The influence of climatic change, fire and species invasion on a Tasmanian temperate rainforest system over the past 18,000 years
Introduction
Climate change represents a significant and long-term stress on biological systems, which is exacerbated by the occurrence of extreme events, such as heatwaves, droughts and wildfires (Harris et al., 2018). For example, wildfires are predicted to increase in many regions in response to climate change (Moritz et al., 2012), compounding the ongoing effects of climate change on the growth, reproduction and recovery of plant species and potentially resulting in fire-driven ecosystem collapse (Bowman et al., 2014; Enright et al., 2015). Moreover, the spread of fast growing and highly flammable plant species, such as Eucalyptus, by people across the Earth threatens to further increase the probability of fire (Fletcher et al., 2020). Indeed, the potential of this threat has motivated a ban on Eucalypt plantations in some regions. There is, thus, an urgent need for data on the role of flammable species invasion in driving fire regime change if we are to effectively manage fire sensitive vegetation into the future.
The analysis of long-term rainforest dynamics in Tasmania, Australia, represents an ideal case study for this important problem. The rainforests in this topographically complex region persist in tiny topographic fire refugia in a landscape dominated by flammable vegetation (Wood et al., 2011). The modern dominance of the landscape by flammable vegetation was established during the Last Glacial-Interglacial Transition (LGIT) in response to landscape burning by the Palawa People of Tasmania (Fletcher and Thomas 2010), the Indigenous owners of this mountainous temperate island. Following the invasion by the British, there has been a marked increase in the occurrence of catastrophic wildfire in this landscape (Marsden-Smedley 1998; Mariani and Fletcher 2016) which has decimated huge swathes of pyrophobic rainforest (Cullen 1987; Cullen and Kirkpatrick 1988b, a; Holz et al., 2015; Holz et al., 2020), presumably due to the shift in the way fire was applied in the landscape and the removal of a sophisticated cultural burning methodology that had a net result of maintaining lower landscape fuel loads (McKemey et al., 2020).
The precariousness of topographic fire refugia in Tasmania in a landscape of increasing fire activity and in a rapidly changing climate has prompted suggestions that the extinction of these very long lived (some tree species live in excess of 2000 years) and hyper fire-sensitive Gondwanan relicts is all but a fait accompli (Rickards 2016). While recent work on high altitude rainforest communities in Tasmania demonstrates that both climate change (Mariani et al., 2019) and Eucalyptus invasion (Fletcher et al., 2020) can both facilitate localised fire-driven elimination of rainforest, little is known about the response of lowland rainforest to these pressures. Here, we use a detailed palaeoecological analysis of lake sediments from within a catchment currently occupied by both fire-sensitive temperate rainforest and Eucalyptus forest to elucidate the response of this vegetation type to changes in climate, fire and species composition over the last 18,000 years (18 kcal yrs) in southwest Tasmania, Australia.
Several factors mediate the local impact of fire on vegetation, such as macro- and microclimate, topography, species composition, the presence of fire-breaks and human intervention. Rainforest in Tasmania is largely restricted to steep south-facing slopes, where a combination of microclimate, topographic fire-breaks, low fuel flammability, and increased sub-canopy humidity form an effective barrier to the spread of fire carried readily across the landscape by flammable plant species (Wood et al., 2011; Cadd et al., 2019). Indeed, fires only occur in Tasmanian rainforest following anomalously low summer rainfall, which allows sufficient drying of fuel loads to support fire (Styger and Kirkpatrick 2015). Data from single fire events within rainforest indicates that post-fire rainforest regeneration does occur in the absence of repeated burning, while post-fire invasion by flammable plant species (Hill and Read 1984), such as Eucalyptus, after repeated fires can alter the post-fire species mix (Fletcher et al. 2014, 2020). The change in vegetation that accompanies an increase in pyrogenic species results in more flammable fuel load that can facilitate a change in fire regime, potentially leading to the localised loss of rainforest vegetation (Fletcher et al. 2014, 2018b, 2020).
While the role of species invasion in changing local ecosystem dynamics and facilitating an ecosystem shift is well recognised (Wilson and Agnew 1992), relatively little is known about the potential of highly flammable and fire-adapted Eucalyptus species to change local fire regimes and eliminate rainforest vegetation. Detailed long-term (palaeoecological) analysis of vegetation and fire in high altitude Tasmanian rainforest indicates that a replacement of rainforest by eucalypt-dominant vegetation can occur following repeated fires over multiple millennia, supporting the contention that an invasion by fire-promoting species can lead to a change in local fire regime and the localised extinction of rainforest vegetation (Fletcher et al. 2014, 2020). The Fletcher et al. (2014, 2020) studies appear to be consistent with models of the evolution of flammability that depict the destruction of competitors via self-immolation (Bond and Midgley 1995), demonstrating that fire and climate alone are insufficient to drive transitions between alternative vegetation states in the absence of flammable plants like Eucalyptus which act as ecosystem engineers (Fletcher et al., 2020).
