mopane vega

(writing in progress)
 
I hypothesise as follows.
 
In the original situation, with the migratory system intact, this vega would have been free of the sagebrush (Pechuel-loeschea leubnitziae) as well as Bothriochloa.
 
With the demise of the migratory system (particularly the great densities seasonally of eland as well as gnu and zebra) the vega has been partly encroached by the sagebrush and the grass stratum has been encroached by Bothriochloa. This is now taken as ‘natural’ because the original migration has bee forgotten so completely that it is not even missed, i.e. scientists don’t even suspect that it once played an important role here.
 
Philenoptera violacea and Vachellia tortilis have not encroached, remaining at more or less their original numbers. This is because the current population density of the elephant is great enough to keep these woody plants in check, given that they are favourite targets of the elephant. The relationship of these two species to the elephant differs: P. violacea tends to be disfigured more and eaten less, while V. tortilis tends to be disfigured less and eaten more. However, the effect is similar in this context and that is that neither of these trees amounted to much in the original situation and neither amounts to much today, in terms of having usurped the habitat of the original sweet grasses.
 
Flueggea virosa has not encroached either; this is interesting because it is only too easy to image this vega choked with F. virosa in the way it is partly choked by the sagebrush. Here, the reason I give is that F. virosa is readily eaten by the impala as well as kudu and probably hook-lipped rhino (although obviously avoided by the elephant) and it is not a plant that thrives under even moderately intense herbivory (relying more on toxicity than on spinescence). Flueggea virosa may have encroached a bit on the knobthorn savanna west of Satara Rest Camp, but there is no sign of such encroachment anywhere in the Letaba area.
 
It is interesting that I did not even see Dichrostachys cinerea in this vega, i.e. it seems to be completely absent here to this day. Whereas D. cinerea seems to have encroached on gabbro in the southern Kruger Park, and it certainly has also partly encroached on basalt e.g. near Lower Sabie Rest Camp, it has completely failed to encroach in this vega. The best explanation I can offer at the moment is that D. cinerea cannot tolerate even slight seasonal waterlogging.
 
I doubt that the lack of migratory ungulates has made much difference to the nutritional regime at the level of what we have been analysing in the soils; I see the difference more at the mechanistic level of biomass-reduction by trampling and eating. In other words, I doubt that the encroachment by the sagebrush and Bothriochloa can be ‘fixed’ by fertilisation; instead what would ‘fix’ it is amelioration of the intensity of grazing and trampling by certain herbivores.
 
Were the elephant to be reduced or removed from this system, I predict that both P. violacea and V. tortilis would encroach on this vega, converting it to a savanna.
 
In terms of the practicalities and tactics of completing a good study, in which various loose ends are tied up, I suggest organising for the following two bits of data to be gathered while we still have a chance.
 
Firstly, we really should establish which species of Bothriochloa it is that grows in this vega. There are at least two possible spp. but it’s really not good enough that at the moment we can only say ‘Bothriochloa sp.’. Perhaps we could ask Tercia or Michele to collect this grass from this location next time they drive past with a game guard? (Also someone should go back to the original description by U. de V. Pienaar to see if he actually names the species of Bothriochloa in his description).
 
Secondly, we really should find out how successful the square-lipped rhino was in this vega during the heyday of this species of rhino in Kruger Park about a decade ago. The square-lipped rhino never became as successful in the Letaba area as farther south in the Park, but it is possible that at one stage before poaching (and possibly even today) this vega was fully inhabited by this species of rhino. Various people in the Park will know the answer and I suggest we ask them before the memory fades. The first person I’d ask is the section ranger, Andrew Desmet, who has an impressively long record in the Park and will probably be able to answer our question right off the top of his head if we make sure we phrase it clearly with sufficient context. The context is not just ‘does the white rhino occur in that drainage line’ but more ‘how common is the white rhino in that drainage line in the scheme of things, i.e. relative to its general occurrence in the northern Kruger Park and relative to its general occurrence in the southern Kruger Park, in both cases before the current spate of poaching?’

