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Letting giants be – rethinking active fire management of old‐growth eucalypt forest in the A ustralian tropics

2014; Wiley; Volume: 51; Issue: 3 Linguagem: Inglês

10.1111/1365-2664.12233

1365-2664

David Y. P. Tng, Steve Goosem, Gregory J. Jordan, David M. J. S. Bowman,

Rangeland and Wildlife Management

Tall old-growth forests are of global social-economic, political and ecological significance. These forests contribute significantly to the global carbon budget and are of high conservation value given sustained logging and clearing over the past two centuries (Tng et al. 2012a). In Australia, these old-growth forests extend from tropical to temperate regions of Australia in areas where rainfall exceeds 1000 mm per year, being characterized by emergent eucalypt trees attaining statures of 30 m to more than 80 m, with canopy and understorey layers consisting of mesophytic broad-leaved trees and treelets, sclerophyllous shrubs and graminoids (Fig. 1). These forests support some of the tallest flowering plants in the world, are important habitats for a unique suite of flora and fauna, and are important forest cover for metropolitan water catchments – values that make giant eucalypt forests a focal point of scientific study and ecotourism (Tng et al. 2012a). Until recently, these eucalypt forests were extensively exploited as a timber resource, but now, most remaining old-growth stands have been set aside for conservation. In some regions containing giant eucalypt forest, native forestry activities either have ceased or are based on short-rotation harvests of regrowth forests, meaning that the trees can never achieve their potential size. Regeneration typically occurs after landscape fires, and fire is also used to initiate regeneration of temperate eucalypts after logging and to reduce fuel loads (Attiwill et al., in press). In temperate regions, fire management of the remaining stands of old-growth giant eucalypt forest is largely based upon fire suppression and fuel reduction burning in surrounding open forests, as fires in giant eucalypt forests are extremely difficult to control because these forests are only flammable under dangerous fire weather conditions (Bowman et al. 2013). In subtropical and tropical forests, fuel reduction burning is used to reduce fire hazard within the giant forests as well as in adjacent open forests and savannas. What constitutes the most appropriate and ecologically sustainable fire management practices of these giant forests remains a controversial issue among scientists, land managers and conservationists. Here, we outline recent advances in landscape ecology theory, palaeoecology and functional biology research on a giant eucalypt forest type in the Wet Tropics region of northeast Australia to explore options to achieve sustainable management of these systems. Given the ecological similarity between Australia's giant eucalypt forests and other old-growth forests in the Northern Hemisphere (Tng et al. 2012a), the insights gleaned herein have implications for a wide range of old-growth forests. Further, the expansion of rain forest into surrounding savannas has implications for the management of savannas and grasslands, where there remains debate as to whether increased woody biomass should be managed using fire or allowed to accumulate (Bond & Parr 2010). Eucalyptus grandis W.Hill ex Maiden is native to the east coast of Australia, extending from the middle of New South Wales (latitude approximately 33°S) to north Queensland (latitude 16°S). The most northern outlier populations are located in the Wet Tropics bioregion of Australia where E. grandis forests reach a height of over 60 m and form a narrow but distinct band at rain forest–savanna boundaries that does not exceed 4 km in width (Harrington & Sanderson 1994; Tng et al. 2012b; Fig. 1). The understorey of the forests forms a spectrum from dense tropical rain forest to an open layer of flammable perennial grasses with scattered shrubs. Over the last 50 years, it has been demonstrated that the aerial extent of rain forest understoreys is increasing in E. grandis forest throughout the Wet Tropics region, at the expense of grass understoreys (Harrington & Sanderson 1994; Tng et al. 2012b). Eucalyptus grandis forests are habitats for a number of endangered bird and mammal species, by providing tree cavities and coarse woody debris used for shelter and nesting (Harrington & Sanderson 1994; Sattler & Williams 1999). Grassy understoreys do not support a unique suite of wildlife. The endangered northern bettong Bettongia tropica Wakefield, which has undergone a dramatic range contraction over the past 100 years, does occur in a few E. grandis forests with grassy understoreys, although this is not the prime habitat for this small ground-dwelling marsupial which prefers the adjoining savanna (Laurance 1997). Eucalyptus grandis is an obligate seeder because it is among the few eucalypt species that lack lignotubers: fire-resistant, underground stem structures that sprout new shoots if the main trunk is killed. It is dependent on high-severity fire disturbance to regenerate and out-compete rain forest trees because these fires (i) stimulate the release of seed from seed capsules, (ii) kill rain forest competitors creating a light environment for shade intolerant seedlings (Duff 1987) and (ii) create a mineral soil seedbed rich