Will Acacia secondary forest become rainforest in the Australian Wet Tropics?

• Mono-dominant Acacia secondary forest do not represent an arrested succession.
• Acacia does not regenerate under its own canopy.
• Richness of other tree species accumulates with age since pasture abandonment.
• Succession rate unaffected by geology, rainfall or extent of surrounding forests.
• Acacia secondary forest represents a passive, low cost means of restoration.

Fragmentation presents a threat to tropical forest biodiversity and restoration can be expensive. Secondary forests regenerating on abandoned pasture are widespread, represent an opportunity to restore rainforest at minimal management cost, but can become arrested in a state dominated by a single tree species. Species richness and diversity was assessed from 26 sites in Acacia secondary forests in the Australian Wet Tropics of varying age since abandonment and the influence of rainfall and soils, and the context of remnant mature forest on succession were assessed. Stand structure indicated a lack of Acacia recruitment. Late successional species richness and diversity increased with age indicating recruitment under the Acacia canopy. The species richness of late successional tree species with fruit size 10 mm or larger also displayed an increasing trend with age, although it was statistically not significant. Forest succession progresses in Acacia secondary forest and large seeded tree species are able to recruit. The enhancement of rainforest succession with fertile geology, increased rainfall or with more remnant forest in the vicinity was not evident in this study. Secondary forest even when dominated by a single species, particularly a nitrogen fixing legume, represents a viable means of tropical forest restoration provided there is sufficient mature forest in the region to act as a seed source.