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In many protected areas in South Africa, invasive Australian Acacia species pose on-going management challenges, perpetuating high long-term management costs. Due to limited availability of resources, conservation actions need to be prioritised within and across Protected Areas (PA). We draw on comprehensive datasets spanning over 20 years from the Table Mountain National Park to model long-term outcomes of clearing Acacia species at different levels of management clearing efficacy. We test a 50 year outlook based on current and 38 incremental levels of management efficacy, ranging from 5 to 100%, to assess under which scenarios a management goal of reducing Acacia density to below 1 plant per hectare for the 22,671 ha protected area is achieved. With the current clearing resources and maximum clearing efficacy (100% control), it would take between 32 and 42 years to attain the management goal. The modelling revealed two main drivers of Acacia persistence. Firstly, germination of seeds added to the seedbank from standing plants made a significantly larger contribution to future clearing requirements than fire stimulated seed germination or the existing (pre-management) seedbank. Secondly the relationship between the number of hectares and management units that could be treated and the efficacy of the treatment was non-linear. When clearing efficacy was decreased from 100% to the current project minimum target of 80% efficacy, the goal was not achieved in all areas, but the area that reached a density of <1 plant per hectare was significantly reduced to 53% of the PA for the simulated 50 years. Results emphasize the need to differentiate between increasing financial resources and increasing efficacy. While increasing financial resources allows for increased effort, this is of little value for Acacia management in the absence of an increase in clearing efficacy, as low quality implementation perpetuates the need for large budgets over time. Conversely, improving efficacy allows for decreased budget requirements over time, allowing fund re-direction to additional areas of alien species management such as the early detection and rapid control of newly introduced species., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Australian Acacia are invasive in many parts of the world. Despite significant mechanical and biological efforts to control their invasion and spread, soil-stored seed banks prevent their effective and sustained removal. In response South Africa has had a strong focus on employing seed reducing biological control agents to deal with Australian Acacia invasion, a programme that is considered as being successful. To provide a predictive understanding for their management, seed banks of four invasive Australian acacia species (Acacia longifolia, A. mearnsii, A. pycnantha and A. saligna) were studied in the Western Cape of South Africa. Across six to seven sites for each species, seed bank sizes were estimated from dense, monospecific stands by collecting 30 litter and soil samples. Average estimated seed bank size was large (1017 to 17261 seed m-2) as was annual input into the seed bank, suggesting that these seed banks are not residual but are replenished in size annually. A clear relationship between seed bank size and stem diameter was established indicating that mechanical clearing should be conducted shortly after fire-stimulated recruitment events or within old populations when seed banks are small. In dense, monospecific stands seed-feeding biological control agents are not effective in reducing seed bank size.

The method of clearing alien species and the nature of the soil seedbank influence the quality of restoration outcomes, particularly in fire‐prone ecosystems heavily invaded with fire‐adapted alien species. One of the challenges encountered is reducing the likelihood of reinvasions when the invading species are equally responsive to restoration treatments. By simulating the fire effects that are required to regenerate native vegetation, the study tested whether the recovery of the native species could be initiated without conducting a prescribed ecological burn. In a case study of South African Cape Flats Sand Fynbos with a heavy invasion of Acacia saligna, the felled acacia were stacked into brush piles, with the litter raked off from the sowing areas and the collected seeds were pre‐treated for germination. Despite the lack of a fire, the sowing of pre‐treated seeds on raked plots led to good recovery of native vegetation over time. This was indicated by the recovery of higher density, cover and richness of native species in sown plots compared with unsown treatments. However, the recovery of native species had not approached the vegetation structure comparable to a reference site after 2 years; that is, only partial fynbos structure was recovered. The recruitment of acacia was less dense without fire, as hypothesised, and independent of treatment. However, over time, control plots had higher acacia cover than other treatments. Despite this sparse recruitment of acacia, the acacia seedbank decreased naturally to about 50% of the initial size over 2 years in control plots. Raking off litter during site preparation removed 50% of the acacia seedbank which decreased slightly thereafter. Consequently, the residual acacia seedbank was relatively similar across treatments after 2 and a half years. In conclusion, circumventing prescribed burns in heavily degraded fynbos ecosystems is a scalable restoration strategy, as recruitment of alien acacia was minimized, its seedbank declined significantly, and good native cover developed after clearing and sowing. [ABSTRACT FROM AUTHOR]

