The Rate of Regeneration of Native Plant Species After Eradication of Invasive Alien Plant Species (Acacia Decurrens Willd.) in the Limpopo Province, South Africa.

Riparian invasive alien plants are known to compete with native plant species for water, space, daylight, and different other resources by decreasing structural diversity of native vegetation and subsequently changing the functioning of the ecosystem. The aim of this study was to investigate the rate of native plant species recolonization after the eradication of A. decurrens. The investigation was done in the Waterberg District Municipality, Limpopo Province in a farm, which is highly infested with A. decurrens. Twenty-four permanent plots of 10 m x 10 m were constructed and the A. decurrens individuals in the plots were removed and the area was monitored for a period of 2 years. The size of quadrats was based on the size and distribution of the invasive alien plants which develop in an aggregated form and have exceptionally small canopies.


Introduction
About 250 thousand acres of wattle trees are propagated for commercial purposes in the Union of South Africa, primarily for the wattle bark, or its extract, which have valuable tanning properties. The bark reaches maturity when the trees are from eight to ten years of age and the timber is used in the gold mining industry and as fuel. Two species of wattle, originally introduced from Australia where they are indigenous, are grown commercially from natural seed; green wattle, Acacia decurrens Willd., and black wattle. A. mollissima Willd. The growth of A. decurrens wattle is mostly thought of as being superior to A. mollissima under unfavourable conditions in South Africa, however, as a result of its bark containing an excessive amount of red colouring matter, the tiny proportion of A. decurrens in cultivation is slowly disappearing in favour of A. mollissima.
The environmental and nancial effect caused by invasive species on native plants in Australia is not broadly reported by barely any contextual investigations, notwithstanding increasing awareness of the potential of indigenous plants as weeds. The effects of invasive alien plant can incorporate disturbance to environmental cycles by sped up biomass accumulation, decreased light in ltration, increased nitri cation, changed re intensity and frequency, altered geo-morphological processes, hybridisation with congeners, which can prompt decreases in the richness of species and diversity. A large number of these effects are like the attack of plants beginning from different nations (alien plants). Nonetheless, measured effect data on biodiversity esteems are only distributed for Pittosporum undulatum Vent. (Mullett and Simmons, 1995;Rose and Fairweather, 1997), Leptospermum laevigatum (Sol. ex Gaertn.) F. Muell. (Lam and van Etten, 2002) and Acacia longifolia (Andrews) Willd.
The environmental effects of Australian invasive plants, particularly acacias, are the most contemplated species in South Africa, it was discovered that thirteen of these plant species are currently naturalized, and eight of these plants cause far reaching alteration of biological communities and ecological systems (Richardson and van Wilgen, 2004). While a comparable size of infestation and effects are still to be revealed from indigenous acacias in Australia, indirect proof demonstrates that the potential is there.
in Western Australia; A. cyclops Cunn. ex Don in South Australia; and A. saligna (Labill.) W.L. Wendl., A. baileyana F. Muell. and A. longifolia in eastern Australia indicate broad-scale impacts could also be inevitable if there is no implementation of acceptable management measures As invasive native plants are increasingly recognised as problematic in natural vegetation (Adair, 2008) many are subject to suppression programs to protect biodiversity values. Control options vary according to life-form, susceptibility, risk of non-target damage, ease of implementation, size of infestation and outcome targets. Control choices for indigenous plants incorporate the utilization of herbicides, managed re, grazing, mechanical eradication, biological control, and coordinated strategies, including the profoundly viable technique (for some woody plants) of 'rolling' infestation with substantial equipment, then using re after a time of drying (Muyt, 2001 (Benson and McDougall, 2001). In territories where it has become naturalised, Acacia decurrens is commonly found along the roads, creek lines and in wasteland areas. It also develops in disturbed locations adjacent to bushlands and open forests (Benson and McDougall, 2001). It was broadly propagated in New South Wale, and it is hard to tell whether it is indigenous or naturalised in zones close to its native rangelands.

