Mechanisms Enabling a Fire Sensitive Plant to Survive Frequent Fires in South-West Australian Eucalypt Forests

Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 26

ReseaRch aRticle

MECHANISMS ENABLING A FIRE SENSITIVE PLANT TO SURVIVE FREQUENT FIRES IN SOUTH-WEST AUSTRALIAN EUCALYPT FORESTS

Neil Burrows1* and Ted Middleton2

1 Department of Parks and Wildlife 17 Dick Perry Avenue, Perth, Western Australia, Australia

2 Department of Parks and Wildlife Brain Street, Manjimup, Western Australia, Australia

*Corresponding author: Tel.:+618-409994880; e-mail: [email protected]

ABSTRACT RESUMEN

A fire sensitive plant, Banksia querci- Una planta sensible al fuego, Banksia quercifo- folia R.Br., that often occurs as thick- lia R.Br., que frecuentemente se halla en forma ets embedded in forest landscapes in de matorrales cerrados integrados a paisajes south-west Australia was exposed to boscosos en el sudeste de Australia, fue expues- repeated broad-scale fires at short in- ta a fuegos repetidos de gran escala en interva- tervals. Fire severity and patchiness los cortos. La severidad del fuego y la frag- was mapped using satellite imagery mentación fueron mapeados utilizando imáge- and the response of the B. quercifolia nes satelitales y la respuesta de la población de population monitored. Over the study B. quercifolia monitoreada. Durante el período period, the mean interval of fire in the de estudio, el intervalo medio de fuego en el landscape in which B. quercifolia oc- paisaje en el cual B. quercifolia se encuentra curred was 1.7 yralmost half the ju- fue de 1,7 añoscasi la mitad del período juve- venile period of the speciesand the nil de la especiey la frecuencia de fuego en el landscape fire frequency was six fires paisaje fue de 6 incendios por década. La po- per decade. The population increased blación aumentó como respuesta a los episo- in response to episodes of fire escape dios del escape de fuego, como así también la and fire-caused mortality and conse- mortalidad causada por el fuego y la consequent regeneration. Unlike surround- cuente regeneración. A diferencia de la vegeta- ing vegetation, immature B. quercifo- ción que la rodea, los matorrales de B. quercifolia thickets were not flammable under lia no fueron inflamables bajo condiciones me- conditions of mild weather and moist teorológicas moderadas y combustibles húme- fuels, so they burnt at a lower fre- dos, por lo tanto se quemaron a una frecuencia quency than more flammable vegeta- más baja que el resto de la vegetación circun- tion in the surrounding landscape, en- dante más inflamable, permitiéndole a la espe- abling the species to persist. When cie persistir en el sistema. Cuando los matorra- the thickets had developed sufficiently les se habían desarrollado lo suficiente como to burn, the plants had reached matu- para quemarse, las plantas alcanzaron la madu- rity and regenerated readily from rez y regeneraron rápidamente de semilla. Sin seed. However, the juvenile period embargo, el período juvenil se incrementó en increased by 58 % following a period un 58 % a continuación de un período de preci- Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 27

of 16 % below average rainfall, which pitaciones 16 % por debajo del promedio, lo has implications for fire management cual tiene implicancias para el manejo del fue- in a drying climate. go en un clima cada vez más seco.

Keywords: fire patchiness, fire sensitive plants, frequent fire, prescribed fire, south-west Australia

Citation: Burrows, N., and T. Middleton. 2016. Mechanisms enabling a fire sensitive plant to survive frequent fires in south-west Australian eucalypt forests. Fire Ecology 12(1): 26–40. doi: 10.4996/fireecology.1201026