Invasive alien plants negatively impact ecosystems, necessitating intricate management actions. In a critically endangered vegetation type within the fynbos biome of South Africa, a study was performed comparing different management interventions over plots invaded by Acacia saligna. A dynamic modeling approach was designed to analyze field data and simulate the effectiveness of several restoration methods. Field data for vegetation recovery rates over the course of 2 years were fed into the model, which allowed the extrapolation of multiple recovery trajectories over a long time‐span, not possible to obtain from traditional short‐term field surveys. Our model simulations show that different treatments in similarly degraded states at the time of clearing can result in vastly different recovery trajectories. Active seed sowing was initially most expensive but resulted in the most successful native shrub recovery, decreasing costs of longer‐term follow‐up acacia clearing. Clearing without burning was cheapest but resulted in limited establishment of both native and acacia cover, providing an opportunity for secondary invasion by alien forbs. In this case, biotic thresholds may have been crossed which prevented recovery of certain vegetation components. Active sowing can partially reverse thresholds by restoring shrub cover but not structural diversity. Therefore, even applying this treatment did not resemble vegetation structure of the reference condition after an extended period of 30 years, but does show how restoration can be improved by native seed sowing compared to passive restoration alone. Our model simulations provide a useful tool to support decision‐making by providing management recommendations for optimizing alien plant‐clearing protocols. [ABSTRACT FROM AUTHOR]

Studies of invasive Australian Acacia have shown that many seeds are still produced and accumulate in soil stored seed banks regardless of the presence of seed-targeting biological control agents. This is despite claims of biological control success, although there is generally a lack of data on the seed production of invasive Australian Acacia before and after the release of the respective agents. We aimed to quantify seed production and seed survival of invasive Australian Acacia currently under biological control. The seed production and survival (proportion of aborted, predated and surviving seeds) of A. longifolia, A. pycnantha and A. saligna were each studied at four to five sites in the Western Cape of South Africa. The relationships between seed production and stand characteristics were determined and the relative effects of seed predation and abortion on seed survival were established. The investigated invasive Australian Acacia produced many seeds that survived the pre-dispersal stage despite long-term presence of released biological control agents. It was shown that seed crop size is the only significant factor influencing seed survival of the studied Australian Acacia species. Furthermore, the seeds surviving per tree and per square meter were related to tree size. No quantitative evidence was found to suggest that seed-reducing biological control agents are having an impact on the population dynamics of their Australian Acacia hosts. This study illustrates the importance of studying the seed ecology of invasive plants before biological control agents are selected and released. [ABSTRACT FROM AUTHOR]

The degree to which invasive Acacia species affect South Africa’s livestock production has received little attention. We investigated the ecological impacts of Acacia mearnsii invasion on forage quality and quantity and on soil resources, along A. mearnsii invasion gradients, on South African rangelands and the subsequent conditions following clearing. Grazing capacity was reduced by 72% in densely invaded sites, whereas clearing improved grazing capacity by 66% relative to densely invaded sites within 5 years. In densely invaded sites total grass species basal cover was reduced by up to 42%. As a result, A. mearnsii reduced grazing capacity, from 2 to 8 ha required to support one large stock unit (ha/LSU) in uninvaded and densely invaded sites, respectively. Soil moisture content was lower in densely invaded sites compared with lightly invaded and cleared sites. Plant litter increased from 1.3% to 4.2%, carbon content of the soil increased from 2.0% to 4.0% and nitrogen concentrations increased from 0.1% to 0.2% in response to invasion by A. mearnsii. Clearing resulted in improved grazing capacity within 5 years. These results also showed that, if left uncontrolled, wattle species can reduce livestock carrying capacity within montane grasslands in South Africa. EDITED BY James Aronson [ABSTRACT FROM AUTHOR]

