variable retention silviculture in tasmania's wet forests: ecological rationale, adaptive...

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Variable retention silviculture in Tasmania's wetforests: ecological rationale, adaptive managementand synthesis of biodiversity benefitsSusan C. Baker a b c & Steve M. Read a ba Forestry Tasmania, GPO Box 207, Hobart, Tasmania, 7001, Australiab CRC for Forestry, Private Bag 12, Hobart, Tasmania, 7001, Australiac School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania,7001, AustraliaPublished online: 15 Apr 2013.

To cite this article: Susan C. Baker & Steve M. Read (2011) Variable retention silviculture in Tasmania's wet forests:ecological rationale, adaptive management and synthesis of biodiversity benefits, Australian Forestry, 74:3, 218-232,DOI: 10.1080/00049158.2011.10676365

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218

Variable retention silviculture in Tasmania's wet forests: ecological rationale, adaptive management and synthesis of biodiversity benefits

Susan C. Baker1•2•3•4 and Steve M. Read1•2

1Forestry Tasmania, GPO Box 207, Hobart, Tasmania 7001, Australia 2CRC for Forestry, Private Bag 12, Hobart, Tasmania 7001, Australia

3School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia 4Email: [email protected]

Revised manuscript received 21 February 2011

Summary

The recognition that biodiversity conservation requires more than a system of reserves has led to the need to consider the outcomes ofland management actions, such as timber harvesting, in the matrix land outside reserves. The design of harvesting systems can be guided by the natural disturbance regime, which in Tasmania's lowland wet eucalypt forests is infrequent, intense wildfire. Clearfell, bum and sow silviculture has been used since the 1960s for harvesting these forests but, while this system is practical and effectively regenerates eucalypts in harvested coupes, it is predicted to lead to losses at the coupe level of late-successional species and structures that would survive into stands regenerating following natural wildfire. Variable retention silviculture is thus currently being implemented as an alternative to clearfelling in wet old-growth forest on public land (state forest) in Tasmania. In contrast to clearfelling, variable retention has the explicit ecological goal of maintaining some species, habitats and structural legacies from the pre-harvest forest into the harvested and regenerating stand. This paper synthesises biodiversity findings from the Warra Silvicultural Systems Trial (SST), established in 1997, and demonstrates that aggregated retention is the optimal form of variable retention for ensuring coupe-scale persistence ('life boating') of mature-forest biodiversity. In addition to providing retained forest, aggregates are also designed to facilitate recolonisation of harvested areas by mature-forest species ('forest influence'), and to provide connectivity across the forest stand. In the last few years, more than 50 aggregated-retention coupes have been harvested in mature forest across Tasmania. Development and implementation of variable retention in Tasmania is an example of active adaptive management, which we describe in relation to five steps for a formalised adaptive management program, indicating how ecological criteria are incorporated in operational guidelines for implementation of aggregated retention.

Keywords: si1vicultural systems; harvesting; regeneration; management; adaptation; old-growth forests; biodiversity; forest influences; eucalypts; Tasmania

Introduction

Landscape-level reserves provide habitat for many late-successional forest species. However, habitat requirements for biodiversity are manifest at multiple spatial scales, with local-scale habitat characteristics also being important for maintaining populations of certain species (Lindenmayer and Franklin 2002; Driscol12008). Some species also have limited ranges that may not be well encompassed by landscape-level reserves. Formal reserve systems alone are thus not able to protect all forest-dwelling species (Munks et al. 2004, 2009), and modem, ecologicallybased forest management seeks to employ approaches at a variety of spatial scales to maintain populations of species at the same time as harvesting timber (Franklin et al. 1997; Forest Practices Board 2000; Lindenmayer and Franklin 2002).

The Regional Forest Agreement (RFA) between the Commonwealth of Australia and the State of Tasmania (Australian Government and Tasmanian Government 1997) is a 20-year agreement that provides for the long-term ecologically sustainable management of Tasmania's forests. Through establishing a Comprehensive, Adequate and Representative (CAR) reserve system (Commonwealth of Australia 1997), the RFA increased the area of the Tasmanian reserve system by almost one-fifth. In parallel, the RFA set a framework for management by prescription of areas available for timber harvesting, and included a commitment to continuous improvement of forest management. Adaptive management is a process whereby practices evolve in response to experience. This can be achieved informally through passive adaptive management. However, a more structured approach of active adaptive management, including policy development, research, monitoring and documentation, has many benefits (Lindenmayer and Franklin 2002).

The wet eucalypt forests of southern Australia have a eucalypt overstorey, and an understorey of rainforest or sclerophyllous species depending on previous fire history (Jackson 1968). The primary natural disturbance regime in this forest type is large wildfires, which usually result in prolific regeneration of eucalypts

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Susan C. Baker and Steve M. Read 219

(Gilbert 1963; Hickey et a/. 1998). Wet eucalypt forests are widespread in Tasmania, currently covering about 821 000 ha ( 12%) of this island state. Of this area, 241 000 ha are classified as old-growth, of which 172 000 ha (71%) are included in the permanent reserve system, and 61000 ha (25%) are potentially available for timber production on public land. In practice, only about 33 000 ha (14% ofthe total area of old-growth wet eucalypt forest in Tasmania) is designated for future wood production.

Clearfell, bum and sow silviculture has been the main harvesting system used in Tasmanian lowland wet eucalypt forest since the 1960s, since it successfully regenerates eucalypts (Attiwilll994; Hickey and Wilkinson 1999b). Forest regenerating following clearfelling ( clearcutting) also provides habitat for most plants and animals that naturally occur in young forest (Attiwill1994; Hickey 1994; Hickey and Wilkinson 1999b; Baker eta/. 2004; Turner and Kirkpatrick 2009). However, compared with clearfelling followed by regeneration burning, wildfires are more patchily distributed and variable in intensity at the site level. This results in regenerating forests that usually contain surviving late-successional species and structures, either as individual trees and stags (snags) scattered throughout regenerating stands, or as unbumt patches (fire skips) of variable size (McCarthy eta/. 1999; Baker eta/. 2004; Turner eta/. 2009). These persisting mature-forest elements are important biological legacies from wildfire that maintain biodiversity and heterogeneity at the stand level (Franklin eta/. 1997). With the exception of small numbers of trees retained in habitat clumps (located on harvest boundaries in wet forest, Forest Practices Board 2000), these structural and biological legacies are removed during clearfelling (Lindenmayer and McCarthy 2002). Although some forest area is inevitably retained unlogged, either because it is unsuitable or because of protection under forest practices legislation, clearfelling nevertheless has a much greater impact on mature-forest biodiversity at the level of the individual coupe (cutblock) than does wildfire.

