WILDLIFE FIRE REGIMES AND THE MANAGEMENT OF BIODIVERSITY IN TEMPERATE AND TROPICAL EUCALYPTUS FOREST LANDSCAPES IN AUSTRALIA
Richard J. Williams CSIRO Wildlife and Ecology, PMB 44, Winnellie, Northern Territory, 0822, Australia
Ross A. .Bradstock Biodiversity Survey and Research Division, New South Wales National Parks and Wildlife Service, Box 1967, Hurstville, New South Wales, 2220, Australia
ABSTRACT
We discuss interactions between fire regimes and biodiversity in the Eucalyptus forests and woodlands of temperate south-eastern Australia, and the savannas of wet-dry tropical northern Australia. Despite the major geomorphic and climatic differences between these regions, we argue that in both parts of the country, fire regimes and the fire ecology of species are governed by a common set of factors: (1) the incidence of severe fire weather, (2) ignition probability, (3) fuel accumulation, and (4) critical life history processes (e.g., juvenile periods, acquisition of fire tolerance, dispersal). The relationships between fire regimes and biodiversity (i.e., the per sistence of species at landscape scales) will reflect differences in these processes between regions. In both regions, however, a major management question concerns fires with a return time of 1-10 years, in terms of their effects on both fuel and the bio41. In the savannas, fires are frequent and spatially extensive, but of relatively low intensity. Fuel reduction burning is prescribed as a general management tool, primarily to minimize the extent and impact of late dry-season fires, which can have deleterious impacts on plants and animals in the savanna. The use of prescribed fire thus has an explicit biodiversity component in the management of landscapes in northern Australia. In temperate Australia, fires are relatively infrequent, and potentially very intense. Fuel reduction burning is used and promoted as a general management tool, primarily for the protection of life and property, rather than the maintenance of biodiversity. Extensive use of prescribed fire is necessary to achieve maximum levels of human protection in south-eastern AustralIa, but such a regime could be incompatible with the conservation of biodiversity. Although there are clearly species and communities in both regions that can persist in the landscape in the face of frequent fires, some plant functional groups, and some small. mammals, are at risk in the face of inappropriate fire regimes. Further research is required to integrate point-based studies of fire impacts with a spatial understanding of ignitions, fire behavior, and ecosystem dynamics. keywords: Australia, biodiversity, Eucalyptus, fire regimes, forest, fuel accumulation, national park, savanna, temperate, tropical, woodland. Citation: Williams, R.I., and R.A. Bradstock. 2000. Fire regimes and the management of biodiversity in temperate and tropical Eucalyptus forest landscapes in Australia. Pages 139-150 in W. Keith Moser and Cynthia F. Moser (eds.). Fire and forest ecology: innovative silviculture and vegetation management. Tall Timbers Fire Ecology Conference Proceedings, No. 21. Tall Timbers Research Station, Tallahassee, FL.
INTRODUCTION represented in extensive and high profile National Parks close to urban centers (e.g., Morton National Park near Sydney, and Kakadu National Park near The landscapes of the Australian continent as a Darwin). A major fire issue in both regions is the ef whole are both fire-prone and biologically diverse (Gill fect of frequent fire (at l-lO-year return intervals) on et al. 1981, Groves 1994). A fundamental scientific and plant and animal biodiversity, and on fuel reduction management question, therefore, is identifying and in and thus the incidence of unwanted fires. We have re corporating the most appropriate fire regimes into the stricted our discussion to the more open eucalypt-dom management of this precious biological diversity. We inated communities, which are common to both re highlight below some of the problems in arriving at an gions. This does not imply, however, that other forest agreed-upon set of appropriate fire regimes at the land vegetation types such as rainforest and tall. eucalypt scape scale for 2 distinctive and extensive Australian forests (sensu Groves 1994) do not present problems forest types: (1) eucalypt woodlands and open forests in fire management, or that fire frequency is not an with a shrubby understory from the temperate south issue for biodiversity conservation. east, and (2) eucalypt savanna woodlands with a grassy understory from the wet-dry tropics of the north. A FRAMEWORK FOR ASSESSING FIRE The regions we discuss, though several thousand kilometers apart, have interesting parallels, and offer REGIMES AND BIODIVERSITY the potential for developing common principles in fire INTERACTIONS IN AUSTRALIA and biodiversity management. Both contain diverse Despite major differences in climate and soils eucalypt-dominated forests and woodlands that are across Australia, we argue that fire regimes and their l39 140 WILLIAMS AND BRADSTOCK impacts .on ecosystems in different regions are gov 80 erned by a common set of factors. These differ from place to place (e.g., tropical versus temperate zones, 60 mesic versus arid environments) in rate or extent, such that each place has a unique combination of factors. The critical processes are: (1) annual incidence of se vere fire weather; (2) rates of ignition, both seasonal 20 and annual; (3) rates of fuel accumulation; and (4)crit MMAXFFDI ical life history processes, e.g., time to flowering, length of juvenile periods, acquisition of fire tolerance, JFMAMJJASOND and