Methodology
The study was conducted in the Waterberg district municipality which is found in the north west of Mookgophong town and highly infested with Acacia decurrens. It lies at 24.22'S and 28.46'E. As shown in gure 1, the Waterberg district municipality is one of the ve district municipalities of Limpopo Province in South Africa. The area is mainly dominated by Northern Sotho and Afrikaans speaking people. The population of Acaciadecurrens was distributed along the stream which passes through the farm. It receives summer rainfall, and the vegetation is that of a Savanna biome and includes species such as Combretum molle, Themeda triandra, Setaria sphacelata, Terminalia sericea and Burkea africanum.
Twenty-four permanent plots of 10 m x 10 m each were constructed along the riparian zone using iron bars as corner posts. The choice of the size of the quadrat was made after assessing the morphology and distribution of invasive alien species under study. The number of alien invasive species and native species present in quadrats were recorded before they were removed. Invasive alien plant species were removed mechanically. No chemicals were used since there are concerns about their adverse environmental implications. The areas were monitored for the re-establishment of alien invasive species so that they could be removed when they reoccur. Only native plant species were left alone once they establish themselves in the study site. All data collected were entered and stored in Microsoft Excel 2010, which were also used in the descriptive statistical analysis of the results. They were then analized using Primer V6 and PERMANOVA.

Establishment
Acacia decurrens can be effortlessly grown from seeds but, similarly as with many acacias, the seeds must undergo some pre-treatment to promote germination. The best strategy is to heat the seeds at 100˚C for 1 minute in a relatively vast volume of water (Richardson et al., 2015). The seeds must then be allowed to cool and consume water for 24 hours. Seeds that oat to the surface are mostly not potent and ought to be disposed of while viable seeds ought to swell and sink. The seeds can likewise be shaken (Wrigley and Fagg, 1996) and mechanical scratching of the seed coat is an alternative. The average viability of A. decurrens seeds, considering laboratory tests of 12 provenances, is 57 000 seeds for every kilogram and optimum temperature for ideal germination is 25˚C.

Species Richness
Signi cant difference in species richness was observed between the sampling periods. The observations of the eradicated sampling plots showed that species richness in the dug area of Acacia decurens was found to be 09  (Table 1).  Figure 4). Highest number of the other species were observed during the same period as the lowest number of A. decurens (2017-Dec).
The MDS plot of species composition clearly indicated the different in species richness between the sampling periods ( Figure 5). More species are observed in 2017-Dec, which is like what is presented on species richness table (Table 1). The similarity between the sampling periods was primarily attributed to species such as A. decurens, Cleome, B. pilosa and S. pinnata which were the dominant plant species among the quadrants during the sampling periods.

Frequency
The ve sampling periods hosted different species before and after digging. Frequency was high after the eradication of A. decurens (Table 2). There was an increase of six species (Cleome sp, Mupatsapatsane, Digitaria sp, Schkurhia pinnata, Purple sp, Hibiscus cannabis, Bidens pilosa, Sida cardifolia, D. liquidios and Congesta). This was because when A. decurens was dug, new species were observed with only A. decurens observed during all the sampling periods, followed by S. cephaselata, Cleome, S. pinnata and B. pilosa observed during four of the sampling periods. These species are the ones that seem to be able to resist or compete with A. decurens. Table 2 Frequency of native plant species following eradication of Acacia decurens The two gures above (Figure 6 (a) and (b)) are showing the study site after two different events had occurred. In Figure 6 (a) A. decurrens litter, after the people from the 'Working for Water' Programme had cut some and ring-barked some of the trees and applied the chemicals. This was before the re occurred and burnt all the litter and (b) the study site just after the re which was said to have occurred accidentally in December 2015. No grasses or any other plant species were seen growing in the same vicinity as A. decurrens except for other invader plant species such as the Eucalyptus. After the eradication, grasses started to colonize the area (Figure 7).
Some of the herbaceous species that occupied the area disappeared during the study, but these grasses remained persistent throughout. This area was covered with only an impenetrable layer of A. decurrens plant species before the experiment was conducted (Figure 8).