Moreover, since clearfelling of a site is intended to occur about every 80-100 years (Hickey and Wilkinson 1999a), forest harvested repeatedly by clearfell, bum and sow silviculture may not provide suitable long-term habitat for some species (Lindenmayer and McCarthy 2002), including those that either prefer or rely on older forest, for example rainforest trees, or birds and mammals that nest in tree hollows (Lindenmayer and Franklin 2002; Beese et a/. 2003). Furthermore, large coarse woody debris (CWD) will exist only as legacy elements after the first clearfelling event, as subsequently the mature trees required for CWD production will not be present (Grove eta/. 2002). Forest landscapes managed predominantly by clearfelling therefore do not contain the biological legacies at the coupe level that are present in wildfire-derived forest landscapes.

In response to community concerns over the biodiversity and aesthetic impacts of clearfelling (Tasmania Together Community Leaders Group 2001 ), and to increased scientific understanding of the impacts of clearfelling on biodiversity conservation, Forestry Tasmania, the government business enterprise responsible for managing Tasmania's state forests, established the Warra Silvicultural Systems Trial (SST) in 1997. The SST tested various forms of variable retention (VR) as alternatives to clearfelling in tall wet eucalypt forests, to meet both ecological objectives and objectives relating to silviculture (e.g. regeneration), economics, safety and biodiversity (Hickey eta/. 2006; Neyland eta/. 2009b ).

A premise of VR is that it can be more ecologically valuable to distribute mature-forest elements throughout coupes and the production forest landscape than to simply add an equivalent amount of mature forest to the large existing reserve system. Recognising that all silvicultural systems will impact biodiversity in the coupe to a certain extent and at least in the short term, the objective was to find an alternative to clearfelling with effects on biodiversity more similar to that of wildfire. One important caveat is that, because there has not been a recent wildfire within the region containing the study area that could be used for direct comparison, interpretation is based on an understanding of the effects of wildfire derived from other research. As a result of the studies at the Warra SST, aggregated retention has recently been formally selected for operational implementation in most old-growth wet eucalypt forest in Tasmanian state forest (Forestry Tasmania 2009).

This paper reviews the goals ofVR silviculture in the Tasmanian context, synthesises the biodiversity findings from the Warra SST, and describes the foundations for operational implementation of VR and monitoring of biodiversity outcomes.

Goals of variable retention

Variable retention (VR) (sometimes called green-tree retention) silviculture is increasingly being applied in temperate and boreal native forests in place of clearfelling (Vanha-Majamaa and Jalonen 2001; Beese eta/. 2003; Bunnell and Dunsworth 2004; Hickey et a/. 2006; Martinez Pastur eta/. 2009). Although implementation varies regionally, the central principle of VR is the retention of structural elements from the previous stand for the long term (usually at least one rotation) (Franklin eta/. 1997). The goal is for these retained elements to provide refuges for mature-forest species and structures, to improve habitat connectivity, and to accelerate recolonisation of the regenerating stand, thereby maintaining a structurally complex and species-rich forest (Franklin eta/. 1997). In addition, VR is intended to produce variability in stand structure and habitat conditions within individual coupes (harvested units), and-by varying the designs of different coupes-also to produce variability in habitat conditions at the landscape level (Mitchell and Beese 2002). Variability in configuration and habitats retained within and between different coupes is more likely to result in suitable habitat conditions for a fuller range of biodiversity over time (Lindenmayer et a/. 2006). The practices that comprise VR are guided by natural wildfires, which usually leave some original forest structures standing and do so with a high degree of spatial variability (Mitchell and Beese 2002). Compared with clearfelling, wildfires produce more patchily distributed habitat at various spatial scales-from individual sites to the landscape (Lindenmayer and McCarthy 2002; Baker eta/. 2004).

Internationally, VR and other green-tree retention practices are proving to be effective systems for maintaining mature-forest species and structures (Rosenvald and Lohmus 2008), and are being widely implemented in western Canada and Fennoscandia, and have some use in USA and in Argentinian Patagonia. The use of VR in Australia is more recent, with research trials in Tasmania (Hickey et a/. 2006) and Victoria (Squire et a/. 2006; Lindenmayer 2007), and initial operational implementation in Tasmania's tall, wet old-growth forests from about 2006 (Forestry Tasmania 2009).

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220 Variable retention and biodiversity

Like other silvicultural systems, VR is grounded in principles relating to good silviculture, such as productivity and regeneration, as well as in economic feasibility and worker safety. The objectives of VR also include greater emphasis on, and more explicit consideration of, forest biodiversity (Mitchell and Beese 2002), with the system being underpinned by knowledge of natural disturbance processes and the composition of forest landscapes. The structure and ecology of forests regenerated following natural disturbance is thus used to guide the design and implementation ofVR silviculture, and within this paradigm Franklin eta/. ( 1997) articulated three objectives for VR. The first objective is to provide refugia for 'lifeboating' species, structural elements and processes over the regeneration phase: refugia provide inocula for re-establishment of species into the harvested area once conditions become suitable. The second objective is to enrich new forest stands with structural features so that conditions suitable for late-successional species are re-established earlier in the rotation; forests with greater structural diversity provide more habitat niches and thus greater carrying capacity for biodiversity, as well as possibly ameliorating local microclimatic conditions. The third objective is to enhance landscape connectivity; structural retention moderates habitat conditions and provides 'steppingstones' to facilitate movement of organisms within the managed forest landscape.

These ecological objectives were adapted to the Tasmanian context as part of providing a formal policy base for implementing VR in Tasmanian state forests. Forestry Tasmania's goals for VR (Box 1, Forestry Tasmania 2009) are articulated around ecological parameters of forest retention and forest influence, in the context of stewardship, productivity and safety.

A very large number of forest-dwelling species have specialised habitat requirements, with critical late-successional forest habitats including hollow-bearing trees (e.g. for hollow-dependent vertebrates), large-size CWD (e.g. for invertebrates and fungi), very old large-diameter overstorey trees or tall, mature understorey trees (e.g. for lichens and bryophytes, and as a future

source ofCWD), and undisturbed soil and leaflitter layers (e.g. for invertebrates and fungi). It is these specific habitat types that VR silviculture is designed to retain. The various forms of VR, such as dispersed or aggregated retention, relate to the amount and spatial arrangement of retained trees, and are selected according to their suitability for the ecological and management characteristics of particular forest types.

Improvement in the ability of poorly-dispersing species to recolonise harvested areas is also a design goal ofVR, a concept known as 'forest influence' (Mitchell and Beese 2002). Recolonisation by late-successional biodiversity of areas disturbed by harvesting or burning is time-dependent but may also be related to proximity to colonisation sources in adjacent undisturbed forest. The 'rule of thumb' is that areas within one tree-height of long-term retention are under forest influence (Mitchell and Beese 2002), with coupes in which more than half the harvested area is within one tree-height of retained forest being regarded as not being clearfells (Keenan and Kimmins 1993). The spatial distribution of retained late-successional elements thus influences the species composition and trajectory of the harvested area as this regenerates, thereby allowing coupes to achieve more rapidly the biodiversity characteristics of older forest (Mitchell and Beese 2002; Beese eta/. 2003).

