Research conducted by Dr Justin du Toit for his PhD in Grassland Science shows that as the climate changes, grassiness in the semi-arid Karoo is on the rise. His study investigated factors influencing the botanical composition of the eastern Karoo, including rainfall, grazing by livestock, and the impact of minimum temperatures on shifts in vegetation, while also examining the potential increased chances of fire.
Du Toit is based at the Department of Agriculture, Forestry and Fisheries’ Grootfontein Agricultural Development Institute in Middelburg in the Eastern Cape, where he conducts and publishes research on vegetation-related aspects of the Karoo. His study was supervised by Professors Kevin Kirkman and Tim O’Connor, and will be relevant to researchers, managers, farmers and conservationists in the Karoo.
He said that the aim of his study was to gain a better understanding of how Karoo vegetation works and thereby contribute new knowledge on the consequences of management interventions such as livestock grazing, natural background drivers such as rainfall and drought, and emergent factors that can be controlled to a point, such as fires.
He explained that the increase in grassiness in the Karoo over the past few decades is generally seen as a positive development. He investigated a number of factors that contributed to this, beginning by using rainfall records dating from 1888 to establish that rainfall in recent decades has been notably higher, and droughts less common. He also found strong evidence of cyclical rainfall patterns, with the clearest cycle about 20 years long.
Combining the effects of livestock grazing with rainfall effects, du Toit used long-term data to reveal that increased rainfall had resulted in the eastern Karoo becoming much grassier and sometimes less shrubby, and boasting healthier veld from the 1960s to the 2010s. Periods of rainfall decline resulted in more pronounced grazing effects, with heavy summer grazing damaging the veld and increasing the amount of bare ground, as opposed to less damaging grazing spread out over the year. Winter grazing resulted in grasses thriving and shrubs becoming less common. Increased rainfall rendered the results of grazing less pronounced; even damaged veld recovered considerably and boasted good botanical diversity. Du Toit concluded that rainfall was the primary driver of vegetation condition, but that grazing has an important secondary effect that is especially pronounced during dry times.
Since rainfall increases the growth of flammable grasses and therefore the chance of fire (an important consideration as climate change is predicted to increase the incidence of wet periods as well as droughts), du Toit examined the influence of an accidental fire at Grootfontein. He found that while fire did not change the overall number of species, it caused major shifts in which species were dominant, noting that many shrubs took many years to regain their original size.
Du Toit also investigated whether minimum temperatures (and frost) correlated with shifts in vegetation, finding that despite background warming resulting in longer growing seasons and a general increase in temperatures, there was no correlation between the length of the growing season and changes in vegetation.
This research has resulted in 12 publications and presentations, including in the African Journal of Range and Forage Science, the South African Journal of Botany and at the Annual Congress of the Grassland Society of Southern Africa.
Du Toit extended special thanks to two people among the many that contributed to his research. Grootfontein’s resident Karoo ecology expert Dr Piet Roux was instrumental in du Toit’s research and in providing insight into the region, while he described co-supervisor O’Connor as providing amazing guidance, direction and insight, and said that it was an honour to work with him.
Words: Christine Cuénod
Photograph: Supplied