the respective floras of caesalps from this ecological viewpoint. I will then try to explain the overall pattern: major in Africa but minor in Australia, with South America intermediate.

All legumes tend to have nutritional niches in which the supply of the protein-forming element, nitrogen, is supplemented from sources other than the soil. What sets caesalps apart from other legumes is that they are the least likely of the three families to form nodules on their roots, in which bacteria symbiotically fix atmospheric nitrogen.

Of the three southern continents, it is Australia that has the least incidence of caesalps.

About 18 genera are indigenous (, but most of these have failed to penetrate the most extensive vegetation types in Australia, and those that have done so tend to consist of small plants despite the drought-tolerance of trees on this continent ( Furthermore, it is the cosmopolitan genera of caesalps that tend to be prominent in Australia (Senna, Cassia, Chamaecrista, Bauhinia, Caesalpinia).

The few genera of caesalps restricted to Australia (e.g. Petalostylis, Labichea, Barklya and are ecologically unimportant in the sense of being small plants making minor contributions to the vegetation.

Several genera are restricted to tropical rainforest, which covers only a small proportion of this continent. Most of these (e.g. Cynometra, Crudia, Maniltoa, Sindora, Storckiella) occur in tropical Asia and have only marginally penetrated Australia. However, it is noteworthy that Cassia grows to the size of a tree adjacent to rainforest in Australia.

The only genera of caesalps which qualify as dominating any vegetation in Australia are Senna (e.g. and and perhaps Lysiphyllum ( and and However, Senna in Australia forms shrubs rather than trees, and possesses nitrogen-fixing nodules; and plants of Lysiphyllum are small for trees.

The genus Erythrophleum ( and and occurs mainly in Africa, and the genus Peltophorum ( occurs mainly in South America. Both have a minor incidence in tropical Australia.

The only genus recorded to be ectomycorrhizal is Intsia ( and, which has an extremely restricted incidence in rainforest in Australia (

In summary, the incidence of caesalps in Australia is slight in several ways. Those species growing as large plants make only a slight contribution to the canopy where they occur. Most of the tree species have penetrated Australia only slightly. And most of the species deeply penetrating the continent are slight in the sense of being shrubs, dominant over only small areas, and replaced successionally by trees belonging to families other than caesalps.

To be continued...

Posted on 06 November, 2021 11:06 by milewski milewski


Not sure I agree with your last statement that "nowhere particularly common". Senna is a major part of the environment in inland arid Australia, throughout the arid parts of South Australia, Northern Territory and Western Australia. Much of this country is arid shrubland dominated by a range of genera - but especially Acacia, Senna and some of the chenopods - Maireana and Atriplex in particular. Senna is one of the more dominant genera in the environments which cover a large percentage of Australia.

With respect to Lysiphyllum - it is quite common and obvious in parts of arid tropical Australia but it is not what I would call dominant in many areas - perhaps codominant with species like Cochlospermum fraseri, Adansonia gregorii, and large shrubs/small trees such as Grevillea, Hakea and Acacia species or occasionally small eucalypts

Posted by arthur_chapman over 2 years ago

@arthur_chapman Many thanks for your valuable comment. I accept your point about Senna and I have edited the text accordingly - thank you for helping me to keep improving my writing. Given that Senna is common in parts of semi-arid Australia and in the current context of an intercontinental comparison, would you say that Senna (and allied genera such as Cassia and Chamaecrista) have a greater incidence in Australia than in Africa or South America, in comparable environments?

Posted by milewski over 2 years ago

@arthur_chapman Apologies to you, Arthur, I think I have been too slow to take up your criticism fully. I now realise that Senna can be dominant in at least small patches of vegetation (e.g. I will change my Post accordingly.

Posted by milewski over 2 years ago

@arthur_chapman Hi again, Arthur, I've made edits accordingly. Are these ok by you?