Biodiversity research findings from the Warra Silvicultural Systems Trial

Introduction to the Silvicultural Systems Trial

Silvicultural experiments provide an excellent basis for testing and monitoring of new silvicultural approaches aimed at ecologically sustainable forest management (Peterson and Anderson 2009). Variable retention approaches have been trialled in a number of long-term silvicultural experiments in the USA and Canada, including the Variable-Retention Adaptive Management (VRAM) experiments of MacMillan Bloedel and its successors in coastal


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British Columbia (Beese et a/. 2005), the Montane Alternative Silvicultural Systems (MASS) trial in Vancouver Island (Beese and Amott 1999), the Ecosystem Management Emulating Natural Disturbance (EMEND) project in north-western Alberta (Work et a/. 2004 ), and the replicated Demonstration of Ecosystem Management Options (DEMO) study in Oregon and Washington (Aubry eta/. 2004, 2009). In Victoria, Australia, a Silvicultural Systems Project (SSP) was initiated in 1986 to develop, research and evaluate alternative silvicultural systems in both regrowth Eucalyptus regnans and lowland mixed-species forests (Squire et a/. 2006). The Victorian SSP included clearfelling, shelterwood, gap cutting and stripfell treatments but not aggregated retention. Squire eta/. (2006) and Campbell ( 1997) summarise some of the results. Aggregated retention is also being tested in wet eucalypt forests in Victoria, Australia, in a replicated experiment with one large or three small aggregates at each site (Lindenmayer 2007' 2009).

The suitability of different approaches to variable retention for Tasmania's wet eucalypt forests needed local testing because of differing ecosystem characteristics, such as the requirement to bum harvesting debris for regeneration of eucalypt forests and fuel reduction. Thus in 1997 Forestry Tasmania established the Warra Silvicultural Systems Trial (SST) (Hickey eta/. 2001, 2006) within the Warra Long Term Ecological Research site (Brown eta/. 2001).

A number of potential alternatives to clearfelling were tested at the SST (Fig. 1, Table 1). The SST was one study area, with (usually) two replicate coupes of each harvesting treatment. The clearfelled sites each contained four understorey islands. Harvested areas away from the understorey islands provided information about traditional clearfell, bum, and sow conditions. Clearfell, bum and sow treatments, and unharvested controls, were used as comparisons. Research studies at the Warra SST

have included investigations into social acceptability (Ford et a/. 2009), worker safety and productivity (Neyland eta/. 2009b ), silviculture and regeneration success (Neyland eta/. 2009a) and economics (Nyvold eta/. 2005). The SST has also been the focus of intensive research into the responses of a number ofbiodiversity elements, and to investigate the effectiveness of the various trialled silvicultural systems in meeting the ecological objectives described above for alternatives to clearfelling.

The published and unpublished research on biodiversity outcomes for individual taxa conducted within the first three years after harvesting at the Warra SST is synthesised in this paper. Data collected to date have mostly allowed assessment of the degree to which mature-forest biodiversity can be maintained within coupes using the different silvicultural systems. As noted earlier, since we were unable to directly compare logged areas with wildfire regeneration of the same age, we used information from the unlogged controls to deduce species that were representative of unlogged forest. Taxa monitored at Warra include birds, groundactive beetles, vascular plants, bryophytes, lichens and fungi-a list of taxa similar to those monitored in studies elsewhere on alternatives to clearfelling (Rosenvald and Lohmus 2008). Mammal surveys at Warra were discontinued because of low rates of detection for most species, with habitat trees being used as surrogates for suitability for hollow-dwelling mammals. Not all taxonomic groups were surveyed at all silvicultural treatments. Because the SST was an operational experiment, silvicultural treatments were allocated to sites where they were considered most suitable for the objectives of a particular system, based on characteristics such as slope and vegetation. Lack of random treatment allocation and underlying site factors may have affected the results for certain biodiversity groups. Results therefore need to be interpreted with this limitation in mind. Subsequent results from a larger number of operational aggregated-retention coupes around Tasmania will be presented separately.

Figure 1. Overview of the Warra SST showing layout of treatment coupes in the trial area. ARN = aggregated retention, CBS-VI= clearfell, bum and sow with understorey islands, CON= unharvested control, DRN =dispersed retention, GS = group selection, SGS = single tree I small-group selection, Strips = stripfells.


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Table I. Treatments at the Warra SST

Treatment

Clearfell, bum, and sow

Clearfell, burn, and sow with understorey islands

Dispersed retention

Aggregated retention

Stripfell

Single-tree I small-group selection

Group selection

Description

Large openings with no structural retention, a high-intensity burn, and aerial application of eucalypt seed. Expected to produce maximum eucalypt regeneration and maintain biodiversity representative of younger successional forest.

Similar to the clearfelling treatment but with 40 m x 20m machinery exclusion zones in up to 5% of the coupe area, with the objective of enhancing local survival ofunderstorey flora in the machinery-free areas that could potentially act as a source of propagules for regeneration of late-successional plants across the coupe.

Retention of I 0-15% basal area of overstorey eucalypts, a low-intensity burn, and natural seedfall for regeneration of eucalypts. By retaining trees for a full rotation, dispersed retention aimed to improve aesthetic outcomes, provide habitat for hollow-dependent fauna and epiphytes, and allow for continuity in supply of CWD.

Retention of30% of the coupe area in patches (aggregates) of0.5-l.O ha, with the majority of the harvested area within one tree-height of retained forest, a low-intensity bum, and natural seedfall. Objectives were adequate eucalypt regeneration, and enhanced biodiversity and aesthetic outcomes by retaining patches of undisturbed forest for a full rotation. Aggregates were intended to provide relatively intact habitat as well as colonisation-sources for late-successional species.

Cable-harvesting 250 m x 80 m openings, a low-intensity bum and natural seedfall, with the objectives of adequate eucalypt regeneration and enhanced biodiversity outcomes by retaining strips of undisturbed forest for half the rotation (-40 years) to provide habitat and seed supply for all species including rainforest trees.

Retention of>75% offorest cover, by harvesting 40m3 timber ha-1 every 20 years using permanent machinery tracks and group openings <one tree-height wide, heaping slash, using mechanical soil disturbance (rather than burning), and allowing for natural seedfall. The objective was to provide enhanced biodiversity outcomes by maintaining a high proportion of tall-forest cover and encouraging development of rainforest elements within regenerating stands, and providing adequate eucalypt and rainforest species regeneration.

Similar to single-tree I small-group selection, but used group and strip openings two tree-heights wide, lowintensity burning and natural seedfall. No more than 30% of the stand to be harvested every 30 years, so that at least I 0% of the stand remains unharvested within a 90-year rotation.

Birds and ground-active beetles post-harvest regeneration burn, bird assemblages were similar to those in harvested areas (Lefort and Grove 2009).

