Phloeonomus Looks at Resprouting: What is it?
In the sections of Alata in Leucadendron, Common Sunshine Conebush Leucadendron salignum and Riverine Conebush - Leucadendron salicifolium were called the Resprouting Sunshine Conebushes. Although not stated,This gives the impression that these are the only two resprouters in the section Alata:
Alas, this is not so! Because resprouting is such a useful feature in identifying proteas [and this is indeed a feature of many of the more common species], we have made use of this opportunity to discuss resprouting in general.
What is resprouting?
The majority of plant species in Fynbos consist of individuals which are killed by fire. Regeneration in these species is by seeds, which have to be protected from the fire, either on the ground, underground or in fire-proof seed-heads on the plant. Some species require many years for plants to mature and produce seeds. These species may be eliminated from areas which get burnt at intervals shorter than this "maturation period": such "fire avoiders" only occur in habitats which very seldom get burnt. However, many species have individuals which are able to survive fires in some way: these are loosely referred to as resprouters.
Thick bark is the first step towards surviving fire. It is a useful strategy when plants are taller than the surrounding vegetation. Thus when the surrounding grass burns, Protea caffra may only suffer some light damage to the lowermost branches. Some species only have a thick bark and can thus only survive fires which do not burn the upper leaves.
[The list below only contains those which regularly survive fire in this manner: occasional plants of many more species may survive cool fires.] A far more efficient strategy is to retain buds under the thick bark. These allow the plant to resprout from the subcortical [under-bark] buds even after a hot fire has burned away all the leaves and killed the thinner stems.
However, these strategies do not help young plants, which have stems[ and bark] too thin to survive near- annual fires. Consequently, young plants need a strategy to survive fires. Thus many proteas which inhabit grassveld and savanna regions initially invest their energies in forming an underground rootstock. Being partly underground a rootstock usually survives a fire, allowing the plant to regenerate from underground subcortical buds: the older the plant, the thicker the bark and the greater the likelihood of survival. These buds develop into erect, usually unbranched stems: such plants are multi-stemmed . After about five to eight years the plant stops investing in the rootstock. A single stem [now big enough to support thick bark] becomes dominant, branches and the plant starts to resemble a small tree. Frequent fires at this stage may keep the plant in a multistemmed form - a period of a few years without fire is required for the plants to assume a single-stemmed tree habit.
Some species have adopted only the multi-stemmed form: this multi- stemmed habit is characteristic of species with a rootstock. After long periods without fire, plants may stop growing and flowering: they become scenescent or moribund. Too long a period without fire and they may die. On the other hand, these species may proliferate in areas burnt at frequent intervals.
Two odd variations of rootstock have developed:
In some species the plants have the rootstock completely exposed above the soil surface. These plants can only survive cool fires: almost all of these species form large, flat mats and shade out other plants beneath them. Consequently even fires that burn the edges fiercely, are thwarted by the lack of fuel near the exposed rootstock, and the plants may survive.
The other strategy is to produce underground stems [rhizomes] as in "The Fern" taught at school. Such species produce little tufts of leaves and blossoms only at the exposed ends of the branches. With the rhizomes hidden from view, it is difficult to determine the size of such plants - plants visible as three or four tufts of leaves, but several metres in diameter are common. Situated beneath the ground, such branches are safe from fire. Yet some species have both an underground rootstock and rhizome. Rhizomatous species appear to fall into two categories: those which flower mainly after a fire and then become scenescent, thus resembling resprouters; and those with a flowering peak many years after a fire which survive for far longer periods before becoming moribund, thus resembling species typically killed by fire. We need a lot more data on flowering time and vegetation age before most rhizomatous species can be confidently ascribed to such categories.
The investment in structures which protect the plant from fire is expensive. Typically plants with a rootstock produce far fewer seeds than those which rely only on seeds [reseeders] for regeneration. This has other implications: it must affect gene flow because, on average, the fewer seeds will not disperse as far as the huge quantities produced by reseeders. The potential for rapid evolution must be far slower, because where populations of reseeders consist of individuals of the same generation, resprouters still have their great, great grandparents to breed with. Overall, resprouting appears to be a strategy that capitalizes on the singlemost weak point in a plants life cycle: that of establishing seedlings. Resprouters can "hang on" in areas where reseeders dominate for long periods, and then capitalize on the occasional cycle where reseeders fail to re-establish themselves.
How does all this help us to identify Proteaceae? All species of Diastella and Vexatorella regenerate from seeds only. Sections of Protea [Dwarf-tufted Proteas] and Leucospermum [Cylindrical Pincushions] contain only resprouting species. Among the Grassveld and Shaving -brush Proteas only P. petiolaris Sickleleaf Protea and P. rupicola Krantz Protea do not resprout from underground rootstocks. Protea subvestita Lipped White Protea is the only non-Cape species which is almost always killed by fire and regenerates only from seed. In most other genera and sections, there is a single resprounting species and a larger group of non-resprouters. Typically, the resprouting species, perhaps because they can survive too frequent fires, have much larger distribution ranges than related non sprouters. Therefore, you will meet them more frequently on your walks.
Of course there are always a few bad apples: Leucadendron thymifolium Malmesbury Imbricate Conebush and P. cordata Heart Leaf Rodent-Protea have exposed rootstocks but are invariably killed by fire. Why do they have a rootstock? Hopefully the data collected by atlassers may provide some answers. Observant atlassers may discover the answers without any need to study the reams of data processed by the atlas.
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