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Secondary forest dominated by Acacia spp. occurs over large areas of abandoned pastures in Australia’s Wet Tropics on areas that were previously species diverse rainforest. The study by Yeo and Fensham was designed to determine whether acacia dominated secondary forests represent an arrested succession.
Long-lived, mono-dominant, early-succession tree species that are able to regenerate under their own shade are particularly likely to result in arrested succession. Where a self-perpetuating, mono-dominant forest inhibits the recruitment of late-successional species the viability of a passive forest restoration strategy is precluded.
Indications of an arrested succession would include evidence of a continuous replacement of the dominant Acacia spp. in the stand structure of a site, and evidence that the floristic composition of secondary acacia forest stands being unrelated to forest age.
The study was also designed to test the hypothesis that:
• succession will be more rapid at high rainfall sites on fertile soils
• the development of late successional species richness and diversity will be enhanced with proximity to remnant forest.
• the richness of species with larger seeds will increase with age of secondary forest.
Twenty-six secondary forest sites with either Acacia cincinnata or A. celsa as the canopy dominants were selected to represent a range of ages since disturbance.
Some of the findings of the research included:
• Stand structure indicated a lack of acacia recruitment.
• Late successional species richness and diversity increased with age indicating recruitment of diverse rainforest species under the acacia canopy.
• The species richness of late successional rainforest tree species with fruit size 10mm or larger displayed an increasing trend with age.
• Mono-dominant acacia secondary forests do not represent an arrested succession.
• Acacia spp. do not regenerate under their own canopy.
• Richness of other tree species accumulates with age since pasture abandonment.
• There was no evidence of any enhancement of rainforest succession on more fertile sites, on sites experiencing greater rainfall or on sites with more remnant forest in their vicinity.
The research concluded that passive management of acacia secondary forests is a cost effective means of restoring tropical forest in the Wet Tropics. Not only is acacia native, it has a functional role in reducing soil erosion precluding invasive weeds, and it allows the establishment of other native species including a succession of tree species typical of mature forest to develop. The association of acacia with ectomycorrhizal fungi and nitrogen fixing bacteria enhances growth on relatively infertile soils—this leads to an increase in organic matter, prevention of soil erosion and enhanced nutrient cycling. This association also stops the growth of exotic woody species in the understory.
The findings of the study suggest that acacia secondary forests will gradually be replaced by other rainforest species including those representing late-successional stages. It could be speculated that once the acacia trees die, the growth rate of the late-successional species will accelerate. However, the longevity of some acacia species means that the succession towards mature rainforest may require more than 200 years.
