Consequences of broadscale timber plantations for biodiversity in cleared rainforest landscapes of tropical and subtropical Australia

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Abstract

In Australia, as in many countries, there has been a shift in timber production from native forests to plantations. While plantations are primarily considered an efficient means of producing timber, there is increasing interest in their potential contribution to biodiversity conservation. Plantations may have both positive and negative consequences for biodiversity, at a range of scales. We compiled a list of these consequences from the literature, and used them to assess plantation scenarios proposed for cleared rainforest landscapes in tropical and subtropical Australia. The scenarios were monocultures of: (i) hoop pine, (ii) exotic pine and (iii) eucalypts; (iv) mixed species plantations; (v) a mosaic of monoculture plantations; and (vi) a mosaic of plantations and ecological restoration plantings. Of these scenarios, plantations of eucalypts and exotic pines have the least positive consequences for biodiversity: they have little or no intrinsic value in rainforest landscapes, provide poor quality habitat for rainforest biota, and (particularly eucalypts) are characterised by a relatively open canopy which in cleared landscapes favours the recruitment of grasses and other weeds. The three scenarios based on plantations of rainforest trees have similar, moderately positive consequences for biodiversity, while a mosaic of plantations and restoration plantings has the most positive consequences for biodiversity in cleared rainforest landscapes. All scenarios may have negative impacts on biodiversity conservation if plantations replace remnant forest, provide habitat for weeds, or the tree species used in plantations or their genes escape into native forests. In practice, the relative importance of positive and negative impacts, and hence the ranking of scenarios may vary with landscape forest cover. Scenarios with strongly positive consequences for biodiversity would be favoured for the reforestation of heavily cleared landscapes, whereas scenarios with few negative consequences for biodiversity would be favoured in well-forested landscapes. Consequently, any plantation of rainforest trees may have acceptable consequences for biodiversity in well-forested landscapes, provided the trees are not invasive or carrying exotic genotypes, and plantations are managed to control weeds and feral animals. With the same caveats, plantations of exotic pines may also be acceptable from a biodiversity conservation perspective in well-forested landscapes. At present, our capacity to design and manage rainforest plantations for both timber and biodiversity objectives is limited by a lack of information on factors affecting timber production, biodiversity values and trade-offs or synergies between these objectives. Obtaining this information will require the integration of large-scale, long-term biodiversity research in broadscale plantation projects.

Introduction

For two centuries after European colonisation of Australia, the continent's tropical and subtropical rainforests provided a rich harvest of cabinet timbers. High value species such as red cedar (Toona ciliata) and the conifers hoop pine (Araucaria cunninghamii), bunya pine (A. bidwillii) and kauri pine (Agathis robusta) were extracted from these forests ahead of, or as part of, the frontier of settlement which converted large areas of rainforest to pasture (Webb, 1966, Dargavel, 1995). As the resource began to dwindle in the early part of the 20th century, foresters began establishing plantations of rainforest trees to meet the projected future demand. Due to conflict with agriculturalists over utilisation of the remaining rainforest tracts, these plantations were mainly established by conversion of native forest. By the 1980s, about 50,000 ha of rainforest timber plantations had been established in eastern Australia, primarily hoop pine, with smaller areas of bunya pine, kauri pine, red cedar, Queensland maple Flindersia brayleyana and other cabinet timbers (Keenan et al., 1997).

In recent decades, in response to increasing community awareness of the outstanding natural values of Australian rainforests, rainforest timber plantations have mostly been established on cleared land that has proved marginal for agriculture (Lamb et al., 2001). These plantations include monocultures of hoop pine and eucalypts, and mixed species plantations of rainforest cabinet timbers, established by joint ventures between state forest agencies and private landholders. In north Queensland, for example, about 2000 ha of mixed species plantations were established on cleared, privately owned land by the Community Rainforest Reforestation Program, a government funded scheme set up in the 1990s following the cessation of logging in state-owned rainforests (Lamb et al., 1997). The size of the plantation estate on cleared rainforest land in tropical and subtropical Australia is still relatively small, but there are proposals to greatly expand the size of the resource in both regions due to the decline of traditional agricultural industries (Spencer et al., 1999, CRA/RFA Steering Committee, 2000, Annandale et al., 2003).

At present, the relative merits of plantation scenarios are routinely assessed against economic criteria (e.g., Herbohn and Harrison, 2000). However, we argue that the potential consequences of plantations for biodiversity also deserve serious consideration from industry proponents. It is reasonable to assume that impacts on biodiversity will have a major bearing on public acceptance of development proposals, with negative impacts being unpopular, contested or possibly circumscribed by law (Wet Tropics Management Authority, 2003). Impacts on biodiversity are likely to affect the willingness of landholders to participate in plantation schemes, and the likelihood that such schemes will be supported by governments or large corporations (Emtage et al., 2001). Impacts on biodiversity may also determine whether projects can attract environmentally-linked funds (e.g., ethical investments, various forms of environmental credits) or sell products at a premium through environmental certification schemes (Binning et al., 2002, Lindenmayer and Hobbs, 2004).