INTRODUCTION monly use plant life histories such as juvenile period to model population survival under a In fire-prone environments, prescribed fire range of fire frequencies and invariably is used to achieve a range of management ob- demonstrate that fire return intervals close to jectives including fuel hazard reduction, habi- or less than the plant’s juvenile period will tat management, catchment management, and likely result in the local extinction (extirpa- for silvicultural purposes (McCaw and Bur- tion) of the population. For example, rows 1989, Walstad et al. 1990, Stephens and Drechsler et al. (1999) used a stage-based Ruth 2004). While these forest ecosystems are model to predict the fate of Banksia goodii well adapted to specific fire regimes, they may R.Br. in south-west Australia and concluded not be adapted to all potential fire regimes that fires should have minimum intervals of 15 (Gill and McCarthy 1998). Of the elements to 20 years to ensure the persistence of the that compose the fire regime, the interval be- population. Similarly, McCarthy et al. (2001) tween fires and fire frequency are of particular modeled the population dynamics of Banksia concern. Plant life histories and vital attri- ornata F.Muell. ex Meisn., a fire sensitive se- butes, especially the juvenile period of species rotinous species with a juvenile period of that are readily killed by fire and depend on about five years, and concluded that the opti- seed for regeneration, have been widely used mal fire interval to maximize population for setting minimum fire intervals below growth was greater than 30 years. which extinction is likely (Keeley and Zedler These models make a number of key as- 1978, Noble and Slatyer 1980, Kruger and Bi- sumptions about fire severity, patchiness, plant galke 1984, Tolhurst and Friend 2001). This mortality, seed bank dynamics, and recruit- has led to debate about whether frequent burn- ment following fire. Critically, the models as- ing to manage fuel hazard and mitigate wild- sume that fire is a uniform or homogeneous fire threat is compatible with long term plant event across the landscape: either it occurred conservation (Cary and Morrison 1995, En- or it didn’t, and if it did, then it was lethal to right et al. 1996, Bradstock et al. 1998, Keeley fire sensitive species in the landscape. Jurskis 2002, Driscoll et al. 2010). et al. (2003) suggest that these assumptions A number of scientists studying ecosys- about plant mortality and the uniformity of fire tems in south-west and south-east Australia may be unrealistic. Of the few published field have used a variety of models to predict the studies on this subject, workers in shrublands fate and viability of fire sensitive, usually se- and savanna woodlands reported that the re- rotinous, species under various fire interval sponse of populations of fire sensitive obligate scenarios (Bradstock et al. 1996, Bradstock et seeders was dependent on fire intensity and al. 1998, Enright et al. 1998, Groeneveld et al. patchiness (Ooi et al. 2006, Oliveira et al. 2008, Nield et al. 2009). These models com- 2015). Most population viability models are Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 28 unable to accommodate the often complex quantify the effects of frequent introduction of patchiness in fire behaviour. Variability of fire fire into a forest landscape that contains popu- behaviour, including whether or not fire will lations of fire sensitive Banksia quercifolia spread, depends on the prevailing topography, R.Br. as part of the vegetation mosaic. Specif- weather, and fuel conditions. Fuel flammabili- ically, the study investigated whether the re- ty, as determined by its moisture content, peated introduction of fire at intervals less than structure, biomass, time since last fire, and the juvenile period of B. quercifolia results in consequent fire behaviour, can vary at both population decline or extirpation. landscape and local patch scales (Whelan 1995, Catchpole 2002, Penman et al. 2008, METHODS Oliveira et al. 2015). South-west forest fire management units treated by aerial prescribed Study Site burning are generally 3000 ha to 6000 ha and contain a mosaic of vegetation types with dif- The study was carried out in London For- ferent fuel characteristics. Landscape flamma- est, an area of 5000 ha about 40 km north-east bility and related fire behaviour reflect the of the town of Walpole in south-west Western vegetation mosaic (Jurskis et al. 2003, Australia (34o 47′ S 116o 59′ E). The vegeta- Wardell-Johnson et al. 2006). In fire-prone re- tion comprised a fine scale mosaic of tall euca- gions experiencing a mediterranean-type cli- lypt forests to 40 m, low open woodlands to 15 mate, such as south-west Australia, the flam- m, and various heathlands to 2 m, reflecting mability of vegetation types can also vary due variability in soil and landform. The major to topographical variation in fuel moisture vegetation complexes as defined and mapped content, especially during spring and autumn. by Mattiske and Havel (1998) were Caldyanup Therefore, the generalisation that repeated (~52 %), which is predominantly heathlands, fire at short intervals leads to the decline or ex- and Collis and Lindesay, which are various eu- tinction of fire sensitive species overlooks the calypt forests and woodlands (~48 %). The re- spatial and temporal complexity and diversity gion experiences a mild climate of cool wet of fire behaviour in the landscape. Introducing winters and warm dry summers with mean an- fire into a landscape at short intervals does not nual rainfall ranging from 900 mm to 1200 necessarily equate to burning the entire land- mm. An automatic weather station was in- scape at short intervals. Here, “introducing stalled on site for the duration of the study. fire” means introducing ignition sources into a B. quercifolia is a woody shrub that grows landscape (in this case, the 5000 ha London to a height of about 2.5 m. It occurs along the Forest) under fuel and weather conditions con- south coast and adjacent hinterland of south- ducive to a proportion of the ignitions result- west Western Australia and its preferred habi- ing in spreading fire, the extent and behaviour tat is on the margins of swamps and in the of- of which will depend on prevailing fuel and ten abrupt ecotone between seasonally wet and weather conditions. densely vegetated heathlands and open euca- By dropping incendiaries on a grid pattern lypt forests. It was chosen for study because it across the landscape, aerial prescribed burning is one of a small number of species in the re- enables the delivery of virtually simultaneous gion known to be fire sensitive: it is non-ligno- ignitions at a density of about one ignition per tuberous, thin barked, and readily killed by 2 ha. This ignition pattern ensures that most fire. It relies on canopy-stored seed for regen- vegetation types in the landscape will experi- eration and, typical of many Banksias, it is se- ence one or more ignitions and have the poten- rotinous (Lamont and Barker 1988, Lamont et tial to burn. The aim of this study was to al. 1991a). Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 29

