Fuels and Landscape Flammability in an Australian Alpine Environment 3

1 2 Fuels and landscape flammability in an Australian alpine environment 3 4 Fraser, Imogen P.1; Williams, Richard J.2; Murphy, Brett P.3; Camac, James S.4; Vesk, Peter 5 A.1 6 1 School of BioSciences, The University of Melbourne, Parkville, Victoria, Australia. Email: 7 [email protected]. Phone: 0430 433 422. 8 2 CSIRO Tropical Ecosystems Research Centre, Winnellie, Northern Territory, Australia 9 3 Research Institute for the Environment and Livelihoods, Charles Darwin University, 10 Darwin, Northern Territory, Australia 11 4 Department of Biological Sciences, Macquarie University, New South Wales, Australia 12 13 Running head: Australian alpine fuels and landscape flammability 14 15 16 Key words: bulk density, fuel load, vegetation structure, fire severity, fire regimes 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 31 Abstract 32 Factors governing landscape-scale flammability are poorly understood, yet critical to 33 managing fire regimes. Studies of the extent and severity of the 2003 Australian alpine fires 34 revealed marked differences in flammability between major alpine plant communities, with 35 the occurrence and severity of fire greater in heathland compared to grassland. To 36 understand this spatial variation in landscape flammability we documented variation in two 37 physical properties of fuel – load and bulk density – at the life-form and plant community 38 scale. We measured load (mass per unit area) and bulk density (mass per unit volume) of fine 39 fuels (<6 mm) at 56 sites across the Bogong High Plains, southeastern Australia. Fine fuel 40 load was positively correlated with shrub cover, and fine fuel bulk density was negatively 41 correlated with shrub cover. Furthermore, fine fuel load and bulk density were accurately 42 predicted using simple measures of canopy height and shrub cover. We also conducted a 43 burning experiment on individual shrubs and snowgrass (Poa spp.) patches to assess 44 comparative differences in flammability between these life-forms. The burning experiment 45 revealed that shrubs were more flammable than snowgrass as measured by a range of 46 flammability variables. Consequently, our results indicate that treeless alpine landscapes of 47 southeastern Australia are differentially flammable due to inherent life-form differences in 48 both fine fuel load and bulk density. If shrub cover increases in these alpine landscapes, as 49 projected under climate change, then they are likely to become more flammable and may 50 experience more frequent and/or severe fires. 51 Introduction 52 53 Fire patterning across landscapes is typically heterogeneous due to variation in flammability 54 – the availability and propensity of fuel to burn (Turner 2010). Landscape flammability is 55 affected by numerous factors, including vegetation, topography and fire weather (Sullivan et 56 al. 2012), all of which vary and interact spatially and temporally at a range of scales 57 (Bradstock 2010). However differential landscape flammability may also be due to intrinsic 58 fuel properties such as fuel load (mass per unit area), chemical composition and bulk density 59 (mass per unit volume), which may vary spatially but are less subject to short- to medium- 60 term temporal variation. 61 62 Plants vary widely in their physical and chemical properties (Sullivan et al. 2012) and the 63 flammability of a vegetation community depends upon the composition, abundance and 2 64 structural array of the constituent plant species (McCarthy et al. 2001; Scarff and Westoby 65 2006). Fuel load and bulk density are fuel properties that vary spatially and are well-known 66 to be critical determinants of flammability at scales of plant parts through to entire vegetation 67 communities (e.g. Murphy and Russell-Smith 2010; Madrigal et al. 2011; Marino et al. 2011; 68 Hoffmann et al. 2012). Both fuel load and bulk density are integral components of classical 69 and enduring fire behaviour models. For example, fire-line intensity is defined as the product 70 of fuel load, rate of spread and the heat of release of the fuel (Byram 1959). Thus, as fuel 71 load increases, so too does intensity; a phenomenon that has been widely demonstrated 72 (Murphy and Russell-Smith 2010; Hoffmann et al. 2012). Rothermel’s (1972) rate of spread 73 equation explicitly includes bulk density as a term in the denominator. As bulk density 74 declines, potential rate of spread increases (Thomas 1971; Davies et al. 2006; Marino et al. 75 2012). Bulk density is inversely related to aeration of the fuel bed and determines convective 76 and radiative heat transfer rates (Fernandes and Cruz 2012). Variation in these physical fuel 77 properties is likely to drive variation in flammability, producing heterogeneous fire patterns 78 (Turner 2010). 