Posted by milewski over 2 years ago

Hi Antoni

Just a few comments - you say " despite the prevalence of trees on this continent generally." but about 1/3 of Australia is arid, semi-arid or desert - very few trees over most of that area. I have recently been in the Tanami Desert and on the Nullabor - and there, there are either no trees, or scattered shrubs (Acacia, Senna, Grevillea, etc.)with very few trees. If you look at the distribution of Acacia in Australia - it is over most habitats, eucalypts (Eucalyptus and Corymbia) on the other hand - the dominant trees are not commo0n in the arid, and what is there is generally restricted to watercourses (trees) or in large swathes as small trees in the mallee (mallee habits). Trees are prevalent in the Tropics (Queensland, Northern Territory and Western Australia), in the east, south east and along the west and south west coasts, but the whole of the inland - extending to the Western Australian coast is arid, desert or semi-arid. The rest looks OK.



Posted by arthur_chapman over 2 years ago

@arthur_chapman Hi Arthur, many thanks for these additional thoughts and you're probably right that my writing could be improved here too. Anyway, I'll reply soon because the question of vegetation height is one that I've been particularly interested in (e.g. and and

Posted by milewski over 2 years ago

Well written and goad suggestions from Arthur. I'm not a botanist or scientist, just a vastly travelled amateur but my one comment would be where you write "and plants of Lysiphyllum are small for trees." I would disagree with the word small. All depends on definition of a tree as a minimum height but I would think that an awful lot of Lysiphyllioum could be classified as medium trees, 6-8m. If you regard that as a large shrub than your statement would be correct.

Posted by gjn over 2 years ago

@gjn Hi Greg, many thanks for your comment. As a vastly travelled amateur you are my favourite kind of reader and I look forward to more commentary from you in future. Size is indeed relative. In the present context, trees in the caesalps generally range from 5 to 30 metres high, with a particularly common and familiar species in Africa being typically 15 metres or more ( So if we broadly characterise trees in the caesalps as small, medium, or large, would you agree that Lysiphyllum (particularly cunninghamii, which seems to be the one most likely to be the main tree in its habitat, e.g. on the dry southern edges of the Kimberley region in Western Australia), as in the (relatively) small category?

Posted by milewski over 2 years ago

Probably yes.

Posted by gjn over 2 years ago

G'day Antoni;
I'm an ameteur who lives in semi-arid South Australia, near Swan Reach Conservation Park, which can be found via google.
So please take my comments with a pinch of salt, and not intended as criticism but expansion of thought.

I think there is an error on your link page
What you have down as Senna artemisioides nothosubsp. coriacea
I think is Senna artemisioides ssp. zygophylla
Here is our treatment of our Senna:
I find id'ing Senna very difficult as the leaf shape changes with age, season & rainfall.

To our South, 1km, are thousands of acres of monoculture of Mallee Trees. Biodiversity is very minimal. It covers clay, limestone and sand dumes.
Our property was damaged by western culture. After the war, return servicemen were given land as a reward to farm. There is a law to clear their parcels of land for farming to produce food. Hmmm.
So Ellura (188 acres) was damaged in this vein, but the result after 100 years is incredible biodiversity. We have identified over 1,300 species (majority are insects).
We are surrounded by thousands of Senna plants (as well as Mallee, saltbush, etc) and feel it is an imperative cover to stop erosion and provide habitat to the forbs/herbs and animals.
If you think in terms of time, 1,000's of years, it seems Senna is an important interum species between destroyed land and Mallee monoculture. I don't have any research to back this up. Just think about it a lot. Without western culture destruction, one surmises the only other sort of destruction would be fire & weather. In full mallee canopy, the trees support each other so wind doesn't have much impact, and it's clearly resistant to drought. So fire seems to be the only contender of natural wide spread destruction.
When sheep are removed from our land, grass establishes/protects the ground. Then the Microphytic crust (which inhibits grass germination), then forbs & bushes (Senna) start to reclaim the land. And eventually mallee trees.
My point is that Senna is incredibly important, in my opinion, not "make only a slight contribution to the canopy where they occur." For many years they'll be the only canopy, and are more biodiversity friendly than Mallee. I certainly wouldn't describe Senna as "cosmopolitan".

