Fire and biodiversity - Society for Conservation Biology · PDF fileFire and biodiversity ... evolution and ecology of terrestrial life, including our own species. ... about the evolution

CHAP T E R 9

Fire and biodiversityDavid M.J.S Bowman and Brett P. Murphy

In a famous passage in the concluding chapter ofThe Origin of Species, Darwin (1859, 1964) invitesthe reader to “contemplate an entangled bank,clothed with many plants of many kinds, withbirds singing on the bushes, with various insectsflitting about, and with worms crawling throughthe damp earth” and “reflect that these elaboratelyconstructed forms, so different from each other,and dependent on each other in so complex aman-ner, have all been produced by laws acting aroundus.” Likewise, let us consider a tropical savannaablaze with hovering raptors catching insects flee-ing the fire-front, where flames sweep past treetrunks arising from dry crackling grass. Withinweeks the blackened savanna trees are covered ingreen shoots emerging from thick bark, woodyjuveniles are resprouting from root stocks, andherbivores are drawn to grass shooting fromgrow-

ing tips buried beneath the surface soil (Figure 9.1).In this chapter we will show that the very sameevolutionary and ecological principles thatDarwinespoused in that brilliant passage relate to land-scape fire. This is so because fire is enmeshed in theevolution and ecology of terrestrial life, includingour own species. This perspective is deeply chal-lenging to the classical view of the “Balance ofNature” that is still held by a broad cross-sectionof ecologists, naturalists and conservationists,mostof who have trained or live in environments wherelandscape fire is a rare event, and typically cata-strophic (Bond and Van Wilgen 1996; Bond andArchibald 2003). Only in the past decade havebooks been published outlining the general princi-ples of fire ecology (Whelan 1995; Bond and VanWilgen 1996) and journals established to commu-nicate the latest findings in fire ecology and

Figure 9.1 A eucalypt savanna recovering from a fire that has occurred in the early dry season in Kakadu National Park, northern Australia. Note theflowering Livistona palm and the strong resprouting response of juvenile woody plants on the still bare ground surface. Photograph by David Bowman.

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wildfire management (see http://www.firecology.net and http://www.iawfonline.org).

9.1 What is fire?

At the most basic level fire can be considered aphysiochemical process that rapidly releases ener-gy via the oxidation of organic compounds, andcan be loosely considered as “anti-photosynthesis”.This physiochemical process if often summarizedin a classic “fire triangle”made up of the three keyfactors to cause combustion: oxygen, fuel and igni-tions (Whelan 1995; Pyne 2007). Atmospheric oxy-gen levels create a “window” that controls fireactivity because ignitions are constrained by atmo-spheric oxygen (Scott and Glasspool 2006). Firecannot occur when levels fall below 13% of theatmosphere at sea level, and under the currentoxygen levels (21%) fire activity is limited by fuelmoisture, yet at 35% even moist fuels willburn. Because of substantial fluctuations in atmo-spheric oxygen, fire risk has changed significantlythrough geological time. In the Permian Period(between 290 and 250 million years ago) for exam-ple, oxygen levelswere substantially higher than atpresent and even moist giant moss (lycopod) for-ests would have been periodically burnt (Scott andGlasspool 2006). However, fire in the biosphereshould not be considered merely a physicochemi-cal process but rather a fundamental biogeochemi-cal process. Fires instantaneously link biomasswith the atmosphere by releasing heat, gases (nota-bly water vapor), and the geosphere by releasingnutrients andmaking soils more erodible and thuschanging the nutrient content of streams and rivers(hydrosphere). Fire is therefore quite unlike othernatural disturbances, such as floods and cyclones,given the complex web of interactions and numer-ous short and long-range feedbacks. Some ecolo-gists have suggested that landscape fires should beconsidered as being “biologically constructed”,and have drawn parallels with herbivory (Bondand Keeley 2005) or decomposition (Pyne 2007).Such tight coupling between fire and life bedevilssimple attribution of cause and effect, and raisesfascinating questions about the potential coevolu-tion of fire and life.

9.2 Evolution and fire in geological time

There is evidence from the fossil record thatwildfires started to occur soon after vegetationestablished on the land surface (about 420 millionyears ago) (Scott and Glasspool 2006). The longhistory of exposure of terrestrial life to fire leadsto the idea that fire is an important evolutionaryfactor, and more controversially, that fire and lifehave coevolved (Mutch 1970). While gainingsome support from modeling (Bond and Midgley1995), this is difficult to prove because adapta-tions to fire cannot be unambiguously identifiedin the fossil record. For example, in many fire-prone environments, seeds are often contained inwoody fruits that only open after a fire event, afeature known as serotiny. However, woody fruitsmay also be a defense against seed predators suchas parrots, and seeds are released once mature,irrespective of fire (Bowman 2000). In most casesit is impossible to know if fossilized woody fruitsare truly serotinous, thuswoody fruit occurrence isnot clear evidence of an adaptation to fire. Muchcare is required in the attribution of fire adapta-tions. For example, microevolution can result inswitching from possible fire-adaptations, such asthe serotinous state. More problematic for under-standing the evolution of flammability, Schwilkand Kerr (2002) have proposed a hypothesis theycall “genetic niche-hiking” that flammable traitsmay spread without any “direct fitness benefit ofthe flammable trait”.

Insights into the evolution of flammabilityhave been gained by tracking the emergence ofhighly fire-adapted lineages such as Eucalyptus.Eucalypts are renowned for their extraordinarilyprolific vegetative recovery of burnt trunks viaepicormic buds (Figure 9.2). Recently, Burrows(2002) has shown that eucalypt epicormics areanatomically unique. Unlike other plant lineages,which have fully developed dormant buds on thetrunks, eucalypts, have strips of “precursor” cellsthat span the cambium layer that, given the rightcues, develop rapidly into epicormic buds. Theadvantage of this system is that should the trunkbe severely burnt the tree retains the capacity todevelop epicormic buds from cells protected inthe cambium. The molecular phylogeny of

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eucalypts, dated using the fossil record, suggeststhat this trait existed before the “bloodwood”eucalypt clade split off from other eucalyptssome 30 million years ago, given that it occursin both these lineages. Such an ancient feature tothe lineage suggests that eucalypts had devel-oped a vegetative response to landscape fire,which appears to have become more common inthe Australian environment associated with a dryclimate and nutrient impoverished soils. This in-terpretation is concordant with the fresh insightsabout the evolution of the Australian biotaderived from numerous molecular phylogeniesof quintessentially Australian plants and animals(Bowman and Yeates 2006).

