A comparison of Sapindaceae in the mediterranean and adjacent arid climates of Australia and southern Africa

The following do not quite qualify for inclusion in this Post:

https://www.inaturalist.org/taxa/892708-Dodonaea-petiolaris

https://www.inaturalist.org/taxa/956862-Diplopeltis-eriocarpa

https://www.inaturalist.org/taxa/370208-Dodonaea-lanceolata

@tonyrebelo @adriaan_grobler @alastairpotts @botaneek

PAPPEA CAPENSIS

The following aspects of the biology of Pappea capensis are noteworthy.

Firstly, it has a somewhat disjunct distribution, localised in a) Namaqualand sense lato (https://www.inaturalist.org/observations/138622531), and b) the Klein Karroo and Eastern Cape, separately from a wide range in the mesic savannas of East and southern Africa. A particularly odd disjunction is the occurrence of a mainly tropical species into zones combining winter-rainfall and aridity.

Secondly, it incongruently possesses features indicating nutrient-richness, and features indicating nutrient-poverty.

Indicating nutrient-richness are its a) association with dry climates and - in mesic climates - termite mounds, b) hedged growth-form (https://www.inaturalist.org/observations/210093221 and https://www.inaturalist.org/observations/241088122 and https://www.inaturalist.org/observations/232319784 and https://www.inaturalist.org/observations/196917873 and https://www.inaturalist.org/observations/196900564), suggesting palatability to herbivores, c) restriction to fire-free vegetation, and d) complex heterophylly/heteroblasty, affecting the size and shape of leaves, which can revert to a distinctive ('juvenile' in the case of saplings) form after shoots are damaged, and also affecting the form of the shoots (compressed/elongated/epicormic)(https://www.inaturalist.org/observations/130093681 and https://www.inaturalist.org/observations/202707854 and https://www.inaturalist.org/observations/244490793).

Indicating nutrient-poverty are its a) semi-sclerophylly, in which the leaves are somewhat lignified, b) tendency to evergreenness, c) lack of stem-spinescence, d) dull green hue of the foliage (https://www.inaturalist.org/observations/241338081), e) slow growth, longevity, and dense wood, even in mesic climates, and f) affinity for lichens (https://www.inaturalist.org/observations/211682038).

Thirdly, its endozoochorous dispersal involves an unusual form of fleshy fruit (https://www.inaturalist.org/observations/169211993 and https://www.inaturalist.org/observations/168305680). This combines a persistent capsular pericarp (https://www.inaturalist.org/observations/150511973), which starts to dehiscence when still green (https://www.inaturalist.org/observations/104095528 and https://www.inaturalist.org/observations/123370364 and https://www.inaturalist.org/observations/224853478), filled with a succulent food-body (arillode, https://www.inaturalist.org/observations/193818101). For its part, the arillode has an odd origin in the micropyle of the ovule.

The ecological nature of P. capensis can perhaps be summarised by the adjectives 'arid/eutrophic', 'bird-dispersed', 'long-lived', and 'megaherbivore-adapted'.

The incongruence in the quality of the foliage of P. capensis is indicated by the following quote from van Wyk (1974, page 360): "In the eastern Cape province the species is regarded as an exceptionally good fodder plant. Wild animals [in Kruger National Park] are not particularly fond of it, however, and up to the present only elephants, giraffe and impala have been observed eating the young leaves and twigs."

Further thoughts on Pappea capensis, stimulated by https://www.inaturalist.org/observations/10324739:

Pappea capensis is ecologically comparable with Sideroxylon inerme, Euclea undulata (https://www.inaturalist.org/taxa/533457-Euclea-undulata), and Schotia afra.

Like those tall shrubs/low trees, it is seldom recorded as seedlings/saplings, despite the obvious mechanisms of dispersal and sowing.

However, the clonal vegetative reproduction in Sideroxylon and Euclea help to explain the persistence of these plants. Is P. capensis known likewise to be clonal, in the sense of suckering (https://www.britannica.com/science/suckering)?

Unlike the other genera mentioned here, Pappea seems to have an induced response in its anti-herbivore strategy. I refer to the heterophylly/heteroblasty, in which damage to shoots results in the replacements having 'juvenile' (serrated) leaves.

Please note that S. afra (https://www.inaturalist.org/taxa/138594-Schotia-afra) resembles P. capensis in its geographical disjunction between a) the lower Orange River, b) the Klein Karroo, and c) Eastern Cape.

Dodonaea viscosa extends to the Richtersveld (https://www.inaturalist.org/observations/183183322 and https://www.inaturalist.org/observations/10934687), whereas Erythrophysa alata apparently does not.

https://onlinelibrary.wiley.com/doi/abs/10.2307/3237199

@tonyrebelo @alastairpotts @adriaan_grobler

ALECTRYON OLEIFOLIUS

Reference: pages 117-120 in Burnside D et al. (1995, https://www.publish.csiro.au/RJ/pdf/RJ9950243 and https://library.dpird.wa.gov.au/pubns/2/)

Also see https://www.inaturalist.org/posts/68618-the-puzzle-of-hakea-as-an-indigenous-woody-weed-in-currantbush-mixed-shrubland-in-western-australia#.

