A number of woody encroacher species are expanding both their distribution and density in the Eastern Cape. Sweet thorn (Vachellia karroo), the most common of these, detrimentally affects the
quality of grazing in the province, writes independent agricultural consultant Dr Louis du Pisani.
Many years ago, people started observing that sweet thorn (Vachellia karroo) was gradually migrating out of its known habitats in a westerly direction in the Eastern Cape. A simultaneous westerly spread of kudu into areas where they were previously unknown was attributed largely to this phenomenon.
The role of climate change
Scientists suspect that global warming, and particularly warmer temperatures, have triggered and accommodated this westerly expansion of V. karroo. The saplings and seedlings of this species are vulnerable to cold nights and are killed off at what is known as the critical elimination temperature. Several nights in succession of temperatures at this level will kill the saplings and seedlings, whereas just one or two nights of these low temperatures will enable a percentage of the plants to survive. With global warming, very low temperatures do occur from time to time, but the number of consecutive nights that reach the elimination temperature has decreased. The result is that a portion of young plants can survive and take advantage of the opportunity to establish themselves and reach maturity. Larger and mature plants are significantly less sensitive to the elimination temperature. V. karroo has also increased in density over large areas of the Eastern Cape. Prof Tim
O’Connor and his co-workers published a paper in 2014 in the Journal of the Grassland Society of Southern Africa, where they presented photographic evidence of the species increasing in density in the King William’s Town area over a period of 53 years. In 2019, the Department of Environmental Affairs estimated that some 300 000ha of Eastern Cape grazing land had been seriously encroached
by woody plants, with V. karroo and paperbark (V. sieberiana) being the the most common species. why carbon dioxide is a culprit Bush encroachment is driven by a range of factors, including incorrect grazing practices, the absence of so-called mega or large browsers (such as elephant and black rhino), a lack or misuse of fire, shifts in rainfall patterns and amount of rain, global warming and climate change. The drastic increase in carbon dioxide (CO2) concentration in the Earth’s atmosphere over the past 100 years is considered to be the main driving force behind current increased levels of bush encroachment. Plants absorb atmospheric CO2during photosynthesis and use the carbon (C) for growth and fodder production. Due to the increase in CO2 levels in the atmosphere,
plants now have more C available and thus grow better. Recent field research indicates that average increases of 17% in above-ground plant production and about 30% in root growth are due to these higher atmospheric CO2 levels. This is called CO2 fertilisation. Not all plants benefit equally from the increase in CO2. Woody plants benefit the most, as the process follows a photosynthetic (C3) pathway, which requires high levels of CO2. The efficacy of CO2 fertilisation also depends on the availability of nitrogen (N) to the plants. A deficiency of N limits the effect, whereas higher levels of N enhance it. Legumes convert atmospheric N into organic N that can be utilised for growth and production, which places them at a particular advantage. V. karroo, being a legume, is one of these species. CO2 fertilisation has benefitted V. karroo to such a degree that it has developed an extensive root system packed with carbohydrate reserves, and is less sensitive to fire, grazing and low temperatures.
Impact on grazing
Over the years, grazing experts from the Döhne Research Station, the University of Fort Hare and elsewhere have conducted groundbreaking research on the effect of bush encroachment in
the Eastern Cape. This has provided farmers with a number of valuable guidelines. V. karroo competes with grazing plants for light and water. It starts to suppress grass production at a density of more than 300 tree equivalents (TE)/ha, which means that grazing for livestock is negatively affected at a bush density of above 300 TE/ha. One TE is equal to a tree with a height of 1,5m. A 3m-high tree is thus equal to 2 TE and a 0,5m-high tree is 0,5 TE. When browsers such as goats are incorporated into the production system, the combined grass and shrub grazing/browsing capacity starts
decreasing at a bush density above 1 000 TE/ha. The bush density levels over large areas of the Eastern Cape are already significantly higher than any of the optimal density norms, and control is necessary to maximise profit. In addition to its negative effect on carrying capacity, bush encroachment also affects the biodiversity of rangeland, specifically the grass sward. It also leads to a reduction in soil cover, which in turn leads to accelerated erosion. All of these factors make it essential to manage bush encroachment. Various control methods are available, ranging from fire to mechanical control with rollers and earthmoving equipment to chemical to proceed. While the exact amount of carbon sequestered through bush encroachment in South Africa is unknown, even if it were substantial, the risk of losing biodiversity and further degrading ecosystem services from allowing bush encroachment to continue unheeded is considered unacceptable […]. It is clear that bush encroachment should be considered a form of land degradation under the UN commitments, and that other, less damaging, emission reduction opportunities should be employed to meet those targets.” The financial cost of using herbicide is also a factor. When land was still relatively cheap (up to about 30 years ago), this method of control was almost impossible to justify economically. The
cost of control was higher than the value of the land, so it was cheaper to buy more land than try to manage the encroachment. Today, the cost of control is significantly lower than the value of
land, and thus control makes economic sense.
If encroachment of sweet thorn continues unchecked, it may reach a point where the damage to grazing is irreversible
This, of course, is in addition to the resulting improvement in carrying capacity. The perceived danger that herbicides pose to the environment is another important aspect. Modern products are significantly safer and more environmentally friendly than in the past. Often, problems arise not with the herbicide itself, but from misusing it. Some people also believe that chemical control
results in the annihilation of all woody plants. This is especially a concern of those who are afraid that rare indigenous plants may be killed in the process. With the correct application, however, it is
entirely possible to control bush highly selectively. It is therefore best to consult a specialist in the field who can provide the best practice. In summary, the encroachment of V. karroo and other bush species adversely affects the production potential of large tracts of the Eastern Cape’s rangelands, with a consequent reduction in the profitability and sustainability of livestock production. If this encroachment continues unchecked, the damage may be irreversible. • Dr Louis du Pisani is the national manager for production advice and development at the National Wool Growers’ Association. He wrote this article on commission from Corteva Agriscience. Phone him on 082 773 9778, or email him at louis@nwga.co.za.
control with herbicide. Bush control is often a controversial topic. Some say that the removal of trees could further enhance CO2 levels in the atmosphere, while others are strongly opposed to the use of herbicide and its perceived negative effects on the environment. Others are against herbicide due to the cost involved. The case for and against control As noted, woody plants remove C from the atmosphere and capture it in their roots and stems. Controlling woody plants would therefore seem to be counterproductive in terms of carbon sequestration. A report published by the department of Environmental Affairs in 2019 investigated this issue and came to the following conclusion: Addressing bush encroachment would outweigh the mitigation benefit of allowing it.