9.3 Pyrogeography

Satellite sensors have revolutionized our under-standing of fire activity from landscape to globalscales. Global compilations of satellite data havedemonstrated the occurrence of landscape fire onevery vegetated continent, yet the incidence offire is not random across the globe (Justice et al.2003). Fire has predictable features regardinghow it spreads across landscapes and the fre-quency and season of occurrence. Such predict-ability has lead to the idea of the “fire regime”.

Key aspects of the fire regime include types offuels consumed (e.g. grass vs. canopies), spatialpattern (area burnt and shape), and consequences(severity relative to impacts on the vegetationand/or soils) (Gill 1975; Bond and Keeley 2005).For example, savanna fires are often of low inten-sity and high frequency (often annual), while for-est fires are often of low frequency (once everyfew centuries) andveryhigh intensity. Fire regimesare part of the habitat template that organizes thegeographic distribution of biodiversity, and, inturn, species distributions influence the spread offire. Some authors have even applied “habitat suit-ability modeling” to predict where fire is mostlikely to occur at the global to local level.

Fire activity is strongly influenced by climatevariability. Fire managers have developed empiri-cal relationships that combine climate data, such asthe intensity of antecedent moisture deficit, windspeed, relative humidity, and air temperature, tocalculate fire danger (see http://www.firenorth.org.au). Mathematical models combining such cli-mate data with fuel loads and topography havebeen developed to predict how a fire may behaveas it spreads across a landscape (Cary et al. 2006).The spread of fire is also strongly influenced byvegetation type (Figure 9.3). For example, grassyenvironments carry fire frequently because of therapid accumulation of fuel while rainforests burn

Figure 9.2 Prolific epicormic sprouts on a recently burnt tall eucalypt forest in eastern Tasmania. Photograph by David Bowman.

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infrequently because of microclimates that keepfuels moist under all but drought conditions. Cli-mate cycles such as the El Niño Southern Oscilla-tion (ENSO) also strongly influencefire activity. Forexample, fire activity typically increases in arid en-vironments after a wet period because of the build-up of fine fuels. Conversely, fire activity increasesafter a long drought period in moist forests.

The satellite record has been extraordinarilyuseful in understanding fire activity in highlyfire prone environments (see http://www.cfa4wd.org/information/Forest_FDI.htm). Yetthe limited time-depth of this record may maskthe occurrence of infrequent fire events that occurin long-lived fire-prone vegetation such as theboreal forests of Canada and Siberia. Under-standing the “fire regimes” of long-lived forestslike those of the boreal zone demands historicalreconstruction such as dendrochronology (tree-ring analysis) to determine the timing of “standreplacing fires” which initiate a cohort of regen-eration to replace the burnt forest. Statistical anal-ysis of forest stand-age structures can be used todetermine the inter-fire intervals (Johnson andGutsell 1994). Dendrochronology has also beenused to date precisely past fire events by identify-ing injuries to growth rings (fire scars) on thetrunks on long-lived trees (Swetnam 1993).

The study by Sibold et al. (2006) captures manyof the above complexities in understanding fireextent and occurrence. They combined tree ringanalyses and geographic information systems(GIS) techniques to identify the influence of vege-tation type and structure, elevation and aspect,and regional climate influences on fire activity inthe Rocky Mountains National Park, Colorado,USA. Their analysis identified the primary impor-tance of ENSO for fire activity, yet this climaticeffect was modulated by landscape setting andvegetation type. Over the 400-year record, fireactivity was common in the dry, low elevationslopes that support fire-prone lodgepole pine(Pinus contorta) forests but at higher elevationthere were large areas of long unburnt mesicspruce-fir (Picea engelmannii) forest. On the moistwestern side of the mountain range were fewer,but larger, fires compared to the drier easternsides of the mountain range. This exampleshows that while climate is a driver of fire risk,the linkage between fire, climate and vegetation iscomplex, frustrating simple attribution of “causeand effect”. Finally, human fire usage has a pro-found effect on fire activity, disturbing “natural”fire regimes. For example, tropical rainforests arecurrently being transformed to pasture by burn-ing (see Box 9.1) yet in some environments, like

Figure 9.3 Landscape scale patterns of fire spread in southwestern Tasmania. Fire spread is controlled by topography, vegetation, and the meteorologicalconditions that prevailed at the time of the fire creating strongly non‐random patterns of burnt and unburnt areas. Photograph by David Bowman.




the forests of thewesternUSA,firemanagers haveeffectively eliminated fire from some fire pronelandscapes.

9.4 Vegetation–climate patternsdecoupled by fire

A classic view of plant geography is that vegeta-tion and climate are closely coupled. RecentlyBond et al. (2005) challenged this view by asking

the question of whether the vegetation of theEarth is significantly influenced by landscapefire. The approach they took was via dynamicglobal vegetation models (DGVMs), which arecomputer simulations of vegetation based onphysiological principles. The effect of landscapefire on global vegetation patterns is implicit inDGVMs because they include “fire modules”that introduce frequent disturbances to modeledvegetation patterns and processes. Such modulesare necessary in order to recreate actual

Box 9.1 Fire and the destruction of tropical forestsDavid M. J. S. Bowman and Brett P. Murphy

Each year, extensive areas of tropical forest areunintentionally burnt by anthropogenic fires,and are severely degraded or destroyed as aresult (see Chapter 4). Enormous conflagrationscan occur in response to drought eventsassociated with ENSO, most notably theIndonesian fires of 1997–1998, which burntaround 8 million hectares of forest (Cochrane2003). Until recent decades, most tropicalforests experienced fires very infrequently,with fire return intervals in the order ofcenturies, although it is now clear that firefrequency has increased dramatically in thepast few decades. Current human land‐useactivities promote forest fires by fragmenting(see Chapter 5) and degrading forests andproviding ignition sources, which wouldotherwise be rare. These three factors can actsynergistically to initiate a series of positivefeedbacks that promote the massive tropicalforest fires that have become common inrecent decades (see Box 9.1 Figure). Forestedges tend to be much more susceptible to firethan forest cores, because they tend to bemoredesiccated by wind and sun, have higher ratesof tree mortality and hence, woody fuelaccumulation and grassy fuel loads tend to behigher. The result is that fire frequency tends toincrease with proximity to a forest edge, suchthat highly fragmented forests have high firefrequencies. Forests degraded by selectivelogging are also at risk of fire due to theirreduced canopy cover, which allows the forestto become desiccated and light to penetrate

