Caesalps on southern continents, part 4

In this series of four posts I have summarised the incidence of caesalps according to the size of individual plants and their appropriation of the canopy, averaged over the entirety of each continent.

This crude estimation suffices to show that the southern continents differ by two orders of magnitude in the incidence of legumes other than peas and mimosas: about tenfold greater in South America than in Australia, and about tenfold greater again in Africa than in South America.

This variation is particularly surprising in view of the fact that various genera (e.g. Hymenaea, Cynometra, Copaifera, Erythrophleum, Bauhinia, Piliostigma, Crudia, Senna, Parkinsonia, Dialium) are indigenous to more than one of these continents.

The incidence of ectomycorrhizal (https://mycorrhizas.info/ecm.html) caesalps varies in the same direction and by similar orders of magnitude, suggesting that the prevalence of caesalps in Africa is linked to a mode of supplementation targeting not only nitrogen but also other nutrient elements.

The intercontinental differences are not merely the result of the drought of Australia and the wetness of South America. Instead, they apply even within categories of climate and arborescence.

Taking tropical savannas for example: in Australia the dominant trees tend to be ectomycorrhizal Myrtaceae (eucalypts), with caesalps relegated to a minor incidence and lacking ectomycorrhizae. By contrast, in Africa the dominant trees tend to be ectomycorrhizal caesalps, of which Brachystegia is representative.

So, if the intercontinental variation ultimately reflects environmental differences, what could these be?

Well, Africa is the continent on which there is the greatest incidence of large herbivores, which has implications for the cycling of crucial nutrients such as phosphorus and zinc (https://onlinelibrary.wiley.com/doi/pdf/10.1111/jbi.12100 and https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2699.2000.00436.x).

It also differs from South America and Australia in the occurrence of fungus-culturing termites (see https://www.inaturalist.org/journal/milewski/59182-comparisons-of-termites-and-termite-eating-animals-in-africa-and-australia-part-1#), which are the most powerful recyclers of nutrients among the social insects.

Furthermore, the three continents form a series w.r.t. the histories of anthropogenic influence on the cycling of nutrients. The human species has farmed in tropical Africa for most of the last ten thousand years, whereas there was no farming in Australia until Europeans settled. As in various other aspects both ecological and biotic, South America is intermediate.

Miombo woodland and other caesalp-dominated vegetation in Africa has been affected continually by proboscideans. The bush elephant (Loxodonta africana) and other large herbivores were hardly affected by the spate of megafaunal extinctions which wiped out proboscideans from the Americas by ten thousand years ago. Although practised in tropical South America for at least five thousand years, farming seems not to have affected the main types of woodland and savanna (llanos, caatinga, chaco, cerrado), leaving these vegetation types largely unaffected by both shifting cultivation (https://en.wikipedia.org/wiki/Shifting_cultivation) and all herbivores heavier than 100 kg.

Because woodlands dominated by ectomycorrhizal caesalps usually occur on relatively nutrient-poor soils, the role of fungus-culturing termites (particularly Macrotermes, https://en.wikipedia.org/wiki/Macrotermes) may be crucial. These termites concentrate trace elements in their mounds (https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.2008.00544.x), which can form patches of sufficient fertility to grow crops. This advantage sets Africa apart from the other two continents and perhaps helps to explain the antiquity and extent of shifting cultivation and the adaptation of caesalps to an anthropogenic regime on a large scale.

Miombo woodland and its equivalent north of the equator are so resilient from slash-and-burn cultivation (https://www.jstor.org/stable/4620417) that these vegetation types can be seen as, in some sense, anthropogenic (https://en.wikipedia.org/wiki/Chitemene#:~:text=Chitemene%20(also%20spelled%20citemene)%2C,agriculture%20practiced%20throughout%20northern%20Zambia.). Disturbance by humans and large herbivores may be integral to the dominance of caesalps.

What looks like pristine woodland may actually be the 'climax' of a successional cycle in which the trees are intermittently broken down by physical means, and their eventual regeneration is facilitated by the patchy suppression of wildfires carried out by farmers and gregarious herbivores.

Posted by milewski milewski, November 08, 2021 07:43

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B L Mitchell (1960, Kirkia 1: 120-128) explained the local dominance of the caesalp Baikiaea plurijuga in southwestern Zambia, as follows. This species grows on nutrient-poor sandy soils and is not eaten by large herbivores, but it depends on the herbivores for its regeneration and establishment. Because of the fortuitous distribution of seasonal pools of drinking water on the plain, the movements of the large herbivores tend to ensure the removal of all fuel for wildfires in certain zones where the trampling is most intense. This temporary alleviation of damage by combustion allows the seedlings of B. plurijuga to establish. The result is vegetation dominated by a tree species too poor to be eaten by the herbivores crucial for its episodic regeneration. Although Baikiaea is not ectomycorrhizal, and is relatively restricted in distribution, a similar theme may apply to the other caesalps capable of dominating woodlands and forests: the mature vegetation is unattractive to large mammals except on the nutrient-rich large mounds built by Macrotermes, but the regeneration ultimately depends on the patchy but intense disturbance, fertilisation, and fire-exclusion enacted by large mammals including the human species.

Posted by milewski 8 months ago (Flag)

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