B. quercifolia commonly occurs as thickets burn in the spring (November) 2002 prescribed or in clumps with little or no overstorey pro- fire in London Forest are shown in Figure 1 vided by other vegetation. There is little or no and included a population of some 1.5 ha of surface or near surface fuel (as defined by mature B. quercifolia on the north-east corner Gould et al. 2011) and, typical of many shrub- of London Forest that was estimated to be lands, the fuel is a discontinuous elevated mix eight years old in 2002, based on fire history of mostly live vegetation, with the proportion and the size of the plants. of dead material increasing with the age of the In early autumn (March) 2003 a wildfire vegetation. This contrasts with the surround- burnt towards London Forest but, because ing shrublands, sheoak (Allocasaurina fraseri- most of it was recently burnt by prescribed ana [Miq.] L.A.S. Johnson) woodlands, and fire, the wildfire did not spread through Lon- eucalypt forests, where fuel is more continu- don Forest. However, it burnt the population ous, hence more flammable, under mild weath- of B. quercifolia that had not burnt during the er conditions. 2002 prescribed fire. This sequence of fire events provided an opportunity to examine the Fire History and Patchiness response of B. quercifolia to an intense early autumn wildfire and subsequent frequent low Since the 1960s, London Forest has expe- intensity broad-scale prescribed fires. rienced low intensity prescribed fires (using the aerial ignition technique) at 6 yr to 8 yr in- Landscape Fire Regime Definition tervals, with the last fires prior to this study being in late spring (November) 1994 and Forman and Godron (1986) defined a 2002. The patchiness and burn severity (loss “landscape” as a heterogeneous area com- of vegetation cover) of fires experienced over posed of a mosaic of interacting ecosystems the period 2002 to 2008 was mapped from 30 and landforms with similar geomorphology m resolution Landsat satellite data using the and climate. Approximating this definition, Normalised Difference Vegetation Index Mattiske and Havel (2002) delineated 29 land- (NDVI) (Garcia-Haro et al. 2001, Li et al. scapes in the south-west forest region of West- 2001). The reliability and limitations of satel- ern Australia, ranging from several thousand lite-derived fire mapping have been well docu- hectares to ten thousand hectares. Fire regime, mented (Price et al. 2003, Oliveira et al. as defined by Gill (1981), relates to the history 2015). of fire frequency, intensity, and seasonality at a The intrinsic spatial variability in structure point in the landscape, so it does not adequate- and biomass of live and dead vegetation, and ly describe the regime experienced at the land- temporal and spatial variability in fuel mois- scape scale, unless the entire landscape burns. ture content associated with topography, re- Characterising the fire regime experienced at sults in variability in flammability and fire be- the landscape scale is more complex due to the haviour potential of vegetation across the land- intrinsic spatial and temporal variability of scape. In south-west Australian forests, this fuel flammability, especially when fires burn variability is most pronounced during mild under mild weather conditions or in discontin- weather conditions in spring and autumn (after uous fuels. In determining fire regime compo- opening rains) when parts of the landscape are nents such as fire interval and fire frequency, it dry while other parts remain damp, and it is is also important to identify the temporal least evident in summer and early autumn bounds that apply to these measures. when almost the entire landscape has dried In this study, we characterise the fire re- (Burrows et al. 2008). Patches that did not gime in London Forest over the time period Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 30

