Even during drought, savannas on basalt and granite in Kruger National Park underutilise their groundwater

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In several Posts, I have reported my observations on the responses of vegetation to the drought of 2016, in Kruger National Park in South Africa. The phenology of the woody plants on three substrates (Ecca, basalt, and granitic) indicates that the height and density of woody plants is not necessarily determined by the the supply of water to deep roots.

Other aspects of the environment seem to be more important than water. I refer in particular to the nutritional regime in the soil, and the associated regime of herbivory and suppression of woody plants by large mammals (https://www.tandfonline.com/doi/abs/10.2989/10220119.2021.1938222 and https://www.tandfonline.com/doi/abs/10.2989/10220119.2021.1938223 and https://www.tandfonline.com/doi/abs/10.2989/10220119.2021.1938224 and https://www.tandfonline.com/doi/abs/10.2989/10220119.2021.1938221).

The nutritional regime on Ecca substrate in Kruger National Park strongly favours woody plants over herbaceous plants. Thus, the biomass there is maximally expressed relative to the rainfall and the dry-season regime: a woodland of trees about 8 m high, dominated by Senegalia burkei, which remained bare of flowers and leaves until the drought broke.

This vegetation, although providing food for the impala (Aepyceros melampus) by day during the rainy season, does not show much obvious damage by the African bush elephant.

Senegalia burkei tends to die if its bole is broken by the proboscidean. However, this breakage seldom happens. Instead, the height and spacing of trees and tall shrubs on Ecca substrate seem to represent what - in old textbooks - might be called the ‘climax vegetation’ for this climate, which is characterised by mean annual rainfall of about 575 mm, with a dry season usually lasting half of the year.

The trees on Ecca substrate grow to the maximum height sustainable under this rainfall on deep soil. The dominant woody plants are fully dormant during dry seasons, after virtually exhausting the groundwater available to their deepest roots.
 
This situation can serve as a ‘standard’ against which to compare two types of savannas in Kruger National Park: savanna on a basalt plain, and savanna on the ridgetops of undulating granitic terrain.

The former is dominated by Senegalia nigrescens, and the latter is dominated by Terminalia sericea and Combretum apiculatum.

In both situations, the African bush elephant routinely persecutes the dominant woody plants, by breaking the boles and, in the case of C. apiculatum, uprooting the whole plant.

The mean rainfall in savanna of S. nigrescens on the basalt plain is about 550 mm, while that on the granitic ridgetops east of Phalaborwa is about 475 mm. Both are similar enough to that on Ecca substrate that we would not expect major differences in the height and spacing of woody plants, given that the average length of the dry season is about six months throughout (https://www.sanparks.org/images/parks/kruger/conservation/scientific/maps/map_images/rainfall.jpg). 
 
What my fieldwork in Sept.-Nov. 2016 showed was that

  • the woody plants in these two savannas are far lower and more spaced-out than those on Ecca substrate, and
  • this is associated with underutilisation of groundwater by the relatively low and open vegetation, even during drought.

In the case of the basalt plain, S. nigrescens retains a single bole, but is often broken by the African bush elephant (tending to survive this disfigurement, unlike its close relative S. burkei). Even the unbroken individuals seem suppressed by herbivory. This is because they seldom exceed 6 m high, despite the species being capable of reaching 15 m under this rainfall.

In the case of the granitic ridgetops we sampled east of Phalaborwa, T. sericea never retains its normal bole, being converted to a multi-stemmed shrub by the African bush elephant. Coexisting C. apiculatum, which has a natural tendency to be multi-stemmed, is so often uprooted by the proboscidean that a considerable proportion of the population lives in a prone position. The branches lie on the ground, but continue to shoot foliage and to produce seeds.
 
Senegalia burkei, and most of the associated woody species on Ecca substrate, remained bare throughout the dry season, until the drought-breaking rains of mid-November 2016. By contrast, on the basaltic plain, there was some anticipation of the rains in the refoliation of S. nigrescens and the associated woody species.

In the case of S. nigrescens, some individuals started to shoot new leaves before the first rains fell. This anticipation occurred even earlier in the case of e.g. Sclerocarya birrea and Combretum hereroense.

In the case of C. apiculatum on granitic substrate, most individuals came into prominent bud just before the first rains, so that within days they rapidly produced both new leaves and the first flowers – in contrast to T. sericea which was relatively slow to respond.

Legumes tended to anticipate the rains on both basaltic and granitic substrates. However, there was an interesting difference between these substrates. This was that there was no member of the mimosoid legumes present in our plots on granitic substrate, other than Dichrostachys cinerea, which anticipated the rains weakly, if at all.

On both basaltic and granitic substrates, Cassia abbreviata (https://www.inaturalist.org/taxa/147000-Cassia-abbreviata) and Ozoroa engleri? (https://www.inaturalist.org/taxa/431394-Ozoroa-engleri) occurred in short forms, suppressed by continual breakage by the African bush elephant. These species strongly anticipated the rains, producing new leaves several weeks before the end of the dry season.
 
What this seems to add up to:
On both basaltic and granitic substrates, the nutritional regime in the soils has effectively constrained the establishment and growth of woody plants, relative to herbaceous plants. At the same time, the nutritional regime, as expressed by all plants, has invited intense activity by megaherbivores, leading to suppression of even those woody plants that have been able to establish in competition with grasses.

Because woody plants require more water per land area than grasses do, the constrained trees and large shrubs on basaltic and granitic substrates did not exhaust the water supply in the soil even in the drought of 2016.