At the same time, the potential for plantations to have positive consequences for biodiversity also deserve consideration by government agencies, organisations and individuals interested in conservation. In Australia, for example, tens of millions of dollars have been spent restoring rainforest patches and corridors to cleared land over the past decade (Erskine, 2002, Catterall et al., 2004). However, only relatively small areas have been reforested using this approach, as ecological restoration plantings are expensive. Without a large increase in government funding or a substantial reduction in the cost of establishment, the extent of restoration plantings is likely to remain small. In contrast, timber plantations have the potential to reforest large areas of land for a financial gain. If plantations can be designed and managed to provide benefits to native biota, they may be a cost-effective means of restoring biodiversity to cleared rainforest landscapes (Lugo, 1997, Lamb, 1998).

In this paper, we consider a range of broadscale plantation scenarios proposed for cleared rainforest landscapes in tropical and subtropical Australia, including monocultures of: (i) hoop pine, (ii) exotic pine, and (iii) eucalypts; (iv) mixed species cabinet timber plantations, (v) a mosaic of monoculture plantations of different species, and (vi) a mosaic of plantations and restoration plantings. Some of these scenarios involve the expansion of existing plantation schemes while others have been promoted in the literature as having both production and biodiversity benefits (e.g., Tracey, 1986, Shea, 1992, Lamb et al., 1997, Lamb and Keenan, 2001, Catterall et al., 2005). We begin by briefly reviewing the consequences of plantation development for biodiversity. We then assess the scenarios proposed for cleared rainforest landscapes in terms of these consequences. Where possible, we base our assessment on surveys of rainforest timber plantations and other types of reforestation in tropical and subtropical Australia (Fisher, 1980, Keenan et al., 1997, Bentley et al., 2000, Borsboom et al., 2002; Kanowski et al., 2003a, Kanowski et al., 2005; Proctor et al., 2003; Catterall et al., 2004, in press; Wardell-Johnson et al., 2005). However, the results of these studies must in most cases be extrapolated to broadscale plantation scenarios, due to the young age of most types of reforestation, the relatively small scale at which reforestation has been attempted to date, and/or aspects of the location, design and management of existing plantations which may not pertain to broadscale plantations. Consequently, we have also drawn on studies of plantations in other rainforest landscapes and elsewhere in Australia, and used reasoned speculation to assess some scenarios. Despite these limitations, we believe such an assessment is opportune, given the proposed expansion of broadscale plantations in tropical and subtropical Australia in the near future.

Section snippets

Consequences of plantation development for biodiversity

Plantations may have a range of positive or negative consequences for biodiversity, at a range of scales (see e.g., Parrotta et al., 1997 and references therein; Lamb, 1998, Norton, 1998, Richardson, 1998, Lindenmayer, 2000, Ashton et al., 2001, Bonham et al., 2002, Hartley, 2002, Lindenmayer, 2002, Lindenmayer and Franklin, 2002, Lindenmayer and Hobbs, 2004; and specifically for rainforest landscapes of tropical and subtropical Australia: Keenan et al., 1997, Lamb et al., 1997, Catterall, 2000

Consequences for biodiversity of plantation scenarios in cleared rainforest landscapes in tropical and subtropical Australia

In this section, we focus on those consequences of plantation development for biodiversity which vary between different scenarios (i.e., consequences associated with attributes of the tree species planted and their characteristic management regimes; and their landscape-scale arrangement). We are mainly concerned with the consequences of plantations for rainforest-dependent biota, the primary focus of conservation and restoration efforts in cleared rainforest landscapes (see also Catterall et

Discussion

The conservation of biodiversity is increasingly considered a legitimate objective of plantation establishment and management, along with other environmental goals (e.g., the control of soil erosion; landscape amenity) and a range of social and economic objectives, notably timber production (Parrotta et al., 1997, Harrison, 2000, Harrison et al., 2000, Hill, 2000, Hartley, 2002, Lindenmayer and Hobbs, 2004, Catterall et al., 2005). Integrating these diverse goals may be difficult, particularly

Acknowledgements

Thanks to Heather Proctor for the mite data, Terry Reis and Elinor Scambler for the bird data, Nigel Tucker, Stephen McKenna and Rob Kooyman for contributing to the plant data, the many landholders, councils and government agencies who provided access to sites, and others who have contributed to the ‘Quantifying Biodiversity Values of Reforestation’ project of the Rainforest CRC.

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