1 Estimating the time since fire of long-unburnt Eucalyptus salubris (Myrtaceae) stands in 2 the Great Western Woodlands 3 4 Carl R. Gosper A,B,C , Suzanne M. Prober B, Colin J. Yates A and Georg Wiehl B 5 6 AScience Division, Department of Environment and Conservation, Locked Bag 104, Bentley 7 Delivery Centre, WA 6983, Australia. 8 BCSIRO Ecosystem Sciences, Private Bag 5, Wembley WA 6913 Australia 9 CCorresponding author. Email: [email protected] 10 11 This is a pre-publication version. The definitive version of the paper has been published 12 in the Australian Journal of Botany on the CSIRO PUBLISHING website. The definitive 13 version can be found here: 14 Gosper, C.R., Prober, S.M., Yates, C.J. and Wiehl, G. (in press) Estimating the time since fire 15 of long-unburnt Eucalyptus salubris stands in the Great Western Woodlands. Australian 16 Journal of Botany doi: 10.1071/BT12212 17 http://www.publish.csiro.au/paper/BT12212.htm 18 19 1 For the definitive version of this paper, go to: http://www.publish.csiro.au/paper/BT12212.htm 20 Abstract. Establishing the time since fire in infrequently burnt, yet fire-prone, 21 communities is a significant challenge. Until this can be resolved for >50 year timeframes, 22 our capacity to understand important ecological processes, such as the periods required for 23 development of habitat features, will remain limited. We characterised the relationship 24 between observable tree growth rings, plant age and plant size in Eucalyptus salubris F. 25 Muell. in the globally significant Great Western Woodlands in south-western Australia. In the 26 context of recent concerns regarding high woodland fire occurrence, we then used this 27 approach to estimate the age of long-unburnt E. salubris stands, and the age-class distribution 28 of Eucalyptus woodlands across the region. Time since fire was strongly predicted by trunk 29 growth rings and plant size predicted growth rings with reasonable accuracy. The best model 30 estimating growth rings contained parameters for trunk diameter, plant height and plot 31 location, although simple models including either trunk diameter or plant height were nearly 32 as good. Using growth ring-size relationships to date long-unburnt stands represents a 33 significant advance over the current approach based on satellite imagery, which substantially 34 truncates post-fire age. However, there was significant uncertainty over the best model form 35 for estimating the time since fire of stands last burnt over 200 years ago. The management 36 implications of predicted age-class distributions were highly dependent on both the choice of 37 what, if any, transformation was applied to growth rings, and the theoretical age-class 38 distribution to which the actual age-class distribution was compared. 39 40 2 For the definitive version of this paper, go to: http://www.publish.csiro.au/paper/BT12212.htm 41 Introduction 42 Establishing the time since disturbance is a significant challenge in investigations of temporal 43 changes in ecosystem function and composition in infrequently-disturbed communities. Fire 44 is a common disturbance affecting vegetation dynamics across much of the world (Bond et al. 45 2005; Verdú and Pausas 2007). Individual fire events can have effects lasting for centuries 46 (Wood et al. 2010), but it is usually difficult to date fires that occurred prior to those 47 documented in contemporary sources, such as long-term historical records or remotely-sensed 48 imagery (Clarke et al. 2010). This is not a trivial problem, as many ecological processes 49 operate over long time scales (Mackowski 1984; Clarke et al. 2010), and space-for-time 50 studies based on estimated times since fire offer one of the few empirical approaches to 51 understanding these processes. 52 Researchers are thus faced with the problem of deriving a time since fire for vegetation 53 that has not been burnt since the earliest available historical records (such vegetation is often 54 referred to as ‘long-unburnt’). Historical records may include remotely-sensed satellite 55 imagery that begins in Australia in 1972, aerial photography that begins in Western Australia 56 in 1948, or written or verbal sources. One approach to deriving a time since fire for long- 57 unburnt stands is to allocate a uniform time to all, either the absolute minimum (e.g. Parsons 58 and Gosper 2011), or an estimated minimum based on the rate at which fire scars of known 59 age become less visible over successive satellite images and assuming similar rates of 60 vegetation recovery applied previously (e.g. Gosper et al. 2012). In both approaches, the time 61 since fire allocated to long-unburnt stands is likely to be substantially truncated compared to 62 their actual time since fire (Clarke et al. 2010). 