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.

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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.

Cyclopia Vent. (honeybush) is a South African herbal tea legume endemic to the Cape Floristic Region of South Africa. At present, knowledge of reproductive fitness (fecundity) between cultivated and undomesticated Cyclopia species is scant. Cultivated genotypes of C. genistoides (L.) R.Br. and C. subternata Vogel were evaluated for their potential to produce fruit and seeds in different localities spanning two years. Fecundity and germination were also determined from natural environment of each species and compared to cultivated species in order to determine the variation between species and between cultivated and non-cultivated species. Seeds collected were weighed to determined seed mass and evaluated for germination rate and cumulative germination per individual site. A fifth of flowers developing into a pod, averaged more initiated and filled seeds per pod in C. subternata than C. genistoides. Germination percentage was directly proportional to rate of germination in both species. Cyclopia subternata had a higher fecundity and germination than C. genistoides in both cultivated and natural environments. The study results assist in the further improvement of the genus through breeding new cultivars for seed with desired qualities for improving commercial characteristics; planning of seed orchards for seed production, selecting of best sites, assisting in choosing progenies for seed production and rehabilitation of degraded sites for the sustainable production of honeybush.