This inadvertent sparing of water may help to explain my findings, viz. that the lower, sparser vegetation types (on basalt and granite) showed some refoliation during the drought, whereas no such refoliation occurred on Ecca substrate, except by a few scarce and small plants such as Commiphora, which seems to store water in its roots, and appeared to have been suppressed by herbivory even in the woodland of S. burkei.

PHENOLOGICAL OBSERVATIONS from Kruger National Park, Sept.-Nov. 2016, with particular reference to anticipation of the rains by plants:
 
This year (2016) there was no widespread anticipation of the rains by flowering acacias. Even Senegalia nigrescens largely failed to flower this year, owing to the drought.
 
Vachellia robusta (https://www.inaturalist.org/taxa/559229-Vachellia-robusta) anticipated the rains extremely conspicuously in Sept. 2016, in its restricted habitat of riverbanks. This involved mainly vigorous, bright new foliage, grown at a time when coexisting plants, such as Diospyros mespiliformis, were actually shedding foliage.

Vachellia robusta is, in Kruger National Park, certainly anomalous phenologically.
 
Vachellia tortilis is, just as I have known for many years now, effectively ‘evergreen’, and a real anomaly.
 
Full-size, mature, tall Senegalia nigrescens on basalt anticipated the rains with foliage growth.
 
The rest of the acacias, including Dichrostachys cinerea, hardly anticipated the rains, in their shooting of foliage.

A possible partial exception was 6 m (not full-size) Senegalia nigrescens on basalt near Satara.

On Ecca substrate near Satara, Senegalia burkei remained bare a week after the first rains, except in the runoff zone next to a tarred road. Vachellia nilotica (https://www.inaturalist.org/taxa/557826-Vachellia-nilotica) and Vachellia grandicornuta seemed similar in this tardiness.
 
Combretum imberbe seems to be more or less the combretaceous equivalent of Vachellia tortilis: effectively evergreen at all ages and sizes.
 
Combretum hereroense and C. apiculatum seem, in some areas, to anticipate the rains in shooting foliage. However, this is variable, and, in some places, failed, at least in this drought year.

By the end of this visit to Kruger National Park, I became certain that Combretum hereroense differs from C. apiculatum phenologically. The former anticipates the rains, whereas the latter does not.

By 20 Nov. 2016, the foliage of C. hereroense was so full and mature that this species looked like an evergreen, whereas in the same areas C. apiculatum was still in bright and small-leafed shoot.

However, puzzlingly enough, C. hereroense on gabbro substrate near Ship Mountain (https://www.krugerpark.co.za/Kruger_Park_Game_Viewing_Routes-travel/voortrekker-road-h2-2.html) was tardy in its shooting of foliage, and individually variable; I would not describe that population as anticipating the rains. I wonder if the form on gabbro might instead be C. zeyheri (https://www.inaturalist.org/taxa/553599-Combretum-zeyheri), a species with which I am unfamiliar (our game guard did call it hereroense, but perhaps he was wrong?).
 
Terminalia prunioides anticipated the rains with foliage growth. So may have Terminalia sericea in some areas, but the latter species in particular was surprisingly variable (unpredictable). I certainly saw stands of T. sericea that did not anticipate the rains, and seemed, if anything, to lag.
 
Colophospermum mopane generally anticipated the rains, being about on a par with S. birrea in the time before the rains when its new leaves started to appear. However, as in the case of T. sericea, I was surprised by the degree of local variation among populations.
 
Cassia abbreviata is unusual in its exemption from herbivory by large mammals once adult. Even its fallen flowers seem to be shunned. This species certainly anticipated the rains in both flowering and shooting of foliage, and in Sept. 2016 was the prime example of this phenomenon, away from the riverbanks.

Sclerocarya birrea is also a clear example of anticipation of the rains, although not as extreme as Cassia abbreviata.

Unlike S. birrea, Lannea schweinfurthii (https://www.inaturalist.org/taxa/340118-Lannea-schweinfurthii) did not anticipate the rains.
 
Trichilia emetica (https://www.inaturalist.org/taxa/595643-Trichilia-emetica) seems to be evergreen in its natural habitat, as are Capparis tomentosa (https://www.inaturalist.org/taxa/342724-Capparis-tomentosa) and Euclea spp.

However, I learned in Nov. 2016, on Ecca substrate near Satara, that even Euclea divinorum can lose its leaves in drought, and it certainly did not anticipate the rains.
 
Some of the deciduous species are tardily and briefly deciduous, e.g. Diospyros mespiliformis and perhaps Schotia brachypetala (https://www.inaturalist.org/taxa/369448-Schotia-brachypetala). The former fruited at the driest time, and the latter, if memory serves, flowered at the driest time. The leaves turned yellow before falling in the former species, but fell in a green state in the latter species. More data are needed.
 
Zanthoxylum humile (https://www.inaturalist.org/taxa/596428-Zanthoxylum-humile) was fully bare by Nov. 2016, and did not anticipate the rains.
 
Ehretia rigida? (https://www.inaturalist.org/taxa/346399-Ehretia-rigida) approaches an evergreen in its behaviour, being at least as ‘evergreen’ as coexisting Euclea divinorum on Ecca substrate near Satara. It has accordingly semi-scelrophyllous leaves, dull and ragged by mid-Nov. 2016. Ehretia obtusifolia? (https://www.inaturalist.org/taxa/343318-Ehretia-obtusifolia) near Skukuza seems different, including in phenology.
 
Phragmites mauritianus (https://www.inaturalist.org/taxa/343087-Phragmites-mauritianus) is evergreen, although it looked ragged in the drought. In this way it is quite different from Phragmites communis (https://www.inaturalist.org/taxa/64237-Phragmites-australis), which I have not seen in Kruger National Park.

Posted on August 07, 2022 05:33 AM by milewski milewski

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