Seed banks of invasive Australian Acacia species in South Africa: Role in invasiveness and options for management

Abstract

Despite impressive efforts at clearing stands of invasive Australian Acacia species in South Africa, insufficient attention has been given to understanding the role of seed banks in the invasiveness and long-term persistence of populations. We review information on seeds of these species, considering seed production, seed rain, and the dynamics of seeds in three layers: leaf litter, and upper and lower seed banks in the soil. Many factors affect the accumulation and susceptibility to destruction of seed banks and thus the opportunities for intervention to reduce seed numbers for each of these components. Reduction of seed banks is crucial for the overall success of the multi-million dollar management initiatives against these species. Classical biological control of buds, flower and young pods has reduced the seed production of many Australian acacias in South Africa. Fire can be applied to reduce seed numbers in the leaf litter and upper seed bank in some cases, although there are serious problems associated with high fire intensities in dense acacia stands. Other options, e.g. soil inversion and solarisation, exist to exercise limited reduction of seed numbers in some situations. There is little prospect of meaningful reduction of seed numbers in the lower seed bank. Preventing the accumulation of seed banks by limiting seed production through biological control is by far the most effective means, and in almost all cases the only practical means, of reducing seed numbers. This must be an integral part of management strategies. Several invasive Australian acacias are already under effective biological control, and further work to identify additional potential agents for all the currently invasive species and potentially invasive alien species is the top priority for improving the efficiency of management programmes.

<i>Acacia cyclops</i> A. Cunn. ex G. Don (Leguminosae) in Italy: first cases of naturalization

“…Attempts to control it have included the use of both herbicides and biocontrol agents (Henderson, 2001), as well as prescribed burning, alone or combined with felling (Holmes, 1989). However, two insects have recently provided a significant reduction in Acacia cyclops seed production (Impson & al., 2004;Post & al., 2010), and biological control appears to be the most effective and cost-saving method for curbing the Acacias reproductive potential and related invasiveness (Richardson & Kluge, 2008). Because of the substantial absence of vegetative propagation (Gill, 1985;Gibson & al., 2011), A. cyclops establishment and invasive success is strongly dependant on a copious production of viable seeds and their subsequent dispersal (Milton & Hall, 1981;Holmes, 1988).…”

“…Because of the substantial absence of vegetative propagation (Gill, 1985;Gibson & al., 2011), A. cyclops establishment and invasive success is strongly dependant on a copious production of viable seeds and their subsequent dispersal (Milton & Hall, 1981;Holmes, 1988). In particular, its seeds with attractive arils are well adapted for bird dispersal whilst they are retained in the canopy (Glyphis & al., 1981), but once the seeds have fallen to the ground, ants and small rodents may also spread them or carry them into the deeper soil layers, thus increasing the likelihood of their being added to the soil seed-bank (Holmes, 1989;Richardson & Kluge, 2008 fery & al., 1988;Holmes, 1989), rather it depends upon chemical scarification, notably after the passage through the gut of birds, or to only moderate heating in order to break dormancy (Glyphis & al., 1981;Gill, 1985;Jeffery & al., 1988). Its known ability to regenerate prolifically following fire from the huge seed-bank stored in the soil (Milton & Hall, 1981), and its dominance over frequently burned areas in South Africa (Glyphis & al., 1981;Richardson & al., 1996), is therefore mainly due to the reduction of interspecific competition caused by fire rather than to a higher seedling emergence and establishment.…”

Soil seed bank of the invasive Robinia pseudoacacia in planted Pinus nigra stands

“…The black lo-cust has a remarkable spreading capacity due to its rapid vegetative propagation, high adaptability and nitrogen-fixing character (SWAMY et al 2002, RICE et al 2004). The invasion success of Robinia and of other woody Fabaceae species is further enhanced by their persistent soil seed bank and the physical dormancy of seeds (RICHARDSON and KLUGE 2008). The life expectancy of Robinia seeds in soil is considerably prolonged by antimicrobial proteins accumulated in the seed tissues, making them resistant to most pathogens (TALAS-OGRAS et al 2005).…”

Ecosystem-wide management of invasive species in the face of severe uncertainty

“…rotundata failed to reduce the population of this shrub (Thomas and Reid 2007). Approaches such as mechanical and chemical removal can effectively decrease invasive populations but re-invasion or an increase in abundance recurs after the control is ceased (Richardson and Kluge 2008).…”