Clearing stinkbean in the Kamma River – August/September 2020

ReStory narrator: Ken Coetzee – Conservation Management Services

Contracted to: the Herbertsdale Mountain Corridor project of the Gouritz Cluster Biosphere Reserve.

The Kamma River lies below the northern slopes of the Langeberg mountains and it drains from East to West into the Gouritz River. It has the distinction of containing the only true peat wetlands in the Little Karoo but these wetlands are unfortunately under threat as a result of invasion by black wattle (Acacia mearnsii) and stinkbean (Paraserianthes lophantha).

These alien trees invade the wetlands, changing both the plant composition and the soil surface stability. This ultimately results in canalisation and increased flow of the water out of the wetland as well as the formation of severe erosion head-cuts, which voraciously eat back into the less protected wetland substrate with each flood. The peat wetlands then begin to suffer the loss of their function as storage systems for water and carbon.

The locally indigenous plants of these wetlands, particularly the dominant palmiet (Prionium serratum), are shaded out by the invasive trees and due to the more rapid loss of water from the system, they lose ground, leaving the top layer of peat exposed to further aridification and erosion.

Donovan Kotze, the project wetland expert, identified the Kamma River wetlands as a conservation priority and during our landscape-scale survey of the wetland areas of the Gouritz River corridor, we found a young infestation of stinkbean, for the first time, in the Kamma River wetlands. Most of the trees were little more than saplings at the time and were well hidden, almost indistinguishable from the denser black wattle trees within which they grew. Most of the younger trees had not yet flowered so we decided to try to prevent the imminent flowering and mass release of seed by mobilising some of the already-existing resources within the Gouritz Cluster Biosphere Reserve (GCBR).

Getting to grips with the stinkbean infestation

Andre Britz, project manager of the Jobs for Carbon project of the GCBR immediately agreed to move some of his spekboom planting teams from Vanwyksdorp into the Kamma River wetlands to eradicate the stinkbean invasion. Steve du Toit, CEO of the GCBR, managed to quickly find the funds needed to purchase a large amount of herbicide gel for the project (the herbicide gel includes a blue dye and because it is applied to the cut stumps with a paint brush, it is extremely target selective and there is thus no negative impact on the wetland).

The local landowners rallied to the call and the owner of Bonniedale Guest Farm on the Kamma River, Nico Hesterman, kindly provided a fully-serviced campsite to house the work teams at no charge. Another landowner, Greg Moore, provided access to the wetland. Both landowners have been gamely fighting back the black wattle invasion in the Kamma River for years at their own cost with truly impressive results.

The work teams cleared away the stinkbean invasion in the wetland within a week and during the following week they cleared away all the stinkbean trees along the Kamma River as far as the Gouritz River, a distance of approximately 17km.

Owing to the fast reaction and complete co-operation of all the people involved, the GCBR, the landowners and the field teams, the stinkbean threat in the Kamma River has been dealt a severe blow and all that is needed now is timeous follow-up clearing. The mass release of stinkbean seed into the wetland and the river has been averted and all the mature and maturing trees have been killed.

The supervision, speed, efficiency and good spirit of the Jobs for Carbon work teams must be experienced to be fully appreciated and it was all captured on film by Ina Scholtz. It was indeed an example of a committed restoration effort when it was most needed.

Stacking the cut stinkbean outside the wetland area