Figure 1. Satellite-derived map of fire severity (extent of vegetation cover removal) following apre- scribed fire in the ~5000 ha London Forest landscape in spring 2002. The location of the study population of Banksia quercifolia is shown. Note: two categories of unburnt are different reflectances of vegetation in unburnt condition. from the spring prescribed burn in November fire interval, and mean landscape fire frequen- 2002 to the spring (November) 2011 assess- cy (Table 1). Landscape fire interval and fre- ment, three years after the last introduction of quency refer to the interval and frequency with fire in early summer (December) 2008. Hav- which fire was introduced to this landscape ing experienced two summers, there was a (London Forest) and resulted in some portion high probability that most of the surviving re- of the landscape being burnt. generation from the 2008 fire would develop to maturity (Enright et al. 1998). A final popu- B. quercifolia Population Assessment lation assessment was made in summer 2015 to confirm that the population had more or less In March 2004, some 12 months after the stabilised following the 2008 fire. To charac- wildfire, permanent sample plots were placed terise the landscape fire regime as it applied to in the burnt B. quercifolia population. The the 5000 ha London Forest, we used measures wildfire defoliated the vegetation and killed all including the proportions of London Forest B. quercifolia plants. A transect composed of burnt at various frequencies, mean landscape a series of fifteen 5 m × 2 m (10 2m ) plots was Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 31

Table 1. The proportion (%) of the area of forest and non-forest vegetation, and of the B. quercifolia study area that burnt following five introductions of fire into the London Forest landscape (5000 ha).

Prescribed fire Wildfire Prescribed fire Prescribed fire Prescribed fire Vegetation spring 2002 summer 2003 autumn 2005 autumn 2006 spring 2008 Forest 84 8 26 38 Non-forest 87 7 11 72 Total London Forest 86 7 5 19 55 Banksia study area 0 100 0 13 61 permanently marked, giving a total sample the number of infructescences collected from area of 150 m2. These plots were assessed in each plant, the number of follicles per infructe- March 2004 and then again several weeks be- scence, and the number of seeds extracted fore and after each introduction of fire in 2005, from each infructescence was determined. 2006, and 2008. Additional measurements In autumn (April) 2005, some three years were made in summer 2010, 2011, and 2015. after the prescribed fire and two years after the For each 10 m2 plot, the following data were wildfire, fire was again introduced to London recorded: Forest. This was performed in two ways: (1) on the ground using drip torches to light • visual estimate of the proportion around the edge of the forest to aid burn secu- (%) of the 10 m2 plot burnt after rity and to attempt to re-burn the young B. each introduction of fire, quercifolia study population that had regener- • number of dead plants, ated following the 2003 wildfire, and (2) by • number of live plants (classified as dropping aerial incendiaries on a grid pattern either seedlings or mature plants), within the forest. As expected, given the • number of plants flowering or young fuel age and the mild weather condi- fruiting, tions, there was little ignition and sustained • number of infructescences per fire spread, with only the most flammable parts plant and number of follicles per of the landscape re-burning (Table 1). Despite infructescence (2004 assessment of attempts to burn the recently regenerated B. wildfire-killed plants only), and quercifolia population using a drip torch, the • height of each live plant. mild conditions and the sparsity of fuel resulted in the population not burning. To obtain information about the juvenile This was repeated in autumn (March) period of B. quercifolia, a nearby population 2006, by which time much of the understorey that had regenerated following the 2003 wild- vegetation and fuel in London Forest was four fire in the adjoining Surprise Forest was moni- years old and the B. quercifolia population was tored annually from 2004 to 2011. Juvenile three years old. In addition to aerial incendiar- period was deemed to be the time after fire ies, a drip torch was again used in an attempt when at least 50 % of the population had to burn the B. quercifolia study population, reached flowering age. To obtain information which, unlike other vegetation types in the about the quantity of seed in the canopy-stored landscape, was difficult to burn because of the seed bank, infructescences were collected sparse fuel. However, with increased effort, from 40 mature plants of varying sizes and some patches did burn, particularly two plots ages outside the sample area. Stem diameter, beneath a large jarrah (Eucalyptus marginata Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 32

