The Phyto-Philes: Fever Tree
Created | Updated Oct 26, 2014
Fever Tree
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
The Fever Tree, Vachellia xanthophloea, is an iconic tree here in South Africa. It is most particularly associated with riverbanks or swamps in hot, low-lying regions. It is one of many thorn tree species growing in South Africa, until recently all put in the genus Acacia although there is currently a tendency of reclassifying them, to the outrage of laypeople who have only just gotten used to using their old scientific names. The rather unpronounceable species name simply is Greek for 'with yellow bark', while 'Acacia' comes from 'akis' meaning sharp-pointed, referring to the thorns. Fever trees occur from the far northeast of South Africa northward, as far north as Kenya. Some of the great game reserves of Tanzania and Kenya have magnificent specimens. Fever trees have been mentioned by Rudyard Kipling in his story 'The Elephant's Child'. In the story, the titular child travels northward and eventually comes 'to the banks of the great grey-green, greasy Limpopo River, all set about with fever trees'. The Limpopo river flows along the northern border of South Africa, and my own province is named for it. It's not really greasy, and rarely grey-green! If the water is muddy, it's rather brownish to reddish, and it is often clear. The river is usually sluggish though, and in dry years broken up over much of its lengths into ponds separated by stretches of sand. But Kipling got the fever trees right! They are indeed common and very noticeable along the banks of the Limpopo, especially in the north and east.
Fever: Cause or Cure?
In olden days people actually thought fever trees caused fever! It is not really the case. But what happened is that in those days, malaria was still a major scourge, and the same warm, moist environments in which fever trees grew, were also ideal for the Anopheles mosquitoes which carry malaria. Because the trees were such conspicuous elements of such environments, the old pioneers came to associate the trees with the fever. Today malaria is largely under control, and now we appreciate the trees for their own sakes!
It is indeed a splendid sight to come across a grove of these trees along a river in the wild. Their smooth, greenish yellow bark is unique. There is usually a thin layer of powder on the surface of the bark. The greenish-yellow colour comes from chlorophyll, the same green substance that in the leaves of plants helps with photosynthesis, the production of sugars using energy from the light of the sun. Like many other South African plants, but unusually among the thorn trees, fever trees apparently carry out a significant part of their photosynthesis using their bark in addition to their leaves. In favourable light, especially that of the setting sun, the trees look as if they're glowing. Indeed, another link to the idea of fever might be these trees looking so surreal that the person seeing them considers him or herself to be delirious! Fever trees are typically tall, upright and sparsely branched, with a light crown of leaves letting much sunlight through. A grove of these trees is therefore a sunny rather than a shady place. They can reach a height of 30m/100' and a comparable spread, but most reach only about half that size.
Being thorn trees, fever trees bear many long, white, straight, sharp-tipped thorn along their twigs amidst the small, soft, twice-compound leaves. Their thorny crowns provide excellent protection for nesting birds. Especially weaver birds of various species hang their finely woven grass nests from the outermost twigs along the periphery of a fever tree's crown, frequently overhanging the water of the river. There, the weavers are very safe: very few predators can make it through the thorns and out to the slender twig tips. Also because the fever tree's canopy is so open, nesting birds have a clear view of predators heading their way either from above or from below.
While the weaver birds are seed eaters, fever trees also attract a great many insect-eating birds. Like other thorn trees, it is a member of the pod bearing plants, the legumes (sometimes all placed in a single family, sometimes split in three). These plants have a symbiotic relationship with bacteria housed amidst their roots. From the air diffusing into the soil, these bacteria extract nitrogen, a naturally very inert gas difficult for plants to absorb, but a vital component of most living tissues. What the bacteria do is to bind the nitrogen with oxygen to form compounds much easier for plants to absorb and transform in chemical reactions. This way, legume plants get an abundance of nitrogen, which they incorporate in nutritious protein compounds. For this reason, legumes like beans and peas are important vegetable protein sources for humans. And leguminous plants also provide these nutritious substances to animals! All of their tissues are high in protein. So it is for fever trees, too. They consequently have a vast host of invertebrates feeding on them: some come for the flowers, some for the pods, some for the leaves, and some bore into the bark and branches. Fortunately a host of insect-eating birds come to the trees as well, using every strategy at their disposal to catch the little bugs wherever they try to hide!
But of course fever trees are eaten by much larger things than insects, too. Those same nutritious pods and leaves are favourites of browsing mammals up to the size of elephants. Some mammals browse the pods off the trees, others eat them after they've fallen on the ground. Since so many pods are parasitized by insects, only few actual seeds are formed, but in nature this is enough for replenishing the populations.
Humans use the wood of fever trees for general purpose timber, but it would be a shame to cut down a large fever tree for that! The wood is pale brown with a reddish tinge, hard and heavy, but must be seasoned before use or it might crack. Unfortunately the trees are targeted by huge wood-boring grubs which, while not killing the plants, cause severe, unsightly internal damage to the wood.
Gardening with Fever Trees
These days fever trees are being planted by humans in great numbers all over the warmer parts of South Africa. They actually flourish well away from their natural distribution. In spite of their warm, moist natural habitat, they can withstand light frost and moderate drought. They are being planted in parks, parking lots, along streets, and in many people's gardens. My neighbour has a good-sized fever tree. They are remarkable garden specimens with their strikingly coloured bark. They also are pretty in spring when covered in their masses of small, fragrant, yellow, puffball-like flowers. Fever trees grow rapidly, often putting on 2m/6'7" of height each year for several years. They need little watering or care. They can survive very soggy conditions, trees in the wild often being seasonally flooded.
The only detriment to growing fever trees is that it is difficult to get undamaged seeds. But some seed suppliers have them. Fever trees can only be grown from seeds, but thankfully, undamaged seeds germinate readily. Germination can be sped up by scratching the tough seed coats and/or by soaking them in water for a dozen hours or so prior to planting. They should be planted in well-draining soil. I would recommend planting seeds individually in fairly tall, narrow bags, since they rapidly form long taproots which are easily damaged, making them tricky to transplant. They should indeed be planted in the soil where they're meant to grow as quickly as possible. Healthy seedlings will shoot up and within three years make nicely shaped trees. They should be protected against cold winds and frost while young. Ideally they should be planted in groups or groves, spaced 3m-4 m/10'-13' apart, since they set off each other so well. Their roots will in general not damage paving or other structures, but seeing as how large they can grow, they should be given plenty of space.