The multi-species assemblage composition of birds and groundactive beetles responded strongly to the treatments at the Warra SST (Baker eta/. 2009; Lefort and Grove 2009). Both birds (Lefort and Grove 2009) and beetles (Baker 2006; Baker eta/. 2009) had very different assemblage compositions in recently harvested forest compared with unharvested control areas, few species being common in both these habitats. Individual bird and beetle species, however, responded differentially to the silvicultural systems, depending on their specific habitat requirements (Baker eta/. 2009; Lefort and Grove 2009).

Bird assemblages in understorey islands and dispersed-retention coupes were equivalent to those in felled areas, suggesting that, in the early years after harvesting, insufficient habitat was retained with these systems to benefit birds associated with mature forest. However, retained aggregates of0.5-l.O ha did support most bird species recorded in unharvested controls (Lefort and Grove 2009). Compared with dispersed retention, which retains only scattered overstorey eucalypts, intact patches of forest retained through aggregated retention provide suitable habitat for most birds from shrub-layer, mid-layer and canopy-layer guilds. Nevertheless, bird assemblage composition was quite variable among aggregates. Most aggregates had bird assemblages similar to those in unharvested controls, although some were of intermediate composition between unharvested controls and harvested areas. However, in areas of aggregates that had been affected by the

With the exception of the only understorey island not burnt in the regeneration burn, beetle assemblages in dispersed retention and understorey island treatments consisted of species affiliated with young forest. However, ground-active beetle assemblages in retained aggregates were similar to the assemblages found in the unharvested control areas, and all the commonly collected beetle species with preferences for mature forest were detected in aggregates (Baker eta/. 2009). In contrast to the situation with birds, beetle assemblages had rather uniform species composition among aggregates. Some species affiliated with young forest were also present, whilst some edge-avoiding species affiliated with mature forest (Baker et a/. 2007) were less common than in undisturbed controls, suggesting that aggregates of0.5-I.O ha comprised predominantly edge-affected habitat.

For beetles, it was also possible to compare the assemblages present in the harvested area of coupes (Baker eta/. 2009). There were trends for the beetle assemblages in the harvested parts of aggregated-retention and dispersed-retention coupes to be slightly more similar to those in the unharvested forest than to those in the clearfelled coupes. This appeared to relate to differences in abundance of species affiliated with young forest rather than an increased abundance of species affiliated with mature forest and may have resulted from the less-intensive burns than those obtained with clearfelling, rather than recolonisation of harvested


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areas from species that survived in aggregates. However, this is speculation, since the trends were not statistically significant. More research is required to clarify local-scale responses of beetles to regeneration burning and the spatial extent and time-scales for recolonisation of harvested areas.

Over time, though, it is expected that mature-forest beetles with poor dispersal abilities will more rapidly recolonise the harvested areas of aggregated-retention coupes than they will colonise the harvested areas of clearfelled coupes, due to closer proximity to source populations ('forest influence'). The timeline for this process is unknown and is the subject of ongoing monitoring. Between one and three years after harvesting, beetle assemblages in harvested areas were continuing to diverge from those in unharvested controls (Baker eta/. 2009), whereas bird (Lefort and Grove 2009) assemblages in harvested areas were becoming more similar to those in unharvested controls by six years following harvesting.

The results for both birds and beetles indicate that, over at least the first three years after harvest, retained aggregates provide habitat suitable for many mature-forest species, are of intermediate or sub-optimal habitat value for others, and are unsuitable for some sensitive species (Baker et a/. 2009; Lefort and Grove 2009). Thus, while aggregates provide mature-forest values and support many mature-forest species, the 0.5-1.0 ha aggregates in the Warra SST may be either too small or too disturbed and edge-affected to provide habitat of equivalent value to that of unharvested forest for many species. An increase in the size of aggregates in operational aggregated-retention coupes to over I ha is occurring (Scott eta/. 2011) and could minimise this apparent reduction in biodiversity value, although species with large home-range requirements could be especially sensitive.

Vascular and non-vascular plants and fungi

Aggregated retention, dispersed retention, and stripfell coupes had reached, or were close to meeting, the commercial eucalypt seedling stocking standard at three years after harvesting, although seedling densities, particularly in aggregated-retention coupes, were much lower than in the clearfelled areas (Neyland eta/. 2009a). Seed-fall from retained trees allowed continuing eucalypt recruitment even in the absence of aerial sowing, as well as contributing to the conservation oflocal gene pools. This will also be the case if the on-site seed is supplemented with aerial sowing of locally-collected seed, as is generally prescribed for most operational aggregated-retention areas.

The regeneration bums at the Warra SST affected both vascular and non-vascular plants, both in the burnt, harvested areas and in those parts of aggregates and understorey islands that were burnt (Neyland 2010). In a study in operational aggregated-retention coupes, disturbance by harvesting machinery and or burning had a strong impact on vascular plant species composition (Hindrum 2009). At the SST, the various VR treatments had differing effects on vascular and non-vascular plants, with the response of vascular plants to harvesting being strongly related to the local pre-harvest understorey species composition (sclerophyll or rainforest) (Neyland and Ziegeler 2008). The vascular plant species composition in areas that carried rainforest understoreys before harvest changed substantially following harvesting. Six years afterwards, the vegetation remained substantially different

from that pre-harvest, being dominated by rainforest species. By contrast, the vascular plant species composition in areas that carried sclerophyllous understoreys before harvest was quite similar to that pre-harvest after six years, being once again dominated by sclerophyllous species (Neyland 2010).

Unburnt aggregates retained relatively undisturbed plant communities, but vascular plant communities in dispersedretention treatments were more typical of communities in clearfelled treatments as only overstorey eucalypts were retained. Some vascular plants are recovering more rapidly in understorey islands, as these machinery-exclusion areas were less disturbed than other harvested areas (Neyland 2010).

Bryophytes and lichens, on the other hand, are highly sensitive to forest disturbance, and very few mature-forest species are found in the early years after harvesting and regeneration burning (Kantvilas and Jarman 2006; Kantvilas eta/. 2008). Many species are also dependent on late-successional structures such as very old trees or large logs (Jarman and Kantvilas 2001 b; Kantvilas and Jarman 2004; Turner and Pharo 2005), and are also very susceptible to changes in microclimate. The impact of the regeneration bum was particularly clear for lichens and bryophytes as such species are mostly consumed by fire (J. Jarman, Forestry Tasmania, and G. Kantvilas, Tasmanian Herbarium, pers. comm., 2009), being without fire-protected organs (underground buds, lignotubers or buried seed) and thus needing to recolonise from pioneering propagules. Longer-term fire impacts on these taxa are not known, but recolonisation may be slow and depend on the distance to source populations as well as on the development of suitable substrates and microclimates (Pharo and Zartman 2007). Unbumt aggregates contained many species of bryophytes and lichens no longer present in harvested areas (Kantvilas et a/. 2008) but, because of their relatively small size, aggregates at the Warra SST were edge-affected and thus of compromised habitat value for late-successional bryophyte species (Strutt 2007).