Donn ex Sm.) tree where leaf litter had re-ac- variability in flammability of the vegetation. cumulated. Other plots did not burn. Follow- Fire frequency in the most flammable and least ing a similar procedure, fire was re-introduced flammable vegetation (fuel) in this landscape in early summer (December) 2008 when most was equivalent to 4.8 fires and 1.2 fires per de- of the B. quercifolia population was now 4.8 cade, respectively, with the most flammable yr old. The build-up of fuel over this time and (burnt most frequently) vegetation being sheo- persistent attempts at igniting the population ak woodlands and the least flammable being using a drip torch ensured that much (61 %) of some moist riparian areas, vegetation on rock the study area burnt, albeit at low intensity. outcrops, and some heathland vegetation types with sparse fuels. RESULTS The proportion of the B. quercifolia study area and of the broader landscape (London Landscape Fire Regime and Fire Patchiness Forest) that burnt during each of the prescribed fires and the wildfire is shown in Table 1. As The variability of fire behaviour during satellite mapping was inadequate at fine spa- low intensity prescribed fires is reflected in the tial scales, the proportion of each of the 10 m2 satellite map of burn severity, or biomass re- sample plots burnt was visually estimated, moval, derived using the pre- and post-fire whereas the proportion of the London Forest NVDI (Figure 1). Over the period from No- landscape (5000 ha) that burnt was determined vember 2002, when the first fire was intro- from satellite mapping. The proportion of duced, to February 2011 (8.3 years later) when London Forest that burnt following each intro- the B. quercifolia population more or less sta- duction of fire (Table 1) reflects the variability bilized after the last fire in December 2008, in flammability of the different vegetation fire was introduced into London Forest and to types. For example, of the area burnt in 2006, the B. quercifolia population five times (Table ~70 % was forest and ~30 % was woodlands 1). From satellite mapping, 1.8 % of London and heathlands. With the exception of two Forest did not burn, 2.9 % burnt once, 8.8 % plots that were affected by jarrah leaf litter, burnt twice, 84.8 % burnt three times, and none of the B. quercifolia plots burnt. 1.7 % burnt four times. Over the same period, the B. quercifolia population burnt three times, Response of B. quercifolia Population but the entire population only burnt once (Ta- ble 1). This regime equates to a London For- Before the 2003 wildfire, the study popula- est landscape fire frequency (the frequency tion of B. quercifolia comprised even-aged with which fire was introduced into the land- mature plants of similar size. There is no evi- scape and some part of the landscape burnt) dence that this species regenerates in the ab- equivalent to six fires per decade and a mean sence of fire (or other disturbance), so the landscape fire interval of 1.7 yr. Fire frequen- pre-wildfire population was likely eight years cy and mean fire interval in the B. quercifolia old. Details about the age of the population in population over the same period (8.3 yr) was 1994, and the behaviour and patchiness of the 3.6 fires per decade and 2.8 yr, respectively. 1994 prescribed fire that burnt and regenerated However, the frequency with which the entire the population are not available. (100 %) B. quercifolia population burnt, as op- The spring 2002 prescribed fire burnt posed to a part of the population, was equiva- much of the surrounding vegetation, but did lent to 1.2 fires per decade. Over this period, not burn the study population (Table 1, Figure the fire frequency and interval at various 1). However, the early autumn 2003 wildfire points in the landscape varied, reflecting the did, killing all plants, which regenerated from Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 33