dispersal (Whelan 1995, Bond and van Wilgen MONTH 1996, Keith 1996). Fig. 1. Seasonal 1500-hour Forest Fire Danger Index (FFDI) These factors affect individual fires (e.g., size and for Jabiru, 250 kilometers east of Darwin, Northern Territory. shape), resultant fire regimes (e.g., intensity and fre MAXFFDI = absolute maximum FFDI for month; MMAXFFDI = quency of fires), and the likelihood of persistence of mean maximum FFDI for month. From Gill et al. (1996). species at landscape scales. Different outcomes may be expected across landscapes of differing topographic strategies (Benson and Howell 1990, Benson and Mc complexity (e.g., size and connectivity of fragments) Dougall 1996). Rainforest or tall eucalypt forest may even within a single region. The nature of land use be found in deep moist gullies but also in other to and administrative boundaries further influences out pographic situations in association with igneous intru comes. sions or beds of shale. The structure, composition, and Our thesis is that these fire, biotic, and landscape variables can be used systematically to examine fire positioning of these vegetation types is likely to have been similar over the past 10,000 years (Benson and regimes and their biotic consequences. By contrasting Redpath 1997). the nature of fire regimes and biodiversity between tropical and temperate regions, we intend to shed light on how management through prescribed burning can Fire Weather, Ignition, and Historical Changes to be focused to achieve multiple land use objectives. Ignition Several fire danger rating systems are used in Aus tralia, based on standard fire weather variables and fuel Fire in Tropical and Temperate Landscapes moisture. The most common is McArthur's Forest Fire The Geographical Setting Danger Index (FFDI) (Luke and McArthur 1978, No ble et al. 1980). FFDI ranges from 0-100, with values The savannas of the Darwin region (ca. 120 S; >50 indicating extreme fire weather. 131 0 E) are the dominant vegetation type in the higher Fire weather in tropical savannas is governed pri rainfall regions of northern Australia, where annual marily by the arrival and departure of the monsoon rainfall exceeds 1,000 millimeters. They typically oc (Gill et al. 1996). During the peak monsoon period cur on light-medium textured soils (sands and loams) (Jan...... ,-.early Mar), when the majority of the year's rain on generally flat land surfaces. They are characterized falls, the FFDI is < 10 (Figure I) and the vegetation is by a tree stratum 15-20 meters tall, and an understory essentially non-flammable. From late March onward, that is usually dominated by species of the tall, annual both atmospheric and soil moisture decline rapidly grass Sorghum (Wilson et al. 1990). The savannas are (Gill et al. 1996). Nevertheless, average maximum the "sea" in which "islands" of non-woodland veg FFDI in the early dry-season (May-Jun) remains <30, etation occur, such as treeless grasslands and some steadily increasing to values of around 40 in Septem heath communities. ber. The most extreme value recorded in 11 years' re Forests and associated woodlands in the Sydney cords at Jabiru (250 kilometers east of Darwin) was region of south-eastern Australia (ca; 340 S; 151 0 E) ca. 60, well below peak levels of ca. 90, which occur vary greatly in terms of structure and floristic com on extreme days in the Sydney region (Gill and Moore position (Gill 1994). The spectrum of forest types cor 1994, 1996). responds to moisture and fertility gradients. Site con Fire is frequent in Australia's savannas. In the ditions, as determined by terrain and parent material, Adelaide Rivers-Kakadu region, Press (1988), determine the overall mix of forest types. Much of the Braithwaite and Estbergs (1985), and Russell-Smith et vegetation is strongly influenced by the predominant al. (1997) reported fire frequencies of approximately 1 sandstone geology. The resultant landscape is dissect year in two or 2 years in three. This high fire frequency ed, with abrupt· valleys and canyons and expanses of results from the combination of predictable rainfall, mainly sandy soils of varying depth but generally low consistent production of biomass in the understory ev fertility. Heaths, shrublands apd woodlands, and low ery wet season, a 5-8-month dry season that cures fine open forests with a heathy understory are found, often fuels every year, the generally inter-annual predict in complex arrays according to soil depth,· drainage, ability in fire weather, and, most importantly, high and aspect. A common feature of these communities rates of ignition, which, at present, is from human is a prominent shrub layer containing a diverse array sources (Gill et al. 1996, WIlliams et al. 1998). Pre of species, with different life forms and regeneration scribed fire is a landscape management· tool for vir- FIRE REGIMES AND THE MANAGEMENT OF BIODIVERSITY IN EUCALYPTUS 141