Similar to birds and beetles, the assemblage composition of macrofungi (based on above-ground fruiting bodies) differed substantially between recently clearfelled areas and mature forest at Warra (Gates eta/. 2005). A study of macrofungi in one Warra SST aggregated-retention coupe found lower fungal species richness in aggregates compared with a nearby unharvested control area (Gates eta/. 2009). However, whilst not providing habitat completely comparable to that of unlogged forests, the aggregates still contained many species of ectomycorrhizal fungi that were not recorded in the harvested areas (Gates et a/. 2009). Ideally, these findings should be confirmed using molecular methods for below-ground fungal species composition at replicated study sites.

Habitat trees and coarse woody debris

Large, hollow-bearing trees and large CWD (burnt and unbumt) are important structural attributes for invertebrate and vertebrate fauna, fungi and epiphytic plants (Jarman and Kantvilas 2001a; Grove eta/. 2002; Yee eta/. 2006; Koch eta/. 2008b ). Although relatively common in old forests, these habitats may be rare or absent in areas repeatedly subjected to clearfell harvesting (Grove eta/. 2002; Munks eta/. 2009). Habitat tree surveys in the Warra SST found that aggregated-retention coupes retained more hollow-bearing trees and stags per hectare of total coupe area than


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either dispersed-retention or clearfelling with understorey islands (Baker et al. 2008). Aggregates also contain both existing CWD and a future supply in the form of standing trees and stags from several tree species. The CWD in aggregates remained largely unaffected by the regeneration bums, in contrast to harvested areas. Thus, aggregates should provide an ongoing supply of relatively undisturbed CWD habitat of a variety of tree species and decay stages for saproxylic species (Grove et al. 2002; Yee et al. 2006). Standing trees retained in dispersed-retention coupes will also provide an ongoing supply of CWD, although only of eucalypts since other species were not retained. Fewer trees were retained in dispersed retention than with aggregated retention, although dispersed retention may result in a more even distribution of CWD across the coupe area. Understorey islands will also contribute small inputs ofCWD in the future, although, because nearly all trees were killed in the regeneration bums, inputs will not occur over as prolonged a period as for dispersed and aggregated-retention treatments (Grove et al. 2008).

Biodiversity synthesis

The results of these various biodiversity surveys are synthesised in Table 2. The table restricts consideration to the silvicultural systems with the greatest amount of data, although expert judgement was used in some cases. Results of stripfell and group-selection treatments are discussed in the text where data were available. The sensitivity to silvicultural system varied amongst biodiversity groups, with lichens being ranked the most sensitive of the taxa investigated. The synthesis illustrates that aggregates in the aggregated-retention system were particularly effective at retaining, within coupes, populations of mature-forest species across a range of taxa and biodiversity attributes. Aggregated retention was better able to achieve the 'lifeboating' and structural objectives of VR when compared to dispersed retention, or to understorey islands in clearfell coupes. The 0.5-1.0-ha aggregates at Warra, although edge-affected, functioned as refuges and habitat for all biodiversity groups studied; mature-forest ground-active beetle assemblages and vascular plant assemblages were similar between aggregates and unharvested controls, while habitat quality was somewhat more compromised for sensitive bird, bryophyte and lichen species.

Surveys of vascular plants (Neyland 20 I 0) and birds (Lefort and Grove 2009) indicated that stripfell and group-selection treatments also had positive short-term benefits compared with clearfelling. However, these were designed as staged harvesting systems, with complete implementation possibly resulting in most of the mature-forest habitat in the coupe being harvested within the (80-1 00 year) time-period of a single clearfell rotation. These systems are also not being widely implemented. Small-group selection, in spite of being advocated by some, is poorly suited to tall, wet eucalypt forests, with very poor regeneration of eucalypts (Neyland et al. 2009a) and poor safety outcomes (Neyland et al. 2009a). The group-selection treatment, with wider harvested fairways enabling a regeneration bum, has scope for use in Special Timber Zones (Forestry Tasmania 2010) to provide an ongoing supply of timber from rainforest tree species. This treatment was not included in the Warra SST until2007, and consequently results of biodiversity surveys are not yet available.

In contrast to the situation with retained aggregates, understorey islands were found to have only limited ability to act as refuges because of their susceptibility to the regeneration bum. Seven of the eight understorey islands were almost completely burnt, and excluding regeneration bums from such small patches is rarely likely to be successful in practice. The vascular plant species composition of hot-burnt understorey islands was similar to that of clearfelled areas, while there was some coppice regeneration in cooler-burnt areas (e.g. for vascular plants, Neyland 2010). Burnt understorey islands also appeared not to act as refuges for birds, beetles, bryophytes and lichens, although the small numbers of retained structures (live and dead trees and future CWD) could provide substrates for future recolonisation. The dispersed retention system was also not effective at providing refuges for mature-forest ground-active beetle, bryophyte or lichen assemblages, although there may be some long-term benefits for these groups because retaining overstorey eucalypts maintains structural diversity and habitat trees, and provides some continuity in supply of CWD.

Table 2. Ranking of responses of mature-forest biodiversity attributes to different silvicultural systems at the Warra SST in the first few years following harvesting. Systems are ranked from I to 4, where 1 is best and 4 worst at maintaining mature-forest attributes within harvested coupes. Treatments rated equally for an attribute were ranked as the lower number (e.g. three rankings of '4' for lichens). Mature-forest attributes were identified from surveys of unharvested control sites. The superscript 'e' indicates that the response ranking is based on empirical evidence: Lefort and Grove (2009) for birds, Baker et al. (2009) for beetles, Neyland (2010) for vascular plants, Gates et al. (2005, 2009) and unpublished data for other groups. The superscript 'j' indicates that the response ranking is based on expert judgement: see acknowledgements. ARN = aggregated retention, DRN =dispersed retention, CBS-UI = clearfell, burn and sow with understorey islands, CBS= clearfell, bum and sow.


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While, overall, the results of Warra SST biodiversity studies provide a promising ecological argument for aggregated retention being the preferred alternative to clearfelling in Tasmania's wet eucalypt forests, these are early results in the context of an intended 80-100 year harvesting rotation. Further, the Warra SST contained only two (or occasionally one) replicates of each silvicultural system in a single study area, and had limited sampling effort in some cases. In spite of these constraints, biodiversity elements exhibited very clear responses to harvesting systems. The Warra SST biodiversity studies also used unharvested mature forest as a reference when comparing the relative abilities of the various silvicultural systems to retain mature-forest species and structures. Regardless of silvicultural system, however, recent harvesting should not be expected to provide habitat of equivalent value to that of extensive unharvested forest, and regrowth forest arising from recent wildfire would have been the appropriate control treatment for comparison with harvesting systems. The lack of a recent wildfire at Warra prevented this natural control being used for the SST, so these comparisons are not currently possible.