63 Alternatively, efforts can be made to estimate the actual time since fire of long-unburnt 64 vegetation through dendrochronology or allometric relationships, carbon dating, and other 65 forms of evidence such as charcoal in lake, marine or bog deposits (Conedera et al. 2009; 3 For the definitive version of this paper, go to: http://www.publish.csiro.au/paper/BT12212.htm 66 Wood et al. 2010). Dendrochronology is one of the few methods of fire regime reconstruction 67 with the appropriate spatial and temporal resolution to be useful in linking with plot-based 68 chronosequence studies. Applications of dendrochronology to dating fire events have usually 69 concerned plant species in which fire scars are overgrown by living tissue (Burrows et al 70 1995; Conedera et al. 2009), allowing the reconstruction of multiple fire events. This 71 functional response to fire is rare among some communities, such as in mallee and woodland 72 in the global biodiversity hotspot of southern Western Australia. On the other hand, obligate 73 seeding plant species are abundant in some communities and offer potential for determining 74 time since the last fire (Conedera et al. 2009; O’Donnell et al. 2010). Such species would 75 need to meet a number of criteria to be suitable. These include high longevity relative to the 76 frequency of stand-replacing fires; consistent mortality after fire; rapid recruitment after fire; 77 and negligible inter-fire recruitment. Potentially suitable species in Australia include Callitris 78 (O’Donnell et al. 2010), some Allocasuarina (Burley et al. 2007) and some Eucalyptus (Rose 79 1993; Wood et al. 2010). 80 Allometric relationships between measures of plant size and age (derived from remotely- 81 sensed imagery, historical record or dendrochronology) may also allow for the ageing of 82 plants or plant parts that cannot be dated by other means (Koch et al. 2008). This has been 83 applied in Australia, with variable success, to mallee, woodland, forest and subalpine 84 Eucalyptus (Rose 1993; Schulze et al. 2006; Koch et al. 2008; Rumpff et al. 2009; Clarke et 85 al. 2010), open-forest Allocasuarina (Burley et al. 2007) and semi-arid Callitris (O’Donnell 86 et al. 2010). All of these studies found a significant relationship between trunk diameter and 87 age, although the accuracy of age predictions can decline as the trunks become older (Rumpff 88 et al. 2009) and age estimates can be less accurate than those based on dendrochronology 89 (Koch et al. 2008). The choice of any transformation applied to age is crucial: age can be 4 For the definitive version of this paper, go to: http://www.publish.csiro.au/paper/BT12212.htm 90 underestimated where untransformed, but age is also highly susceptible to minor variation in 91 plant size if square- root or log transformations of age are used (Clarke et al. 2010). 92 Eucalyptus salubris F. Muell. (gimlet) is a thin-barked tree that is killed by complete 93 canopy scorch. It is widespread across dry and semi-arid Mediterranean-climate south- 94 western Australia, including the 16M ha region known as the Great Western Woodlands 95 (GWW). The woodlands of this region are typically fire sensitive, and are at risk from 96 inappropriate fire regimes in a potentially warming and drying climate (Prober et al. 2012). 97 Uncertainty concerning the time since fire of long-unburnt woodlands, and hence the scale 98 over which temporal changes in woodland dynamics occur, currently constrains 99 understanding as to whether the relatively frequent recent incidence of large wildfires (DEC 100 2010; Parsons and Gosper 2011) represents a significant departure from the historical fire 101 regime or a long-term threat to mature woodland ecosystems. Towards a better understanding 102 of the fire ecology of GWW woodlands, we aimed to characterise the relationship between 103 observable tree rings, plant age and plant size in E. salubris . We then tested the viability of 104 this approach for estimating the time since fire of long-unburnt E. salubris stands and the time 105 since fire age class distribution of Eucalyptus woodlands more broadly. 106 107 Materials and Methods 108 Survey plots 109 The study was undertaken in E. salubris woodlands in three districts along the western edge 110 of the GWW, south-western Australia: Karroun Hill (30˚14 ′S, 118˚30 ′E); Yellowdine 111 (31˚17 ′S, 119˚39 ′E) and Parker Range (31˚47′S, 119˚37′E) (Fig. 1). These areas have a semi- 112 arid Mediterranean climate, with mean annual rainfall in Coolgardie and Merredin (the 113 nearest long-term weather stations) of 270.5 and 325.9 mm respectively, with the highest 114 mean rainfall months in winter. Mean monthly daily temperature maxima range from 16.1 to 5 For the definitive version of this paper, go to: http://www.publish.csiro.au/paper/BT12212.htm 115 33.3˚C (Coolgardie) and 16.2 to 33.8˚C (Merredin), and mean monthly minima from 5.2 to 116 17.0˚C (Coolgardie) and 5.4 to 18.0˚C (Merredin) (Bureau of Meteorology 2012).