Silver Tree Leucadendron argenteum
The characteristic silver sheen of the leaves is due to thousands of hairs which cover the leaf and protect the plant from desiccation and herbivory. The intensity of the sheen varies with temperature and is most pronounced in hot, dry weather when the hairs lie flat on the leaves. During wet weather the hairs stand more erect allowing free circulation of air around the leaves and the leaves are relatively drab.
These leaves were pressed, dried and used by artists for painting scenes for sale as souvenirs. Leighton suggests that boys collecting leaves may have been a major factor in the extermination of the Silver Tree on the north slopes of Table Mountain near Kloof Nek. Since no documentation of the quantities involved exist, this can only be conjecture.
The distribution of the Silver Tree is something of a mystery. There is a school which maintains that it is a Peninsula endemic, and that the populations at Paarl Mountain (first recorded in 1844), Simonsberg and Silverboomkloof (Helderberg) are all planted. However, even today, the vast majority of plants are still confined to Table Mountain. The largest populations are those at Kirstenbosch (more than half of all natural plants grow here), Wynberg Hill and Lion's Head. Smaller populations occur above Cape Town, Rhode's Memorial, Newlands Forest, Orangekloof, Hout Bay Corner, and Eagles Nest, with a possibly planted population at Noordhoek Estates. With the single exception of the Rhode's Memorial site on shale, all extant populations are confined to granite deposits on cool southerly slopes.
In many respects the Silver Tree is a forest margin species. It prefers areas which do not burn too frequently: despite its very thick bark (over 20 mm) it is killed by fire. Perhaps with a grassy understory the thick bark might allow it to survive fires, but such conditions seldom occur. By contrast, nearer forests where vegetation may survive for 40-plus years between fires, Silver Trees thrive.
The seeds also appear to favour situations adjacent to forests. Some contentious data from a pine plantation and the emergence of seedlings after 60 years suggest that seeds might survive in the soil for many decades after the adults have disappeared from an area. Similarly, the Kirstenbosch population expanded dramatically after the removal of the Gum plantation above the gardens, but it is not clear whether these seeds blew in or grew from soil-stored seeds. The fruit are not exclusively stored for long periods on the plant as one might expect from a fire-adapted species. Rather, a few fruit are released during berg-wind conditions. These fruit, with their helicopter-like appearance (a cross of blade-like plumes above a round nut) float down to the ground. Although most are eaten by rodents, sufficient obviously survive to allow plants to germinate. Squirrels also eat the seeds from cones in the process of opening.
Perhaps the most characteristic feature of Silver Tree populations are the conspicuous dead plants. Leighton attributes death of silvertrees to beetles. However, old age and the need for regular fires are probably more acceptable explanations.
Seedlings are extremely susceptible to Phytophthora (root rot) infection, especially under cultivation. In 1981 19 000 seedlings died after a Phytophthora infection in a Cape Town City Council nursery. However, under natural conditions only about 1-5 per cent of the plants die each year. Following infection the plants appear healthy until warm weather when the leaves droop. This drooping may promote heating of leaves increasing water loss by evaporative cooling. During unseasonably hot spells plants may suddenly be water-stressed and, with damaged roots unable to supply water, can wither and die within hours.
Generally by the time above-ground symptoms become obvious the roots have rotted badly, and the tree is unlikely to survive. Severe pruning may help in garden situations, but it is best to choose a species which is less susceptible to Phytophthora. Indeed, it is stated that any planted Silver Tree will certainly die due to Phytophthora, it merely being a matter of how long it will take. Soil disturbance and highly competitive grasses (such as kikuyu) are implicated in increasing infection, but the data are sparse. Certainly, an undisturbed area with no digging, no (lawn) fertilizers and a granitic soil will guarantee the best results.
For a while the Silver Tree Borer (also known as the Protea Jewel Beetle, Sphenoptera sinuosa) was implicated in the number of dead Silver Trees seen in the wild. Although tunnels in roots may promote Phytophthora infection, this insect probably plays a minor role in Silver Tree deaths.
Such is the appeal of the Silver Tree that Parks and Forestry plant some 1 000 per year on the Cape Peninsula. However, few of these are planted under ideal conditions and most of these die within a few years. Trees can readily be obtained from nurseries and may grow to 8 m within 10 years. They tolerate most conditions in well-drained soils, but need winter watering and some protection from frost. Dead plants can easily be replaced: perhaps this explains why there has been no attempt to develop Phytophthora-resistant strains.
Few people realize that the genus Protea, and indeed the family Proteaceae, were originally named after the Silver Tree, by Linnaeus in 1753, as Protea argentea. Only in 1771 did Linnaeus redefine Protea, the start of the concept of Protea as it is used today.
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