Woody plants surrounding treeless lawns on sodic substrates in southern Kruger National Park, part 2: adaptations to intense herbivory at the edges of the grazing lawns

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...continued from https://www.inaturalist.org/journal/milewski/68005-woody-plants-surrounding-treeless-lawns-on-sodic-substrates-in-southern-kruger-national-park-part-1-floristic-composition#

WOODY PLANTS IN THE TREELESS PATCHES OF LAWN:

The following species of woody plants occurred in the plots in treeless vegetation, on the sodic patches along the N’waswitshaka River in Kruger National Park.

All woody plants in these plot were <10 cm high, except where otherwise specified.

I include an unidentified faboid legume, although this might not be regarded as a woody plant. My visit was during drought, preventing identification. @troos any ideas?

The species recorded would not seem to be a candidate for a woody plant, had I seen it in other vegetation systems in Kruger National Park.

I exclude a pliable, quasi-karoid perennial that was frequently encountered in these plots (as well as the treeless plots on Ecca sediments near the Orpen Road southwest of Satara). The latter is probably about as ligneous as the faboid sp., but was invariably extemely low (<10 cm).

I could not distinguish Vachellia grandicornuta from V. exuvialis in the extremely suppressed form of acacias found in these treeless plots. However, I assume V. grandicornuta, simply because it was this species that was prominent in the adjacent woody vegetation.

After each plot number I give the total woody cover, assessed as a percentage of the surface area of the plot.

Plot 9 (approx. 1 % as usual including Indigofera): Vachellia grandicornuta, faboid sp., Grewia bicolor (40 cm high), Grewia hexamita (40 cm high) (also see notes of Jessica & Zurelda) 
10 (see notes of Jessica & Zurelda) 
11 (< 1 %): faboid sp., Vachellia
12 (< 1 %): Gardenia volkensii, Dichrostachys cinerea
13 (< 1 %): Dichrostachys cinerea, faboid sp. 
14 (5 %): Dichrostachys cinerea (up to 20 cm high), Vachellia grandicornuta, faboid sp. 
15 (0 %): no woody plants whatsoever 
16 (0 %): no woody plants whatsoever 
17 (< 1 %): Vachellia grandicornuta, faboid sp. (up to 40 cm high)  
18 (0 %): no woody plants whatsoever 
19 (< 1 %): Dichrostachys cinerea, Vachellia grandicornuta 
20 (< 1 %): Vachellia grandicornuta, faboid sp. 
22 ( 1.5 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three spp. totalling 27 individuals in plot), faboid sp. 
23 (2.5 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three acacias totalling 14 individuals in plot), faboid sp. (particularly numerous in this plot) 
24 (1 % for acacias and 2 % for faboid sp., totalling 3 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three acacias totalling 47 individuals), faboid sp. (hundreds of individuals) 
25 (1 %): Vachellia grandicornuta, Dichrostachys cinerea (10 individuals) 
26 (0 %): no woody plants whatsoever 
27 (< 1 %, or more precisely <0.1 %): Dichrostachys cinerea (only one individual, <10 cm high as usual) 
28 (< 1 %): Vachellia grandicornuta (6 individuals), Indigofera sp.
29 (< 1 %): Vachellia grandicornuta, Dichrostachys cinerea (approx. 10 individuals), faboid sp.

These results mean the following.

The tallest woody plant in these plots was only 40 cm high, and most individuals were only <10 cm high.

The greatest canopy cover of woody plants was 5 %, and several plots scored 0%.

My estimate of the average canopy cover for these 20 plots would be <1 %, and this consisted of plants generally 5-10 cm high.

This reflects the extreme suppression of the acacias, and the marginally woody (= arguably herbaceous) status of faboid sp. I do not know how tall this faboid species is capable of growing if protected from herbivory. To estimate canopy cover excluding the faboid sp: this would be about half of that including the faboid sp. However, the exact figure matters little, as it would remain as <1 %.

In summary so far:
It is noteworthy that there is no ‘woody flora’ – even a distinctive flora of small shrubs - associated with this treeless lawn vegetation on sodic patches along the N’waswitshaka River. The only woody species (apart from the questionably woody faboid sp.) are suppressed individuals of species common in the adjacent shrubland/savanna/woodland/forest.

ANTI-HERBIVORE MODIFICATION OF WOODY GROWTH FORMS:

One of the most intriguing findings of this study is the vegetative modifications seen in, and at the edges of, the treeless patches.

These reflect the suppression of woody plants by herbivores, particularly the impala.

In the case of V. grandicornuta, I am surprised how miniaturised the plant can remain in its suppressed form on the lawns.

The stipular spines of this species grow to several inches long in fully-formed specimens, browsed by the greater kudu (Strepsiceros strepsiceros) and the hook-lipped rhino (Diceros bicornis). By contrast, the miniatures have tiny spines and leaves, even when the specimen is plainly old enough to have a small woody rootstock. The plant is suppressed not only in size, but also in form. (I have noticed a similar phenomenon, elsewhere, also for Vachellia exuvialis.)

This miniaturisation applies also to S. nigrescens. However, this is less striking, because the prickles are not as miniaturised as are the stipular spines of V. grandicornis.

Dichrostachys cinerea warrants particular mention.

The species frequently occurs as tiny individuals in the treeless plots, as in the case of the true acacias. However, these tiny ‘saplings’ are spineless. The spines of D. cinerea are essentially extremely stiff twigs. It stands to reason that these cannot be miniaturised, as can the stipular spines of Vachellia.

However, the result is a considerable difference in form between these suppressed ‘saplings’ and grown-out individuals: seemingly structurally defenceless against the impala in the case of the former. It is only with practise that D. cinerea can even be recognised in the treeless plots.

In the case of S. africana, the growth-form is much more interesting than conveyed by any simplistic portrayal of the species as chemically defended.

The form of spinescence is similar to that in Dichrostachys. However, it is induced by browsing, disappearing as the plant surpasses the reach of the southern giraffe (Giraffa giraffa).

There is a certain scope for miniaturisation in this ‘quasi-spinescence’, which exceeds that in D. cinerea.

The 'spines' are sharp-pointed, leaf-bearing twigs at approximately right angles to the stems bearing them. These ‘spines’ are smaller at height <0.5 m than at height >1.5 m, showing remarkable adaptive plasticity.

Furthermore, the form of the whole plant (probably just a ramet, https://en.wikipedia.org/wiki/Clonal_colony)  is surprisingly modified/distorted/suppressed near the edges of the treeless vegetation. Here, S. africana ‘pioneers’ as virtual ‘hummocks’, in some cases only 0.5 m high, and far broader than tall, shaped and maintained by the impala.

I observed a similar pattern in Terminalia prunioides (https://www.inaturalist.org/taxa/430533-Terminalia-prunioides). This species did not occur in our sample plots, but was noted in passing, in the vicinity. Please see https://www.inaturalist.org/observations/99213130 and https://www.inaturalist.org/observations/99213199 and https://www.inaturalist.org/observations/99213100.

In the case of Euclea divinorum, E. natalensis and Diospyros mespiliformis, there is no noticeable capacity for miniaturisation. Although individuals can be suppressed in the size of the plants, this is not reflected in the growth-forms. These species are defended from herbivory chemically (e.g. by means of tannin), not structurally.

Posted by milewski milewski, August 06, 2022 00:38

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