Because aggregated retention retains unharvested areas somewhat akin to areas skipped by fire, and early results demonstrate that aggregates effectively 'lifeboat' mature-forest species and structures at the coupe scale, it can be argued that this type ofVR is most likely to result in communities of plants and animals similar to those following a natural wildfire at both early and subsequent later stages of regeneration. An ongoing wildfire chronosequence study (Turner et a/. 2007) comparing several previous wildfire events up to and including 1967, as well as previous comparisons between wildfire and clearfelling (Hickey 1994; Baker eta/. 2004; Turner and Kirkpatrick 2009), will provide natural disturbance controls for the SST harvesting treatments as these reach 40 years after harvest.

Previous fire history has a large effect on forest structure and biodiversity. Although many of the overstorey Eucalyptus obliqua trees within the Warra SST area are mature (> 110 years old), the most recent wildfire that burnt much of the SST area at varying intensity (in 1934) has resulted in a forest that does not contain an 'old-growth' species composition of bryophytes and lichens (J. Jarman, Forestry Tasmania, and G. Kantvilas, Tasmanian Herbarium,pers. comm., 2009). Hence, conditions in the aggregates retained at Warra may not be typical of conditions in aggregates in aggregated-retention coupes in other mapped old-growth areas.

The results of the Warra SST biodiversity surveys to date suggest that three years after harvesting is too early to detect significant influences of aggregates on the harvested area. Previous research by Tabor et a/. (2007) found that regeneration of the four dominant rainforest tree species in clearfelled coupes, from seed originating in adjacent unharvested mixed forest, increased with coupe age over at least the first 15 years. However, this regeneration declined rapidly with distance from mature-forest seed sources, with relatively little rainforest tree regeneration beyond 50 m from the forest edge excepting for celery-top pine (Phyllocladus aspleniifolius) which can also have bird-dispersed and soil-stored seed. Relative to clearfelling, aggregated retention can be predicted to allow higher densities of rainforest trees to establish throughout a larger proportion of the coupe within the approximately 80-1 00-year harvesting rotation, because in this system most of the harvested area is within one tree-height of

retained forest. It is similarly anticipated that aggregates will enable a range of animals, plant propagules and fungi to colonise the adjacent forest regenerating in the harvested area.

Operational implementation of aggregated retention in wet old-growth forests

The Tasmanian Community Forest Agreement (TCFA) (Australian Government 2005; Australian Government and Tasmanian Government 2005), a supplement to the Regional Forest Agreement, established a target ofnon-clearfell silviculture in a minimum of 80% of the annual old-growth harvest area on state forests by 2010. Based on preliminary findings from the Warra SST, Forestry Tasmania (2005) recommended the future use ofVR (in particular, aggregated retention) as part of a mixed-silviculture approach to enhance biodiversity and aesthetic outcomes associated with harvesting tall, wet old-growth eucalypt forest by non-clearfelling methods; other silvicultural systems would continue to be used in other forest types. The operational implementation of aggregated retention in wet old-growth forests across Tasmania then commenced in 2006, and by June 2010 Forestry Tasmania had harvested and conducted regeneration bums in 38 aggregated-retention coupes (1396 ha including aggregates) across Tasmania, with further coupes being harvested currently.

Although improved ecological outcomes in the Warra SST were critical in selecting aggregated retention for operational adoption over other VR systems (Table I), adequate performance against other criteria was also essential. For example, although dispersed retention, or a combination of aggregated and dispersed retention, is widely implemented in other ecosystems (Rosenvald and Lohmus 2008), it posed too high a risk to worker safety in Tasmania's tall forests (Hickey eta/. 2006; Neyland eta/. 2009b). Old-growth eucalypts are tall (often over 50 m high), with large crowns that usually contain some dead branches and are often interconnected with the crowns of other trees. The forests generally have very dense tall understoreys (because eucalypts allow light through the canopy) which impede visibility and operability (and escape routes) for workers. The tree stems are often also highly decayed. Combined, these factors make directional falling of trees into the small gaps between retained dispersed trees unacceptably dangerous. A recent review of the operational roll-out of aggregated retention (Forestry Tasmania 2009) thus included a combined assessment of safety, economics, social acceptability, ecological and silvicultural outcomes, fire management, and timber supply and management implications.

Planning of coupes for VR needs specific consideration of biodiversity objectives if the desired ecological outcomes of this silvicultural system are to be fully realised. Formalised operational guidelines for implementation ofVR were therefore articulated by Forestry Tasmania (Forestry Tasmania 2009), grounding Tasmanian VR practices in the principles of natural disturbance dynamics, good silviculture and biodiversity outcomes (Box 2). The Goals (Box 1) and Guidelines (Box 2) for VR necessarily cover operational, safety and silvicultural matters as well as biodiversity outcomes, and are bounded by practical considerations.


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It is a key requirement of VR implementation in Tasmania that most of the harvested area be within one tree-height of forest that is to be retained unharvested for the entire rotation; this is the metric that distinguishes VR from clearfelling (Keenan and Kimmins 1993) and ensures structural and habitat diversity at the coupe scale. However, the ecological effects of 'mature forest influence', and the relevance of the 'one tree-height from retention' threshold for facilitating recolonisation by various biodiversity groups, are poorly understood. This is the subject of a current research project, the results of which may guide further refinement ofVR practices in the future.

There is a requirement, where practical, to locate aggregates so they include the key habitats present before harvest in individual coupes. By anchoring aggregates on the variety of habitats and structural conditions present in sites, and by varying their sizes, configurations and connectedness to the surrounding landscape, aggregated retention is more likely to create habitat conditions for plants and animals comparable to a forest regenerated by natural disturbance. These eucalypt forests have adapted to wildfire, and thus the impacts of regeneration burns on aggregates is analogous to wildfire in standing forest. It adds structural diversity to the coupe as a whole, and also potentially caters for fire-dependent species (Harrison 2007). However, as noted previously, unbumt habitat is necessary to maintain many mature-forest species. Thus, while noting that some fire in aggregates is an acceptable and perhaps even desirable outcome, the guidelines advise that aggregate sizes should be sufficiently large to minimise both edge : interior ratios and impacts of regeneration bums.

Integral to the operational roll-out of aggregated retention across Tasmania has been the development of new procedures and planning tools: a VR manual (Scott eta/. 2011 ), a GIS 'influence calculator' planning tool for determining whether coupes meet

the> 50% influence target (Scott 2008), and new 'slow burning' prescriptions for regeneration bums in aggregated-retention coupes (Chuter 2007). Biannual meetings of a Variable Retention Implementation Group of scientists, managers and planning and operational staff involved with implementation greatly assisted the adaptive management process, and a set of meetings of a Variable Retention Advisory Group of managers, forest industry members, contractors and safety experts ensured on-ground issues were assimilated into the planning process. Aggregates and other areas of 'forest providing influence' over the harvested area are zoned in Forestry Tasmania's GIS system as being unavailable for harvest over the next rotation, to ensure that the values of retained areas for lifeboating and influence are maintained in the long term.