The western Gascoyne region (https://en.wikipedia.org/wiki/Gascoyne and https://www.agric.wa.gov.au/rangelands/rangelands-western-australia) of Western Australia is relevant to this Post.

This is despite the subtropical location, and because rain (mean annual 226 mm) falls mainly in winter (https://www.climate.top/australia/carnarvon/graphs/).

The climate here differs from the same latitude in southern Africa (https://en.wikipedia.org/wiki/Maltah%C3%B6he), where the even sparser rain falls in summer instead (https://en.climate-data.org/africa/namibia/hardap-region/maltahoehe-169737/).

Alectryon oleifolius is one of the three most common 'low trees' in 'Currant Bush Mixed Shrub', a vegetation type occurring on the 'less sandy', duplex soils over more than 4000 square kilometres.

This vegetation type "occupies the land systems flanking the erodible river floodplains in the Carnarvon area" (https://en.wikipedia.org/wiki/Carnarvon,_Western_Australia).

The other two of the most common 'low trees' are Acacia xiphophylla (https://www.inaturalist.org/taxa/964520-Acacia-xiphophylla) and Santalum spicatum (https://www.inaturalist.org/taxa/793376-Santalum-spicatum).

"While [Scaevola spinescens, https://www.inaturalist.org/taxa/421592-Scaevola-spinescens] is the most distinctive of the palatable species and is locally dominant within stands in good condition, the general vegetation comprises a rich mixture of both low and tall shrub species".

My commentary:

'Currant Bush Mixed Shrub' vegetation is particularly relevant to this intercontinental comparison. This is because this community, which contains an approximate counterpart for the southern African species Pappea capensis, resembles a southern African rather than an Australian ecological syndrome, in several ways.

I refer to a tendency to a) relative nutrient-richness (alluvial), b) freedom from intense wildfire, c) stem-spinescence (mainly in S. spinosa), d) palatability to herbivores, e) drought-deciduousness in some plants or fleshy foliar texture in others, and f) endozoochorory (with fleshy fruits/'arillate' diaspores in Alectryon, Scaevola, Stylobasium, Solanum, Acacia, Chenopodium, Enchylaena, Atriplex, Rhagodia, Santalum, and Eremophila).

INTRASPECIFIC VARIATION IN PAPPEA CAPENSIS

Reference: Van Wyk (1974, https://www.abebooks.com/9780360001589/Trees-Kruger-National-Park-Volume-0360001580/plp and https://catalogue.nla.gov.au/catalog/1052708 and https://books.google.com.au/books/about/Trees_of_the_Kruger_National_Park.html?id=2DctAQAAIAAJ&redir_esc=y)

Kruger National Park (https://en.wikipedia.org/wiki/Kruger_National_Park) has a summer-rainfall climate, Here, two distinct forms of P. capensis have been called 'lowland' and 'mountain'.

The lowland form, which resembles that common in e.g. Eastern Cape, is restricted in Kruger National Park to the region south of the Tropic of Capricorn.

The mountain form occurs on rocky/sloping terrain over the full latitudinal span of the Park. It differs from the lowland form in being shrubby, multistemmed, and deciduous, with simple, thin branches; the foliage is sparsely distributed on the branches, and consists of exceptionally large (usually 9 cm and up to 17 cm long) leaves (https://www.inaturalist.org/observations/204134534 and https://www.inaturalist.org/observations/199680523).

The lowland form flowers in Sept.-Nov., fruiting in summer; by contrast, the mountain form flowers in Feb.-March and fruits in late winter and early spring.

https://www.inaturalist.org/observations/117999595

Reference: Beard J S (1990, https://catalogue.nla.gov.au/catalog/1877392 and https://books.google.com.au/books/about/Plant_Life_of_Western_Australia.html?id=b6bwAAAAMAAJ&redir_esc=y)

Page 176:

The Nullarbor Plain (https://en.wikipedia.org/wiki/Nullarbor_Plain) is largely devoid of both trees and large shrubs. The extensive treeless vegetation is dominated by amaranthaceous shrubs, about 0.5 m high.

However, Alectryon oleifolius occurs extremely sparsely as a small tree, 3-5 m high. It is accompanied by the following other small trees, of similar size: Acacia papyrocarpa (https://www.inaturalist.org/taxa/499405-Acacia-papyrocarpa), Myoporum platycarpum (https://www.inaturalist.org/taxa/787861-Myoporum-platycarpum), Pittosporum angustifolium (https://www.inaturalist.org/taxa/349255-Pittosporum-angustifolium), and Casuarina pauper (https://www.inaturalist.org/taxa/787663-Casuarina-pauper).