seed in the first winter post wildfire. Based on fructescences. Plants with a stem diameter the number of infructescences per plant and less than about 2 cm rarely bore fruit. the number of follicles per infructescence (two An assessment of the London Forest study seeds per follicle), the ratio of seeds shed after population in 2015 found that 47 % of plants the wildfire to the number of seedlings mea- that regenerated following the last fire in De- sured in spring 2004 was calculated to be cember 2008 had not reached flowering age 32.4:1 (i.e., ~3 % of the seed bank successfully after 6.2 yr, which is about 1.6 times the juve- developed into seedlings by 18 months after nile period of the plants that regenerated fol- the wildfire). The density of parent plants lowing the early autumn wildfire in 2003. pre-wildfire, hence the density of seedlings These plants were also significantly smaller post-wildfire measured in the fifteen 10m2 than the plants that regenerated following the sample plots, was highly variable. Post wild- wildfire (Figure 3). Rainfall in 2003 was 1117 fire, the mean ratio of seedlings-to-parent mm and in 2009 it was 758 mm, 24 % higher plants was 9.2:1 (SE = 0.99) (Figure 2). and 16 % lower, respectively, than the average rainfall over the study period (904 mm). Im- portantly for seedling establishment, rainfall in the first three months after post-fire seedfall was 337 mm following the March 2003 wildfire but only 61 mm following the December 2008 prescribed fire.

Figure 2. The number of mature (parent) Banksia quercifolia plants in each of the fifteen 10 m2 sample plots before the summer 2003 wildfire versus the number of seedlings measured after the wildfire in spring 2004.

The juvenile period for B. quercifolia pop- Figure 3. Mean height of Banksia quercifolia ulation in Surprise Forest that regenerated fol- plants with age for the 2003 and 2008 cohorts of lowing the early autumn (March) wildfire in post-fire regeneration when the rainfall in the first three months post fire was 377 mm and 61 mm, 2003 was 3.9 yr, and by ~5.9 yr post fire, about respectively. 90% of the population had flowered and set fruit. The larger plants in the even-aged co- The fluctuation in the number of plants in hort, as measured by stem diameter at ground each of the 10 m2 sample plots over the study level, matured earlier than smaller plants. period is shown in Figure 4. Plant numbers Similarly, there was a direct linear relationship fluctuated due to episodes of fire-caused mor- between plant stem size and the number of in- tality and post-fire regeneration. As a result of fructescences per plant, with the largest plants a mean landscape fire interval of 1.7 years, B. (stem diameter ~10 cm) carrying ~150 in- quercifolia was extirpated (0 plants) in two Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 34

Figure 4. Population fluctuations of Banksia quercifolia in fifteen 10 2m plots in response to a wildfire (WF) and four prescribed burns (PB) from 2002 to 2011, resulting in a mean landscape fire interval of ~2 yr. The spring 2005 prescribed burn, which burnt ~5 % of the landscape (London Forest) and none of the study plots, is not included.

plots, remained unchanged in one plot, de- were about equal numbers of mature and juve- creased in two plots, and increased in ten plots. nile plants. Cycles of fire escape, and fire-caused mortality and regeneration, resulted in an increase in the DISCUSSION study population from the 2002 baseline of 126 plants, to 428 plants in 2015. The popula- An even-aged population of mature B. tion trends graphed in Figure 4 show a 14 % quercifolia embedded in forest and heathlands overall decline in plant numbers from 2011 to in south-west Western Australia escaped a low 2015. intensity patchy spring prescribed fire but was As a result of the frequent exposure to fire then burnt and killed by an intense summer since 2002, the population changed from an wildfire. The population escaped the pre- even-aged cohort to one of three age classes. scribed fire because, unlike the surrounding In 2011, 56.5 % of the population was two vegetation, the discontinuous fuel typically as- years old, 6.5 % was five years old, and 37 % sociated with these populations was not flam- was eight years old, representing the three mable under spring conditions of relatively fire-stimulated regeneration events. The high fuel moisture content and mild weather. changing density of juvenile and mature plants In fire-prone environments, fire sensitive plant over time is shown in Figure 5. By 2015, there species are usually restricted to less flammable Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 35