Decreased canopy cover

Increased amount of edgeIncreased desiccationIncreased tree mortalityIncreased fuel loads

Increased desiccationIncreased grassy fuels

LoggingDeforestation

Forest degradation

Forest fire

DroughtClimate change

Fragmentation

Box 9.1 Figure The synergistic effects of habitat fragmentationand degradation on the occurrence of tropical forest fires. Adaptedfrom Cochrane (2003).

and encourage grass growth. The wastebiomass from logging operations can alsodramatically elevate fuel loads. Similarly,forests degraded by an initial fire tend to bemore susceptible to repeat fires, furtherenhancing the feedback loop.The negative impacts of frequent, intense

fires on tropical forest biodiversity are likely tobe enormous, given the existing threats posedby the direct effects of deforestation (Chapter4) and overharvesting (Chapter 6). Intense fireseasily kill a large proportion of tropical foresttree species, and repeated fires can beespecially detrimental to species regeneratingvegetatively or from seed. Generally, repeatedfires lead to a loss of primary forest tree species,with these replaced by an impoverished set ofpioneer species (Barlow and Peres 2008). Theeffects of fire on forest animals are less well

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vegetation patterns. Bond et al. (2005) found thata world without fire had very different vegetationzones compared with the actual vegetation geog-raphy. For example, when fire was “switchedoff”, dense (>80%) tree cover increased from27% to 56% of the vegetated Earth surface andmore than half (52%) of the current global distri-bution of tropical savannas were transformed toangiosperm-dominated forests. The core messageof this analysis is that fire causes the “decou-pling” of vegetation patterns from climate.

Arguably the most well known decoupling ofvegetation and climate concerns the geographicdistribution of forest and savanna. Savannas areamong the most fire-prone biomes on Earth, andare characterized by varying mixtures of both treeand grass biomass. The question of how both trees

and grasses can coexist in the long-term has longpuzzled savanna ecologists. Conventional ecolog-ical theory of plant succession suggests that highlyproductive savannas are unstable and shouldgradually progress toward closed canopy forest.While it seems that in less productive savannas,such as in low rainfall areas, tree biomass is indeedconstrained by the limitation of resources, such aswater, recent research suggests that in more pro-ductive savannas, recurrent disturbance plays animportant role in maintaining a tree–grass balance(Sankaran et al. 2005). Given the high flammabilityof savannas, it seems that disturbance due to fire isof particular importance.

The most widely accepted explanation of howfrequent fires limit tree biomass in savannas as-sumes that a “tree demographic-bottleneck”

Box 9.1 (Continued)

understood, although studies following the1997–1998 Indonesian fires suggest severeimpacts on many groups, especially thosereliant on fruit‐trees and arthropodcommunities in leaf litter (Kinnaird and O’Brien1998). On Borneo, endangered orangutan(Pongo pygmaeus) populations suffereddeclines of around 33% following the1997–1998 fires (Rijksen and Meijaard 1999).In many tropical regions, climate change is

expected to exacerbate forest fires. There isevidence that extreme weather events, such asthe ENSO droughts that triggered the 1997–1998 Indonesian fires, and tropical storms, maybecome more frequent (Timmermann et al.1999; Mann and Emanuel 2006). Additionally,we can expect strong positive feedbacksbetween forest fire occurrence and climatechange, because tropical forest fires result inenormous additions of greenhouse gases to theatmosphere, leading to even more rapidclimate change. For example, the 1997–1998Indonesian fires released 0.8‐2.6 Gt of carbon tothe atmosphere, equivalent to 13–40% ofglobal emissions due to burning fossil fuels,making a large contribution to the largestrecorded annual increase in atmospheric CO2

concentration (Page et al. 2002).

REFERENCES

Barlow, J. and Peres, C. A. (2008). Fire‐mediated diebackand compositional cascade in an Amazonian forest.Philosophical Transactions of the Royal Society ofLondon B, 363, 1787–1794.

Cochrane, M. A. (2003). Fire science for rainforests.Nature, 421, 913–919.

Kinnaird, M. F. and O’Brien, T. G. (1998). Ecological effectsofwildfire on lowland rainforest in Sumatra.ConservationBiology, 12, 954–956.

Rijksen, H. D. and Meijaard, E. (1999). Our vanishing rela-tive: the status of wild orang‐utans at the close of thetwentieth century. Tropenbos Publications, Wagenin-gen, the Netherlands.

Mann, M. E. and Emanuel, K. A. (2006). Atlantic hurricanetrends linked to climate change. Eos, Transactions of theAmerican Geophysical Union, 87, doi:10.1029/2006EO240001.

Page, S. E., Siegert, F., Rieley, J. O., Boehm, H. D. V., Jaya,A., and Limin, S. (2002). The amount of carbon releasedfrom peat and forest fires in Indonesia during 1997.Nature, 420, 61–65.

Timmermann, A., Oberhuber, J., Bacher, A., Esch,M., Latif, M., and Roeckner, E. (1999). IncreasedEl Niño frequency in a climate model forcedby future greenhouse warming. Nature,398, 694–697.




occurs. It is accepted that fire frequency controlsthe recruitment of savanna trees, particularly thegrowth of saplings into the tree layer. Unlikemature trees, saplings are too short in stature toavoid fire-damage and unlike juveniles, if theyare damaged they cannot rapidly return to theirprevious size from root stocks (Hoffman andSolbrig 2003). Thus saplings must have the abili-ty to tolerate recurrent disturbance until theyhave sufficient reserves to escape through a dis-turbance-free “recruitment window” into thecanopy layer where they suffer less fire damage.Recurrent disturbance by fire can stop savannatree populations from attaining maximal treebiomass by creating bottlenecks in the transitionof the relatively fire-sensitive sapling stage to thefire tolerant tree stage (Sankaran et al. 2004). Inthe extreme case, a sufficient frequency of burn-ing can result in the loss of all trees and thecomplete dominance of grass. Conversely, fireprotection can ultimately result in the recruit-ment of sufficient saplings to result in a closedcanopy forest.

