Long-term field trials in the Eastern Karoo yield valuable observation data

The Bergkamp grazing trial shows the long-term effects of summer (left side of fence) versus winter (right side of fence) grazing. (Picture: Justin du Toit)

Justin du Toit is a production scientist at the Grootfontein Agricultural Development Institute, and principal investigator of the SAEON-DAFF collaborative project. (Picture: Yolandi Els)

Responding to the increased rainfall, farmers seem to have introduced more cattle into a region traditionally grazed by sheep only.

Yolandi Els, Coordinator, SAEON Arid Lands Node

Historically, most rangeland scientists in South Africa -- especially in the semi-arid and arid regions -- have focused on rangeland change (vegetation and soil) owing to degradation or desertification.

The more recent perspective has shifted the focus to the role played by global change (such as changes in climate and land use) on rangeland resources, thereby acknowledging that rangelands are not only important due to their ability to supply forage for livestock, but also due to their effect on a number of our country’s critical ecosystem goods and services, including sustainable provision of high quality water, carbon sequestration and biodiversity conservation.

In order to assess the impact of global change on rangeland ecosystems, South Africa needs high quality, consistent and accessible long-term environmental data and models which describe how changes occur in response to various impacts. The numerous long-term field experiments established by various natural resource agencies and universities in South Africa, are therefore ideally suited to provide valuable information in order to understand current and future environmental changes.

The longest running grazing trials in the world

The longest running of these trials was initiated in the early 1930s by the then Grootfontein College of Agriculture in the Karoo region. These are probably the longest running grazing trials in the world and are located at the Grootfontein Agricultural Development Institute (GADI) just outside the town of Middelburg in the Eastern Cape.

The data associated with these trials are of high value due to the uniform and uninterrupted management of experimentally well-designed grazing trials, as well as accurate and thorough record keeping over the years.

Researchers from SAEON and the Department of Agriculture, Forestry and Fisheries (DAFF) have joined forces in a collaborative project to secure and analyse the data associated with these trials. The project is being led by principal investigator Justin du Toit, a pasture scientist at GADI, and SAEON’s Arid Lands Node, which incorporates the expertise and valuable scientific input of Professor Tim O’Connor, SAEON’s observation science specialist.

Masses of long-term data (such as climate, vegetation and soil) associated with four selected trials are being collated, inventoried and archived. In addition, re-sampling of the vegetation component of the selected trials was initiated in 2011, with the set of four trials to be completed by 2013.

This large area in the north-eastern upper Karoo is interposed between the Grassland and drier Nama-Karoo Biomes. Historically, post-colonial invasion was blamed for the conversion of 32 200 km² of this area’s grassland into a veld type dominated by karoobossies (dwarf shrubs). John Acocks (1953) described it as “the most spectacular of all changes in the vegetation of South Africa”.

It is for this reason that the long-term trials at GADI were established, aimed at identifying suitable systems of rangeland management which would enable maximum utilisation of the veld whilst ensuring resilience of the system to prevent further degradation. These trials have been singularly influential in developing national agricultural policy for arid lands, also attracting considerable scientific interest over the years.

A changing landscape

Travelling through the same parts today, the picture seems to have changed. Many areas have shifted from Karoo veld to semi-arid grassland dominated by perennial grasses, with only a fraction of the earlier recorded shrub component. In the Karoo, rainfall is the main driver of primary production and several studies in the region have emphasised the importance of variation in rainfall over that of grazing in controlling growth form composition (e.g. O’Connor & Roux, 1995). The long-term rainfall patterns of the area offer a plausible explanation for the increased “grassiness” of the region.

Justin du Toit is currently analysing long-term rainfall records for the region in combination with vegetation composition data from four selected long-term grazing trials at GADI. His analysis will reveal whether there is indeed a climate-induced spatial shift from karoo to grassland vegetation. In addition, one of the long-term trials situated in mountainous terrain (Bergkamp) reveals a stark contrast in woody vegetation due to seasonal grazing systems, possibly revealing an impact by increased atmospheric CO₂ levels as well.

Significant rise in midsummer rain

Preliminary analysis of the rainfall patterns around Middelburg for the past 20 years shows a significant rise in the amount of midsummer rain (Figure 1). Historically, rain peaked annually in late summer, leading to enhanced growth of karoo shrubs due to their C3-metabolic pathway which enables growth during cool seasons. Current trends in midsummer rains are favouring the C4-metabolic pathway grasses which grow, reproduce and proliferate during the hotter seasons.

Figure 1. The rainfall patterns over the last 20 years (1991-2011) in the area around Middelburg, indicate a significant rise in the amount of midsummer rain received in comparison to the historical rainfall peak occurring in late summer. (Source: Justin du Toit)

Figure 2. Map showing the presence of dense stands of Themeda triandra (red grass) inside the Camp 6 grazing trial. This trial was erected in 1934. Based on the long-term vegetation monitoring records, red grass was first recorded here in 1957. (Source: SAEON)

The dense stands of perennial grasses in many of the long-term trials at GADI provide evidence for the perceived changes in long-term rainfall patterns. Fine-scale mapping shows dense stands of Themeda triandra (red grass) inside the Camp 6 grazing trial when sampled in 2011 (Figure 2).

Impact

What impact does the change in rainfall patterns and subsequent vegetation composition in the Karoo have on farmers and other land users in the area?

Farmers seem to have responded by introducing more cattle into a region traditionally grazed by sheep only. Cattle thrive on the bulk provided by grass, while sheep prefer the higher quality (but lower volume) diet offered by dwarf shrub species. In addition, many farmers see the shift to cattle as beneficial in that livestock losses due to predators such as jackal and caracal are no longer a problem.

Challenges and risks

However, increased rainfall brings a number of challenges and risks to which farmers in the region have not been exposed in previous years. Livestock diseases can become more common in the area as vector organisms -- such as mosquitoes in the case of Rift Valley fever -- have more favourable conditions to breed. For humans this also poses a threat as some livestock diseases can result in human infections due to direct or indirect contact with the blood or organs of infected animals.

The risk of fire also increases due to a bigger fuel load provided by the larger stands of grass. The effect of more frequent fires on the eastern Karoo’s vegetation has not been well researched, and farmers will now have to include this variable in their planning.

It is also possible that the suitability of tried and tested rangeland management systems in the region will have to be revisited in the light of imminent changes in climate.