79 80 The alpine vegetation of mainland south-eastern Australia (Williams et al. 2014) provides an 81 excellent case study of how variation in community-level flammability may affect patterns of 82 fire activity at a landscape scale. The Australian alps were burnt by extensive wildfires in the 83 decade 2000-2010. In Victoria, approximately 60% of Victoria’s 6,474 km2 Alpine National 84 Park was burnt in the fires of 2003 (Esplin 2003). Most major forest, woodland and treeless 85 vegetation types were burnt and there were clear community-level differences in fire 86 occurrence and severity across the landscape (Williams et al. 2006; Murphy et al. 2013). In 87 the alpine vegetation of the Bogong High Plains, plant community-level differences in the 88 patterns of burning were very pronounced (Williams et al. 2006). Fire occurrence was 89 greatest in closed heathland (87% of closed heathlands burnt), intermediate in open heathland 90 (59% burnt), and lowest in grassland (15% burnt); closed heathland also burnt more severely 91 than open heathland. On this basis, Williams et al. (2006) proposed that the physical fuel 92 properties of alpine plant communities and their constituent species were critical 93 determinants of the patterns of severity and extent of the 2003 fires. They concluded that 94 alpine shrubs provided the fuels for fire to propagate, and that heathlands were more 95 flammable than grasslands, not only because of the greater abundance of shrubs, but because 96 shrubs were intrinsically more flammable than snowgrass, in part due to differences in fuel 97 structure (Williams et al. 2006). 98 3 99 The fire regimes of the Australian alps are also likely to be sensitive to climate change, 100 through the combined effects of climate change on fire weather and fuels (Williams et al. 101 2014). The bioregion has shown a positive relationship between area burnt and maximum 102 temperature during fire years over the period 1975-2009 (Bradstock et al. 2014). The climate 103 of the treeless, alpine landscapes has warmed and dried since the 1970s (Wahren et al. 2013). 104 On the Bogong High Plains, in Victoria, increases in shrub cover have been documented 105 between 1936 and 1980 (McDougall 2003), and from 1979-2010 (Wahren et al. 2013). 106 Warming and drying is also expected to lead to an increase in the abundance of shrubs within 107 the alpine heathland-grassland vegetation complexes of the mainland alps (Camac et al. 108 2015). 109 110 Thus, understanding how variation in the physical properties of fuels derived from the 111 dominant alpine shrubs and grasses may affect landscape flammability is important for 112 understanding the fire ecology and management of Australian alpine environments, both at 113 present and in the future, as argued by Williams et al. (2006). We investigated how variation 114 in these community-level fuel attributes may affect landscape flammability. We quantified 115 community-level variation in fuel load and structure in the dominant grassland and heathland 116 vegetation types. We also performed a simple burning experiment on the dominant shrubs 117 and snowgrass species to test various measures of flammability. We hypothesized that (1) 118 fuel load and bulk density will vary between heathland and grassland; (2) community-level 119 fuel load will vary positively with shrub cover, but community-level bulk density will be 120 negatively related to shrub cover (Fig. 1); (3) the differences in fuel structure make shrubs 121 more flammable than snowgrass and that (4) the differences in fuel physical characteristics 122 between grassland and heathland are consistent with the observed patterns of burning 123 documented on the Bogong High Plains by Williams et al. (2006). 124 125 Methods 126 127 Field survey 128 129 Study area 130 This study was conducted across the Bogong High Plains in the Alpine National Park (37° S, 131 147° E; about 1600–1860 m altitude), northeastern Victoria, Australia (Fig. 2). Major fires 132 occurred in 1998, 2003, 2006/07. The vegetation is a mosaic of eucalypt woodland and 133 treeless communities, including grassland, open heathland, closed heathland, herbfield and 4 134 wetland (McDougall and Walsh 2007). In this study we focused on the most extensive 135 treeless communities: grassland, open heathland and closed heathland (Suppl. 1 ;McDougall 136 and Walsh 2007). Grassland (shrub cover <20%) is dominated by the snowgrass Poa 137 hiemata. Open heathland is typically dominated by the shrub Grevillea australis with Poa 138 hiemata the dominant inter-shrub species; shrub cover is 20–70% and shrub height is 20–60 139 cm (as in Williams et al. 2006). Closed heathland (shrub cover 70–100%; shrub height 50– 140 150 cm) is dominated by several tall shrub species, including Prostanthera cuneata, Orites 141 lancifolia and Bossiaea foliosa. 142 143 To quantify landscape variation in fuel load and bulk density, we surveyed fuels at 56 144 randomly selected sites that were originally surveyed by Williams et al. (2006) immediately 145 after the 2003 fires. The heathland sites were also resurveyed by Camac et al.

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