~ Aspect Q) 100000 50 oa E ~ u.,., 10000 ~ 40 8 ::Co OJ:) Nlw E-<~ 30 .£ 1000 '-' S ~~ lZl lZlO 20 :>< 100 10 ~ 10 ~~ 0 ~ 0 Z 1 0 10 20 30 40 50 60 70 10 20 30 40 50 60 >60 FFDI CATEGORY FFDI CATEGORY Fig. 3. The relationship between % days with fires reported (a Fig. 2. Long-term 1500-hour FFDI for Sydney, New South measure of ignition probability) and. 1500-hour FFDI for 3 loca Wales, based on records from 1955-1995. tions (squares, diamonds, triangles) in the Sydney region. tually all land management agencies and land uses son (March), increased in frequency until about July, (Andersen 1996, Kakadu Board of Management and decreased in frequency until the end of the dry season, Australian Nature Conservation Agency 1996, Parks then increased again (Braithwaite 1991). The incidence and. Wildlife Commission of the .Northern Territory of late dry-season fires appears to have increased fol [NT] 1997). lowing European settlement of the Top End of the NT Fire weather in the Sydney region, like most of until the end of the 1980's (Press 1988). In Kakadu south-eastern Australia, is influenced by the passage National Park, however, late dry-season fires appear to of frontal systems traveling generally in an easterly have occurred less frequently since the early 1980's direction (Speer etal. 1996). These fronts result in (Russell-Smith et al. 1997). periods of strong winds (>50 kilometers per hour), In south-eastern Australia, quantitative measures which can shift rapidly from north to west and south. of changes in the incidence of fire are not available Prior to the passage of fronts, high temperatures and generally, although a number of case studies in the low relative humidity «20%) may occur as northerly mountainous country are available (Williams and Gill winds draw warm, dry air across from the interior of 1995) mainly through dendrochronological methods. the continent. Analysis of fire weather over a 40-year These studies indicate distinct and differing periods of period (Figure 2) revealed that for the vast majority of fire activity after European settlement began. Evidence the period FFDI was < 10. There was on average 1 suggests that in some cases the incidence of fire in Extreme day (FFDI > 50) per year (Gill and Moore creased in association with grazing activities (e,g., 1994, 1996). Leigh et al. 1987). In the mid-twentieth century the Most fires are attributable to human sources, with incidence of fire subsequently declined. The general lightning causing < I % of fires in this region (New applicability of this scenario is unknown, but unpub South Wales [NSW] National Parks and Wildlife Ser lished records in the Sydney region indicate highly vice, unpublished records). Significant lightning variable trends in the overall incidence of fire (both caused fires associated with severe drought currently planned and unplanned) in the last 20 years. occur on about a 20,-30-year cycle (NSW National Parks and Wildlife Service, unpublished records). Im Fuels portantly, the incidence of fire appears to be related to FFDI. In major conservation reserves in the Sydney Fuel in the tropics is produced consistently each region, the proportion of days on which unplanned year. Most of the fine fuels are grass. The proportion fires occurred was positively related to 1500-hour of leaf and twig components from both evergreen and FFDI (Figure 3). A similar trend was found by Gill et deciduous trees increases as the dry season progresses al. (1987) across the state of Victoria. Most unplanned (Williams et al. 1997). Annual fine fuel produCtion is fires in these reserves are attributable to arson or neg of the order of 3-5 metric tons per hectare, and fuel ligence (Conroy 1996, NSW National Parks and Wild loads tend to remain within this domain with annual life Service, unpublished records). However, it is not burning. Equilibrium fuel loads are of the order of 8- known whether this trend is due to more ignitions un 10 metric tons per hectare, which can be achieved der more extreme fire weather conditions, or a greater within 2-3 years without fire (Figure 4) (Cook et al. chance of successfulignition and spread. 1995). Thus, fine fuels are produced each year to lev Fire regimes have probably shifted in both regions els that can sustain fire, and can effectively reach near since prehistoric times. In the savannas, before Ab maximal levels in 2 or 3 years without fire. original people arrived, fire probably occurred during In contrast, equilibrium fuel loads in temperate the transition period.between the dry and wet seasons. vegetation communities around Sydney are of the or This period is the only one during the year that has der of 20-30 metric tons per hectare (Figure 5), both dry, combustible fuel, and lightning. Aboriginal reached 10-20 years after burning. Heaths and shrub people modified the timing of fire by burning during lands show the highest rates of accumulation with a the dry season. Fires commenced in the early dry-sea- substantial component of fine fuel being contributed 142 WILLIAMS AND BRADSTOCK
30 tZl '"""' -+- shrubland 00'-'" 10 • Total ~~~ ~5~ 8 ~. • Of-
Table 1. A comparison of the broad fire weather, fuel, and fire behavior characteristics of the landscapes of the Darwin Region and the Sydney Region. Darwin Sydney Peak fire weather period September/October November Maximum FFDI 60 100 Equilibrium fuel load 10 metric tons per hectare 30 metric tons per hectare Major fuels Grass Leaf and twig Ladder fuels No Yes Maximum fire intensity 20,000 kilowatts per meter 50,000 kilowatts per meter Landscape relief 10-100 meters 50-500 meters Crown fires No Yes FIRE REGIMES AND THE MANAGEMENT OF BIODIVERSITY IN EUCALYPTUS 143 of the wet season following fire (R.J. Williams, un or elimination of a population. The development of a published data). Adult eucalypts and other canopy sub fire-resistant lignotuber, capable of vegetative regrowth dominants can flower in the season following fire (Wil after fire, determines the minimum interval between liams 1997). The eucalypts are non-serotinous; seed fires that can be sustained by populations in the long release occurs each year, as fruits ripen and dry, rather term. If fire recurs before the lignotubers of new re than periodically in response to fire, as is common in cruits are sufficiently well developed, they will die. south-eastern Australia (Dunlop