Phenology of woody vegetation on basalt in Kruger National Park, showing water surplus where woody growth constrained by nutritional regime favouring grasses

(writing in progress)

The question is: what evidence is there that woody vegetation on basalt near Satara in Kruger National Park, which is relatively low and open, is limited by competition with herbaceous plants (and by the damage inflicted by large herbivores supported partly or mainly by the herbaceous plants) rather than by sheer lack of water? I.e. what evidence is there that it is the nutritional regime, rather than the climate, that constrains the height and density of the woody cover?
 
Herbaceous plants, other things being equal, need less water than woody plants do; this is largely a function of body sizes and absolute surface areas, together with the exponential increase in wind speed with height above ground.
 
Therefore, if the nutritional regime (mediated partly by the actions of herbivores) favours herbaceous plants (particularly grasses) over woody plants (particularly trees) then one would predict that the water supply in the soil would not be exhausted even in drought, and that deciduousness of relatively deep-rooted plants in the community would be a phenological tactic rather than a dormancy enforced by an inability to afford the water costs of transpiration.
 
On this basis, I would predict the following if the height and density of woody plants is constrained mainly by the nutritional regime on basalt in Kruger Park.
 
1) Some woody plants should be evergreen even if all herbaceous plants are deciduous.
 
2) Some woody plants should anticipate the rains in shooting either flowers, or new foliage, or both, rather than waiting for the first rains to fall.
 
3) The woody plants showing either evergreen tendencies or anticipation of the rains should include small or relatively shallow-rooted species, or shrubby species, or individuals suppressed by physical damage by large herbivores.
 
4) Some woody plants should be able to produce flowers and/or foliage even in times so dry that all the grasses and other herbaceous plants are fully dormant.
 
Now let’s examine the reality of what I found on my two visits to this woody vegetation, on 15 and 17 Nov. 2016, 3-6 days after the first drought-breaking rains that wet the clay-rich soil to a depth of about 5 cm.
 
The vegetation to which I refer consists of an upper stratum dominated by Senegalia nigrescens over a lower woody stratum dominated by Flueggea virosa, over a grass stratum dominated by Bothriochloa. There are various other species in the mix in all three strata. Most individuals of S. nigrescens are only 6 m high or lower, despite the potential of this species to exceed 15 m high; and the stand of S. nigrescens is open, with the individuals on average >?20 m apart (it may be possible to estimate this from Google Earth?).
 
Prediction 1):
 
It is indeed true that several woody plants in this vegetation are evergreen, and this evergreenness is of several different types. Euclea natalensis and Diospyros mespiliformis are evergreen in the most prosaic way, having semi-sclerophyllous, long-lived, simple leaves; however these species are scarce here, fail to reach their full size, and may not be reproductively mature. Peltophorum africanum is evergreen despite having bipinnately compound leaves which look tired by the end of the dry season. Vachellia tortilis continues to grow bipinnately compound leaves through the dry season and did not become bare even by the end of this severe drought. Since all these species are < 3 m high here and the root system of the last-mentioned species in particular is shallow and spreading, this is evidence that water remains available throughout the year and the drought cycle. Lastly, Combretum imberbe (including suppressed saplings) retains leaves continually, even though it belongs to an otherwise deciduous genus and differs from the Ebenaceae and Peltophorum in tending to be quasi-spinescent as a sapling.
 
Prediction 2):
 
Various woody species did indeed anticipate the rains in producing flowers and or new foliage. The clearest examples were Sclerocarya birrea (both tall, mature individuals and suppressed saplings) and tall, mature individuals of S. nigrescens. However, even the somewhat suppressed individuals of S. nigrescens, about 6 m high, tend to produce flowers at the height of the dry season, when most deciduous trees in Kruger Park are still bare. Other spp. anticipating the rains in the shooting of foliage were Ximenia caffra, Combretum hereroense, Vachellia exuvialis, Vachellia robusta, Philenoptera violacea, Cassia abbreviata, Strychnos spinosa, and possibly Peltophorum africanum. It is noteworthy how many of the species which produce fresh foliage during drought are legumes (or, in the case of Ximenia caffra, a root hemiparasite which ostensibly parasitises legumes); however, it is also noteworthy that one species of legume, namely Dichrostachys cinerea, remained completely bare and moreover had suffered noticeable mortality in this area.
 
Prediction 3):
 
It was indeed true that those woody spp. showing green foliage during dry conditions included suppressed individuals with body sizes far inferior to their potential. This applied particularly to V. tortilis, Cassia abbreviata (of which no mature, full-size individual was seen in this sampling area), P. violacea and S. birrea.
 
Prediction 4):
 
It was indeed true that the shooting of new foliage by various woody spp. during drought was unaccompanied by any shooting of new foliage by any grass or other herbaceous plant. The main grass apparent in the sampling area was Bothriochloa, which remained dry and dormant even six days after the first drought-breaking rains. During my second visit on 17 Nov. 2016, several deciduous woody species (e.g. Ximenia caffra, Sclerocarya birrea, and V. tortilis) had grown much of their foliage but the only green apparent in the herbaceous stratum was tiny, scattered, freshly-germinating seedlings of unidentified dicotyledonous herbs, probably annuals. Furthermore, the most conspicuous members of the lower woody stratum, namely Flueggea virosa and Grewia sp. indet. (bicolor?), remained completely bare during both my visits. Thus it can be said that there was a great disparity between the tallest plants and the shortest plants in this vegetation phenologically, with the former clearly anticipating the rains but the latter tending not to anticipate the rains and, in the case of the herbaceous stratum, showing no anticipation of the rains whatsoever. It is understandable that there is some incentive for the woody plants to conform to this prediction because the dry season is a time when the woody plants can avoid the superior competitiveness of the (relatively shallow-rooted but metabolically more powerful on a per cell basis) herbaceous plants for nutrients. However, the failure of D. cinerea to capitalise in this way – and indeed to a large extent even to survive in this area - was one of the noteworthy findings of this study. What emerged was an interesting contrast between D. cinerea and V. tortilis in phenology and persistence: both spp. were suppressed in this area and had sparse populations, but the former failed to produce any foliage or flowers during this drought, and showed evidence of much mortality, whereas the latter proved not only resilient but also functionally evergreen, something unusual among ‘acacia’ spp. and not attributable to a deep rooting system.
 
(writing in progress) 

Posted on August 07, 2022 06:44 AM by milewski milewski

Comments

Highveld grasses even in the presence of water in the soil, and in the re-activation of woody plants in the savannas even during the season of deciduousness. In their ways, the brownness of Phragmites in a vlei in winter and the re-greening of marula weeks before the end of a severe recent drought in Kruger Park are manifestations of a single pattern: that it is after all not water that matters the most in shaping vegetation.

Posted by milewski almost 2 years ago

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