Large herbivores may also interact with fireactivity because high levels of grazing typicallyreduce fire frequency, and this can enable woody

plants to escape the “fire trap”, and increase indominance (Sankaran et al. 2004; Werner 2005).For example, extensive woody plant encroach-ment has occurred in mesic grassland and savan-na in Queensland, Australia, and has beenattributed to cattle grazing and changed fire re-gimes (Crowley and Garnett 1998). This trend canbe reversed by reduced herbivory coupled withsustained burning—a methodology used by pas-toralists to eliminate so called “woody weeds”from overgrazed savannas. Bond and Archibald(2003) suggest that in southern African savannasthere is a complex interplay between fire frequen-cy and herbivory. Heavily grazed savannassupport short grass “lawns”, dominated byspecies in the sub-family Chloridioideae, whichdo not burn. These lawns support a diversity oflarge grazers including white rhino (Ceratother-ium simum), wildebeest (Connochaetes spp.), impa-la (Aepyceros melampus), warthog (Phacochoerusafricanus), and zebra (Equus spp.) (Figure 9.4).Under less intense grazing, these lawns canswitch to supporting bunch grass, in the sub-family Andropogoneae, which support a less di-verse mammal assemblage adapted to gazing tallgrasses, such as African buffalo (Syncerus caffer).

Figure 9.4 Zebra and wildebeest grazing on a ‘lawn’ in a humid savanna in Hluhluwe‐Umfolozi Park, South Africa. Bond and Archibald (2003)suggest that intense grazing by African mammals may render savannas less flammable by creating mosaics of lawns that increase the diversity of thelarge mammal assemblage. Large frequent fires are thought to switch the savannas to more flammable, tall grasses with a lower diversity of largemammals. Photograph by David Bowman. See similar Figure 4.6.



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The high biomass of the bunch grasslands renderthese systems highly flammable. Bond and Archi-bald (2003) propose a model where frequent largefires can result in a loss of lawns from a landscapewith corresponding declines in mammal diversi-ty. The mechanism for this is that resprouting bygrasses following fire causes a lowering in overallgrazing pressure across the landscape. Fully un-derstanding the drivers of the expansion ofwoody vegetation into rangelands, including therole of fire and herbivory, remains a major eco-logical challenge (see http://ag.arizonal.edu/re-search/archer/research/biblio1.html).

How savanna vegetation evolved is unclear.Some authors suggest that falling atmospheric car-bon dioxide (CO2) concentrationsmay have stimu-lated the development of grasses that nowdominate tropical savannas (Bond et al. 2003).Tropical savanna grasses have the C4 photosyn-thetic pathway that is highly productive in hot,wet climates, and under low CO2 concentrationsthese grasses have a physiological advantage overwoody vegetation that has the C3 photosyntheticpathway. The production of large quantities offine and well-aerated fuels may have greatlyincreased the frequency of landscape fire disad-vantaging woody plants and promoting furthergrassland expansion. The development of mon-soon climates might have also been as importanta driver as low atmospheric concentrations of CO2

(Keeley and Rundel 2003). The monsoon climate isparticularly fire-prone because of the characteristicalternation of wet and dry seasons. Thewet seasonallows rapid accumulation of grass fuels, while thedry season allows these fuels to dry out and be-come highly flammable. Furthermore, the dry sea-son tends to be concluded by intense convectivestorm activity that produces high densities oflightning strikes (Bowman 2005).

9.5 Humans and their use of fire

Our ancestors evolved in tropical savannas andthis probably contributed to our own species’mas-tery of fire. Indeed, humans can be truly describedas a fire keystone species given our dependence onfire; there is no known culture that does not rou-

tinely use fire. For example, the Tasmanian Abor-igines always carried fire with them, as it was anindispensable tool to survive the cold wet environ-ment (Bowman 1998). The expansion of humansthroughout the world must have significantlychanged the pattern of landscape burning by eitherintentionally setting fire to forests to clear them oraccidentally starting fires. How prehistoric humanfire usage changed landscape fire activity and eco-system processes remains controversial and thisissue has become entangled in a larger debateabout the relative importance of humans vs. cli-mate change in driving the late Pleistocene mega-faunal extinctions (Barnosky et al. 2004; Burney andFlannery 2005). Central to this debate is theAborig-inal colonization of Australia that occurred some40 000 years ago. Some researchers believe thathuman colonization caused such substantialchanges tofire regimes and vegetation distributionpatterns that the marsupial megafauna weredriven to extinction. This idea has recently beensupported by the analysis of stable carbon isotopes(d13C) in fossil eggshells of emus and the extinctgiant flightless bird Genyornis newtoni in the LakeEyre Basin of central Australia. Miller et al. (2005a)interpreted these results as indicating that sus-tained Aboriginal landscape burning during colo-nization in the late Pleistocene caused thetransformation of the central Australian landscapefrom a drought-adapted mosaic of trees, shrubs,and nutritious grasslands to the modern fire-adapted desert scrub. Further, climate modelingsuggests that the switch from high to low leaf-area-index vegetation may explain the weak pene-tration of the Australian summer monsoon in thepresent, relative to previous periods with similarclimates (known as “interglacials”) (Miller et al.2005b).

Yet despite the above evidence for catastrophicimpacts following human colonization of Austra-lia, it is widely accepted that at the time of Euro-pean colonization Aboriginal fire managementwas skilful and maintained stable vegetation pat-terns (Bowman 1998). For example, recent studiesin the savannas of Arnhem Land, northern Aus-tralia, show that areas under Aboriginal firemanagement are burnt in patches to increase kan-garoo densities (Figure 9.5; Murphy and Bowman




2007). Further, there is evidence that the cessationof Aboriginal fire management in the savannashas resulted in an increase in flammable grassbiomass and associated high levels of fire activityconsistent with a “grass–fire cycle” (see Box 9.2).It is unrealistic to assume that there should onlybe one uniform ecological impact from indige-nous fire usage. Clearly working out how indige-nous people have influenced landscapes

demands numerous studies, in order to detectlocal-scale effects and understand the underlying“logic” of their landscape burning practices (e.g.Murphy and Bowman 2007). Also of prime im-portance is study of the consequences of prehis-toric human colonization of islands such as NewZealand. In this case, there is clear evidence ofdramatic loss of forest cover and replacementwith grasslands (McGlone 2001).