Operational VR coupes differ significantly from the two Warra SST research coupes with regards to layout, design and management, as the system has evolved in response to research outcomes and improved local and international experience, and as operational constraints were overcome. Most current operational aggregated-retention coupes have wider harvested fairways, less 'influence' of retained trees over the harvested area (although still greater than 50% influence), larger aggregates, and aggregates more likely to be located contiguous with coupe edges ('edge aggregates' rather than 'island aggregates'). Operational aggregated-retention coupes thus do not have the uniform pattern of small island aggregates characteristic of the research coupes at Warra (Fig. 1 ), with the layout of operational coupes being very variable, and tailored to local topography, habitat and existing reserves (Fig. 2).

Implementation of aggregated retention in Tasmania has been strongly guided by experiences in the Pacific Northwest (PNW) of USA and Canada, including expert guidance from PNW


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Figure 2. Variation in the layout of operational aggregated-retention coupes across Tasmania. The red line shows in each case the harvesting boundary that would have applied had the coupe instead been clearfelled; forest inside this line comprises the retained aggregates.

scientists and practitioners on repeat visits (Forestry Tasmania 2009). The range of approaches to retention forestry worldwide was a focus of the Old Forests, New Management conference in Tasmania in 2008 (www.oldforests.com.au), timed to coincide with review ofthe potential role of aggregated retention for use in Tasmanian state forests. At the same time, aggregated retention has been tailored to Tasmanian environmental conditions and

forest management constraints, which has resulted in some differences from PNW forms of this silvicultural system. In particular, the need to use regeneration bums influenced a move to much larger aggregates in Tasmania than is usual in the PNW and to a much higher proportion of retained aggregates being located on or contiguous with coupe edges. Such configurations (Fig. 2) make it easier to bum harvested areas successfully and facilitate eucalypt regeneration, whilst reducing fire damage to retained aggregates. Thus, while Tasmanian aggregated-retention coupes with largely edge retention may not meet PNW criteria for dispersal of retention throughout the harvested area (Zielke et al. 2008), adherence in the Tasmanian model to the minimum forest influence target ensures appropriate dispersal of retention across the coupe.

Increasing the degree of'old-growthness' of managed forests can be achieved through a mix of reservation, structural retention and restoration (Bauhus et a/. 2009). Although aggregated retention is currently being implemented almost exclusively in formally mapped old-growth areas in Tasmania, there is also a need to consider how VR could be used to restore old-growth elements in those regrowth forest landscapes that are depauperate in mature forest elements (Forestry Tasmania 2009). Creating, then retaining, aggregates for one or more rotations in landscapes with an intensive harvesting history could help restore important old-growth structures such as hollow-bearing trees that rarely develop within the time-frame of a single harvesting rotation (Koch eta/. 2008a). Modelling and landscape planning will be required to identify those managed forests where old-growth elements are now sparse, and prioritise management for restoration of these elements (Yee eta/. 2008).

Biodiversity monitoring in operational aggregated-retention coupes

A continued ecological research and monitoring program has been incorporated into the roll-out ofVR from the Warra SST research coupes into operational coupes across Tasmania. This has initially focused on assessing the lifeboating ability of aggregates, as indications from the Warra SST and one study of vascular plants in operational coupes (Garandel eta/. 2009) are that several years need to elapse following harvesting before influence effects of aggregates over the harvested area may be detected.

Ground-based surveys have been conducted to compare the habitat value of aggregates in operational coupes with that of nearby unharvested control areas. The results indicate that aggregates, especially those not affected by regeneration bums, are effective at maintaining late-successional species and structures including mature rainforest trees and old-growth eucalypts (Garandel et a/. 2009). Helicopter-based surveys were used to count habitat trees in aggregates and control areas, and map the penetration of the regeneration bum into aggregates and forest surrounding the coupe. Across eight aggregated-retention coupes burnt in 2007, about 1400 habitat trees were present in retained aggregates, an average of 3.6 mature trees and stags with visible hollows per hectare of gross coupe area (Forestry Tasmania 2009). The aggregates also included large numbers of regrowth trees, ensuring future recruitment of a continuing supply of important structural features.

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Results from the bum-impact mapping (22 coupes, 2007-2009) indicated that small island aggregates were particularly susceptible to regeneration burns, with 29% of island-aggregate area burnt as opposed to 5% of edge-aggregate area (McElwee and Baker 2009). Changes to coupe design (Fig. 2), with a decrease in the proportions of island aggregates and of smaller aggregates, and application of 'slow-burning' prescriptions, meant that a much lower proportion of aggregate area was burnt in 2008 and 2009 compared with 2007 (McElwee and Baker 2009). While retention of unburnt aggregates will be beneficial to most species, certain species are thought to either require or be favoured by burnt conditions (Harrison 2007). The operational objective is therefore to minimise but not exclude any effect of regeneration burns on retained aggregates. Trees killed by the regeneration burn should not be salvaged as they contribute to structural diversity and habitat, and provide an ongoing supply ofCWD.

Adaptive management of VR

Adaptive management is a process of management, planning and decision-making in the face of uncertainty, to acquire and use knowledge as this is created, learn from successes and mistakes, and modify practices to better achieve management goals (Walters and Holling 1990; Lindenmayer and Franklin 2002; Stankey et al. 2005). Although adaptive management can work as an informal 'trial-and-error' approach, a more formalised approach increases the likelihood that new knowledge will be generated and incorporated into management practices (Lindenmayer and Franklin 2002). The research program and implementation ofVR in wet eucalypt forests includes ongoing monitoring and continuing assessment and modification of operational practices, and contains the system of linked steps characteristic of a formal, active adaptive management process as outlined in Lindenmayer and Franklin (2002). The approach relating to each ofLindenmayer and Franklin's (2002) five steps (briefly summarised below in italics) for a formalised adaptive management program is outlined as follows:

Step 1. The first step in the adaptive management process is to gather available information about the system, clarifY policies on approaches to meet management goals and create alternative management models.

Forestry Tasmania recognised in the 1990s the need to look for an alternative to clearfelling for management of old-growth tall, wet eucalypt forests. A formal literature review and discussions with international and Australian experts (including those involved with the Victorian SSP) was thus undertaken, to assess potential systems for inclusion in the Warra Silvicultural Systems Trial (Hickey et a!. 200 l ).

Step 2. The second step is to create a small set of testable hypotheses for different management options.

A subset of potentially practical silvicultural systems was selected, according to expected benefits to biodiversity (Hickey et a!. 200 l ). Key possible advantages of each system were specified, providing testable hypotheses and allowing the alternative systems to be assessed and compared against defined objectives

and criteria, including social and economic criteria (Hickey et al. 200 l, 2006).

Step 3. The third step is to establish a robust experimental design and monitoring program that specifies which components should be measured to assess the success of different management options.