and Webb 1991, Set Typical juvenile periods of obligate seeders in the flora terfield and Williams 1996). Importantly, there are of the Sydney region are 2-4 years for legumes, and very few species of obligate seeder shrubs or trees in 4-10 years for many species of Proteaceae, Alloca tropical savannas, e.g., < 1 % of the woody species of suarina, and Callitris (Benson 1985, Keith 1996). the savannas of Kakadu National Park (Brennan 1996). The prominence of shrub species in woodlands The 1-2 year fire-return period, coupled with the rapid and forests of south-eastern Australia is of twofold sig postfire responses of the vegetation, means that grad nificance for biodiversity and fire management. First, ual, postfire successional patterns in the vertebrate fau many species are characteristic of, or endemic to, the na, which are typical of the fire responses of many area. They have conservation significance in their own vertebrates in southern Australia, do not occur in the right. Second, the shrub layer contributes to habitat for savannas (Braithwaite and Werner 1987, Woinarski many animals. For mammals the prevailing paradigm 1990). in south-eastern forests and woodlands is the positive Open forests and woodlands of south-eastern Aus relationship between habitat complexity, and the abun tralia contain a wide array of plant and faunal func dance and diversity of animals (Catling 1991, Catling tional groups with a correspondingly wide spectrum of and Burt 1995, Cork and Catling 1996). If the under fire regime requirements. Shifts in fire regimes at both story is diminished or removed, the abundance and the point and landscape scales will affect diversity and diversity of mammals may decline. High frequencies structure. Typically, overstory Eucalyptus and Ango of fire can result in such habitat simplification by dis phora species of either single-stemmed or multi rupting plant regeneration processes. stemmed habit (mallees) can resprout from a basallig Changes in diversity, richness, and abundance of notuber. Arborescent species of Eucalyptus may lose invertebrates in relation to changes in forest structure this basal resprouting capability once trunks develop, have also been described (York 1995). Complex veg relying instead on insulated epicormic stem buds to etation contains specialist species of invertebrates not initiate crown recovery after loss through fire. Similar present in habitat with open structure. Thus, structur modes of recovery occur in many of the understory ally complex forests may offer a greater range of hab shrubs. Although eucalypts that lack the capability to itat conditions for animal species than forests with resprout are generally absent from low-open forests simpler structure. and woodlands in south-eastern Australia, many such species of understory shrubs, herbs, and sedges are common ill these communities and may constitute up THE PRESCRIBED BURNING AND to 50% of the flora (e.g., Nieuwenhuis 1987, Morrison BIODIVERSITY PROBLEM et al. 1995, 1996, Bradstock et al. 1997). Reproductive and seedbank characteristics vary Tropical Eucalypt Savannas among strata in the vegetation. Tree species typically The savannas are extremely rich in flora and fauna retain seeds in woody capsules for .2:1-2 years (La (Braithwaite and Werner 1987, Bowman et al. 1993, mont et al. 1991, Pook et al. 1997). Adult trees of Brennan 1996). Within the mesic savannas, diversity Eucalyptus and Angophora species are usually able to across virtually all trophic levels is higher in the sa flower within 2-3 years of fire even after sustaining vannas than in monsoon rainforest, or grasslands of extensive crown damage (Gill 1997). Similar flower the fertile cracking clay soils of the floodplains (Wil ing patterns have been observed in resprouting shrubs. liams et al. 1995). The World Heritage Kakadu Na Obligate seeder shrub and herb species exhibit a sim tional Park contains about 1,800 species of vascular ilar range of seedbank characteristics. Species with on plants, >33% of the 4,500 species for the NT as a plant storage of seeds in serotinous fruits, and within whole (Brennan 1996). Historically, Australian savan soil storage are particularly common (Keith 1996). nas are unlikely to have been derived by recent (late Fire triggers germination either through the release of Pleistocene) burning of rainforest (Bowman 1999) and the aboveground seedbank or through cues that break are likely to have always been both rich biologically, the dormancy of buried seed. Species incapable of re and fire prone. Australia's savannas are also relatively sprouting depend on such mechanisms for recovery intact ecologically (Williams et al. 1996) because for following fire. estry and intensive agriCUlture have been of limited The juvenile period of species, whether resprout extent both spatially·· and historically. The saVannas ing or obligate seeders, is important. Typically, estab also represent important habitats for several groups of lishment of seedlings is linked to fire as a result of vertebrates whose range and/or abundance have con direct and indirect stimuli and other effects such as tracted since European settlement of Australia (Woin predator satiation (O'Dowd and Gill 1984, Bond and arski and Braithwaite 1990). van Wilgen 1996). However, recurrence of fire during Given the 2-3-year return time of fires in the land the juvenile period may result in the immediate decline scape, and the increase in potential intensity as the dry 144 WILLIAMS AND BRADSTOCK season progresses, management questions regarding such as extra food resources allow additional repro fire regimes and biodiversity