Box 9.2 The grass–fire cycleDavid M. J. S. Bowman and Brett P. Murphy

D’Antonio and Vitousek (1992) described afeedback between fire and invasive grassesthat has the capacity to radically transformwoodland ecosystems, a process they describedas the “grass–fire cycle”. The cycle begins withinvasive grasses establishing in nativevegetation, increasing the abundance ofquick‐drying and well‐aerated fine fuels thatpromote frequent, intense fires. While theinvasive grasses recover rapidly from these firesvia regeneration from underground buds orseeds, woody plants tend to decrease inabundance. In turn, this increases theabundance of the invasive grasses, furtherincreasing fire frequency and intensity. The lossof woody biomass can also result in driermicroclimates, further adding momentum tothe grass–fire cycle. Eventually the grass–firecycle can convert a diverse habitat with manydifferent species to grassland dominated by afew exotics.The consequences of a grass–fire cycle for

ecosystem function can be enormous. Theincrease in fire frequency and intensity canresult in massive losses of carbon, both directly,via combustion of live and dead biomass, andindirectly, via the death of woody plants andtheir subsequent decomposition orcombustion. For example, invasion ofcheatgrass (Bromus tectorum) in the GreatBasin of the United States and theestablishment of a grass–fire cycle has led to aloss of 8 Mt of carbon to the atmosphere and islikely to result in a further 50 Mt loss in comingdecades (Bradley et al. 2006). During fires,nitrogen is also volatilized and lost in smoke,

while other nutrients, such as phosphorus, aremade more chemically mobile and thussusceptible to leaching. Thus, nutrient cyclesare disrupted, with a consequent decline inoverall stored nutrients for plants. This changecan further reinforce the grass–fire cyclebecause the fire‐loving grasses thrive on thetemporary increase in the availability ofnutrients.

An example of an emerging grass–fire cycle isprovided by the tropical savannas of northernAustralia, where a number of African grassescontinue to be deliberately spread as improvedpasture for cattle. Most notably, gamba grass(Andropogon gayanus) rapidly invades savannavegetation, resulting in fuel loads more thanfour times that observed in non‐invadedsavannas (Rossiter et al. 2003). Such fuel loadsallow extremely intense savanna fires, resultingin rapid reductions in tree biomass (see Box 9.2Figure). The conversion of a savanna woodland,with a diverse assemblage of native grasses, toa grassland monoculture is likely to haveenormous impacts on savanna biodiversity asgamba grass becomes established over largetracts of northern Australia. Despite the widelyacknowledged threat posed by gamba grass, itis still actively planted as a pasture species inmany areas. Preventing further spread ofgamba grass must be a management priority,given that, once established, reversing a grassfire–cycle is extraordinarily difficult. This isbecause woody juveniles have little chance ofreaching maturity given the high frequency ofintense fires and intense competition fromgrasses.

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Agricultural expansion is often enabled byusing fire as a tool to clear forests, a pattern thathas occurred since the rise of civilization. Current-ly, this process is occurringmost in the tropics. Thefire-driven destruction of forests has been studiedin close detail in the Amazon Basin, and is char-

acterized by an ensemble of positive feedbacksgreatly increasing the risk of fires above theextremely low background rate (Cochrane et al.1999; Cochrane 2003; see Box 9.1). Recurrent burn-ing can therefore trigger a landscape-level trans-formation of tropical rainforests into flammable

Box 9.2 (Continued)

Box 9.2 Figure An example of a grass‐fire cycle becomingestablished in northern Australian savannas. African gamba grass ishighly invasive and promotes enormously elevated fuel loads andhigh intensity fires, resulting in a rapid decline in woody species.Photograph by Samantha Setterfield.

REFERENCES

Bradley, B. A., Houghtonw, R. A., Mustard, J. F.,and Hamburg, S. P. (2006). Invasive grassreduces aboveground carbon stocks in shrublandsof the Western US. Global Change Biology, 12,1815–1822.

D’Antonio, C. M. and Vitousek, P. M. (1992). Biologicalinvasions by exotic grasses, the grass/fire cycle, andglobal change. Annual Review of Ecology and System-atics, 23, 63–87.

Rossiter, N. A., Setterfield, S. A., Douglas, M. M.,and Hutley, L. B. (2003). Testing the grass‐firecycle: alien grass invasion in the tropical savannasof northern Australia. Diversity and Distributions,9, 169–176.

Figure 9.5 Traditional land management using fire is still practiced by indigenous people in many parts of northern and central Australia. Recentwork in Arnhem Land suggests that skilful fire management results in a fine‐scale mosaic of burnt patches of varying age, which is thought tobe critically important for maintaining populations of many small mammals and granivorous birds. Photograph by Brett Murphy.




scrub and savanna, exacerbated by the establish-ment of a “grass–fire cycle” (see Box 9.2).

9.6 Fire and the maintenanceof biodiversity

9.6.1 Fire-reliant and fire-sensitive species

Many species in fire-prone landscapes are notonly fire tolerant, but depend on fire to completetheir life-cycles and to retain a competitive edgein their environment. Such species typicallybenefit from the conditions that prevail follow-ing a fire, such as increased resource availabilityassociated with the destruction of both livingand dead biomass, nutrient-rich ash, and high

light conditions (see Box 9.3). For example, fireis critically important for the regeneration ofmany plant species of the fire-prone heath com-munities typical of the world’s Mediterraneanclimates (e.g. South African fynbos, southwest-ern Australian kwongan, Californian chaparral).Many species in these communities have deeplydormant seeds that only germinate followingfire, when normally limited resources, such aslight and nutrients, are abundant. Many hard-seeded heath species, especially Acacia speciesand other legumes, are stimulated to germinateby heat, while many others are stimulated bychemicals in smoke (Bell et al. 1993; Brown1993). Other species in these communities typi-cally only flower following a fire (e.g. Denhamand Whelan 2000).

