Phenological evidence of relationship between water and height of vegetation on basalt in Kruger National Park

Please see https://www.inaturalist.org/journal/milewski/68753-vegetation-on-basalt-in-kruger-national-park-does-not-exhaust-its-water-supply-in-drought-even-where-there-are-trees-6-m-high#.

I turn now to the relatively treeless area on basalt, north of Satara in Kruger National Park. This continues my documentation and discussion of phenological patterns, as revealing ‘surplus’ water in the soil, even during the drought of 2016.
 
See https://www.tandfonline.com/doi/abs/10.2989/10220119.2021.1938222.

As far as I know, the first rain fell at the same time in both of our sampling areas on basalt in the vicinity of Satara, on 11-12 Nov. 2016.

I attempted to verify this, by checking how deeply the rain had penetrated the soil. I found that a scratch in the soil showed penetration to be about 5 cm, in both sampling areas.

Despite this commonality in the timing of the drought-breaking rainfall, the phenological state of the woody plants was relatively advanced in the relatively treeless area.

This can perhaps be seen as consistent with the even greater ‘surplus’ of water in the soil here than in the woody vegetation on basalt (detailed in a recent Post).
 
The specific observations are as follows.
 
The following species were already in some leaf during my visit on 15 November 2016:

The species of Grewia common in the woody vegetation on basalt – which I suspect to be G. bicolor - was uncommon in this relatively treeless vegetation on basalt. However, here it had already started to shoot leaves, and flowers had appeared. By contrast, on the same day (15 Nov. 2016) I observed it as completely bare in the woody vegetation on basalt.

Dichrostachys cinerea had already started to shoot leaves (from the base of the stem). This means that this species seemed more advanced here than in the woody vegetation on basalt; I even saw a few inflorescences here, already.

Typical evergreenness was not exemplified by any plant in this relatively treeless area area. However, Combretum imberbe was somewhat evergreen. Even a suppressed ‘sapling’, only 1 m High, of C. imberbe (observed in plot 2), was in leaf, not bare.

In the case of Vachellia tortilis, the usual pattern was apparent.

This species had not become fully leafless regardless of the drought, being ‘evergreen’ in a way more dynamic (repeatedly producing and shedding small leaves) than the usual pattern of evergreenness.

We observed the hook-lipped rhino (Diceros bicornis) here, right out in the open, foraging on a suppressed individual of V. tortilis.

This defiance of the drought by V. tortilis is important, because V. tortilis is one of the most important woody plants in this relatively treeless area on basalt.

It is hard to generalise about Senegalia nigrescens in this relatively treeless area. This is because it was so scarce here. However, it seems significant that the two individuals I found in or near the plots, both of them tiny (= ‘suppressed saplings’ <10 cm high), were already in leaf, i.e. apparently advanced relative to their conspecifics in the woody vegetation on basalt.

Flueggea virosa and Terminalia prunioides were still largely bare. However, in contrast to the same species in the woody vegetation on basalt, F. virosa retained a few old leaves, and had already shot some new leaves by the time of my visit.

So, I can say the following with certainty:
Flueggia virosa remained completely bare in the woody vegetation on basalt, where it was so common as to constitute a stratum of the savanna. By contrast, the same species, growing more scattered in the relatively treeless vegetation on basalt, had not become completely bare - and was already starting to produce new leaves.
 
The anticipation of the rains by woody plants in this relatively treeless area on basalt was not a major phenomenon. What I saw during my visit was, After all, just a small proportion of the full foliage, and there was individual variation within each species.

However, the fact that fresh leaves were apparent at all, on most of the woody plants, shows

  • a difference from comparable plants in the woody vegetation on basalt, and
  • some anticipation, even if only by a week or so, of the rains, rather than merely a response to the actual fall of rain.

The amount of new tissue produced by each species may seem minor. However, the sheer consistency of the difference across the floristic spectrum makes for a clear result, overall.

Not a single species of woody plant here failed to show green leaves on at least some individuals, at a time when this shooting could not have been merely responsive to the first rains.
 
It is important to note that I found virtually no green grass in any of these plots in the relatively treeless area on basalt. The only exception was a few fresh leaves on a few dry, worn tussocks of Bothriochloa. This was a species uncommon in the relatively treeless vegetation on basalt, despite its dominance in the woody vegetation on basalt.

(Even though Bothriochloa was uncommon here in the relatively treeless vegetation, it was if anything ahead of its conspecifics in the woody vegetation on basalt, in responding to the rain that fell in both areas, three days previous to my visit.) 

It is significant that the woody plants were ahead of the grasses phenologically. This implies some degree of anticipation of the rains, and in turn some degree of underutilisation in the water supply in the soil.
 
To summarise:
 
Yes, the woody plants in the relatively treeless vegetation on basalt did indeed anticipate the rains to some extent in their shooting of foliage, and yes, this pattern was more marked than in the woody vegetation on basalt.

All of this certainly supports the idea that there was still water available in this ‘black cotton soil’ at the end of a severe drought, in which the lawn grasses virtually disappeared from view even as dry remnants.

And, likewise, it supports the interpretation that the ‘bareness’ and ‘openness’ and ‘arid appearance’ of this relatively treeless plain north of Satara was not a consequence of drought in either the sense of a dry season, or the sense of an unusual drought. Instead, it resulted from a combination of

  • the ‘outcompeting’ of woody plants by lawn-forming grasses, and
  • the suppression of woody plants by large herbivores including the African bush elephant, both species of rhinos, and the southern giraffe (Giraffa giraffa).

The bottom line seems to be this:

If anyone assumes that the basalt plain north of Satara has such ‘open’ vegetation because this is a dry part (either climatically or edaphically) of Kruger National Park, they are probably mistaken.

This relative treelessness has little to do with the water supply, and more to do with nutritional regimes, mediated by herbivory.

The green leaves apparent on woody plants during my visit, right at the end of a severe drought, although small in absolute quantity, are revealing because they prove that water is not ‘limiting’ in this ecosystem, as far as woody plants are concerned. As a corollary, the deciduousness of woody plants, such as Cordia and Dichrostachys, is a ‘tactic’ rather than something imposed absolutely by the hydrological regime.

Posted on August 07, 2022 07:09 AM by milewski milewski

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