Invasive Australian Acacia seed banks: Size and relationship with stem diameter in the presence of gall-forming biological control agents

Abstract

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.

Pine Stand Density Influences the Regeneration of Acacia saligna Labill. H.L.Wendl. and Native Woody Species in a Mediterranean Coastal Pine Plantation

“…Resistance to fire events is also highly influential for Mediterranean ecosystems [65]. A. saligna could have reached its maximum potential soil seed bank in the Platani nature reserve as this is related to the stem diameter reaching 6 cm [68], a value commonly found in our field surveys.…”

“…While less than 10% of seeds germinate in the first year, the remaining seeds help to build up a consistent and long-lasting soil seed bank [65,66]. In South African invaded stands, more than 44,000 seeds per square meter may be found in the seed bank, including the soil and litter layers [67,68]. This huge amount depends on the long viability of the seed in the soil, resisting up to 50 years in the absence of mechanical scarification [65] and even 10 years after clearing [62,66,68].…”

“…In South African invaded stands, more than 44,000 seeds per square meter may be found in the seed bank, including the soil and litter layers [67,68]. This huge amount depends on the long viability of the seed in the soil, resisting up to 50 years in the absence of mechanical scarification [65] and even 10 years after clearing [62,66,68]. Resistance to fire events is also highly influential for Mediterranean ecosystems [65].…”

“…This is not simple as A. saligna reaches reproductive maturity extremely quickly: We observed individuals at just one meter tall already fruiting in the study area. It is also essential to eliminate seedlings that may emerge after a disturbance event [68]. The absence of continuous forest interventions may lead to the comeback of the seed bank at pre-intervention levels in only seven years [67].…”

Peer Review #2 of "Seed survival of Australian Acacia in the Western Cape of South Africa in the presence of biological control agents and given environmental variation (v0.2)"

“…At 258 each sampling point, the trees within a circle (1.5 m radius), with the seed trap as midpoint were 259 counted (Strydom et al, 2017). If the canopy of trees not inside the circle crossed the seed trap, 260 these trees were also counted (Strydom et al, 2017). It was also noted whether each counted tree 261 had pods.…”