 
I think we can eliminate the simple explanation I suggested.

We cannot explain the commonness of Bothriochloa in the mopane vega north of Letaba as a manifestation of any sort of phosphorus-poverty. The reality is that here we have a situation where a sour grass has come to be abundant in a sweet environment. This is analogous to a partly run-down farm, i.e. an intensely disturbed environment where, for one reason or another, the most is not being made of the soil by the regime of consumption by animals, and a relatively unproductive and ‘stagnant’ plant has been allowed partly to ‘take over’ along the lines of an indigenous ‘noxious weed’. I’m come full circle to the explanation I suggested already more than four months ago, when I first saw how choked-up with Bothriochloa the mopane lands on basalt are in Kruger Park. This explanation is that the northern part of Kruger Park is no longer grazed as Nature intended, by a migratory system, and that as a result the most nutrient-rich environments such as this vega have languished somewhat, their productivity being partly stifled by what amounts to ‘mismanagement’. I.e. Bothriochloa is here not as a ‘healer of overgrazing’ (although that is likely to be the case in places in the southern half of Kruger Park, where there is still something closer to the original migratory system). On the contrary, Bothriochloa is here as a symptom of not enough grazing; through our extirpation of a whole migratory system of herbivory we’ve managed to sour a sweetveld, and none of the burning regimes maintained in the mopane lands in the last half-century have compensated for the basic problem that the best lands in northern Kruger Park are no longer grazed intensively enough during the green season. I suggest to you that in the original situation, a thousand years ago and perhaps even several centuries ago, this vega north of Letaba would have had so many large grazers on it, in the form of not only gnu and zebra but also eland and possibly an extinct gazelle, that Bothriochloa would have been absent. For that matter, the sagebrush would have been virtually absent too, for its proliferation is in line with that of Bothriochloa even though its niche is far more restricted in Kruger Park. 
Subject: simpler explanation for: corrected version of: Kruger analysis: first set
 
On second thoughts perhaps the rationale could be as simple as this, in order to answer the question: ‘why was Bothriochloa, a sour grass, so common in an area P-rich enough to exclude trees?’
 
Does the following answer make sense? The P/B ratio is great enough to exclude trees, but the absolute availability of P is not enough to support ‘sweet’ grasses.
 
Is it the case that the major cause of treelessness in the Kruger Park, which seems to be P-richness, could not possibly apply to ‘sour’ grasslands in southern Africa? These grasslands, common on the Highveld, cannot conceivably be P-rich given that the grasses are so nutrient-poor. So I presume that a major cause of treelessness in sour grassland is a too great P/B ratio, although the absolute values for P and B will be much smaller than those in Kruger Park?

One take on the Pienaar grasslands is that P as a catabolic nutrient and B as an anabolic nutrient are controlling the major difference in veg height between the grassland and the tall mopane.

This is perhaps the most mundane/prosaic possible result, lacking any kind of conceptual elegance along the lines of trace elements, catalysis, etc. However, the picture is all the more meaningful for that. Phosphorus is in a sense the ‘master nutrient’ (as indicated by the name ‘superphosphate’ and its effects have played out in the difference between the relatively sterile mopane country and the Pienaar grassland (which I do not think of as a dambo because I don’t see it as seasonally waterlogged). Fertilise the mopane soil with enough P and the grasses ‘outcompete’ the mopane, attracting large mammals which keep recycling the phosphorus. The role of B is a wonderful eye-opener.

It is a surprise to find that basalt is relatively P-poor and relatively B-rich, because this is the opposite of what I would have predicted, based on a) my general concept that basalt is a P-rich rock and b) the fact that basalt can be 20-fold poorer than shale in B.