in the savannas have to duction and recruitment (Griffiths and Christian 1996). do with both fire frequency and fire intensity. The ap How do the impacts of early dry-season fires com parent increase in the incidence of extensive, relatively pare with those of late dry-season fires? Evidence is intense late dry-season fires since European occupa accumulating, especially from the Munmarlary and tion is of particular concern, and the prevention of Kapalga fire experiments (Bowman et al. 1988, An such fires, especially repeat late dry-season fires, is a dersen et al. 1998), that early dry~season fires are ei general management objective (Russell-Smith 1995, ther relatively benign or indeed may benefit some el Russell-Smith et al. 1997). ements of the biota. These studies have indicated rel The impact of late dry-season fires, both as indi atively few impacts on understory composition (Bow vidual fires and repeat fires, can be dramatic. Their man et al. 1988), tree mortality (Williams and Cook effects are most profoundly displayed in the tree stra 1998, Williams et al. 1999), tree reproductive phenol tum. Individuals of fire-sensitive species, such as na ogy (Williams 1997), and water quality (Townsend tive cypress (c. intratropica) die following complete 1997). Some elements of the fauna, such as arid-adapt canopy s~orch. Callitris woodlands have contracted in ed ants (Andersen 1991), herpetofauna in general area in the past 50-100 years because of an increased (Trainor and Woinarski 1994), and frill-neck lizards in incidence of late dry-season fires (Bowman and Panton particular (Griffiths and Christian 1996) benefit from 1993). Some vegetation types, such as monsoon rain early dry-season fire. There is also a broad suite of forest (Bowman 1991) and heathlands with a high pro birds in savannas that show short-term positive re portion of obligate seeder species (Russell-Smith et al. sponses to fire. These species are attracted to recently 1997) may undergo local contraction or substantial burned areas and are not necessarily disadvantaged by changes in species composition under a regime of re frequent fire (Woinarski 1990). peated, extensive late dry-season fires. However, some evidence suggests that for certain Within the eucalypt-dominated savannas, species groups and landscape processes, annual early dry-sea vary in susceptibility to late dry-season fires (Lonsdale son prescribed fires may not be so benign. Such fires and Braithwaite 1991, Williams 1995, Williams and may reduce seed output in one of the dominant euca Cook 1998, Williams et al. 1999). The deciduous, lypts (E. miniata) (Setterfield 1997), and limit seedling broad leaf trees (e.g., Terminalia), and the bloodwood recruitment in this species and another common eu group of eucalypts (e.g.,E. porrecta) are more sensi calypt, E. tetrodonta. The abundance of some guilds tive to intense, late dry-season fires than are the dom of small mammals, including rare and endangered spe inant eudesmid group of eucalypts (e.g., E. miniata). cies for which northern Australia represents an ex Late dry-season fires also reduce the floral and fruit tremely important refuge, may be reduced by repeat reserves across all functional groups of trees (Setter early dry-season fire. These species would appear to field 1997, Williams 1997). This reduction has impli require protection from fire for about 2-3 years for the cations not only for tree regeneration and diversity, but maintenance of population structure, and interspecific also for fauna, such as mobile nectarivorous birds, diversity (R.W. Braithwaite, unpublished data). Several which depend upon the floral resour~es of trees, es species of birds are disadvantaged by high-frequency pecially in the dry. season (Franklin 1997). Granivo fire, but they persist in the landscape by exploiting the rous birds may also be disadvantaged by late dry-sea mosaic nature of the landscape, by dispersing into un son fires because such fires may reduce the reserves burned areas (Woinarski 1990). of grass seeds (Woinarski 1990). The nutrient capital Frequent fire, including low-intensity early dry of the savanna may also be reduced by late dry-season season fire, may represent a bottleneck to recruitment fires, via the incremental effect on tree mortality, and (Werner 1986). Fensham and Bowman (1992) argued therefore biomass (Cook 1994, Hurst et al. 1996). against this process, however, on the basis of stand Some elements of the landscape do not appear to structure in the tall mesic savannas on deep soils on suffer unduly from late dry-season fires. The compo Melville Island. Where site quality is low, perhaps be sition of ground stratum vegetation was little different cause of low-nutrient soils, or seasonal inundation, between late-burned, early-burned, and unburned treat then frequent fire may suppress woody species, and ments over a 14-year period at Munmarlary (Bowman prevent the development of a tree stratum (Wilson and et a1. 1988). A similar pattern for these regimesoc Bowman 1993). curred at Kapalga over a 5-year period (R.J. Williams, Fire exclusion in savannas may affect biodiversity. unpublished data). Late dry-season fires benefit some There are some, albeit rare, patches of savanna that faunal groups, such as arid-adapted groups of ants have been unburned for 5-20 years. Such decadal ab (Andersen 1991). Late dry-season fire may also in sence of fire appears to result in a more diverse tree crease nutrient pulses into sD;lall streams at the com stratum (Bowman et a1. 1988, Fensham 1990, Gill et mencement of the wet season, with consequent in a1. 1990, Braithwaite 1995a, b). However, the absence creases in the diversity of aquatic invertebrate biota of fire for 10 years does not appear to result in a suc (M. Douglas, NT University, and S. Townsend, NT cession towards closed rainforest, unlike some savanna Power and Water Authority, unpublished data). Even areas in Mrica (Bowman et al. 1988, Menaut et al. frill-neck lizards, which can suffer 30% mortality dur 1996). In contrast to tree diversity, herb diversity may ing late dry-season fires, are not unduly disadvantaged decline in the longer-term absence of fire (Fensham by late dry-season fires because compensating factors 1990). FIRE REGIMES AND THE MANAGEMENT OF BIODIVERSITY IN EUCALYPTUS 145
