Misleadingly similar solanums with different relationships to wildfire, in Australia vs southern Africa

@plantperson7654 @wojciech @aguilita @bryanconnolly @mjpapay @troos @botaneek @benjamin_walton @richardgill @sedgesrock @wynand_uys @venturefoth @adriaan_grobler @alastairpotts

Acknowledgement: Many thanks to Jeremy Gilmore @jeremygilmore (see comment in https://www.inaturalist.org/journal/milewski/72788-flora-of-fleshy-fruits-at-nature-s-valley-western-cape-south-africa#) and Tony Rebelo @tonyrebelo for information crucial to this Post.

Solanum symonii of southwestern Australia and Solanum guineense of southwestern South Africa are comparable. This is because both are centred on the mediterranean-type climate, and sandy substrates, mainly coastal.

They are similar in growth-form and fruit-form within the genus, despite belonging to different subgenera.

Solanum symonii is strictly confined to the mediterranean-type (winter-rainfall) climate. Solanum guineense is more versatile, extending somewhat into winter-rainfall semi-arid (Namaqualand) and bimodal rainfall (Eastern Cape). However, this is not a major difference.

Both are short-lived perennials growing to 1.5-2 m high. Solanum symonii tends upright with long leaves, S. guineense somewhat lax with shorter leaves. However, the differences seem unimportant.

Both spp. seem to lack spines on the leaves and stems (for relevance, see https://www.jstor.org/stable/2399203).

Both have smooth-skinned, succulent berries of approximately oval shape, and about 1.5 cm diameter. Solanum symonii tends to ripen to purple, whereas S. guineense tends to ripen to orange.

However, in the context of the great diversity of fruit-forms in genus Solanum (https://www.jstor.org/stable/23872220), this difference seems minor, and is unlikely to indicate a categorical difference in the animals, or range of animals, serving to disperse and sow the seeds.

Given these similarities, why is it that there is such a difference in the number of observations in iNaturalist?

There are currently only 10 observations of S. symonii, compared to 224 of S. guineense.

This may provide a clue to the real difference: that S. symonii is a post-fire (ashbed) specialist, whereas S. guineense bears no particular relationship to wildfire, and tends to occur in vegetation naturally protected from fire.

Rippey and Hobbs (2003, https://researchrepository.murdoch.edu.au/id/eprint/17081/1/effects_of_fire_and_quokkas.pdf) describe S. symonii as a 'fire ephemeral', living only about five years and then disappearing until the next fire.

To understand this fully, it is important to realise that the regimes of wildfire tend to differ in the relevant parts of the two continents. In southwestern Australia, the vegetation is relatively tall and 'resinous', burning intensely every several decades (https://www.academia.edu/7565234/Ecology_of_Australia_the_effects_of_nutrient_poor_soils_and_intense_fires). In southwestern South Africa, the vegetation is relatively short, burning with limited intensity on a relatively frequent basis - and tending to spare the littoral dunes with which S. guineense is particularly associated.

This difference is partly explained by the more profound and widespread nutrient-poverty in Australia than in southern Africa, exacerbated by the great difference in the incidence of large herbivores (https://www.researchgate.net/publication/229774801_Why_are_very_large_herbivores_absent_from_Australia_A_new_theory_of_micronutrients and https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2699.2000.00436.x and https://www.jstor.org/stable/2656292).

This difference implies that there is a difference in the nature of the seeds: durable in the soil in the case of S. symonii, compared to non-durable in the case of S. guineense.

Although the agents of dispersal remain to be documented, I suspect that a significant difference is the role of Dromaius novaehollandiae (https://www.inaturalist.org/taxa/20504-Dromaius-novaehollandiae).

Please see https://www.malleeconservation.com.au/blog/emu-poo and https://data.environment.sa.gov.au/Content/Publications/JABG01P321_Symon.pdf.

This species differs fundamentally from Struthio camelus (https://www.inaturalist.org/observations/88050285) in its gastrointestinal system. Emus digest food extremely superficially (https://www.academia.edu/24722483/Emus_as_non_standard_seed_dispersers_and_their_potential_for_long_distance_dispersal). By contrast, ostriches digest their food so thoroughly that few seeds, including the small seeds of solanums, are likely to be defecated intact.

In summary, S. symonii seems to be a natural, indigenous 'fireweed' on a continent run by fire, whereas S. guineense has no particular relationship to fire, instead depending on a combination of relatively phosphorus-rich substrates and relatively intense disturbance by large animals.




















Scroll through images in https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:819387-1/images






Posted on November 22, 2022 08:23 PM by milewski milewski


The mycorrhizal association found in Solanum is vesicular-arbuscular (Lamont 1982).

This is in line with most other genera of fleshy fruits, including Asparagus, Prunus, Cassine, Chionanthus, Diospyros, Myrsine, Hibbertia, and Podocarpus.

Persoonia is the only genus of Proteaceae with fleshy fruits beyond rainforests. This genus is also odd in lacking cluster ('proteoid') roots. Persoonia also lacks vesicular-arbuscular mycorrhizae, which means that this genus is extremely unusual for its flora (kwongan and related types) in lacking any particular rhizal adaptations.

Leucopogon is another kwongan-related genus bearing fleshy fruits in certain species. This may possibly be correlated with the possession of vesicular-arbuscular mycorrhizae, instead of the usual 'ericoid' mycorrhizae.

Posted by milewski 4 months ago (Flag)

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