Box 9.3 Australia’s giant fireweedsDavid M.J.S Bowman and Brett P. Murphy

Australian botanists have been remarkablyunsuccessful in reaching agreement as to whatconstitutes an Australian rainforest (Bowman2000). The root of this definitional problem lieswith the refusal to use the term “rainforest” inthe literal sense, which would involve includingthe tall eucalypt forests that occur in Australia’shigh rainfall zones (see Box 9.3 Figure). This isdespite the fact that the originator of the term,German botanist Schimper, explicitly includedeucalypts in his conception of rainforest. Thereason why eucalypt forests are excluded fromthe term “rainforest” by Australians is thatthese forests require fire disturbance toregenerate, in contrast to true rainforests thatare comparatively fire‐sensitive. Typically,infrequent very intense fires kill all individualeucalypts, allowing prolific regeneration fromseed to occur, facilitated by the removal of thecanopy and creation of a nutrient‐rich bed ofash. Without fire, regeneration from seed doesnot occur, resulting in very even‐aged stands ofmature eucalypts.The gigantic (50–90 m tall) karri (Eucalyptus

diversicolor) forests of southwestern Australiaunderscore the complexity of the term

“rainforest” in Australia. These forests grow ina relatively high rainfall environment (>1100mm per annum) with a limited summerdrought of less than three months duration.Elsewhere in Australia, such a climatewould support rainforest if protected fromfire. However, in southwestern Australiathere are no continuously regeneratingand fire intolerant rainforest species tocompete with karri, although geologicaland biogeographic evidence point to theexistence of rainforest in the distant past.The cause of this disappearance appearsto be Tertiary aridification and theaccompanying increased occurrence oflandscape fire. For example, a pollen corefrom 200 km north of Perth shows that by2.5 million years ago the modern characterof the vegetation, including charcoalevidence of recurrent landscape fires,had established in this region, althoughsome rainforest pollen (such as Nothofagusand Phyllocladus) indicates thatrainforest pockets persisted in thelandscape at this time (Dodson andRamrath 2001).

continues



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Even within fire-prone landscapes, there maybe species and indeed whole communities thatare fire-sensitive. Typically these occur in partsof the landscape where fire frequency or severityis low, possibly due to topographic protection.For example, when fire sensitive rainforest com-munities occur within a flammable matrix ofgrassland and savanna, as throughout much ofthe tropics, they are often associated with rockygorges, incised gullies (often called “gallery for-

ests”), and slopes on the lee-side of “fire-bearing”winds (Bowman 2000). Several factors lead to thisassociation: fires burn more intensely up hill, es-pecially if driven by wind; rocks tend to limit theamount of grassy fuel that can accumulate; deepgorges are more humid, reducing the flammabili-ty of fuels; and high soil moisture may lead tohigher growth rates of the canopy trees, increas-ing their chances of reaching maturity, or a fire-resistant size, between fires.

Box 9.3 (Continued)

The gigantic size of karri and a regenerationstrategy dependent upon fire disturbance,including mass shedding of tiny seeds with

Box 9.3 Figure Giant Eucalyptus regnans tree in southern Tasmania.The life‐cycle of these trees depends upon infrequent fire to enableseedling establishment. Without fire a dense temperate Nothofagusrainforest develops because of the higher tolerance of rainforestseedlings to low light conditions. Photograph by David Bowman.

limited reserves onto ashbeds, suggestsconvergent evolution with other, distantlyrelated, eucalypts such as mountain ash(E. regnans) in southeastern Australianand E. grandis in northeastern and easternAustralia. Such convergence suggests thatall have been exposed to similar naturalselection pressures and have evolved tocompete with rainforest species by usingfire as an agent of inter‐specific competition(e.g. Bond and Midgley 1995). Theextraordinary diversity of the genus Eucalyptusand convergent evolution of traits suchas gigantism in different lineages in thisclade, and similar patterns of diversificationin numerous other taxonomic groups, leadsto the inescapable conclusion that fire hadbeen an integral part of the Australianenvironment for millions of years beforehuman colonization. Aborigines, therefore,learnt to live with an inherently flammableenvironment.

REFERENCES

Bowman, D. M. J. S. (2000). Australian rainforests: islandsof green in a sea of fire. Cambridge University Press,Cambridge, UK.

Bond, W. J. and Midgley, J. J. (1995). Kill thy neighbor—anindividualistic argument for the evolution of flammability.Oikos, 73, 79–85.

Dodson, J. R. and Ramrath, A. (2001). An Upper Pliocenelacustrine environmental record from south‐WesternAustralia—preliminary results. PalaeogeographyPalaeoclimatology Palaeoecology, 167, 309–320.




Somewhat counter-intuitively, many fire-sensi-tive species in fire-prone landscapes are favoredby moderate frequencies of low intensity fires,especially if they are patchy. Such fires greatlyreduce fuel loads and thus the likelihood oflarge, intense fires. In addition, because low in-tensity fires are typically more patchy than highintensity fires, they tend to leave populations offire-sensitive species undamaged providing aseed source for regeneration. Such an example isprovided by the decline of the fire-sensitive en-demic Tasmanian conifer King Billy pine (Athro-taxis selaginoides) following the cessation ofAboriginal landscape burning (Brown 1988;http://www.anbg.gov.au/fire ecology/fire-and-biodiversity.html). The relatively high frequency oflow-intensity fires under the Aboriginal regime ap-pears to have limited the occurrence of spatiallyextensive, high intensity fires. Under the Europeanregime, no deliberate burning took place, so thatwhen wildfires inevitably occurred, often startedby lightning, they were large, intense, and rapidlydestroyedvast tracts ofKingBilly pine.Over the lastcentury, about 30% of the total coverage of KingBilly pine has been lost.