Excluding fire frQm year to year in particular of 10-20 years are typically associated with high lev patches of the mesic savannas is a potential fuel man els of floristic diversity and provide a complex vege agement problem, given that fuel loads can effectively tative structure. Although experimental evidence sug double from 2-5 metric tons per hectare, to near-rtlax gests that plant species regeneration in these commu imalloads of 5-10 metric tons per hectare within 2-3 nities is generally a positive function of fire intensity, years of fire exclusion (Gill et al. 1990, Cook et al. there is some suggestion that long intervals between 1995). If, within a given patch of savanna, the specific fire, e.g., >30 years (Bradstock ,et al. 1995), may lead management goal includes short term (2-3 year) and toa decline in diversity due to the senescence of both longer-term (decadal) protection from fire, then con aboveground plants and dormant seed stores in the soil sideration must be given to the consequences of a pre (e.g., Auld 1987). However, this prediction requires dictable, rapid accumulation of fuel to near-maximal more general validation. Areas in such condition are levels in 2-3 years. This accumulation of fuel can pre relatively rare due to prevailing fire regimes. dispose the savanna to late dry-season, high-intensity In the Sydney region, fire management for con fires, which are likely to occur eventually in such fire servation is made considerably more complex than in protected sites. This scenario happened during the Ka northern Australia by the existence of an extensive, palga Fire Experiment (Williams and Cook 1998, Wil complex interface between bushland and urban devel liams et al. 1999) because of the combination of fuel opment. Sizable areas of such bushland fall within Na accumulation to equilibrium levels, and eventual oc tional Parks, Nature Reserves, Local Government re currence of human ignition nearby during the late dry serves, or Qther forms of tenure that include conser season. vation as a management objective. Prescribed burning Thus, there is aneed for early dry-season burning of bushland is perceived as one of the major fire man in savannas, as a tool for both fuel reduction and bio agement tools for protecting people and their property diversity conservation. Application of early dry-season in the region. Debate about prescribed burning is on fires in northern Australia is relatively easy, due to going and is usually most vigorous following major predictable fire weather patterns in the savanna biome, fire seasons, such as 1993-1994 and 1997-1998. and the non-urbanized nature of the landscape. The Opinion in the community is polarized between effectiveness of early dry-season burning in reducing those who oppose burning and those who strongly fa the extent and frequency of late dry-season fires has vor extensive, frequent burning. In part such debate been demonstrated for the Darwin--Kakadu region reflects confusion about the actual function of pre (Press 1988, Russell-Smith et al. 1997). Such fuel re scribed burning in fire management (Gill and Brad duction burning needs to be of relatively low intensity stock 1999). Essentially 3 views represent various op « 1,000 kilowatts per meter) if potential impacts on tions for prescribed burning available to managers: tree stratum complexity, stem survival, and seed out puts in the tree stratum are to be minimized. An al 1. The chief role of prescribed burning is to stop either ternative fuel reduction tool may be wet-season burn the ignition or the spread of unplanned fires in se ing (Stocker and Sturtz 1966, Williams and Lane vere weather in a passive manner, i.e., without the 1999), which can reduce fuel loads substantially by additional input of suppression. The argument is reducing the abundance of native species of annual that frequent, extensive applications of prescribed Sorghum. However, consideration also needs to be giv fire are needed to deliver maximum fuel reduction, en to the spatial application of such fires so that those and hence maximum protection of people and their elements of the biota that appear to be sensitive to assets from fire. The amount of protection is as annual fires are not disadvantaged. Further research is sumed to be linearly correlated to the area treated required. by prescribed burning. 