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Why do Sugarbushes not Burn Up?

After some fires at Potberg all that was left was some skeletons of sugarbushes and conebushes. All the heaths, reeds and other proteas had burned up completely and left no skeletons. This made it easy to identify them, but of the pincushions and spiderheads there was not a trace. Why do they not also leave tell tale stems? Protea Atlas Logo

The temperature of a fire can be told by the size of stem it consumes. Very hot fires will consume thick sticks. Cool fires will only burn fine material. Near the ground where the fires are hottest, thicker wood is consumed and higher up in the canopy where fires are cooler quite thin wood may remain. Thus a familiar sight would be that near the ground all the stems thinner than say 15 mm diameter have been consumed; emergent Pr repens have been killed even though their leaves are only scorched; and, Pr laurifolia plants above 3m tall are still alive with healthy leaves above the fire line.

Close inspection reveals that things are not quite as simple. True, remaining stem diameter is related to fire temperature. But, no matter how hot the fire, almost all those plants with standing skeletons are serotinous (i.e. store their seeds on the plant for release after fire). And almost all of the non-serotinous plants are consumed to beyond recognition, even after quite cool fires. This makes sense - there is no point in protecting one's seeds against birds and insects, storing them on the plant for years, only to have the seedheads fall during the fire into the hot zone where they will be consumed. This obviously requires a stem that must be thick enough, and dense enough, to survive the hottest natural fires. Note that the serotinous Heath (Erica sessiliflora), Blacktips (Bruniaceae), Ceders (Widdringtonia spp.) and Phaenocoma prolifera (Daisy Family) also have standing stems, but all of the non-serotinous members of these families are consumed by fire. What is surprising is that many proteas that have equally thick stems (e.g. Ld tinctum) disappear without trace after a fire.

Some non-serotinous species are not consumed even after hot fires. However, they invariably fall into two categories. They either survive the fire by extending into the fire-safe zone above the normal vegetation layers (e.g. Ls conocarpodendron), or alternatively, they produce thick stems and thick bark and sprout from buds beneath the bark (only Pr nitida in the Cape, but almost all tree-sized proteas in the grassveld). All serotinous proteas have wind dispersed seeds (hairy in Sugarbushes, flat wings in Conebushes). Thus after the fire has passed and the ground has cooled, the seeds are blown around unimpeded by vegetation or even skeletons.

The same thing applies below ground. Cool fires do not penetrate deep into the soils and seeds which lie on the ground may survive. These tend to be weedy species such as Renosterbos, Slangbos and Sewejaartjies. But hot fires penetrate deeper into soil and kill off all those species with small surface seeds (the deeper into the soil that seeds are buried, the more reserves they need to get to the soil surface, hence these are usually bigger). Thus pincushions survive - indeed, they thrive best - under very hot fires which kill the smaller seeds.

Of course, all of this is inconsequential to serotinous species. The fire temperature does not determine seed survival. At least, not until release. Cool fires will mean that: wind-dispersed seeds are dispersed shorter distances (they will entangle in the unburned stems); more seeds will be consumed by rodents (which are heavily eaten by birds of prey when their shelter is burned off); and, seeds will have problems germinating (the debris may impede their germination and the reduced light levels may slow their growth allowing the weedy species to overtop them). So fire temperatures may determine plant community composition. It is not surprizing, therefore, that serotinous species do not leave their seed dispersal and survival to chance. The plants guarantee that the one feature that they can control - the safe storage and ideal positioning of seed for dispersal after release - is a constant that they can rely on.

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