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 yralmost half the ju- fue de 1,7 añoscasi la mitad del período juve- venile period of the speciesand the nil de la especiey 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.

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