Discussion
The fundamentally lower native species spread for controlled sites is the same with other different investigations where invasive alien vegetation was eradicated from riparian zones (Harms and Hiebert, 2006;Galatowitsch and Richardson, 2005). As anticipated, the reference zones had less invasive alien species spread than infested and controlled regions. The lower native species spread inside controlled sites was in uenced by the presence of alien species. Since the investigation assessed the early recuperation of species (1 to 4 years), it is foreseen that the moderate establishment of riparian species that have endured invasion and control will increase their spread as time advances. Much of the time, restoration of invasive alien invaded riparian vegetation in South Africa is genuinely crude and its objective is basically to return key components of ecosystem functioning. This investigation indicated that there is potential for self-repair, even in regions that have been intensely infested. Not all components normally return unaided. These regions need re-introduction of species to complete the restoration procedure. The bene t of maintaining a seed source for the woody, animal-dispersed riparian species cannot be neglected (Wassie and Teketay, 2006). It is proposed that in localities where such seed sources are inadequate with regards to re-introductions through planting, propagated specimens ought to be a part of the post-disturbance restoration process. Without this fundamental step, restoration is probably going to be fruitless, and may result in the re-establishment of invasive alien plants (Vosse et al., 2008).
Herbaceous and grass species were seen colonizing the study site after the clearing of alien invasive plant species. The brief timeframe of this investigation makes it hard to completely investigate the seral (changes in species composition through time) changes in the native vegetation. The signi cant fact with respect to the restoration of post-clearance is that recolonization of vegetation is generally by native species instead of the unpleasant condition of complete secondary invasion by the same or other alien species. Where clearing of invasive alien plant species was not performed, there was no establishment by any other plant species except for Acacia decurrens.
This demonstrates that A. decurrens affects the indigenous plant populations recovery, as it hinders their existence in its vicinity. However, the large amounts of discovered total native re-establishment shows that the mechanism demonstrates a very important level of exibility to disturbance by invasive alien plants. Flexibility is explained as the capacity of a vegetation to recover to its previous condition after a disturbance or pressure (Wali, 1999). Numerous riverine species are naturally exible because of regular and severe disturbance (Richardson et al., 2007) and have distribution and formation mechanisms, for example, the capacity to colonize virgin landscape and hostile clonal development that allows for quick recuperation following a disturbance (Naiman and Decamps, 1997). Continued monitoring and eradication of invasive alien plants by 'Working for Water' programme is a vital component that equips this exibility in the mechanism, as the repetitive clearing exhausts the invasive alien species seed pools and keeps invasion at levels that are generally easier to control provided it is done before they reach reproductive stage.
More than 1000 native plant species are pressurized by invasive alien species (Raimondo et al., 2009), and if invaders somehow managed to achieve total degree of their possible dissemination, the total biological diversity in the Cape oristic region (CFR) could be lowered by an estimated 40% (Van Wilgen et al., 2008). The greater part of the regions' watersheds exists in protected zones, where progressing invasion by trees and bushes is compromised, thereby decreasing surface water over ow by as much as 36% (whenever permitted to achieve the full degree of their possible dissemination), with signi cant nancial effects (Van Wilgen et al., 2008). Considering worries about the loss of water reservoirs and biological diversity, the South African Department of Water Affairs initiated an expansive programme to cut weeds in 1995.
This programme 'Working for Water' which works nationwide and inside the Cape Floristic Region (CFR), gives funding to the control of invasive alien plants both within and outside the protected regions. In sites where the programme has been active in the CFR, there are signs that the site infested by invasive plants has been lowered by practically half (McConnachie et al., 2016). However, the program has just achieved a little amount (4-13%) of the total infested region (Van Wilgen et al., 2012). At the CFR's scale of protected regions, there has been no endeavour up to so far to precisely measure the size of the issue, or the expense of management, nor has it been conceivable to evaluate advancement regarding lowering the infestation because of the shortage of a follow-up observation scheme (Van Wilgen and Wannenburgh, 2016).

Conclusion And Recommandations
This study indicates that much can be learned about the success and direction of the recolonization path at an early-stage of restoration. In order to correct ecosystems that are either recovering slowly or are going in a different direction off the desired recolonization path, early-stage evaluations must be conducted to gain insight into the status and path that the recolonization project is currently on.
At early-stages of recolonization, success is mostly dependent upon the survival and establishment of native vegetation, either through (active restoration) plantings or (passive restoration) natural regeneration and is therefore largely dependent on the management techniques used to promote ecosystem recovery. Longer-term success is dependent on the status of earlier-stage recolonization results, the application of ongoing management techniques, and the presence (or absence) of further ecosystem disturbance. Although complete success is yet to be determined, evaluating early-stage results provide a chance for midcourse correction, where unsuccessful recolonization can be sped up or steered back on track by altering management techniques and on-site ecosystem conditions.
In conclusion, from the results obtained during the investigation, it is possible for recolonization by indigenous plant species to occur in the regions previously infested with invasive alien plant species such as Acacia decurrens, though follow-up monitoring of the controlled areas needs to be practiced after the eradication of invasive alien plant species.   Picture showing a dense population of Acacia decurrens trees at the study site before initial clearing.