This study demonstrates the importance of spatial variation in fuel flammability and associated local fire behaviour to the survival and persistence of fire sensitive species such as B. quercifolia embedded in flammable landscapes. The introduction of fire at intervals shorter than the juvenile period of B. quercifolia did not result in the extirpation of the population because, unlike many other vegetation communities, the entire population did not burn each time fire was introduced. While the mean landscape fire interval was 1.7 yr, most Figure 5. Plant density and maturational structural of the B. quercifolia population did not burn changes in a population of Banksia quercifolia ex- until it was 5.7 yr old when about 61 % of the posed to a summer wildfire and four prescribed population burnt, even though attempts were fires from 2002 to 2011. made to burn the population about every two years. As the population aged, its composition parts of the landscape or form a less flamma- and structure as fuel also matured. In the early ble fuel complex (Gill and Bradstock 1995, years following fire-stimulated regeneration, Russell-Smith 2006, Burrows et al. 2008) and subsequent fires burning in the surrounding, this is exemplified by B. quercifolia thickets. more flammable vegetation under mild spring After escaping a low intensity prescribed fire, conditions did not burn the B. quercifolia pop- an intense summer wildfire burnt and killed all ulation because of a lack of fuel. However, by plants in the population, but also triggered the the time the population was 5.7 yr old, the fuel release of seed, resulting in dense seedling re- had developed to the extent that about 61 % of generation following winter rainfall. Of the the population was burnt and killed during a total amount of seed estimated to have been low intensity prescribed fire. By this time, a released, about 3 % had germinated and sur- high proportion (~90 %) of the population had vived as seedlings when measured 18 months reached maturity, resulting in post-fire seed- after fire, which is similar to other serotinous ling regeneration. seeders in this region (Enright and Lamont The amount of rainfall in the first 12 1989, Lamont et al. 1991b). Despite subse- months after fire had a strong influence on the quent re-introductions of fire into London For- growth and maturation of post-fire regenera- est landscape at short intervals, the B. querci- tion of B. quercifolia. The juvenile period in- folia population persisted through cycles of creased significantly from 3.9 yr following the fire escape and fire-caused mortality andre- 2003 wildfire to 6.2 yr following the 2008 fire. cruitment. The extent of successful regenera- The 12-month post-fire rainfall was 19 % tion following spring and autumn fires was above annual average following the 2003 fire sufficient to ensure that the population in- and 16 % below average following the 2008 creased despite a mean landscape fire interval fire. This finding is consistent with Enright et of 1.7 yr with a landscape fire frequency of six al. (2014) who, working in shrublands in a fires per decade. By 2011, the estimated num- lower rainfall (~500 mm annum-1) region of ber of seeds in the population’s seed bank was Western Australia, found that a 20 % reduction about 14 % less than the mature population be- in post-fire winter rainfall resulted in a 50% fore the 2003 wildfire due to the increased pro- increase in the juvenile period of serotinous portion of young plants in the uneven-aged species. population. Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 36