A similar situation has resulted in the decline ofthe cypress pine (Callitris intratropica) in northernAustralian savannas (Bowman and Panton 1993).Cypress pine is a fire-sensitive conifer foundacross much of tropical Australia. Mature treeshave thick bark and can survive mild but notintense fires, and if stems are killed it has verylimited vegetative recovery. Seedlings cannot sur-vive even the coolest fires. Thus, it is aptly de-scribed as an “obligate seeder”. Populations ofcypress pine can survive mild fires occurringevery 2–8 years, but not frequent or more intensefires because of the delay in seedlings reachingmaturity and the cumulative damage of fires toadults. Cessation of Aboriginal land managementhas led to a decline of cypress pine in much of itsformer range, and it currently persists only in rain-forest margins and savanna micro-sites such as inrocky crevasses or among boulders or drainagelines that protect seedlings from fire (Figure 9.6).Fire sensitive species such as King Billy pine andcypress pine are powerful bio-indicators of alteredfire regimes because changes in their distribution,

density, and stand structure signal departure fromhistorical fire regimes.

9.6.2 Fire and habitat complexity

A complex fire regime can create habitat com-plexity for wildlife by establishing mosaics ofdifferent patch size of regenerating vegetationfollowing fires. Such habitat complexity providesa diversity of microclimates, resources, and shel-ter from predators. It is widely believed that thecatastrophic decline of mammal species in centralAustralia, where clearing of native vegetation foragriculture has not occurred, is a direct conse-quence of the homogenization of fine-scale habi-tat mosaics created by Aboriginal landscapeburning. This interpretation has been supportedby analysis of “fire scars” from historical aerialphotography and satellite imagery. For example,Burrows and Christensen (1991) compared firescars present in Australia’s Western Desert in1953, when traditional Aboriginal people still oc-cupied the region, with those present in 1986,when the area had become depopulated of its

Figure 9.6 Recently killed individuals of cypress pine (Callitrisintratopica), a conifer that is an obligate seeder. Changes in fire regimefollowing the breakdown of traditional Aboriginal fire managementhave seen a population crash of this species throughout its range innorthern Australia. Photograph by David Bowman.



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original inhabitants. In 1953, the study areacontained 372 fire scars with a mean area of 34ha, while in 1986, the same area contained a sin-gle fire scar, covering an area of 32 000 ha. Clear-ly, the present regime of large, intense andinfrequent fires associated with lightning strikeshas obliterated the fine-grained mosaic of burntpatches of varying ages that Aboriginal peoplehad once maintained (Burrows et al. 2006). Thecessation of Aboriginal landscape burning in cen-tral Australia has been linked to the range con-traction of some mammals such as the rufoushare-wallaby (Lagorchestes hirsutus) (Lundie-jen-kins 1993). Recent research in northern Austra-lia’s tropical savannas, where small mammalsand granivorous birds are in decline, also pointsto the importance of unfavorable fire regimes thatfollowed European colonization (Woinarski et al.2001). A prime example is the decline of the par-tridge pigeon (Geophaps smithii). This bird is par-ticularly vulnerable to changes in fire regimebecause it is feeds and nests on the ground andhas territories of less than 10 ha. Their preferredhabitat is a fine-grained mosaic of burnt and un-burnt savanna, where it feeds on seeds on burntground but nests and roosts in unburnt areas(Fraser et al. 2003). Aboriginal landscape burninghas been shown to produce such a fine-grainedmosaic (Bowman et al. 2004).

9.6.3 Managing fire regimes for biodiversity

The contrasting requirements of different speciesand communities within fire-prone landscapeshighlights the difficulties faced by those manag-ing fire regimes for biodiversity conservation.How does one manage for fire-reliant and fire-sensitive species at the same time? Lessons canclearly be learnt from traditional hunter-gatherersocieties that extensively used, and in some casesstill use,fire as a landmanagement tool.While it isunlikely that the enormous complexity of tradi-tional fire use can ever be fully encapsulated infire regimes imposed by conservation managers,it is clear that spatial and temporal complexity ofthe regimemust bemaximized to ensure themax-imum benefits to biodiversity. Clearly, in the caseof fire regimes designed for biodiversity conser-

vation, one size can’t fit all. The quest for sustain-able fire regimes demands trialing approachesand monitoring outcomes while balancing biodi-versity outcomes against other priorities such asprotection of life and property. This quest forcontinuous improvement in land managementhas been formalized in a process known as “adap-tive management”. This iterative process is mostapplicable when faced with high levels of uncer-tainty, and involves continually monitoring andevaluating the outcomes of management actions,and modifying subsequent actions accordingly.

9.7 Climate change and fire regimes

There is mounting concern that the frequency andintensity of wildfires may increase in response toglobal climate change (see Chapter 8), due to thegreater incidence of extreme fire weather. Whilethe effect is likely to vary substantially on a globalscale, regions that are likely to experience sub-stantial increases in temperature and reductionsin rainfall are also likely to experience moreextreme fire weather. Indeed, such a trend isalready apparent in southeastern Australia(Lucas et al. 2007) and the western United States(Westerling et al. 2006).

In addition to the effects of climate change, anincrease in atmospheric CO2 concentration is like-ly to affect the abundance and composition of fuelloads, and hence the frequency and intensity offires. Elevated CO2 concentration is likely to in-crease plant productivity, especially that of spe-cies utilizing the C3 photosynthetic pathway(mainly woody plants and temperate grasses),such that there have been suggestions that fuelproduction will increase in the future (Ziska et al.2005). Further, elevated CO2 concentration maylower the nitrogen content of foliage, slow-ing decomposition and resulting in heavier fuelbuild up (Walker 1991). However, to state that anincrease in CO2 concentration will increase fuelloads, and hence fire frequency and intensity, islikely to be a gross over-generalization; the effectsof elevated CO2 are in fact likely to vary substan-tially between biomes. For example, in tropicalsavannas, it is likely that increases in CO2




concentration will strongly favor woody plants,especially trees, at the expense of grasses andother herbaceous plants (Bond and Midgley2000). A shift from highly flammable grassyfuels to fuels based on woody plants is likely toreduce fire frequency and intensity in savannas.Indeed, Bond and Archibald (2003) have arguedthat managers should consider increasing fire fre-quencies to counteract the increase in growthrates of savanna trees that would result in highertree densities due to a weakening of the “treedemographic bottle-neck”. In contrast, in morearid biomes where fire occurrence is strongly lim-ited by antecedent rainfall (Allan and Southgate2002), an increase in productivity is indeed likelyto increase the frequency with which fires canoccur with a corresponding decrease in woodycover.

