While many farmers consider the common sweet thorn (Vachellia karroo, formerly Acacia karroo) an unwelcome indigenous invader, researchers point out that it is one of the best sources of feed and shelter in dry regions.
Its nutritious leaves and pods are eagerly eaten by livestock and game, while many farmers regard its yellow, ball-shaped flowers as a type of natural ‘vitamin pill’. Sheep and goats eat them off the ground, and even small quantities can help keep animals in good condition under dry conditions, when there is little greenery in the veld.
So much so, that sheep and goat farms with large numbers of these trees, which do best along water courses, tend to cope better in dry times than farms with few or none of them.
V. karroo provides plenty of shade on hot days, and thickets serve as highly effective windbreaks during cold snaps. However, they need to be controlled by browsers. Where only cattle are kept, the trees tend to form thick stands.
Why so many?
The species has proliferated over the past few decades, growing in thickets as well as singly where they were hardly seen before.
According to scientists, the growth of these trees and many other species has been spurred by increased carbon dioxide levels in the atmosphere.
This is supported by the results of an experiment at Rhodes University, in which researchers exposed sweet thorn specimens to various levels of carbon dioxide. Some of the saplings in the trial were subjected to carbon dioxide levels typical of pre-industrial conditions, while others were exposed to the high levels of the late 1990s.
The latter grew more than three times the biomass, developing massive root systems with increased starch concentration.
Higher levels of carbon dioxide in the atmosphere make the gas more readily available to trees, especially where plant growth is not limited by factors such as shade or low nutrient supply. The trees can therefore take in more carbon dioxide for the same amount of energy expenditure.
Trees store the extra carbon in their roots or stems. Increased storage means they can resprout and recover more quickly after fire or browsing. In addition, seedlings grow faster and have a greater chance of survival.
Higher levels of atmospheric carbon dioxide also result in more carbon being channelled to the plant’s defences, such as spines and tannins.
Another reason that farms with extensive sweet thorn growth are more drought- resistant is the fact that the trees host the parasitic mistletoe, commonly known as voëlent.
Mistletoe (Agelanthus natalitius) grows in clusters high up in the trees that become almost as heavy as lucerne bales and are equally nourishing. Plucked out of the branches by means of long hooks, they make a nutritious and palatable green feed that soon gets the rumens of sheep and goats working well.
Unfortunately, details about the real feed value of mistletoe are not available. But harvesting this plant is one of the most cost-effective drought survival measures available to stock farmers.
Researchers from the University of KwaZulu-Natal’s School of Biological and Conservation Sciences have even established that the mistletoe has a higher nitrogen concentration than its host tree.