Collaborative research aims to broaden understanding of Nama-Karoo rangelands

By Lisa Hebbelmann, PDP* PhD Candidate, SAEON Arid Lands Node Tweet

PDP* PhD student Lisa Hebbelmann carrying out field surveys at the Hereford section of the Afrikaner-Hereford grazing trial as part of her project titled "Factors limiting shrub populations in the Nama-Karoo" (Photograph by Brandon Slater, 2017)

Karoo dwarf shrubs are flagship species of the Nama-Karoo, an important livestock farming area in central South Africa.

In a collaboration, researchers at the Grootfontein Agricultural Development Institute (GADI) and SAEON are working together to understand how grazing systems and Karoo vegetation respond to livestock grazing and stocking rate; especially in anticipation of changing climatic conditions.

Long-term grazing trials at GADI, in Middelburg in the Eastern Cape, provide a useful reference for livestock farmers and researchers in the Nama-Karoo. Such trials can be used to investigate individual plant, plant population and plant community responses and changes over time in response to livestock type, grazing duration, season and stocking rate.

Afrikaner-Hereford grazing trial

The Afrikaner-Hereford grazing trial was started in 1988 to demonstrate the effects of stocking rate and animal type on Karoo veld. In 1988 Angora goats, Merino and Dorper sheep were stocked at low (3 ha/SSU), high (1 ha/SSU) and medium (2 ha/SSU) stocking rates at two sites with a replicated experiment.

In 2009 the trial was suspended, but in 2017 the Pasture and Crop Science section at GADI took the decision to reinstate the trial. Interestingly, after seven years of rest and above-average rainfall years there are still stark differences between treatments, especially at the Afrikaner trial (see image below), indicating a lasting effect of livestock management on Nama-Karoo rangelands.

Aerial photograph of the Afrikaner trial showing differences in Karoo dwarf shrub density between stocking rate and animal type treatments

Current research projects

Three research projects are underway at the Afrikaner-Hereford trials. As part of a PhD project which aims to determine factors limiting dwarf shrub populations in the Nama-Karoo, supervised by Prof. Tim O’Connor (SAEON National Office) and Prof. Ed Witowski (Animal, Plant and Environmental Sciences department, WITS University), PDP^* PhD candidate Lisa Hebbelmann aims to describe how grazing different small stock breeds at three stocking rates affects Karoo dwarf shrub populations, their demography, distribution and growth form.

During the summer of 2017, all treatment camps at the trial were sampled and quantified. Shrub species composition showed a strong response to animal type and which was further influenced by stocking rate. Dorper sheep, over all stocking rates, have had the largest impact on Karoo dwarf shrubs.

Reflecting the stark visual differences (see image below), results show that when grazed at the high stocking rate, Dorper sheep removed nearly all adult Karoo dwarf shrubs. This is attributed to the relative non-selective nature of their browsing patterns.

Where stocked with at an intermediate stocking rate of Merino sheep, the plant size distribution of the Eriocephalus ericoides population, the dominant shrub in the area, conforms to a normal distribution, suggesting that under moderate stocking rates of Merino sheep, populations of this species can persist for long periods of time.

Fence-line contrast between a camp stocked with Angora goats at one hectare per small stock unit (right) and a camp stocked with Dorper sheep at one hectare per small stock unit (left) (Photograph by Justin du Toit, 2016)

Minette Van Lingen, scientist in the Crop and Pasture Research section at GADI, is investigating how overall species diversity and richness have changed over time using historical data and data collected in 2017. Together with the analysis of rainfall and soil type for the area, she aims to describe species composition changes over time.

Justin du Toit, scientist in the Crop and Pasture Research section at GADI, has established fixed-point photography sites for photographic monitoring. Justin’s research focus is vegetation change in the Nama-Karoo and he is a regular contributor to RePhotoSa.

The Afrikaner trials is also the site of an eddy covariance flux tower, run as part of a collaboration between ArsAfricaE and GADI that aims to measure how a degraded landscape (Dorper treatment) assimilates carbon over time.

Repeat photographs at the Afrikaner trial showing changes in vegetation between 1993 (left), taken during a drought, and 2016 (right), taken during an average rainfall year (image [left]: Anon; image [right] by Justin du Toit, GADI)

Results from these studies will be useful for determining how livestock management in the Nama-Karoo contributes to vegetation change over time, providing a baseline for understanding how grazing might interact with other factors which limit plant populations, such as climatic variation including rainfall and frost, fire and interspecific competition. Specific studies on plant populations and demography are important as they provide insight on whether species can persist in an ecosystem over time in response to various limiting factors.

* The Professional Development Programme (PDP) of the Department of Science and Technology and the National Research Foundation aims to accelerate the development of scientists and research professionals in key research areas.