The Phyto-Philes: Umbrella Thorn
Created | Updated Apr 28, 2013
Umbrella Thorn
Willem says: 'The Phyto-Philes are for plant lovers of every size and shape, colour and flavour. As with my Colours of Wildlife column, I'll be featuring one species per article, illustrated with sketches, paintings, and/or photos. Over time I hope to be showcasing the amazing diversity of weird and wonderful plants that occur in South Africa, while also from time to time looking at the flora of other countries. While featuring many spectacular species, I'll not be neglecting the smaller, more humble kinds that are nevertheless fascinating in their own right.'
Today I have for you a truly iconic tree of Africa – the Umbrella Thorn, Acacia tortilis! The species actually grows as far as Iraq and Syria in the Middle East, and throughout the Arabian Peninsula. But it is most strongly associated with Africa, where it grows throughout the hot, dry regions, even being found in the Sahara Desert! It is only absent in the moist rainforest regions and in cold highland areas. In the driest deserts it remains small and shrubby but can become a large and beautiful tree, reaching a height of 21 m/7 ft and a spread much exceeding that. It practically defines the African savannah, with its low, flat to rounded, spreading, densely-thorny crown, and can be seen in the big game reserves and national parks such as the Serengeti, Maasai Mara, or Kruger National Park. In Afrikaans we call it a ‘Haak en Steek’ (hook and sting) because it has hooked as well as straight thorns. These thorn pairs, the hooked ones being short and the straight ones being long and white, are distinctive of the species, making it easy to recognize even when it is not growing in the characteristic umbrella shape.
In my own region this tree is abundant. It is by far the dominant species in our Municipal Game Reserve, and some fine specimens can be seen there. For animals this is a very important species. It is one of the trees that grows best in dry regions and as such is able to make very efficient use of scant water resources. Its roots penetrate deep and can access subterranean water. In turn it turns water and soil nutrients into nutritious leaves and pods. It is browsed by almost all large mammals such as giraffes, black rhinos, and many others. The pods are twisted into tight coils; indeed its species name ‘tortilis’ means ‘twisted’. These pods and the seeds in them are high in protein and are favoured by browsers. They are also eaten off the ground after they fall. Pods on the tree or on the ground are often infested by weevils and other insects. But the tree makes so many pods and sets so many seeds that there are always myriads of seeds germinating and new seedlings establishing themselves.
The small seedling trees initially have a tough time. Everything will try to eat them, and in addition in many regions every few years they will face a bush fire. Not many of them survive this. When they have reached a couple of metres in height they will be resistant to most fires but will still be heavily browsed. Only when they finally exceed five or six metres will their top leaves at last be out of reach even for giraffes, and then they can put on some good height. But the spreading habit of the dense crown also means that only the leaves on the margins are available for browsing; those in the centre are protected. Also this spreading crown is great at capturing sunlight and by photosynthesis converting it to nutrients.
But this dense spreading crown, which benefits the tree, also benefits animals! The umbrella thorn is the best shade tree in the savannah – something that is of extreme importance in regions where temperatures can exceed 40 or 50 degrees Celsius on the hottest of summer days! Even in the relatively mild climate of my home town of Polokwane you will see the big game resting up in the shade of the umbrella thorn trees during the hot hours of mid-day. My photo here shows a couple of Tsessebe, Damaliscus lunatus, taking it easy around a small thorn tree.
The dense crown is also a secure refuge for birds. A vast host of small insect-eating bird species will explore these trees, probing around the leaves and the corky bark for insects and spiders. The pods with their various parasites are also much favoured; birds such as tits with their stout bills will open these up to see what’s inside. The dense thorns protect these little birds from most large predators. Many birds make their nests amidst these thorns; some even build their nests right on top of the flat crown, most notable being secretary birds but also some eagles and vultures.
Additionally this thorn tree is also one of the most valuable sources of wood in dry regions. This is mostly used for fuel, but large trees can supply wood for fence poles, furniture and general woodworking purposes. The trees usually grow rather crooked, and the wood is more suited for rough articles than for fine work. It also needs to be treated to protect it against borers. But the old Israelites apparently used it in constructing the Tabernacle and the Ark of the Covenant. The tree also exudes an edible gum; it is eaten by children and also used as Gum Arabic, a stabilizer used in the food industry, although it is not the main source. The fibrous bark can be used for making rope and yields tannin, used for tanning animal hides into leather.
This and other thorn tree species belongs to the Legume, Pea or Bean Family, the resemblance being evident in its pods. Its genus, Acacia, is huge and includes more than a thousand species, most being Australian but a great diversity also inhabiting Africa. It seems though that this genus will be split up … I know many people here and elsewhere in Africa will rebel against the name changes. Whichever way, it does not detract from this species’ importance.
Umbrella thorn trees can be grown with fair ease, but are not suitable for small gardens. They need room to spread; also they throw off their thorns, carpeting the ground below them, which means children running barefoot will get them in their feet. But I lived around these thorn trees as a kid, I did get pricked, I did get a few thorns deeply embedded in my flesh, but my parents extracted them for me and I survived, and learnt to be careful! Still, this species is actually best suited for large, open areas such as parks, along with other spreading savannah tree species. It is a rather craggy character rather than a delicate beauty, but when covered in its creamy-yellow flowers it can be quite attractive – here is one flowering in the municipal game reserve.
On farms and game reserves, if they do not already occur in abundance, they can be planted. Their root systems are deep and extensive; not only does this allow them to make best use of water, it is also helpful for binding the soil and protecting it against erosion. This makes the tree quite valuable for desert reclamation. It can and should be used in wide-spread projects in Africa to reverse desertification. It is extremely heat and drought resistant, can grow in alkaline, salty, sandy or stony soil, can tolerate light frosts, and can even withstand sand-blasting winds in desert areas.
To grow the umbrella thorn, collect LOTS of pods to ensure at least some are not parasitized. The seeds can be soaked prior to sowing; if you are handling a small number of seeds you can scratch them on sandpaper to weaken their tough coatings and make it easier for the roots and shoots to emerge. Sow the seeds in a sandy soil directly in black nursery bags, keep moist and warm. Plant them in their intended positions as soon as possible: they rapidly make long taproots and do not thrive while kept in bags. This species is fairly slow-growing. Seedlings must be protected against frosts for the first two years. Although very drought resistant, this species does grow faster if given adequate water in summer.