in available phosphorus and free of pathogens (D.Y.P. Tng, D.P. Janos, E. Weber, G.J. Jordan & D.M.J.S. Bowman, unpublished data). Frequent low-intensity fires that maintain grassy understoreys do not initiate E. grandis regeneration. Eucalyptus grandis seedlings and saplings out-compete other rain forest species which also regenerate following fire (Duff 1987; Williams et al. 2012) through extremely rapid growth. Young E. grandis saplings are vulnerable to frequent low-intensity fires because they lack thick bark and their short stature means that their crowns will be burnt by ground fires. Mature E. grandis trees, on the other hand, can tolerate multiple small fires due to their thicker bark and greater height, although frequent burning may reduce the bark's protective capacity (e.g. McArthur 1968). Moreover, trees with hollows and cavities used by fauna appear to be more adversely affected by fire than undamaged trees (Eyre 2005). Fire events in giant eucalypt forests with well-developed rain forest understoreys are likely to be rare because these forests are not highly flammable due to the combined effects of the fire-retardant nature of rain forest species and the moist microclimate resulting from the presence of a dense rain forest understorey (Little et al. 2012). Low-intensity fires, as practiced by forest managers, do not eliminate rain forest, as many rain forest species have the ability to resprout after such fires (Williams et al. 2012). Land Management Agencies (e.g. Queensland National Parks, Wet Tropics Management Authority) tasked with conserving the entire, complex mosaic of habitats that comprise the Wet Tropics World Heritage Area. The region's rain forests were central to the listing of the region as World Heritage. The establishment of rain forest species within the giant E. grandis forests has been hypothesized as a threatening process to the regeneration and thus continual persistence of the E. grandis forests, demanding a programme of high-frequency burning to progressively eliminate undesirable tree, shrub or herb life-forms from the understorey (Harrington & Sanderson 1994). However, such management objectives and prescribed burning practices are highly contentious because of the destruction of rain forest species; loss of early regenerating stages in the life cycle of E. grandis; the longer-term impacts on E. grandis forest successional dynamics; and concerns about anthropogenic CO2 emissions and the global carbon budget (Bowman et al. 2013). Such a debate demands evaluation of the ecology and evolution of the giant forests. Recent landscape ecology research in the Wet Tropics of Queensland suggests that rain forest and savanna dynamics can be explained by reference to a fire driven ‘alternative stable states’ model (Warman & Moles 2009). Rain forest is therefore unlikely to change into savanna and vice versa unless there is a significant change in the environmental drivers underlying these biomes. This view is supported by an analysis of functional traits (i.e. plant traits that relate directly to the ecological functioning of a plant) of representative rain forest, E. grandis forest and savanna tree and shrub species. Tng, Jordan and Bowman (2013) showed that E. grandis forests, even with grassy understoreys, are functionally similar to rain forests and dissimilar to savannas (i.e. E. grandis forest is functionally a rain forest). Under the current climatic regime, rain forest expansion is taking place in many areas where E. grandis grows, in spite of the presence of landscape fires, and it is possible that this region-wide expansion is due to increased atmospheric CO2 (Tng et al. 2012b). Rain forest expansion results in a positive feedback loop because of the reduced flammability (Little et al. 2012) and changes in soil chemistry associated with rain forest expansion (Warman, Bradford & Moles 2013). However, this process of change is very gradual. Even with the fastest estimated rate of rain forest expansion reported for the region (i.e. 1 m year−1: Unwin 1989), rain forest would take over 2000 years to completely engulf the current extent of giant eucalypt forest in the region (Tng et al. 2012b). Over such long time-scales, the chance occurrence of large-scale disturbances such as cyclones and large landscape fires in the region should provide localized opportunities for E. grandis forest regeneration (e.g. Duff 1987; Tng et al. 2012b). Aboriginal tribes inhabited Wet Tropics region, and there is some evidence of their use of fires in rain forest habitats (Haberle et al. 2010); however, little is known about how or whether they actively and specifically managed E. grandis forests. If they did, it is unlikely that they burnt the entire extent of E. grandis forest throughout the Wet Tropics in a systematic way. Clearly, the frequency of burning was appropriate to maintain areas of E. grandis. The contemporary fire management objective of high-frequency burning is justified as mimicking aboriginal landscape management, despite uncertainty regarding the prehistoric fire regimes in E. grandis forests. Pollen analyses of volcanic lake sediments in the upland regions of the Australian Wet Tropics and soil charcoal analyses from a range of localities show that the boundaries of rain forest and open vegetation were very dynamic, putatively responding to both climate and fire regime shifts (Haberle et al. 2010). For example, rain forest