Explaining the extreme growth-form of Gardenia in the Serengeti

Gardenia volkensii grows as isolated large shrubs - one individual here and another last week - in open savanna in the Serengeti ecosystem. It would hardly be noticed were it not for its extreme sculpting by large mammals:

https://www.istockphoto.com/photo/masai-giraffe-giraffa-camelopardalis-tippelskirchi-masai-mara-park-in-kenya-gm1262783396-369525685 and https://photos.com/featured/giraffe-browsing-acacia-tree-manoj-shah.html?product=fleece-blanket&blanketType=blanket-coral-50-60 and https://www.istockphoto.com/photo/masai-giraffes-fighting-gm1191233110-337981747 and https://www.shutterstock.com/nb/image-photo/maasai-giraffe-mara-national-reserve-1863458419 and https://www.istockphoto.com/photo/masai-giraffes-fighting-gm1191233151-337981921 and https://fineartamerica.com/featured/reticulated-giraffe-grazing-on-small-jean-michel-labat.html and https://www.inaturalist.org/observations/8077390.

It is hard to describe this growth-form but 'green coralloid', 'botanical statue' and 'compound bonsai' come to mind. Here we see a combination of gnarled stemwork (e.g. https://www.inaturalist.org/observations/88581599) and tight-set foliage, making for a 'caulifoliar' plant.

Various species of woody plants are under pressure by large mouths, and various species of Gardenia occur in various other environments without being sculpted. So what is it about this species, in this ecosystem, that has led to a natural form of topiary?

Here is general information on Gardenia volkensii:

http://apps.worldagroforestry.org/usefultrees/pdflib/Gardenia_volkensii_KEN.pdf and https://treesa.org/gardenia-volkensii/ and http://pza.sanbi.org/gardenia-volkensii#:~:text=Distribution%20description,KwaZulu%2DNatal%20in%20the%20southeast and https://www.zimbabweflora.co.zw/speciesdata/species.php?species_id=169130 and https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:751323-1 and https://www.randomharvest.co.za/South-African-Indigenous-Plants/Show-Plant/PlantId/159/Plant/Gardenia-volkensii?Filter=All and https://www.seedsforafrica.co.za/products/gardenia-volkensii-transvaal-gardenia-indigenous-south-african-shrub-tree-10-seeds.

Gardenias are indigenous to Africa, Asia and Australia, and have become horticultural favourites. However, it seems to be only in the Serengeti that this genus is obviously shaped by large mammals.

Here are additional photos of G. volkensii in the Serengeti:

https://www.inaturalist.org/observations/66401903 and https://www.inaturalist.org/observations/93189557 and https://www.alamy.com/kenya-masai-mara-game-reserve-gardenia-tree-in-blossom-and-wildebeest-grazing-image246151349.html and https://www.naturepl.com/stock-photo-6-month-lion-cub-panthera-leo-up-in-gardenia-tree-masai-mara-gr-kenya-nature-image01034053.html and https://www.tripadvisor.com/LocationPhotoDirectLink-g294209-d804877-i55550011-Entim_Camp-Maasai_Mara_National_Reserve_Rift_Valley_Province.html and https://www.agefotostock.com/age/en/details-photo/savannah-gardenia-gardenia-volkensii-fruit-masai-mara-game-reserve-kenya/FHR-52727-00001-029 and https://www.naturepl.com/stock-photo-six-month-old-lion-cub-in-gardenia-tree-masai-mara-kenya-nature-image01034052.html and https://www.naturepl.com/stock-photo-six-month-old-lion-cubs-in-gardenia-tree-masai-mara-kenya-nature-image01034054.html and https://www.naturepl.com/stock-photo-lion-panthera-leo-6-months-cub-in-gardenia-tree-masai-mara-kenya-nature-image01034050.html and https://www.naturepl.com/stock-photo-lion-panthera-leo-6-months-cub-in-gardenia-tree-masai-mara-kenya-nature-image01034048.html and http://www.imagesafaris.com/2012-06-18-masai-mara-nr-kenya/.

In trying to understand the niche of any organism, a challenge is to distinguish cause and effect, and to infer the central 'life-history strategy'.

So here are some observations and inferences applying to the genus Gardenia, the species G. volkensii, and the population in the Serengeti.

Gardenias are surprisingly prone to nutrient-deficiencies (as frequently noticed in horticulture, https://www.ecoorganicgarden.com.au/gardening-tips/how-to-grow-gardenias/) for plants that grow their stems, leaves and fruits slowly even under favourable conditions. Because such supplement-dependent plants would be expected to benefit from the nutrient-recycling by megaherbivores, their presence in the Serengeti ecosystem - albeit in small numbers - is unsurprising.

Instead of defending themselves with spines, gardenias tend to withdraw their foliage and stems. This is seen in extreme form in those species which remain largely underground (https://www.inaturalist.org/photos/1532475 and https://www.inaturalist.org/observations/1172045 and https://www.inaturalist.org/observations/1163083 and https://repository.up.ac.za/bitstream/handle/2263/49453/Maurin_Savanna_2014.pdf;jsessionid=1AF15650A3AD79CE5529E41C1B07E767?sequence=1 and https://www.cambridge.org/core/journals/journal-of-tropical-ecology/article/abs/geoxylic-suffrutices-of-african-savannas-short-but-remarkably-similar-to-trees/F7069F10757C299528F3649D8D9FA971).

Although individual plants maintained in sculpted form are unlikely to produce fruits, it is noteworthy that
G. volkensii is unusual in its genus in being specialised for the sowing of its seeds by 'megafauna'.

