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7 August 2019
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Fynbos: Famine in Paradise FOLLOW US ONLINE
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Writer Dr Anina Lee
R
eaders probably don’t need to be told that fynbos is one of the most biodiverse types of plant life on earth. Our own tiny Fernkloof Nature reserve is only 1 800 hectares, yet is home to more species of plants than the entire United Kingdom. In the words of world-renowned botanist and conservation scientist, Prof Richard Cowling: “At first glance, it appears incongruous – a botanical wonderland in an area where essential nutrients are very scarce; where summer drought dries out the topsoil for months; where recurring fires raze all in their path; …where howling winds pummel ceaselessly for days; …and where the amount of plant material produced is only marginally greater than in some deserts. These are definitely not the features that most people associate with paradise.”
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But fynbos plants flourish in these conditions. They have unique “adaptations against adversity” which have evolved over millions of years. One of the major adaptations is to the very infertile soils in which fynbos grows. To quote Prof Cowling, there is indeed “famine in paradise”. All plants need a basic cocktail of mineral nutrients. But fynbos soil is especially poor in phosphorus, as well as nitrogen, sulphur, calcium, potassium and others. Fynbos soils are so nutrient-poor because most of them are derived from quartzites and sandstones of the Table Mountain and Witteberg groups which are themselves poor in minerals. Moreover, they give rise to sandy soils from which minerals can easily “wash out”.
Adapting to adversity The question is how have fynbos plants evolved over the eons to thrive with so little in the way of nourishment? Given enough time, organisms can adapt to just about anything. The major adaptation that we all know about is to fire. A fire burns the vegetation to ash, which contains all the minerals that the plants have managed to scrounge from the poor soil. The ash in turn fertilises new plant growth. But if the minerals are so difficult for the plants to find in the first place, how do they do it? They do what we would regard as very sensible – they form alliances with other organisms. Let’s have a look at some of these successful alliances, called symbiotic relationships. Living together Many fynbos plants have symbiotic relationships with fungi or bacteria. Bacteria in the soil stimulate many proteas to produce dense clusters of rootlets, called proteoid roots. These dense roots improve the uptake of poorly soluble ions, especially phosphorus, iron and manganese. These dense, sponge-like protea roots have another vital function – they soak up rainwater and release it slowly back into the environment. Bacteria also stimulate the production of nodules (small gall-like outgrowths) on the roots of legumes (plants of the Fabaceae or pea family). The bacteria that live in the nodules have a special enzyme that is able to “fix” atmospheric nitrogen, turning it into an organic form that both the bacteria and the host plant can use. The benefits work both ways – the plant gets usable nitrogen and the bacteria get sugars that the plant makes by photosynthesis. Plant-bacterial symbiosis is a win-win strategy. Plants you may know in the pea family include Psoralea pinnata (Fonteinbos) and Podalyria calyptrata (Keurtjie).
Much of the soil is also acidic; it has lots of hydrogen ions and these positive ions displace the positively-charged ions of some essential minerals that cling to the sand particles, which makes it easier for them to leach out in water.
Fungi also have symbiotic relationships with plant roots forming mycorrhizae, meaning “fungus-roots”. The fungi in this relationship absorb phosphorus and other nutrients from the soil and exchange them for sugars and shelter from the plant. The “roots” of a mycorrhizal plant are a tangle of delicate fibres – some are roots and some are fungal hyphae –
Mystropetalon thomii
Harveys capensis
clutching masses of soil particles. Over 75% of fynbos plants form some kind of mycorrhizal association, sometimes to the extent of the plant losing all roots of its own. Erica species are particularly dependent on this symbiotic relationship with fungi. This is why it can be very difficult to grow fynbos species in soils that don’t provide these benefits. The idea that fynbos can be “rescued” from infrastructure development sites and replanted elsewhere is simply a fallacy. Carnivores Some fynbos plants have evolved to be more independent and catch insect prey, rich in proteins and other nutrients they need. Think of the sundews that trap insects on their sticky leaves, then fold the leaf and secrete digestive enzymes, allowing the plant to absorb the nutritious “soup”. My firm favourite is a specialised sundew found in Fernkloof called Roridula gorgonias. Roridula catches the insect prey, but doesn’t “eat” it. Instead, a little beetle that lives on the plant (and nowhere else) consumes the prey, digests it and defecates on the plant, so providing the plant with its fertilizer. This is a truly remarkable case of symbiosis. Both the plant and the beetle get their reward. Parasites Then we have the case of the plants that are just plain greedy, take what food they need and give nothing in return. These are the parasites of nature. They grow on the roots of host plants and extract nutrients from them, offering nothing in exchange except the fact that they don’t actually kill their host. It would not be in their interest to do so, and millions of years of co-evolution have allowed them to strike this balance. Because the parasites don’t need to make their own food, they can dispense with the need for leaves and photosynthesis. But they do need to make seeds to propagate, so they often produce spectacular flowers to attract pollinators. Examples of these parasites include the “Aardroos” (Mystropetalon thomii) and “Inkblom” (Harveys capensis). It occurs to me that plants are not that different from people.
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