Climate change is set to make fire managementeven more complicated, given that climatechange simultaneously changes fire risk, ecosys-tem function, and the habitat template for mostorganisms, including invasive species. A recentreport by Dunlop and Brown (2008) discussingthe impact of climate change on nature reservesin Australia succinctly summarizes the problemconservation biologists now face. They write:

“The question is how should we respond tothe changing fire regimes? Efforts to main-tain ‘historic’ fire regimes through hazardreduction burning and vigorous fire sup-pression may be resource intensive, of limit-ed success, and have a greater impact onbiodiversity than natural changes in re-gimes. It might therefore be more effectiveto allow change and manage the conse-quences. The challenge is to find a way todo this while ensuring some suitable habitatis available for sensitive species, and simul-taneously managing the threat to urbanareas, infrastructure, and public safety.”

Again this demands an adaptive managementapproach, the key ingredients of which include: (i)clear stated objectives; (ii) comprehensive firemapping programs to track fire activity across thelandscape; (iii) monitoring the population of biodi-versity indicator species and/or condition and

extent of habitats; and (iv) rigorous evaluation ofthe costs and benefits ofmanagement interventions.

An important concept is “thresholds of potentialconcern” which predefines acceptable changes inthe landscape in response to different fire regimes(Bond and Archibald 2003). Bradstock and Kenny(2003) provided an example of this approach forassessing the effect of inter-fire interval on speciesdiverse scherophyll vegetation in the Sydney re-gion of southeastern Australia. This vegetationsupports a suite of species that are obligate seederswhose survival is held in a delicate balance by fire-frequency. Fire intervals that are shorter than thetime required formaturation of plant species resultin local extinction because of the absence of seedswhile longer fire intervals also ultimately result inregeneration failure because adults die and seed-banks become exhausted. Bradstock and Kenny(2003) found that to sustain the biodiversity ofsclerophyll vegetation, fire intervals between 7and 30 years are required. Monitoring is requiredto ensure that the majority of the landscape doesnot move outside these “thresholds of potentialconcern”.

Fire management is set to remain a thorny issuefor conservation biologists given the need to devisefire regimes to achieve multiple outcomes that onthe one hand protect life and property and on theother maintain biodiversity and ecosystem ser-vices. The accelerating pace of global environmen-tal change, of which climate change is but onecomponent, makes the quest for sustainable firemanagement bothmore critical andmore complex.The current quest for ecologically sustainable firemanagement can draw inspiration from indige-nous societies that learnt to coexist with fire tocreate ecologically sustainable and biodiverselandscapes (also see Box 1.1). Modern solutionswill undoubtedly be science based and use space-age technologies such as satellites, global position-ing systems, computer models and the web.

Summary

· TheEarth has a longhistory of landscapefire given:(i) the evolution of terrestrial carbon based vegetation;(ii) levels of atmospheric oxygen that are sufficient to



1

support the combustion of both living and dead or-ganic material; and (iii) abundant and widespreadignitions from lightning, volcanoes and humans.

· There is a clear geographic pattern of fire activityacross the planet reflecting the combined effects ofclimate, vegetation type and human activities. Mostfire activity is concentrated in the tropical savannabiome.

· Fire activity shows distinct spatial and temporalpatterns that collectively can be grouped into “fireregimes”. Species show preferences for different fireregimes and an abrupt switch in fire regime canhave a deleterious effect on species and in extremesituations, entire ecosystems. A classic example ofthis is the establishment of invasive grasses, whichdramatically increase fire frequency and intensitywith a cascade of negative ecological consequences.

· Climate change presents a new level of complexityfor fire management and biodiversity conservationbecause of abrupt changes in fire risk due to climatechange and simultaneous stress on species. Further,elevated atmospheric CO2 concentration may resultin changes in growth and fuel production due tochanges in growth patterns, water use efficiencyand allocation of nutrients.

· Numerous research challenges remain in under-standing the ecology and evolution of fire including:(i) whether flammability changes in response to nat-ural selection; (ii) how life-history traits of bothplants and animals are shaped by fire regimes; and(iii) how to manage landscape fire in order to con-serve biodiversity.

Suggested reading

· Bond, W. J. and Van Wilgen, B. W. (1996). Fire andplants. Chapman and Hall, London, UK.

· Bowman, D. M. J. S. (2000). Australian rainforests: islandsof green in a land of fire. Cambridge University Press,Cambridge, UK.

· Flannery, T. F. (1994). The future eaters: an ecologicalhistory of the Australasian lands and people. Reed Books,Chatswood, New South Wales, Australia.

· Whelan, R. J. (1995). The ecology of fire. Cambridge Uni-versity Press, Melbourne, Australia.

· Gill, A. M., Bradstock, R. A., and Williams, J. E. (2002).Flammable Australia: the fire regimes and biodiversity of acontinent. Cambridge University Press, Cambridge, UK.

· Pyne, S. J. (2001). Fire: a brief history. University of Wa-shington Press, Seattle.

Relevant websites

· Online journal of the Association for Fire Ecology:http://www.firecology.net.

· International Journal of Wildland Fire, journal of theInternational Association of Wildland Fire: http://www.iawfonline.org.

· North Australian Fire Information: http://www.fire-north.org.au.

· Forest Fire Danger Meter: http://www.cfa4wd.org/information/Forest_FDI.htm.

· Proliferation of woody plants in grasslands and savan-nas – a bibliography: http://ag.arizonal.edu/research/archer/research/biblio1.html.

· How fires affect biodiversity: http://www.anbg.gov.au/fire_ecology/fire-and-biodiversity.html.

· Kavli Institute of Theoretical Physics Miniconference:Pyrogeography and Climate Change (May 27–30,2008): http://online.itp.ucsb.edu/online/pyrogeo_c08.

Acknowledgements

The Kavli Institute for Theoretical Physics Mini-conference: Pyrogeography and Climate Change meeting(http://online.itp.ucsb.edu/online/pyrogeo_c08) helped usorganize our thinking. We thank the co-convener of thatmeeting, Jennifer Balch, for commenting on this chapter.An Australian Research Council grant (DP0878177) sup-ported this work.

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