Keep up the good work.



Posted by ellurasanctuary over 2 years ago
Posted by milewski over 2 years ago

@ellurasanctuary Hi Brett, how wonderful to hear of your work in restoring the land, and how good it is to be able to discuss the nitty-gritty with someone with so much practical experience. Thank you for the correction re the classification of Senna artemisioides. I take your points that Senna is crucial in the regeneration of mallee ecosystems, that the successional stage featuring Senna is far richer in species than mature mallee, and that the word 'cosmopolitan' is inept in describing Senna in the context of this regenerative role. I guess that where there may be some possibility of confusion is the different angles from which a particular topic can be approached. What prompted me to write this series of Posts is the observation that, for all the importance of e.g. Senna in Australian ecosystems, 'caesalps' in tropical Africa wind up dominating vegetation on a far larger scale than anything seen in Australia, whether in the tropics or in the temperate latitudes. Perhaps I can try to summarise this difference as follows, and thank you for helping me to clarify my thinking by putting me on the spot a bit. Please bear in mind that, while biodiversity is important, the focus of this series of Posts is more on the collective ecological incidence of a whole botanical category (i.e. all caesalps as opposed to peas or mimosas) in terms of the size of the plants and the degree to which they have excluded other plants. In Africa, much of the continent is covered by caesalps in the form of trees (usually taller than 10 metres), which are the most abundant trees in their vegetation types. Caesalps tend to dominate, in this sense, in Africa, by achieving a combination of plant size and a competitive success that persists into the 'mature' stages of succession. In Australia by contrast, caesalps nowhere achieve this kind of ecological importance (as measured by a combination of plant size and competitive superiority). Eucalypts instead show an incidence comparable with that of caesalps in Africa, which suggests some important difference in the environmental regimes because eucalypts are so biologically different from caesalps. The one situation in which caesalps do have a considerable incidence, in the context I've stipulated, is the successional stage with which you're so familiar in your area. Here it is indeed true that a caesalp dominates vegetation by forming the largest plants and by being competitive with all other plants, but Senna still falls far short, in a relative sense, because: a) the plants are only shrubs, not trees taller than 10 metres, b) this genus forms trees as well as shrubs on other continents, some reaching up to 15 metres (e.g. c) Senna fixes nitrogen in nodules and in this sense is more similar to peas and mimosas than to other caesalps, and d) the stage of vegetation dominated by this genus in mallee ecosystems is merely successional to eucalypts and acacias, which means that plants other than caesalps come to dominate. By calling the genus Senna 'cosmopolitan', what I mean is that it and closely related genera differ from the important caesalps in Africa by being far more widespread in Earth, both intercontinentally and across the latitudes from the equator to e.g. southern Australia. This is significant inasmuch as it is consistent, at a generic level, with the weediness of relatively small, short-lived plants occupying temporary roles in patchy succession for relatively brief periods. Does this help to explain? With best wishes from Antoni

Posted by milewski over 2 years ago

The following show that species of Senna indigenous to continents other than Australia can grow to trees: Senna spectabilis ( and and Senna surattensis ( and Although the closely related genus Cassia also grows into trees in Australia (see, as far as I know the species of Senna indigenous to Australia remain as shrubs.

Posted by milewski over 2 years ago

G'day Antoni;
Sorry, wasn't meaning to put you on the spot. And I have absolutely no idea about too much outside of South Australia. Qld, eg, may as well be another country in terms of it's ecology, ha ha :-)
So was just relaying to you how your discussion fits the world around here. We (Marie & I) a lot of time trying to get locals to see the value of small plants, etc, and their importance on the local environment.
The traditional European concept of "big trees are better" doesn't really work so well out here.
My interpretation of "cosmopolitan" generally means "spread by man accidentally in cities". I guess introduced species are a big problem here, introduced via cities into the country and rather than be open and say something is introduced, reports often want to down play/hide the issue by saying it's naturalised, or cosmopolitan.
Again, my appolgies, just offering a different view.

Posted by ellurasanctuary over 2 years ago

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