Aim We explored morphological and ecophysiological traits that enable invasive Australian acacias to compete with native species for resources (light, water and nutrients) necessary to support the substantial growth associated with successful invasions. Location Global. Results Invasive Australian acacias grow large and seed prolifically in invaded regions. The greater capacity for vegetative growth is underpinned by their ability to acquire and efficiently use resources in non-native habitats. Key biological traits that enhance acquisition include (1) rapid and substantial allocation to root mass (up to 6-fold more than co-occurring native species) directed towards deep roots (at least 50% longer than those of natives) and to extensive shallow root networks; (2) heteroblasty, in most species, conferring high relative growth rates as bipinnate seedlings but long-lived, nutrient-conserving phyllodes as adults and (3) strong N2-fixation abilities. Main conclusions The ecophysiological traits that govern the competitive interaction of invasive Australian acacias with native species are an important component of the recognized suite of factors including introduction history, human use and enemy release that combine to produce successful invasions. Traits interact to give Australian acacias competitive advantage over many native species. One such interaction is that of N2 fixation, which when coupled with slow decomposition of sclerophyllous phyllodes results in alteration of soil nutrient cycling. The lasting legacy of soil N-enrichment hinders the competitive ability of native species and further enhances invasions. The importance of edaphic factors and competitive interactions in determining invasive success should be considered in predictive modelling of species distributions. [ABSTRACT FROM AUTHOR]

• Secondary invader germination is not influenced by available soil nitrogen. • Elevated available soil nitrogen favours secondary invader growth. • Secondary invader species respond differently to available soil nitrogen. • Secondary invasion can be managed through a reduction in soil nitrogen levels. Invasive alien nitrogen-fixing species, such as Australian acacias, often leave a legacy of elevated available soil nitrogen after their removal. This legacy effect can facilitate secondary invasion by other alien species, thereby preventing natural restoration of areas being managed. To restore viable native plant communities in ecosystems where secondary invasion is a barrier to restoration, it is important to understand the soil legacy effects of invasions. Using Acacia saligna (Labill.) H. L. Wendl. (Fabaceae) invasions in the South African fynbos as case study, we determined (1) the extent to which levels of available soil nitrogen influence the germination and growth of secondary invaders; and (2) how this differs between secondary invader species. We chose five of the most common species that have been identified as secondary invaders after clearing invasive A. saligna in the Cape Flats Sand Fynbos: (i) Avena fatua (L.) (Poaceae), (ii) Briza maxima (L.) (Poaceae), (iii) Bromus diandrus (Roth.) (Poaceae), (iv) Hypochaeris radicata (L.) (Asteraceae), and (v) Raphanus raphanistrum (L.) (Brassicaceae). Using proportional fertigation, we created soil nitrate levels similar to those found in non-invaded areas (1mg/kg), and the lowest (3 mg/kg), median (7.5 mg/kg) and highest (12 mg/kg) levels typically found in areas previously invaded by A. saligna up to ten years after clearing. For each soil nitrate level, we germinated secondary invader seeds (five seeds per petri dish) in an incubator (five species × four soil nitrate levels × five replicates = 100 petri dishes). Furthermore, for each soil nitrate level, we grew secondary invaders (one plant per pot) in a greenhouse tunnel for five months (five species × four soil nitrate levels × five replicates = 100 pots). There was no significant relationship between germination success and soil nitrate level for any species. However, root and shoot dry mass were significantly positively correlated to soil nitrate level for all species. The relationship was nonetheless only linear for Bromus diandrus, Hypochaeris radicata , and Raphanus raphanistrum. These results indicate that the legacy of elevated available soil nitrogen does not have an effect on the germination of secondary invaders, but favours their growth. Therefore, secondary invasion can be managed through a reduction in soil nitrogen levels. However, secondary invader growth does not decrease at the same rate for all species in response to a decrease in available soil nitrogen. Thus, the use of soil nitrogen reduction as a tool for the management of secondary invasions should take into account the species being managed. However, given that multiple secondary invader species often dominate a restoration site, restoration efforts should reduce soil nitrogen to non-invaded levels to account for the different response rates to available soil nitrogen, and simply management efforts. [ABSTRACT FROM AUTHOR]