Our grasslands: blowing in the wind or are they ‘a changing’?

Long-term experiments with good baseline data of vegetation are shedding light on possible changes in a critical resource such as our grasslands The splendour of our grasslands is revealed when the other plant species it harbours flower following spring rains and a winter burn MSc student Collen Rabothatha at work measuring grassland at Nooitgedacht (top) and Athole (bottom) Landscape view of Hyparrhenia patches at Nooitgedacht (top) and Hyparrhenia (bottom) grassland at Athole Richness of forbs in the burnt camp

An earlier SAEON eNews article described some of the amazing vegetation changes that have taken place in the environs of Middelburg in the Northern Cape.

A major shift - from a typical Karoo mix of dwarf shrubs and relatively short-lived grass species to grassland dominated by long-lived perennial grasses - has taken place over the past 30 years. A concomitant shift in rainfall amount and seasonality appears to be the main agent responsible for this change.

Whether these changes are transient or directional in nature remains to be seen. An obvious question which arises is whether the Karoo example is localised or whether similar, quite stark changes might be taking place elsewhere in the country.

Drawing upon a rich scientific legacy

The Karoo example was uncovered owing to the rich scientific legacy which has been bequeathed on us by the long-term experiments set up by various organisations. A starting point was to revisit some of the other long-term experiments which had good baseline data of vegetation.

Professor Kevin Kirkman of the University of KwaZulu-Natal conducted his PhD research on a set of grazing trials at the Nooitgedacht and Athole research stations of the now Mpumalanga Department of Agriculture during the 1990s. One trial was initiated in the late 1980s; the other in the early 1990s. The first site is located immediately outside Ermelo on the Highveld plateau, while the second is situated further east toward the escarpment edge at lower elevation. Ermelo is known to some for the severity of its winter temperatures; Athole is a little milder.

Widespread change in a critical resource

Each trial consisted of a number of paddocks in order to examine the effect of different grazing systems. Each paddock at Athole was one hectare in size, while those at Nooitgedacht were 0,25 ha, such that the Nooitgedacht and Athole trials covered a total of 6 and 16 hectares respectively. During the 1990s, the grassland was described as a Themeda triandra (red grass) dominated grassland. Grazing treatments had affected botanical composition, but the fundamental character of this grassland was maintained.

Red grass dominates a large part of the grassland biome in South Africa, which covers much of the Highveld as well as a large portion of the Eastern Cape and KwaZulu-Natal. Grassland dominated by this species is described as medium-height grassland, and is known for the rich diversity of other plant species it harbours. Indeed, the splendour of our grasslands is revealed when these species flower following spring rains and a winter burn.

Although the grazing experiments are no longer maintained, the sites have been subjected to uniform grazing management and the paddocks remained. Twenty years on and there appears to have been a considerable degree of compositional change.

Studying the extent and nature of change

Collen Rabothatha of the Mpumalanga Department of Agriculture is conducting his MSc research on the extent and nature of this change, and is attempting to identify responsible agents. He is being supervised by Professor Kirkman, with co-supervision by Tim O’Connor of SAEON.

At Athole, the dominant position of red grass has been replaced by the far taller Hyparrhenia or thatching grasses. This change is evident across all grazing treatments, and indeed evident for surrounding grasslands on this and other properties. At the cooler Nooitgedacht, large patches of thatching grass have established, but there is still a matrix of red grass veld. Similarly to Athole, the changes are apparent across treatments and across properties.

The widespread nature of these changes, and the difference in extent of change between the two sites, are obviously strongly suggestive of an external variable being responsible for the change. A benefit of research stations is that they have maintained climate records. These are being interrogated by Collen in search of any possible climate-related explanation of the shifts. Has temperature increased or rainfall pattern altered in a manner that would benefit thatching grass?

Impact on ecological processes

A change from a medium-height to tall grassland would potentially have a strong influence on a number of ecological processes. Grasslands are maintained by fire, but the available fuel load and pattern of release of heat energy would be changed.

The rich flora found in stands of red grass would likely be disadvantaged in the taller stands of thatching grass. Will these changes herald the loss of plant diversity? Collen’s MSc will address some of these questions, but the consequences of this change will be further investigated.

As with the Karoo example, an agricultural trial that was established for a specific purpose is proving to have far greater value than originally intended. Without the detailed measurement of vegetation that Professor Kirkman undertook for his PhD, Collen would not have been in a position to undertake a study of this nature.

It not only informs Agriculture about possible changes in a critical resource, but informs the country that our world may be changing.