Sweet thorn encroachment in the Eastern Cape

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.

- 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.