2. Prescribed burning is a tool for moderating inten sity of unplanned fires to a level where active sup Temperate Eucalypt Woodlands pression is effective (Luke and McArthur 1978). This more selective or strategic approach, involves The problem of fire frequency and biodiversity in the Sydney region reflects the importance of obligate identifying and treating accessible buffers where prescribed burning can enhance the effectiveness of seeder shrub species in most vegetation types. These suppression in difficult conditions. species are major components of both the flora (hence 3. Prescribed burning should focus intensively on the biodiversity) and the structure (hence fuel) of the interface between developed land and bushland so woodlands, forests, and overstory of adjacent heaths that both ember attack on structures (Ramsay et al. and shrublands. The key management problem is thus 1996) and fire intensity are moderated to manage the impact of a high-frequency, low-intensity fire re able and non-threatening levels close to properties. gime on floristic diversity and structural complexity of animal habitats. Bradstock et al. (1998b) investigated the relation Recent studies have verified that this fire regime ship between the extent of prescribed burning and risk diminishes floristic diversity (Cary and Morrison of uncontrollable fire at the interface between built as 1995, Morrison et al. 1996, Bradstock et al. 1997). sets and bushland. Their model predicts that very high Such effects occur when intervals between fires are <5 levels. of burning (about 40% of the interface annually) years. They may become acute when such short inter are needed to minimize risk of uncontrollable fire (av vals occur successively. Moderate fire-return intervals erage of 1 day per year) at the interface. A 50% re- 146 WILLIAMS AND BRADSTOCK
100 iness of individual fires and resultant fire regimes with respect to species' characteristics. The model can be reparameterized to examine ef e. fects of the alternative strategy of using prescribed burning as an active aid to suppressing unplanned fire ._Option 1 (Figure 6). Under this option (Figure 6, Option 2), •••••• where suppression is simulated as being totally effec tive in all areas subjected to prescribed burning, a sub • stantial reduction in the size of unplanned fires is in • _Option 2 dicated. In this option, prescribed burning is more ef ••••• ficient than Option 1, with a substantial reduction in 20 40 60 80 100 the extent of unplanned fire without high risk of ex- tinction. This result supports the use of selective pre TARGET PRESCRIBED FIRE scribed burning as a strategic management tool. SIZE(%OF LANDSCAPE) Results from modeling exercises such as those de scribed for Figure 6 strengthen the conviction that ex Fig. 6. Modeled relationship between % of landscape burned tensive prescribed burning does not offer the best com annually by prescribed fire (x-axis) and the size of unplanned fires (y-axis) in Sydney region. Option 1 (upper curve) assumes promise between protection of humans and biodiver only passive effects of prescribed burning on the extent of un sity conservation in temperate eucalypt forests. Thus, planned fires (see text). Option 2 (lower curve) includes additive several general principles can be derived that define effects of suppression and prescribed burning on the extent of the conundrum of protection versus biodiversity and unplanned fires. point toward a strategic solution for temperate plant communities in south-eastern Australia.
duction in burning would lead to a lO-fold increase in 1. Prescribed burning to reduce available fuels and the risk, given the nature of rapid fuel accumulation, an intensity and size of unplanned fires is necessary to nual occurrence of severe weather, and fairly high-re protect people and property. lief terrain typical of the Sydney area. The study in 2. Fuel dynamics in conjunction with weather neces dicates that protection levels even within a spatially sitate intervention at a frequency that will reduce restricted approach to prescribed burning will be high biodiversity. ly sensitive to availability of resources. 3. Unplanned fires increase overall fire frequency in Frequent prescribed burning on a broad scale in sites subjected to prescribed fire. vegetation communities with a well developed shrub 4. The biodiversity problem is potentially "manage layer will tend to increase the area subject to frequent able" at the landscape scale provided the number fire through an additive effect with unplanned fires. of "points" or "patches" subject to such adverse Modelling has been used to gain insight into this prob fire regimes is kept below a critical level. Further lem at the landscape scale. Bradstock et al. (l998a) research is necessary to determine these levels. investigated the use ,of prescribed burning in simulated 5. High levels of protection of people and property, landscapes containing vegetation, fuels, and weather but not necessarily maximum levels, may be main typical of the Sydney region. The aim was to under tained when prescribed fire in the landscape re stand how passive control of unplanned fires through mains below this critical level. prescribed burning would affect concurrent protection 6. Local losses of biodiversity at "points" or in (size of unplanned fires) and conservation (probability "patches" will result, but landscape-level losses of extinction) objectives. will be avoided if the conditions in Point 5 are The level of burning necessary to minimize the achieved. size of unplanned fires resulted in high frequencies of fire under the conditions of fuel and weather charac Despite extremes of debate, actual management teristic of the region (Figure 6). To minimize the size has tended to steer a middle course. The use of pre of unplanned fires, about 30% of the landscape needs scribed fire has generally been strategic, through the to be burned each year (Figure 6, Option 1). Such creation of buffers designed to enhance suppression frequencies of fire are predicted to lead to a high risk effort, often along roads or other fuel breaks, and treat of extinction of serotinous and leguminous obligate ment of the urban interface. This approach has evolved seeders at the landscape