eNews

#05 2024

Expanding grasslands: How climate and livestock changes have contributed towards a grassier Eastern Karoo

By Gina Arena, Conservation South Africa and Helga van der Merwe, SAEON Arid Lands Node

In South Africa’s vast semi-arid regions, a quiet transformation is underway. The eastern Karoo, known for its dry shrublands, is experiencing an increase in grass cover, driven by changing climate patterns and shifts in livestock management. 

A recent study by Arena et al. (2023) unveiled how these ecological changes are reshaping the landscape, with potential implications for biodiversity, agriculture and conservation efforts.

A region in flux: understanding the eastern Karoo  

The Nama-Karoo and Grassland biomes merge along a complex transition zone in the eastern Karoo. This region, which spans from the central interior of South Africa eastward, is a mosaic of dwarf shrublands, grassy plains and ridges. Historically, the eastern Karoo has been dominated by low-lying dwarf shrubs, but over the last several decades, this vegetation has gradually been replaced by grasses, signalling a subtle shift towards a grassier environment.

This change may sound like a subtle difference in vegetation, but it has far-reaching consequences for ecosystem function, rangeland productivity and even fire risk. The study by Arena et al. (2023) sought to understand what was driving this transformation and what it might mean for the future of the region.

Nama-Karoo Biome. Left: Perdekamp on Grootfontein Agricultural Development Institute, Middelburg, Eastern Cape. Old grazing trials where Eriocephalus ericoides (Kapokbos) was the dominant plant cover and very little, if any, grass cover was present (Top left: Roux, 1 October 1960). The grazing trials were discontinued after the 1960s and the camp is currently lightly grazed by sheep. Vegetation is comprised equally of grasses (Aristida congesta subsp. barbicollis) and dwarf shrubs (E. ericoides) (Bottom left: Arena and Hoffman, 13 November 2018). Right: Goodhope, Steynsburg. 31.9 km ENE of Steynsburg. Tenaxia disticha persisting on hillside (Top right: Acocks, 25 July 1946). A fire burned through the site in 2014. Tall shrubs, mostly Searsia erosa, on the hillslope have grown but cover has remained stable. Tenaxia disticha remains a dominant grass species (Bottom right: Arena and Hoffman, November 2018).

The role of rainfall and livestock  

Two key factors have emerged as main drivers of the increase in grasses: changes in rainfall patterns – specifically a trend toward increasing annual rainfall in recent decades – and a considerable reduction in livestock numbers at the district-level. Since the 1980s, rainfall in the region has increased significantly, with more rain falling earlier in the summer, extending the growing season for perennial grasses that thrive in wetter conditions. This increase in rainfall has allowed grasses to outcompete the more drought-tolerant shrubs, pushing the boundaries of the Grassland Biome westward by approximately 100 km.

At the same time, livestock numbers in the districts studied have declined, particularly sheep, which have been the dominant grazing animals in the region for decades. Overgrazing had once contributed to vegetation degradation, leading to a landscape dominated by bare ground, and less or unpalatable shrubs. But, with fewer livestock grazing the land, grasses have had the opportunity to recover and spread.

Tracking change: a look at the data  

The study used a combination of historical vegetation surveys and landscape repeat photography to document changes in plant cover over a 60-year period. By revisiting 27 sites first surveyed in the 1960s, Arena et al. were able to compare current vegetation with historical records. These data were supported by repeat photography, where old landscape photos were re-taken from the same vantage points, providing a visual record of how the landscape has changed.

The findings were striking. Perennial grass cover had increased by as much as 32% in some areas, while the cover of bare ground had significantly decreased. Interestingly, palatable shrubs also significantly increased in cover over time, particularly in Karoo sites, while unpalatable shrubs remained constant. The overall rise in grass cover on the plains, however, far outweighs that of the shrubs.

These findings align with earlier studies and highlight a persistent transformation in the landscape. Despite these structural vegetation shifts, the original species composition of the region has remained remarkably intact.

Grassland biome. Left: Hillside farm, Springfontein. (Top left: Roux, February 1962). Xhariep Karroid Grassland, little change has occurred in total cover and the proportion of grasses and dwarf shrubs has remained much the same. The dominant grass cover is Themeda triandra (Bottom left: Arena and Hölscher, 14 February 2018). Right: Grootvlei farm, Graaff-Reinet. Survey area 26 looking west across dongas to mountain slope. Mountain areas and the vegetation plot area were dominated by Tenaxia disticha. Mountain vegetation predominantly comprised T. disticha, Dicerothamnus rhinocerotis and Euryops sp. (Top right: Roux, ~1960). Dry Highveld Grassland, the immediate foreground, which is a hillslope, has changed significantly with an increase in the cover of D. rhinocerotis. The distant basal pediments have undergone the least change. Vegetation cover is relatively high and the grass:dwarf shrub ratio indicates slightly higher dwarf shrub cover relative to grasses. (Bottom right: Arena and Hoffman, 10 January 2018).

Grasslands and rangeland health  

The shift from shrubland to grassland has also influenced the condition of these rangelands. As the cover of perennial grasses and palatable shrubs has increased, so has the rangelands’ grazing capacity. With more grasses available to support livestock, the region is more productive for farmers.

However, this shift towards higher grass cover may also reduce the presence of drought-tolerant shrubs. While grasses flourish in wetter conditions, during frequent droughts in the Karoo, the reduced abundance of drought-tolerant shrubs means there is less forage for livestock. This could have significant implications for livestock production in dry years, when grasses are scarce, and shrubs, traditionally a reliable forage source, are less available.

The study also highlighted a potential downside to this transformation: the risk of increased fire frequency. As grass cover expands, it provides more fuel for veld fires, which could pose a new challenge for land management in the region. Historically, fire has been rare in the Karoo, but with the increase in grass fuel loads, fire management planning may need to become a priority.

Furthermore, Karoo vegetation is not adapted to fire. Over time, increasing fire occurrence could lead to significant changes in the vegetation structure, composition and cover, with potentially severe consequences for rangelands and livelihoods.

Implications for conservation and land use  

The findings of this study offer important insights for conservation and land use planning in the eastern Karoo. On the one hand, the expansion of grasslands is a positive development for biodiversity and ecosystem health, as grass-dominated landscapes tend to support a wider variety of species than shrublands. For farmers, the increased carrying capacity might be a welcome change, offering the potential for more sustainable livestock management.

However, during dry years, the grasses will be of little value compared to the drought-tolerant shrubs. Furthermore, the increased fire risk means that landowners must be proactive in developing fire management plans to protect human communities, livestock and wildlife.

Conclusions  

As climate change continues to impact ecosystems globally, studies like this are critical for understanding how local landscapes are responding to broader environmental trends. The eastern Karoo’s shift from shrubland to grassland is a prime example of how climate and human activity can interact to drive unexpected changes.

Moreover, the long-term effects of rising CO2 levels and increasing temperatures on vegetation remain uncertain, especially as global efforts to curb fossil fuel use continue to lag. Therefore, Arena et al. (2023) emphasise the need for ongoing monitoring and adaptive management to balance agriculture productivity, biodiversity and the risks associated with ecological shifts, such as increasing fire frequency and reduced drought tolerance.