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 in tropical and subtropical Australia. These types include unmanaged regrowth, young and old monoculture plantations, cabinet timber and ecological plantings. Patterns of species richness across reforestation type differed between the tropics and subtropics, although all reforestation types supported fewer species than forest reference sites. In the tropics similar numbers of introduced 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 native 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 in all strata) varied in their resemblance to rainforest reference sites in the different types of reforestation in the two regions. However, there was a tendency for wide-spaced plantings (monoculture and cabinet timber plantations) to be most dissimilar to rainforest reference sites. Old plantation sites in the tropics were an exception, with their similarity to rainforest reference sites being associated with establishment regimes, close proximity to remnants and limited management intervention. 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 biodiversity future under the different types of reforestation. These species demonstrated the individuality of species behaviour under different types of reforestation. However, they also suggest the likelihood of a weedy future for reforestation in the tropics. The history of landscape-scale disturbance as well as the management history of the site will influence the future course of the synthetic vegetation of a region. Rainforest timber plantings can lead to positive outcomes for biodiversity if they are designed to facilitate the colonization of rainforest taxa (all elements), and through management regimes (e.g. plant spacing, careful species choice and early weed control) that favor processes associated with the development of a rainforest environment. Negative outcomes for biodiversity follow the establishment of environmental pests and weeds, and taxa (e.g. non-rainforest species) or processes (e.g. persistent high groundstorey light levels) not associated with a rainforest environment. Management and design to minimize the need for ongoing intervention will be important economic considerations in future reforestation efforts.