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Afrikaanse Inligting
Seed Colour in Serotinous Cape Proteas
Amongst the three serotinous genera of Protea, Leucadendron and Aulax, there are some interesting patterns in seed colour. Species with winged seeds (some sub-sections of Conebushes) tend to be black and species with hairy seeds (mainly Protea and Aulax) tend to brown. There is variation amongst Proteas; some are very lightly coloured, whereas some are a dark brown.
Why are they all not one colour? Presumably, black winged seeds are difficult to find amongst the charcoal of the post-fire environment. After hitting the ground, winged seeds are probably moved by water to their final resting place and so may come to be mixed with fine charcoal. Hairy seeds are probably moved around by wind, not water, and so bump up against larger obstacles such as stems and leaves. It looks like seeds mimic the background they disperse to. Even within Conebushes this pattern is maintained. Hairy serotinous seeds tend to be brown (e.g. Ld galpinii, Ld album). Ld platyspermum is a brown-seeded winged species in a strongly serotinous sub-section, in which all other species are black. I don’t think it breaks the rule because its seeds are not released by female plants until after they have germinated. It will thus not be dispersed to charcoal-filled micro-wiers. Another exception is the light-coloured winged seeds produced by some Leucadendron females. Stephen Gibson and I found that species growing on limestone, such as Ld meridianum, produce more light morphs than dark morphs. Presumably seeds will be dispersed to sites with a very white background in these situations. Conebushes that are not serotinous have brown seeds, presumably because they drop their seeds annually, and not just after fire, into brown litter.
The problem with these patterns is that we are not sure what the seed-predator is that is exerting such a strong selective power. Rodents are mostly nocturnal, colour-blind and live in a smell-dominated world. Birds can’t smell, but can see well and in colour. Was or is there some bird which predates seeds in the days and weeks after fire and which may miss some seeds if they are well hidden? Jeremy Midgley, Botany UCT
Burned areas are visited by canaries and doves, usually in roving flocks. Although these flocks are not large by African standards, given the carrying capacity of Fynbos, they are sizable. What proportion seed they eat probably depends not only on camouflage, but also on seed patchiness in the landscape (clumped seed is easier to find and eat) and on the time interval between fire and rains (spring burns mean at least six months exposure, autumn fires only a few months).
But more interesting is the importance of duds. Conebushes produce no duds – almost all the fruit are fertile and tasty. Whereas in Sugarbushes between 70% and 99% of the fruit are duds – lookalikes that contain no sustenance for either birds are mammals. Do proteas need to be hidden? This is complicated by protea seeds blowing into litter and ridges and forming dense clumps in these wind traps. Could this clumping make proteas such a good food source that plants have evolved duds to make it unprofitable for seed predators?
And why is it that researchers have such an aversion to our aliens? Needlebushes have winged fruit that disperse like Sugarbushes (with a wing), but are black. Banksias are also large fruited, wind dispersed and serotinous, but do not produce dud seeds. What colour are their fruit? Surely black or very dark brown – brown is not good enough camouflage unless you can afford to produce duds! What mechanism do they have to stop them from clumping in wind traps (surely they dare not without having duds)? What are the implications for the two strategies for surviving the interval between fire and rain – do Conebushes survive spring fires better than Sugarbushes?
Tony Rebelo
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