Gardenias have technically fleshy (zoochorous) fruits and the seeds are thought to be dispersed by mammals of various body-sizes
(e.g. see http://ntfieldnaturalists.org.au/site/assets/files/1547/martine.pdf and https://www.researchgate.net/figure/Developing-fruit-of-Gardenia-fucata-growing-on-sandstone-outcrop-near-Cahills-Crossing_fig2_309385368 and http://tropical.theferns.info/viewtropical.php?id=Gardenia+thunbergia). In the case of G. volkensii, the fruits are particularly large, leathery, long-lasting, and sturdily attached - and thus seemingly attuned to the 'megaherbivory' of the Serengeti.

These greyish fruits, when 'ripe' (and incongruously fragrant), seem designed mainly for the bush elephant (Loxodonta africana) and the eland (Taurotragus oryx). They are difficult to detach from the stems, and too big to fit into the gentle, pursed mouth of the Maasai giraffe (Giraffa tippelskirchi); they are likely to be out of reach of the hook-lipped rhino (Diceros bicornis); and they would probably be cud-chewed to destruction of the seeds by the greater kudu (Strepsiceros zambesiensis), which in any event is absent from this habitat in the Serengeti (see https://www2.palomar.edu/users/warmstrong/ecoph42.htm#:~:text=According%20to%20Rudolf%20Marloth%20(The,when%20partially%20decomposed%20by%20fire.).

The bush elephant, when foraging on the foliage, is likely to break branches to some extent despite the flexibility of the wood of gardenias (see https://www.tandfonline.com/doi/abs/10.1080/10295925.1998.9631190 and https://prota4u.org/prosea/view.aspx?id=5483). Where free of such damage, the growth-form of G. volkensii is similar to https://www.inaturalist.org/observations/99477343 and https://www.inaturalist.org/observations/36994728.

The 'juvenile foliage' possessed by saplings of G. volkensii (https://www.inaturalist.org/observations/14124710) is a form of heteroblasty (https://en.wikipedia.org/wiki/Heteroblasty_(botany)) shared with various other woody plants. However, this species seems unusual in also possessing a second kind of heteroblasty at a different stage of its life, i.e. diminutive leaves crowded on to nodes, in response to repeated defoliation of old stems.

Thus an explanation may be found in the following combination of factors:

  • a particularly intense regime of pressures exerted by the bush elephant and the Maasai giraffe
  • in open vegetation where the few woody plants tend to be targeted repeatedly
  • partly owing to a bimodal climate (two rainy seasons each year) in which leaves tend to be present on this technically drought-deciduous plant for most of the time.

Because G. volkensii prefers nutrient-enriched soils, its leaves are somewhat attractive to folivores. Because it is 'hardwired' to grow slowly even in response to losses, it has a 'retractive' defence against defoliation; it may capitalise on faeces and urine but it is not stimulated by defoliation to replace its shoots rapidly. And because no member of this genus, as far as I know, has evolved spines, this species has found an unusual way to hug its leaves so close to its stems that plucking them is time-consuming for large mouths.

Based on the above, do readers share with me an impression that the genus Gardenia can be characterised by a combination of three modes: nutrient-inefficiency, slow growth, and spinelessness? And that this generic syndrome is in a way exposed in the sculpting of G. volkensii in the Serengeti ecosystem?

Publicado por milewski milewski, 26 de noviembre de 2021


Thanks @milewski so much for ths very interesting article.

Publicado por botswanabugs hace alrededor de 2 meses (Marca)

In Kruger National Park, where the regime of 'megaherbivory' is not as intense as in open savanna in the Serengeti, Gardenia volkensii looks hedged (e.g. https://www.inaturalist.org/observations/84281746) but not to the point of topiary.

Publicado por milewski hace alrededor de 2 meses (Marca)

@botswanabugs I'm glad you found it interesting. Do you know if the donkey accepts the fruits as food?

Publicado por milewski hace alrededor de 2 meses (Marca)

@milewski Ill keep an eye open on the tree near my house. We have plenty of donkeys, goats cattle and ill test out the leaves on my rabbit. His nose is quite choosey.

Publicado por botswanabugs hace alrededor de 2 meses (Marca)

The following of an Indian species of Gardenia shows how large the leaves can be in this genus: https://www.inaturalist.org/observations/83528736.

Publicado por milewski hace alrededor de 2 meses (Marca)

The 'ripe' fruits of gardenias are dull-hued compared with most types of fleshy fruits attractive to seed-dispersing birds. One of the brightest-hued is Gardenia jasminoides (see https://upload.wikimedia.org/wikipedia/commons/6/60/Gardenia_jasminoides_fruit.jpg and https://www.ajc.com/lifestyles/home--garden/gardenias-can-grown-from-seed/TJmklgd6rmKTOixd2EoH1M/).

Publicado por milewski hace alrededor de 2 meses (Marca)

Here are further illustrations of Gardenia volkensii being attended by the Maasai giraffe in the Serengeti: https://fineartamerica.com/featured/dinner-for-two-csilla-florida.html and https://fineartamerica.com/featured/table-for-three--color-mike-gaudaur.html.

Publicado por milewski hace alrededor de 2 meses (Marca)

@milewski at first I thought maybe your account was hacked and was now posting spammy links, but I realized these are actually professional photos of giraffes eating. Could you add some content/explanation when you post links to commercial sites, for clarity?

Publicado por carrieseltzer hace alrededor de 2 meses (Marca)

@carrieseltzer Hi Carrie, Many thanks for alerting me to this. The potential problem had not occurred to me. I will adjust accordingly over the next day or two, and could you please monitor this and let me know if my method is sufficient? from Antoni

Publicado por milewski hace alrededor de 2 meses (Marca)

It's much clearer now that you've edited the comments above, thanks!

Publicado por carrieseltzer hace alrededor de 2 meses (Marca)
Publicado por milewski hace alrededor de 2 meses (Marca)

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