pollen peaks at the end of the ice age (12 000 years ago) in response to warmer and wetter conditions that were associated with a regional increase in fire activity, as evidenced by microscopic charcoal (Haberle et al. 2010). Eucalypt-derived charcoal of late Pleistocene age (between 12 000 and 125 000 years ago) is found in the innermost parts of current rain forest patches, possibly reflecting drier and cooler conditions that favoured more fires (Hopkins et al. 1993). These studies demonstrate that climate and aboriginal burning interacted in controlling the extent of rain forest and brings into question some of the rationale for broad-scale prescribed burning as ‘restoring’ aboriginal fire regimes. Synthesizing modern trends in landscape ecology, palaeoecology and functional biology, we propose thinking of E. grandis forest as a secondary rain forest and accepting that E. grandis behaves like a long-lived pioneer that regenerates en masse after infrequent large fire events (Fig. 2a). The inherently more exposed conditions and higher fire risk where rain forest and savanna interface will provide favourable conditions for the establishment of rain forest pioneers like E. grandis. The establishment of rain forest species in the understorey of these forests can be seen as a process of succession (Fig. 2b). Therefore, rather than being threatened by succession, the existence of E. grandis forest may be inextricably tied to the dynamics of rain forest–savanna boundaries. Conceiving of E. grandis forest as a secondary rain forest (Fig. 2a) also accords well with functional biology studies showing that rain forest and giant eucalypt forest form a functional continuum (Fig. 2c). The understanding that E. grandis forest is functionally akin to rain forest (Fig. 2c) calls for a reappraisal of the need for broadscale prescribed burning of these forests to control the ‘invasion’ of rain forests, a term commonly applied to the expansion of rain forest species into E. grandis forests. Most managers and scientists use the term ‘invasive’ to describe species that are introduced, establish, spread and eventually ‘cause harm’. Calling rain forest invasive in this way promotes a misleading idea that rain forest expansion is certain to have a negative impact on eucalypt forests, akin to the invasion by environmental weeds. Unlike in southern Australia, catastrophic bushfire disasters do not occur in the giant tropical eucalypt forests (due to lower forest fire danger), so the focus of fire management is more directed to controlling grass fires rather than reducing the risk of forest forests (Little et al. 2012). Broadscale burning may interfere with natural feedback mechanisms between the biota and the environment. For example, current rain forest expansion may be part of complex global vegetation–climate feedbacks that results in carbon storage. Further, some areas of grassy understoreys in E. grandis may owe their existence to burning to create cattle pasture, increased fire activity after selective logging or both. Any burning of E. grandis forests with grassy understoreys should be more targeted at localized areas to achieve specific biological or management outcomes, such as maintaining grassy understoreys in localities where the northern bettong persists, although specific research will be needed to access the effectiveness of such interventions. Conserving a mosaic of habitats in the Wet Tropics World Heritage Area is an important objective. These landscapes are inherently dynamic, and in the case of giant eucalypt forests, ecological processes operate on time-scales far exceeding the life span of individual researchers or land managers. Effective conservation of landscapes must therefore be coupled with long-term monitoring of multiple permanent study sites throughout the region and interrogation of the palaeoecological record. The resources (both in terms of field interventions and policy debates) currently invested in trying to combat rain forest expansion into E. grandis forest are better redirected in setting up long-term fire experiments in selected priority sites occupied by endangered animal populations (e.g. Laurance 1997). The national AusPlots project (White et al. 2012) for instance is currently establishing permanent plots in giant eucalypt forest across the Australian continent with the view of monitoring long-term vegetation dynamics. These permanent plots, in addition to fire experiments, will undoubtedly be highly informative for the future conservation of these forests. In summary, we suggest that for the purposes of ecosystem health, the most parsimonious and ecologically sensible way of managing giant eucalypt forests in the Wet Tropics is to let them be – allow natural fuel loads to build-up and rely on natural, random fire events to shape the system, as has been suggested for similar giant eucalypt forests in southeast Australia (Bowman et al. 2013). Given the ecological similarities of these forests to old-growth forests, elsewhere, it may also be beneficial to use our study system as an example to show how synthesizing landscape ecology, palaeoecology and functional biology can be used to frame ecologically sound management policies. We thank Ellen Weber and Jeremy Little for stimulating discussions and the editors and reviewers of our manuscript whose comments have helped to improve the manuscript. The authors declare no conflict of interest.