scale. Under high frequencies in spite of widespread debate for a number of reasons. of fire, there is a lower proportion of "points" or First, resources for prescribed burning are limited, "patches" in the landscape with long intervals be which favors smaller rather than large operations. Sec tween fire. The spatial heterogeneity (patchiness) of ond, opportunities for burning are limited by appro individual fires may be insufficient to maintain plant priate weather and dependence on volunteer labor, species in landscapes when the overall fire regime which is readily available only on weekends (Gill et reaches a critical level. Extinction will not be avoided al. 1987). Third, the rugged terrain requires intensive by patchiness alone of some fires, prescribed or oth use of resources; thus, prescribed burning is expensive. erwise, in the face of a high-frequency fire regime. The situation of houses on ridges above narrow can Thus, there are critical and non-critical levels of patch- yons leaves little margin for error; therefore, operation FIRE REGIMES AND THE MANAGEMENT OF BIODIVERSITY IN EUCALYPTUS 147 managers are usually conservative in the choice of is the integration of point-based studies of fire impacts weather for burning. with a spatial understanding of ignitions, fire behavior, and ecosystem dynamics. CONCLUSION ACKNOWLEDGMENTS In the eucalypt forests and woodlands of both south-eastern and northern Australia, the management We thank A. Andersen, G. Cook, D. Keith, and of biodiversity is closely linked to appropriate fire re M. Gill for comments on the draft manuscript. Help gimes. In south-eastern Australia the management of in preparing the manuscript was given by H.G. Nelson biodiversity is linked more closely to fire frequency and W. Waggitt. Parts of the manuscript were compiled than to fire intensity. In northern Australia, the man when one of us (RJW) was a visitor at the Tall Timbers agement of fire intensity in the landscape is seen as Research Station in 1998; L.A. Brennan, J. Noble, T.L. the critical management issue. In both regions, fuel Pruden, J. Walker, and R. Zimmerman are thanked for accumulation rates are rapid enough for the landscape their efforts during this time. This is Tropical Ecosys to carry fire at short return intervals-annually in tems Research Centre Publication Number 1086. northern Australia, and every 2-4 years in the south east. In both regions, prescribed fuel reduction burning is seen as the primary tool by which unplanned fires LITERATURE CITED may be managed. In south-eastern Australia, the most Andersen, A.N. 1991. Responses of ground-feeding ant com heavily populated sector of the country, the primary munities to three experimental fire regimes in a savanna rationale for fuel reduction burning is the protection forest of tropical Australia. Biotropica 23:575-585. of life and property. Management in northern Australia Andersen, A.N. 1996. Fire ecology and management. Pages 179-195 in C.M. Finlayson and I. von Oertzen (eds.). Land more explicitly recognizes biodiversity conservation as scape and vegetation ecology of the Kakadu Region, North a major reason for prescribed burning in the overall ern Australia. Kluwer Academic Publishers, Dordrecht, The management of the landscape. In both regions, how Netherlands. ever, vulnerable species, both plant and animal, could Andersen, AN., RW. Braithwaite, G.D. Cook, L.K. Corbett, RJ. be at risk from prescribed burning regimes designed Williams, M.M. Douglas, S.A. Setterfield, and W.J. Muller. to deliver maximum levels of fuel reduction. This risk 1998. Fire research for conservation management in tropical savannas: introducing the Kapalga fire experiment. Austra is particularly apparent in south-eastern Australia, lian Journal of Ecology 23:95-110. where obligate seeding plants and some small mam Auld, T.D. 1987. Population dynamics of the shrub Acacia suav mals could become locally extinct in the face of a re eolens (Sm.) Willd.: survivorship throughout the life cycle, gime of prescribed fires every 3-5 years. In northern a synthesis. Australian Journal of Ecology 12:139-151. Australia, the primary risk to biodiversity appears to Benson, D.H. 1985. Maturation periods for fire sensitive shrub be from repeat, high-intensity late dry-season fires. species in Hawkesbury sandstone vegetation. Cunning hamia 1:339-349. Annual-biennial early dry-season prescribed fires can Benson, D.H., and J. Howell. 1990. Sydney's vegetation 1788- reduce the incidence of such late dry-season fires. 1988: utilization, degradation and rehabilitation. 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Chap nor likely to reflect the nature of ignition rates and man and Hall, London. Bowman, D.M.J.S. 1991. Recovery of some northern Australian their relation to weather before occupation by Euro monsoon forest species following fire. Proceedings of the peans. Reducing the rate of human ignitions under se Royal Society of Queensland 101:21-25. vere weather remains part of the solution, with addi Bowman, D.M.J.S. 1999. Tansley Review No. 101. The impact tional benefits such as reducing the attendant risk of of Aboriginal landscape burning on the Australian biota. fire frequencies adverse to biodiversity conservation. New Phytologist 140:385-410. More innovative thinking is needed about how to pre Bowman, D.M.J.S., and W;J. Panton. 1993. Decline of Callitris intratropica RT. Baker and H.G. Smith in the Northern vent such ignitions. Territory: implications for pre- and post-European coloni No single fire regime is appropriate in either re zation fire regimes. 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