Vachellia karroo, or more famously Acacia karroo (sweet thorn), is one of Melozhori Private Game Reserve’s most common species of tree that serve as food for browsers. The species’ name comes from the sweet-tasting gum which is exuded from ‘wounds’ on damaged trees. This species plays an enormously important role for Melozhori, in terms of repairing the veldt and supplying food for various organisms. We are happy to notice that these trees are becoming more abundant and spreading well through the veldt as Vachellia karroo is one of the species that possess amazing veldt repairing qualities.
One of their veldt-conserving qualities stems from the thorny canopy which prevents animals from overutilizing the grass and forbs growing under it while also creating shade that maintains moisture in the dry Karoo heat. Most importantly, Vachellia karroo is one of the trees belonging to the legume family – these plants possess nitrogen-binding bacteria which grow on root nodes. The bacteria live in symbiosis with the tree roots and fixate atmospheric nitrogen into ammonia which can be easily metabolized by most organisms.
Nitrogen is imperative for regrowth and plant revitalization. Have you wondered why the grass is so green after a rainstorm? Lightning is one way in which nitrogen is fixated in the air and transformed into a usable source for most organisms. The fixated nitrogen helps the trees to grow faster than surrounding plants giving them a considerable advantage over other plants in the area – this is one of the main reasons acacia trees grow well in disturbed or nutrient-deficient soil.
The combination of nitrogen, shade and moisture creates ideal conditions for various forbs and grass species to grow under the sweet thorns -essentially creating an entire microenvironment for other organisms to thrive in.
These trees are pioneer species, which means that they grow for a period and then die, releasing their nitrogen, to create the ideal compost and environment for other, more tasty plants. Each tree typically lives for 30-40 years, depending on the conditions.
Wherever these trees grow, there is a supply of underground water made accessible due to their deep taproots. For this reason, it’s easy to spot old river beds at Melozhori as there are always green sweet thorn trees in their place.
It puts in perspective the idea that the veldt is like a living organism itself. The question arises, what is protecting these valuable plants from overutilization by browsers? To put it simply, these thorny trees have evolved to such an extent that they have built-in self-preservation mechanisms. As soon as an animal starts damaging the tree or its branches, they start secreting tannin acids. These tannins cause the leaves to turn bitter and the animals to move to more edible food.
Tannin acids cause indigestion, almost like poison. These acids bind with protein causing the animal not to be able to digest their food. Browsers generally avoid this by moving on to other areas where the leaves are sweet and don’t cause them harm. However, as one tree starts excreting these acids, it communicates with the trees around it, which will then also start secreting tannin acids.