SAEON Ndlovu scientists restore fog collector at Haenertsburg

By Tony Swemmer and Rion Lerm, SAEON Ndlovu Node Tweet

Next to the village of Haenertsburg, on the edge of the Transvaal Drakensberg mountains, a pristine piece of montane grassland persists among vast plantations of exotic pine and gum trees.

Like the other fragmented patches that remain of this once extensive grassland type, the Haenertsburg grassland supports exceptionally high biodiversity (630 plant species within an area of 240 hectares).

These fragments are also a source of rare medicinal plants for traditional healers and may still be important for regulating the flow of the Letaba River (the sole source of freshwater supply for many of the eastern parts of Limpopo Province, as well as its capital, Polokwane).

The extent of Woodbush Granite Grassland (orange) in the headwaters of the Letaba River has decreased drastically since the 1950s (Fig. 2.1 of Corne Niemandt's MSc thesis, WITS University, 2015).

While land-use change has drastically decreased the area of these grasslands, climate change poses a threat to the persistence of the remaining patches. With a mean annual rainfall of well over 1 000 mm, forests could potentially grow across the entire area. Yet when the area was first settled, it was an extensive grassland, with trees restricted to kloofs and deep valleys.

A historical photo from the area, circa 1940. Courtesy of Sylvie Kremer-Kohne. A patch of Afromontane forest nestled within the grassland. Expansion of such patches is likely in response to global change.

Fire is a natural factor which prevents the encroachment of trees into grassland, but rising atmospheric CO₂ and climatic change are likely to diminish the positive impacts of fire (see Change is in the Air for why). This poses a threat to high-rainfall grasslands throughout the world, and Haenertsburg was selected as an ideal site for long-term research needed to detect such changes and refine predictions.

Repeated vegetation sampling at the site by SAEON scientist Dave Thompson has already revealed that parts of the Haenertsburg grassland that are not burnt every year are invaded by shrubs. This will ultimately lead to a conversion of the grassland to either thicket or forest.

However, changes in precipitation and temperature are now starting to occur, and the impacts of these are not well understood. Warmer temperatures could lead to more water stress in the dry winter season and limit the growth of forest trees. On the other hand, increased rainfall predicted for the Escarpment might further promote the growth of trees, particularly if winter rainfall increases significantly.

Mist is common in the area, and the contribution of this to water availability in the dry season could be a critical factor. While SAEON has maintained an automated weather station at the site since 2011, this has only measured precipitation that falls as rain. A mist collector was therefore set up in 2016, but this was vandalised shortly after.

Recently, a high-precision Campbell weather station was purchased for the site, and a new mist collector set up. A new location for these valuable meteorological instruments was found on the edge of the village, in the garden of SAEON student Sylvie Kremer-Kohne.

SAEON technician Rion Lerm alongside the weather station he recently erected at Haenertsburg. The mist collector is to his right. Monthly rainfall and mist recorded by the SAEON weather station at Haenertsburg.

Data from the original station and more recent data from the new station show regular precipitation from mist. While this is low in most months, it could be significant towards the end of the dry season, with mist comprising 35% of total precipitation in August of 2016.

Many more years of data will be required to determine the average contribution of mist to precipitation, as well as to detect changes in mist precipitation. The maintenance and security of the new station is therefore critical, as is the conservation of the nearby grassland.

The imminent proclamation of the Haenertsburg Grassland as a provincial nature reserve will be significant progress in the development of this long-term research site.