The bugweed debated
To clear or not to clear, this has been the debate around bugweed. Natives of South America, bugweed and lantana are two of the most problematic alien invasive plants in South Africa. Having invaded large tracts of land especially in the high rainfall areas of South Africa, outcompeting and dominating native vegetation, reducing biodiversity and rendering the invaded land virtually useless.
They are tough survivors, tolerant of drought, fire and frost, have no natural enemies (in South Africa) and are prolific producers of seed that is widely dispersed by birds and small mammals.
The prime areas of invasion by these alien plants is disturbed land, such as alongside roads and railways, abandoned or neglected crop lands, gaps in natural forest edges damaged by fire and in and around commercial forestry land.
They seldom infest intact natural vegetation. Traditional control methods have focused on mechanical and chemical control and has not achieved the desired results when it comes to the eradication and management of bugweed and lantana, instead they tend to come back worse after clearing.
Eminent forest ecologist Dr Coert Geldenhuys sparked widespread interest in the theory that if left untreated bugweed will eventually thin out and die. Geldenhuys has reported that indigenous tree species growing beneath plantation trees and bugweed canopies located close to the edge of natural forest patches indicate the potential for the natural vegetation to regenerate over the long term, with minimal or no management intervention.
However, these theories have never been tested against empirical evidence. A group of scientists from the Forest Biodiversity Research Unit, School of Life Sciences at the University of KwaZulu-Natal have set up two fascinating research studies aimed at testing these theories. The studies are part of the work being undertaken through the BLU project focused on achieving biodiversity goals within agricultural production landscapes, particularly timber.
One study is being undertaken at Ferncliff Nature Reserve outside Pietermaritzburg and is aimed at providing data and evidence to inform the rehabilitation of heavily infested alien plant stands and exited commercial timber areas in the KwaZulu-Natal (KZN) midlands and beyond.
The key questions that the research is seeking to clarify is if a hands-off (or partially cleared) approach to the control of bugweed in the KZN midlands is a viable alternative to the traditional mechanical and chemical clearing approaches. It will also examine the nurse-plant role that bugweed may provide for planted indigenous tree seedlings.
It will do this by examining the effect that clearing the alien invasive plant canopy has on the survival and growth of indigenous tree seedlings over the next ten years or so. The survival and growth of the planted seedlings was recorded over an initial period of 18 months by Nhlakanipho Ngubane, a post-graduate UKZN student.
His results show a clear initial pattern of survival between the treatments, with many more seedlings dying in the exposed sites (78 deaths) compared to the partially cleared (33 deaths) and uncleared plots (14 deaths). This is most likely due to the harsh initial conditions of the exposed sites and the overwhelming regrowth of alien plants there.
The second research study is being undertaken at the Impendle Nature Reserve near Bulwer in KZN. This study compares bugweed succession and natural forest recovery in cleared and uncleared infested areas after 20 years. The results of this study was very striking indeed! Bugweed areas that had been cleared in 2000 were densely packed with an impenetrable thicket of woody shrubs interwoven with numerous scramblers, creepers and climbers forming a canopy cover of 60-95%, all but smothering any seedlings growing underneath.
In contrast, the areas that were previously infested by bugweed and were left untreated had self-thinned and maintained a fairly open structure, with a few scattered, dead bugweed stems and a much more diverse understory. Canopy cover in this plot was 15-30%, allowing a lot more light in, but there was almost no growth of alien plants.
These trends suggest that the effort and resources required to treat the initial bugweed infestation ultimately had no significant advantage vs the plots that were left alone with no clearing. The self-thinning of the bugweed achieved similar end result as the cleared plots.
However, the disturbance of the initial clearing allowed the tangle of creepers, climbers and scramblers to establish in the previously cleared plots. This very dense thicket would in all likelihood alter the structure and composition of the vegetation going forward.
Adie and Lechmere-Oertel in the full article published in SA Forestry magazine, stated: “Our work shows that, if the goal of the clearing of bugweed is forest restoration, and if time is not an issue, then it may be best just to leave the bugweed infestation to follow a natural successional process. If there is need and budget, strategic localised planting of indigenous plants, and selective clearing around existing indigenous trees should be considered.”
Some of the take home points of the study definitely supports Dr Coert Geldenhuys’ theory.
These studies have the potential to change the way land managers approach the control of IAP species like bugweed and lantana. However, further, long-term research is essential as forest successional processes occur over long periods of time.
Please read full article published in SA Forestry magazine.