If we take these mopane vegas as essentially alluvial, somehow what seems to have happened is that the P in the surrounding mopane soils has been washed into and concentrated in the vega (which is essentially a drainage line) but the boron has not! Why not? It is not just the concentration of P in the vega, but also the corresponding ‘depletion’ of the B, that has allowed grasses to ‘outcompete’ woody plants in the vega, not so? But which could be the geomorphic/landform processes leading to this inversion of the P/B ratio between the general, surrounding mopane on basalt and the drainage line? The nutrient picture we’ve discovered is simple, but all the more remarkable for that because it seems that such a simple and gross characterisation of the soils has escaped all other ecologists in Kruger Park? What we’ve found here is that the difference between mopane and grassland is a matter of the P/B ratio. One could put it differently as follows: what is wrong with most of the mopane lands, which leads to them being choked up with mopane instead of having treeless grassland attractive to grazing mammals, is that they are too P-poor (and too B-rich).
 
There are at least three other aspects of these mopane vegas that may be worth considering. Firstly, this was one of the few situations where I spotted signs of the mole-rat, Cryptomys, in Kruger Park. Note the agricultural analogy, with this rodent ‘ploughing’ the soils in a way that it is presumably not doing in the surrounding mopane. Secondly, an important component of the vegetation in this ‘mopane vega’ was the daisy shrub Pechuel-loeschea leubnitziae, which is an obscure plant for even those who are otherwise knowledgeable about the flora of Kruger Park but which is quite significant biogeographically and ecologically. This species is effectively a ‘sage’ in the sense of ‘sagebrush’ in North America, i.e. an aromatic daisy shrub associated with nutrient-rich soils. Because daisies are generally associated with soil disturbance, it is significant that, in a land of grewias and acacias and flueggeas etc. etc. etc. it is instead a ‘sage’ bush that forms a ‘buffer’ between the nearly treeless grassland and the mopane. This indicates how distinctive this nutritional situation is, of having a switch in the P/B ratio from the mopane to the drainage line. As I’ve previously mentioned, part of the fascination of this sage bush for me is having observed the plains zebra eat it with gusto during a drought. It is  obvious that the elephant, the hook-lipped rhino and most of the bovids are not eating this shrub and there is little surprise there because the thing about daisies is that they tend to be aromatic and thus unpalatable. But the zebra has its own niche in all of this and the valid generalisation that equids are specialised grazers hides this interesting nuance, this interesting ability of the monogastric grazer to exploit ‘sage’ as if it were ‘the grass you have when you don’t have grass’. I now realise how much we’ve lost in not being able to observe how the extinct quagga grazed the Karoo lands, where it no doubt remained mainly a grazer but also ate certain asteraceous shrubs, perhaps Pentzias, as part of its diet at certain times. Thirdly, it seems highly significant that even in this extremely fertile situation it was still Bothriochloa that patchily dominated. I would assume that these mopane vegas were more heavily grazed, seasonally, before say 1800 than they have been for the last century; I suspect that a former migration of zebra, gnu, eland and perhaps an extinct species of gazelle vanished undocumented from the mopane lands of what is now Kruger Park, and that one of the results of this loss has been the increase in Bothriochloa as a sign of ‘stagnation’ in the ‘natural’ regime of consumption on these exceptionally fertile soils. I.e. I might be able to understand that the mopane soils, with much B but relatively little P, have become choked with Bothriochloa; but to find this same grass so common on the vega soils really tells me that something is wrong with the modern regime of herbivory in Kruger Park.

If we take Bothriochloa as a sign of ‘healing’ of the veld, what could it possibly be healing from? The alternatives are a) overgrazing and b) stagnation, which are essentially opposites. I don’t see the Letaba area as overgrazed (in the way I might see the Satara area), so stagnation seems the more likely problem. How can a soil be so fertile as to convert mopane to treeless grassland, yet retain an aspect of infertility that leads it to be ‘wasted’ on a grass as poor as Bothriochloa? Where is the dystrophy in this soil that leads it to produce a rubbish grass instead of just ‘sweet’ grasses and lawning grasses?]

(writing in progress)

Posted on 07 August, 2022 01:34 by milewski