Osteospermum moniliferum: odd in being a branch-deciduous shrub

(writing in progress)

Please see https://www.inaturalist.org/journal/milewski/61996-plasticfruits-part-1-how-an-ordinary-daisy-becomes-extraordinarily-fruity#.

Acknowledgement: Thanks to Richard Knight (see https://open.uct.ac.za/handle/11427/23268 and https://www.jstor.org/stable/2844796), who was kind enough to discuss this species in detail in September 1998.

Osteospermum moniliferum (https://www.inaturalist.org/taxa/404420-Osteospermum-moniliferum) is shrubby, and always perennial.

This species regenerates germinatively rather than vegetatively, but is capable of a limited amount of vegetative regeneration, e.g. after mild wildfire.

Its growth is always in response to some sort of disturbance. The species likes open environments, where seedlings are free of competition

In fynbos, it is suited to sandy substrates and post-fire stages, and does not persist in the 'climax' stage of succession.

The species does not tolerate waterlogging. However, it is adaptable to various conditions, and is not a habitat-specialist. It is typical of road-verges. It is often found on sand-blowouts, growing on previously shifting sand. Here, the succession does not necessarily proceed.

It prefers alkalinity over acidicity, in the sense that it tolerates the spectrum of pH, but grows fastest on alkaline to neutral than on acidic soils.

The preference of O. moniliferum for coarse-grained, deep, well-drained substrates (whether acidic or alkaline) is clearest in the Eastern Cape, where rain falls mainly in summer. Here, the species seems restricted to coastal dunes, where it functions as a pioneer.

Osteospermum moniliferum is fast-growing and -transpiring, even in renosterveld, where the soils are loams. It photosynthesises more rapidly, and grows faster, than the sympatric and ecologically comparable shrub, Muraltia spinosa (https://www.inaturalist.org/taxa/526214-Muraltia-spinosa). The latter differs from O. monilifera in emphasising the conservation of water.

The leaves are thickened rather than sclerophyllous. In some situations they qualify as succulent in a non-halophytic sense. In combining fleshy leaves with succulent fruits, it resembles sympatric Lycium (e.g. https://www.inaturalist.org/taxa/181615-Lycium-ferocissimum).

The foliage is noticeably pale, similar to e.g. Acacia cyclops (https://www.inaturalist.org/taxa/75249-Acacia-cyclops). Given the olive-green typical of fynbos and strandveld, the plant thus looks somewhat like a an introduced rather than indigenous plant, even in its natural environment.

The relatively bright hue of green seems consistent with rapid growth and transpiration.

The wood is soft, as in most forms of Asteraceae.

It seems to have fairly shallow and narrow-spread roots, despite its large demand for water. It shows signs of water-stress, with wilted leaves sometimes noticeable in summer. This may help to explain its peculiar habit of branch-deciduousness (see below).

Osteospermum moniliferum does not respond to limited resources by being stunted in form. Instead, it grows branches, and then withers and discards them.

Osteospermum moniliferum is the only species under the mediterranean-type climate in South Africa that shows branch-deciduousness. It aborts whole branches. This occurs particularly after fruiting.

Over a period of a few months, the leaves on a particular branch wither and turn brown. The branch then falls off. Because nutrients seem to be withdrawn in the process, this qualifies as a form of deciduousness, as opposed to what is seen in e.g. Eucalyptus - which sheds branches in a green state (https://www.abc.net.au/local/stories/2014/02/12/3943191.htm).

This can amount to the abscission of a quarter to a third of the crown, in the form of two or three branches, including large branches on some occasions (Richard Knight, pers. comm.).

The uniqueness of O. moniliferum in branch-deciduousness, in the Cape Floristic region, may possibly be related to the fact that this species is often the shrub with the fleshiest (juiciest) and least waxy leaves in the plant communities in which it occurs.

Osteospermum moniliferum dominates the vegetation in certain situations. However, the plants tend not to crowd each other. One seldom sees one individual touching another.

Osteospermum moniliferum tends not to coexist with Morella cordifolia (https://www.inaturalist.org/taxa/527591-Morella-cordifolia), which is associated with even more unstable sand.

The fruits of O. moniliferum are produced over a short period, relative to Muraltia spinosa. They also tend to be produced earlier in the year than those of most sympatric plants with fleshy fruits.

In the vegetation typically associated with O. moniliferum, most species with fleshy fruits tend to be poorly-attended by fruit-eating animals, so that about 75% of the ripe fruits remain, i.e. fail to be dispersed. In the case of O. moniliferum and Sideroxylin inerme (https://www.inaturalist.org/taxa/362175-Sideroxylon-inerme), most of the fruits are promptly removed. Chironia baccifera (https://www.inaturalist.org/taxa/426224-Chironia-baccifera) is intermediate in this respect.

Columba arquatrix prefers Solanum mauritianum (https://www.inaturalist.org/taxa/133287-Solanum-mauritianum) over O. moniliferum (Richard Knight, pers. comm.).

Osteospermum moniliferum is not subject to parasitism by Cassytha ciliolata (https://www.inaturalist.org/taxa/461729-Cassytha-ciliolata), which characterises the edges of fynbos and thus differs in habitat-preference from O. moniliferum.

However, it is somewhat subject to parasitism by Viscum capense (https://www.inaturalist.org/taxa/119585-Viscum-capense). However, this is less than in the case of Metalasia muricata (https://www.inaturalist.org/taxa/527592-Metalasia-muricata) - possibly because the latter has the deeper roots.

Viscum capense tends to parasitise deep-rooted plants (including Searsia as well as Metalasia). This may help to explain why it appears mainly on older, larger individuals of O. monilifera, which presumably have relatively deep roots for this species (Richard Knight, pers. comm.).

In summary, O. moniliferum uses water liberally. However, when the soil dries out, it compromises by simply reducing the size of the crown, replacing shed branches during the following rainy season by means of the characteristically rapid growth.

Many species with fleshy fruits, sympatric with O. moniliferum, are associated with clumps of tall shrubs. Their sowing by birds places them in a microenvironment where there is competition and shading, balanced by some protection from wildfire. Osteospermum moniliferum is ecologically odd, because it is dispersed by birds but is best sown in the open, and on bare ground.
(writing in progress)

Publicado el diciembre 23, 2022 12:59 MAÑANA por milewski