Clearing by a Jobs4Carbon team in September 2021 of invasive alien trees threatening the Kamma 3 wetland

ReStory narrator: Donovan Kotze

This report was originally compiled for the Gourtiz Cluster Biosphere Reserve (GCBR), Riversdale.

Background

Owing especially to its relatively low rainfall, the northern areas of the Langeberg mountains between Garcia Nature Reserve in the west and Ruitersbos Nature Reserve in the east support limited wetlands. However, the Kamma River catchment, which feeds the Gourtiz River 18 km north of Herbertsdale town, holds three of the largest wetlands in this entire area, Kamma 1, 2 and 3 wetlands (Figure 1). Kamma 2 and 3, the only known peatlands in this area north of the Langeberg, are both under a high level of threat of erosional degradation. Also, in both cases, Invasive Alien Plants (IAPs), in particular black wattle, have significantly amplified this threat. The areas in a wetland most vulnerable to erosion and destabilization by black wattle are generally where naturally diffuse flow concentrates into channelled flow, as takes place near the outlets of both Kamma 2 and 3 wetlands.

Black wattle is less well adapted to the prolonged wetness typical of wetlands and wet bank of rivers, and root more shallowly than typical indigenous wetland trees such as the common stream conebush (Leucadendron salicifolium) and are single stemmed and “top heavy” rather than the shorter growing and are multi-stemmed indigenous trees, which are able to bend over with the force of the water so that the plant remains in place rather than being toppled over and uprooted, which is characteristic of the black wattle, and which has a destabilizing effect on wetland sediments. Black wattles further contribute to erosion by suppressing of key species controlling erosion, notably palmiet. Furthermore, where black wattle are present across a portion of the valley floor, the hydraulic resistance to floods from the wattle trees may potentially concentrate flood flows in the remaining portion of the valley floor, where the threat of erosion would be increased (Kotze 2020).

The primary erosion problems in the Kamma 2 wetland are in the lowermost portion of the wetland immediately upstream of the outflow, where since 1989 an erosion headcut has advanced 680 m through the wetland. This resulted in severe degradation of 8 ha of wetland, with approximately a third of the peat physically washed out as a result of the advance of the major gully. As result of the desiccating effect of the gully, further peat was lost as a result of increased decomposition rates and peat fires, with the latest beginning in the fire of 2018 and continuing to smoulder underground until it was extinguished by Cape Nature in 2020 (Kotze 2020).

An erosion headcut has also propagated through a section of Kamma 3 wetland from its outlet, and to which the black wattle again appear to be an important contributing factor. However, where the headcut of the Kamma 2 wetland advanced more than 300 m between 2000 and 2019, in Kamma 3, the advance during this period was 10 m. The wattle infestation at Kamma 3 wetland is also less advanced than Kamma 2 wetland. Nevertheless, the headcut in Kamma 3 is identified as a major threat being amplified by the extensive wattle infestation growing in the headcut area (Figure 2 and 3). In a sense, Kamma 3 in September 2021 was where Kamma 2 was in the early 1990s, and the importance and urgency of reducing the erosion risk to Kamma 3 by clearing these IAPs was identified as being of the highest in a survey of wetlands in the broad Herbertsdale area of the Langeberg Mountains (Kotze 2020).

The clearing operation

Based on the great urgency for IAP clearing in the headcut area of Kamma 3 wetland, described in the Motivation, Andre Britz of Jobs4Carbon secured for funding to undertake the clearing in September 2021 of the area surrounding the main headcut in the wetland (Figure 4 and 5).

On 30 August 2021, I accompanied three representatives from Jobs4Carbon to the site to run through the specifications of the required work, which are follows:

• All black wattle, stink bean and hakea should be cut, with stink bean and wattle needing to be treated with herbicide but hakea not requiring treatment, but just to ensure that there are no remaining branches with leaves (even a very small branch with a few leaves can grow up to continue the life of the hakea tree).

• In the area of the headcut (i.e. the upstream half of the priority area) it is very important that all of the cut material be carried out of the valley floor and stacked above the area likely to be flooded. If remaining, this material can potentially concentrate flows and promote erosion as well as potentially supressing the recovery growth of the indigenous vegetation. At the downstream end of the priority area away from the headcut, as much of the material as possible should be carried out, but if this turns out to be too time consuming and some remains, it is less crucial than in the upstream half of the priority area.

• When cutting and removing the trees, it is very important that the indigenous trees not be cut by mistake or damaged. The most common is Leucodendron salicifolium (riviertolbos, common stream conebush).

• When working on the headcut care should be taken not to cause any collapse of soil at the headcut face.