The Warra SST was established to test different silvicultural systems for wet eucalypt forest (Hickey et al. 2001, 2006). Monitoring indicators were derived for the ecological, social and economic criteria. Initially, indicators for biodiversity were restricted to species richness and abundance of vascular and non-vascular plants and invertebrates (Hickey et al. 200 I), but the scope of the biodiversity surveys was later expanded as responsive components of the ecosystem were better understood (Table 2). Logistical reasons limited the long-term research at the SST to one site and two replicates of most harvesting systems. However, the operational implementation of a potentially suitable harvesting system (aggregated retention) allowed coupe designs then to evolve rapidly in response to developing research and practical experience, with a research and monitoring program also being established in operational coupes.

Step 4. The fourth step is to implement management changes based on the results of the experiments. This is combined with continued monitoring, review and modification of management practices.

A formalised review of the various assessments of alternative silvicultural systems, in particular aggregated retention, was published (Forestry Tasmania 2009), informing the Tasmanian and Australian Governments of the feasibility and benefits of using aggregated retention in most tall, wet, old-growth eucalypt coupes on state forest. A further formal review is planned for 20 15. Meetings of a Variable Retention Implementation Group and a Variable Retention Advisory Group ensured relevant information is shared between researchers, operational staff and policy makers. Aggregated retention has been increasingly used for harvesting operational coupes since commencing in 2004, and is continuing to be modified in response to monitoring and research outcomes (cf. Fig. 2 with Fig. 1). Monitoring ofWarra SST and operational coupes is ongoing, particularly with regard to burn outcomes, aggregate condition, harvest area regeneration, forest influence and coupe-level biodiversity, allowing review and modification of aggregated retention to continue via changes to future coupe planning and design. Multilateral communication, combined with working groups targeting specific operational challenges, has been central to the adaptive management process.

Step 5. The fifth step is to document the adaptive management program with information about all the steps of the process.

The process of assessing potential alternatives to clearfelling, establishing the Silvicultural Systems Trial, and implementing aggregated retention operationally has been documented in the numerous publications cited here as well as in industry technical reports, and this process is on-going. This paper, for example, is the first of a series reviewing biodiversity outcomes.


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Conclusions

Synthesis of results from the research coupes at the Warra SST and from monitoring of operational coupes across Tasmania allows consideration of the advantages and disadvantages of aggregated retention for maintaining biodiversity in tall old-growth forests used for wood production (Box 3 ). Aggregated retention as implemented in Tasmania provides refugia within coupes for old-growth species and enriches regenerating stands with structural features, better retains elements of mature-forest biodiversity at the stand level than does clearfelling, and also achieves better biodiversity outcomes than do the various other VR systems tested in the Warra SST. It can thus be concluded to be effective at meeting the stated management objectives for old-growth forest stands and the goals articulated for alternatives to clearfelling (Hickey et al. 200 I; Lindenmayer and McCarthy 2002; Beese et al. 2003; Hickey et al. 2006; Rosenvald and Lohmus 2008).

Using VR as part of a landscape-level planning approach can thus be predicted to benefit biodiversity elements that are dependent on mature forest. Ecologically sustainable forest management requires that some habitat for biodiversity is retained throughout the production-forest matrix, rather than relying solely on the

formal reserve system for conservation (Lindenmayer and Franklin 2002). A specific premise ofVR is that it can be more ecologically valuable to distribute mature-forest elements throughout coupes and the production forest landscape than to simply add an equivalent amount of mature forest to the large existing reserve system. However, more research would be useful to guide managers as to the relative benefits of different reservation strategies for different ecosystem types and landscape contexts relating to the intensity of previous management history. Use ofVR silviculture in appropriate landscape contexts thus complements various other retention requirements of the Tasmanian forest practices systems, and together with the networks of larger reserves across the forest estate comprises the reservation and retention components of forest management for 'old-growthness' (Bauhus et al. 2009). Aggregates are not expected to cater for the full range of species that would occur in larger reserves (that is the role of the formal reserve system), since certain species are sensitive to edge and or area effects and may have large home-ranges in unharvested forest. Instead, VR silviculture is aimed at complementing existing reserve systems with conservation efforts at the coupe scale. Future research combining landscape analysis with biodiversity survey data could assess the complementary values of retention within coupes

AustralianForestry 2011 Vol. 74 No.3 pp.218-232

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compared to retention at the landscape level. The third component offorest management for 'old-growthness', forest restoration, can be potentially addressed by extending the use ofVR into regrowth forests when these are harvested.

The initial biodiversity research findings for VR in Tasmanian wet old-growth forests are thus that retained aggregates provide viable habitat in harvested coupes, over at least the short term, for many species affiliated with mature forest, as well as key structural features. Future research, both in the on-going long-term studies in the Warra SST and in operational coupes (both recent aggregated retention and older clearfells), will explore the anticipated influence of aggregates on the regenerating forest that could enable a range of mature-forest-associated animals, plant propagules and fungi to colonise harvested areas more rapidly. Together, retention and influence are thus predicted to ensure that forest regenerating from harvesting develops over the rotation period a suite of biota more equivalent to that in wildfire-regenerated forest.

Acknowledgements

Funding for this program was provided by Forestry Tasmania and the Tasmanian State and Australian Federal Governments under the Tasmanian Community Forest Agreement.

Forestry Tasmania's Goals and Guidelines for VR (Boxes and 2) were developed in a workshop with participation by the authors plus Simon Grove, Robyn Scott, Mark Neyland, John Hickey and Tim Wardlaw. The synthesis of biodiversity findings at the Warra SST (Table 2) was assembled based on the research and expert judgements of the following specialists: Paul Lefort and Simon Grove (birds), Sue Baker, Lynne Forster and Simon Grove (beetles), Mark Neyland (vascular plants), Jean Jarman (bryophytes), Gintaras Kantvilas (lichens), Genevieve Gates and David Ratkowsky (ectomycorrhizal fungi), Sue Baker and Sarah Munks (habitat trees), and Simon Grove (coarse woody debris).

We thank the Forestry Tasmania researchers who have been instrumental in establishing and maintaining the Warra SST and developing policy and practices for VR. These include John Hickey, Mark Neyland, Robyn Scott, Tim Wardlaw, Leigh Edwards, Dave McElwee and Simon Grove amongst others. We also acknowledge the enormous effort of many Forestry Tasmania operational staff over the last decade. Their willingness to be involved with testing and implementing these new silvicultural systems has been essential to their success. The advice of the International Science Panel (Bill Beese, Tom Spies, Jack Bradshaw, Jiirgen Bauhus and Ivan Tomaselli) and other international experts, including Ken Zielke and Bryce Bancroft, was of great benefit.

We are very grateful to Mark Neyland, Amy Koch, Simon Grove and Tim Wardlaw for reading and commenting on a draft of this manuscript as well as for helpful suggestions from the anonymous referees.

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variable retention silviculture in tasmania's wet forests: ecological rationale, adaptive...

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