The species was extirpated from two of the hookeriana Meisn. was able to persist even if fifteen 10 2m sample plots as a result of repeat- fires occurred at two- to three-year intervals, ed fires at short intervals burning these plots provided that the fires only burnt a small frac- before the plants had reached maturity. These tion of the population (i.e., the fires were plots burnt at short intervals because of a lo- patchy). That extirpation occurred in two of calised accumulation of flammable jarrah leaf the fifteen plots studied here also supports the litter originating from a single large tree near conclusions of Groeneveld et al. (2008) that the plots. This highlights the importance of the frequency of lethal fires at the patch level fine-scale variation in fuel continuity, quantity, needs to exceed the juvenile period of (seroti- and, hence, flammability to the survivability of nous) species if the species is to persist. fire sensitive plants. This variability in fuel Species persistence or population viability flammability and consequent fire behaviour is models that are not spatially explicit, or that most evident when fires are burning under do not take into account the spatial and tempo- mild weather, higher fuel moisture content ral variability of fire behaviour and subsequent conditions, and in recently burnt fuels, and patchiness, will result in erroneous predictions least evident or non-existent under extreme about the likelihood of plant extinctions under weather, low fuel moisture conditions, and various fire regimes. As noted by others (Jur- long unburnt fuels when virtually all vegeta- skis et al. 2003, Penman et al. 2008, Jurskis tion types are likely to burn. 2011, Oliveira et al. 2015), assuming all fires In one of a few published field studies to are lethal to fire sensitive obligate seeders is explore the persistence of fire sensitive plant largely based on observations of intense wild- species in relation to fire patchiness, Ooi et al. fires rather than low intensity fires.- Per (2006) reported that fire sensitive Leucopogon sistence or population viability models that R.Br. species in the Sydney region of Australia don’t take into account associations between were able to persist under a regime of frequent the fire sensitivity or fire resilience of plant low intensity, patchy fires. Although Leuco- species, and the level of risk of ignition and pogons, unlike B. quercifolia, are not seroti- combustion of their habitats, may result in er- nous but have a soil-stored seed bank, this roneous conclusions. finding is consistent with the findings of the In the current study, with the exception of current study. Ooi et al. also reported that low two plots that were affected by the localised intensity fires were patchier than high intensity build-up of eucalypt leaf litter, immature pop- fires and that the proportion of plants killed by ulations of B. quercifolia were not able to burn fire decreased with increasing patchiness. Al- under mild weather and fire behaviour condi- though this relationship was not quantified, it tions. This mechanism is unlikely to function is consistent with observations made in the effectively when the fuel structure of these current study. Similarly, Oliveira et al. (2015) populations, which usually occur as clumps or reported the importance of variability in fire thickets with little or no overstorey, has been severity and patchiness on the persistence of modified by the presence of an overstorey, by fire sensitive plant species in northern Austra- disease, or by weed invasion. Large, fast lia and, in the same paper, cited studies spreading wildfires burning in long unburnt demonstrating how fauna species also benefit vegetation and under severe weather condi- from patchy fires. tions may also threaten immature, isolated, Field verification that populations of fire small populations of B. quercifolia embedded sensitive taxa are able to persist under a re- in flammable fuel structures such as forests gime of frequent fire provided that the fires are and heath. patchy is also consistent with modeling by En- At relatively small spatial scales, infre- right et al. (1996) who reported that Banksia quent, intense summer fires result in even- Fire Ecology Volume 12, Issue 1, 2016 Burrows and Middleton: Fire Sensitive Plant Survival Mechanisms doi: 10.4996/fireecology.1201026 Page 37 aged populations of B. quercifolia, whereas the landscape at short intervals compared with frequent, low intensity patchy fires result in the juvenile period of the species. This regime uneven-aged populations. This is consistent resulted in relatively low fuel quantities at the with Ooi et al. (2006) who found that patchy landscape scale, which, together with moist fires created uneven-aged populations of fire fuel and mild weather, resulted in a high level sensitive Leucopogon species. The south-west of burn patchiness. Under these conditions, Australian forests have been burnt by frequent the more flammable parts of the landscape low intensity prescribed fires, as well as occa- burnt frequently at low intensity, but the less sional high intensity wildfires, for almost 60 flammable parts, including the B. quercifolia years (Burrows and McCaw 2013) and popu- population, burnt less frequently. Time to ma- lations of B. quercifolia and other fire sensitive turity in relation to the interval between fires species persist. This history, and the findings that burn and kill populations of fire sensitive of the current study demonstrate that, while species such as B. quercifolia is important for fire sensitive species may not depend on fre- assessing the persistence of populations in re- quent, low intensity patchy fire, they are able lation to fire. We have demonstrated that this to persist under such a regime. is strongly influenced by post-fire rainfall with juvenile period increasing for plants that ger- CONCLUSION minated following spring fires and that experienced below average rainfall over the next 12 Understanding the importance of patchi- months. In a drying climate, the likelihood of ness in fire behaviour associated with frequent, below average rainfall following a fire, and the low intensity fires burning under mild weather consequent increase in the maturation time of conditions has important implications for the post-fire regeneration, needs careful consider- management of plant species and communities ation by fire managers. Although the frequent that are sensitive to fire, and for the manage- introduction of low intensity patchy fires re- ment of hazardous fuels in the context of the duces the risk of burning less flammable, fire protection of human communities and other sensitive vegetation types in the landscape, assets from wildfire. We have demonstrated this could be off-set by an increase in the juve- how a fire sensitive plant species persisted un- nile period. der a regime of regular introduction of fire into

ACKNOWLEDGEMENTS

We thank the staff in the Frankland District of the Department of Parks and Wildlife (DPaW) for implementing the prescribed fires; L. Shu, Fire Management Services DPaW, who prepared the burn severity maps from satellite imagery; and I. Abbott and L. McCaw, who made helpful com- ments on an earlier draft.

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