A portion of the extensive intact wetland upstream of the erosion headcut which is under immediate threat if the heacut propogates further

The land-owner, Petrie Beukes, also accompanied me to the site prior to the clearing. This provided a very valuable opportunity to discuss the site and its importance to the long-term ecological health of the wetland, and for him to share how he had seen the erosion some years back and that it was of concern to him. In addition, on 14 September I also had a useful opportunity to discuss with the Jobs4Carbon team the significance of the work that they were doing for the catchment and the wetland and its valuable store of peat/carbon.

I visited again on 15 September when the team were busy with the clearing operations (Figure 5) and then on 7 October after completion of the work (Figure 3b). Based on observations of the team during the clearing and especially of the site after the clearing, it is concluded that the work was well executed and done fully according to specifications outlined above. No invasive alien trees could be seen which were missed, except for a single hakea tree with an actively nesting sunbird. Cutting of IAPs, herbicide application and stacking appeared to be carried out well, and just one minor point was that two hakea trees were noted with the unnecessary application of herbicide. In addition, Margaret Lowies of Working for Wetlands and Martin Kleynhans of Zutari, who I was with on 15 September, noted how well motivated the team were in carrying out their work.

Some positive ecological effects of the clearing

The clearing has had significant positive impacts on-site within the 0.7 ha which was cleared as well as having benefits downstream and upstream. For on-site, it is important to note that much of this area was recently infested and thus the native vegetation is still reasonably intact, and therefore its recovery potential is good.

An important benefit downstream is increased water supply as a result of the higher transpiration rates of the black wattle than the native wetland/riparian vegetation (Dye and Jarmain 2004; Görgens and Howard 2016). This is of benefit to downstream agricultural water users as well as to downstream wetlands and aquatic ecosystems and their biota, including Red-listed fish species occurring in the Kamma River (Kotze 2020).

A key benefit upstream is the contribution that the clearing has had to reducing the risks of further headward erosion of the headcut. There are no obvious natural controls within the approximately 20 ha of intact upstream wetland, throughout which an erosion headcut could potentially propagate. If this occurred, it is likely to result in substantial loss in ecological condition and biodiversity value of the wetland. In addition, based on preliminary sediment sampling it is estimated that at least 12 ha of the intact 20 ha contains peat deposits, generally ranging from 0.5 to >2.0 m deep, which contributes greatly to its carbon storage service. If a conservative depth estimate of 0.5 m is taken throughout the 12 ha then this translates into 60 000 m2 of peat and its associated carbon which will be protected by measures to prevent the advance of the erosion headcut.

Recommendations for further action

Further clearing of IAPs upstream and downstream of the headcut. Although the most obvious and highest priority area has been cleared, there would be much value to be gained by clearing further upstream as well as downstream. Upstream of the headcut, are some scattered black wattle trees, mainly on the margins, which should be cleared before they increase in extent so as to compromise the indigenous vegetation and natural water flows through the system.

Immediately downstream of the headcut is a large pool potentially contributing somewhat to reducing erosion risk, but the dense indigenous vegetation which is having a damming effect at the downstream end of the pool is potentially under threat from black wattle invasion. Thus, any compromising of this indigenous vegetation may have negative implications upstream.

Secure the commitment of the land-owner. Building on the landowner’s indication of a concern about the erosion, it will be very important to work with him in terms of securing his commitment to follow-up maintenance clearing of IAPs in the Kamma 3 wetland.

Strategic planting of indigenous vegetation for erosion control. Robust wetland plants, including palmiet (Prionium serratum) and the restio Platycaulis callistachyus, have begun to establish in some of the channels immediately downstream of the headcut, and measures are recommended to promote their further establishment and expansion in this habitat. This will promote sediment deposition and back-flooding into the “base” of the headcut, thereby helping to reduce the erosion risk.

A hard intervention to work in concert with the above. The ongoing invasive alien plant control and promotion of indigenous vegetation, as described above, will probably be inadequate in the long term to halt erosion, particularly given that the scale of the headcut and its level of advance, together with the aggravating pressures of extreme events (droughts and floods) anticipated with climate change. Thus, a hard intervention is recommended to work in concert with the with above. Encouragingly, an opportunity has recently emerged through Working for Wetlands for undertaking this rehabilitation and it would be very valuable for GCBR and Jobs4Carbon to work in partnership with this initiative.

References

Dye P, Jarmain C, 2004 Water use by black wattle (Acacia mearnsii): implications for the link between removal of invading trees and catchment streamflow response. South African Journal of Science 100:40–44.

Görgens A, and Howard G, 2016. The impacts of different degrees of alien plant invasion on yields from the Western Cape water supply system: Final Report Document produced for CSIR. Aurecon, Cape Town.

Kotze DC, 2020. Ecological assessment and recommendations for the Herbertsdale mountain wetlands. Unpublished report submitted to Gourtiz Cluster Biosphere Reserve, Riversdale.