Rainforest timber plantations and the restoration of plant biodiversity in tropical and subtropical Australia

Abstract

We compared the species richness, growth forms and assemblages of vascular plants in five types of rainforest reforestation with pasture and forest reference sites in tropical and subtropical Australia. These types include unmanaged regrowth, young and old monoculture plantations, young rainforest cabinet timber species plantations and plantings designed to restore natural rainforest communities. Patterns of species richness across these reforestation types differed between the tropics and subtropics, although all reforestation types supported fewer species than natural rainforest reference sites. In the tropics similar numbers of introduced (i.e. non-native) species occurred in all types of reforestation (with the exception of old plantations which included few introduced species) and pasture reference sites. This contrasts with the subtropics where the greatest numbers of introduced species were associated with cabinet timber plantings. Greater diversity of growth forms (including epiphytes and vines) occurred in rainforest reference sites than in any type of reforestation. The assemblages of canopy trees (including both planted species and recruits) varied in their resemblance to rainforest reference sites in the different types of reforestation in the two regions. However, there was a tendency for young plantations to be most dissimilar to rainforest reference sites. On the other hand, old (ca. 60 years) plantation sites in the tropics were similar to natural rainforest reference sites. This was due to their close proximity to remnants and low intensity management regimes.Because species richness and growth form obscures the importance of particular species in reforestation, we targeted eight common species (four native and four introduced) as exemplars of the possible biodiversity future under the different types of reforestation. These species demonstrated the individuality of species behaviour under different types of reforestation. Rainforest timber plantations can lead to increased biodiversity if they are designed to facilitate the colonization of rainforest taxa, and managed to favour processes associated with the development of a rainforest environment. Negative outcomes for rainforest biodiversity follow the establishment of non-rainforest species or processes (e.g. persistent high understorey light levels) not associated with a rainforest environment. Management and designs to minimize the need for ongoing intervention will be important economic considerations in future reforestation efforts aimed at restoring biodiversity.