We use high-resolution pollen and charcoal analyses to reconstruct vegetation and fire dynamics over the last 18 kcal from Lake Vera (42°16′25.03"S, 145°52′52.70"E, 570 masl) in southwest Tasmania, Australia, to service the following aims: 1- to investigate how lowland rainforest in Tasmania responded to the well-described changes in climate and fire activity over the past 18,000 years (18 kcal); and 2- to test whether Eucalyptus also acts as an ecosystem engineer in this system, as observed in higher altitude rainforest in Tasmania.
Section snippets
Study region
Tasmania (41–44° S) is a cool temperate continental island that is bisected by northwest-to-southeast trending mountain ridges which intercept mid-latitude westerly winds and create a steep orographic precipitation gradient across the island (decreasing west-to-east). The temperature regime of Tasmania’s southwest is cool (5–7 °C June to August; 14–16 °C December to February), and precipitation exceeds evaporation for most of the year (Sturman and Tapper 2006). The southwest region is underlain
Core collection and sampling
A 105.5-cm-long core (TAS1108SC1) was taken from the centremost point of the lake (49.5 m depth) using a 6-cm-diameter polycarbonate tube attached to a Universal Corer in 2011. Recovery included the mud-water interface. A Livingstone corer was used to retrieve sediments from a 12 m-deep submerged bench northeast of the short core site, 3.2 m of sediment was recovered in 1 m continuous sections (TAS1108LCA) in 2011 (Fig. 1). TAS1108SC1 was sub-sampled at 0.5-cm intervals and TAS1108LCA was
Chronology
The results of the 210Pb and radiocarbon analyses are presented in Table 1, Table 2, along with their calibrated radiocarbon age and age uncertainty. Unsupported 210Pb activity reached background at 5 cm (Beck et al., 2017). We selected the Constant Initial Concentration (CIC) modelled 210Pb ages (Appleby 2001) as the deep anoxic conditions at Lake Vera (see Beck et al., 2018) imply that sedimentation rates would be relatively linear. No age reversals were evident in the age-model
Long-term climatic change and rainforest vegetation dynamics
Our results indicate replacement of grassland by montane rainforest taxa (Cupressaceae and N. gunnii) at ca.17.8 kcal BP, synchronous with warming in Antarctica (Fig. 7) and with the expansion of rainforest taxa across the southern mid-latitudes (Vandergoes et al., 2013; Moreno et al., 2015). These trends indicate a tight coupling between high- and mid-latitude temperature change and vegetation dynamics across the Southern Hemisphere at that time. Despite evidence for increased fire activity
Conclusions
We record a shift from a grass-dominant landscape in the late Pleistocene to rainforest-dominance in this steep-sided and topographically sheltered catchment in southwest Tasmania during the Last Glacial-Interglacial Transition, a time when flammable vegetation became the dominant regional vegetation type in response to cultural burning by the Indigenous Palawa people of Tasmania. We demonstrate a highly responsive cool temperate rainforest system to climatic change in the absence of
Funding information
Australian Research Council grants DI110100019, IN140100050, IN170100062, IN170100063 (Fletcher) and DP110101950 (Bowman). AINSE award ANALGRA13529, AINSE PGRA scholarship #12039 and Allan Gilmour Science Award (Mariani).
Author statement
Michael-Shawn Fletcher: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing – original draft, Writing – review & editing, Project administration, Funding acquisition. David Bowman: Resources, Writing – review & editing. Cathy Whitlock: Resources, Writing – review & editing. Michela Mariani: Formal analysis, Writing – review & editing. Kristen Back: Formal analysis, Writing – review & editing. Laurie Stahle: Formal analysis, Writing – review & editing. Felicitas
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge that our work was conducted on Tasmanian Aboriginal lands and thank the Tasmanian Aboriginal community for their support. Research was supported by Australian Research Council grants DI110100019, IN140100050 and DP110101950, and AINSE AWARD (ANALGRA13529). Michela Mariani was also supported by an AINSE PGRA scholarship (#12039) and the John and Allan Gilmour Science Award (Faculty of Science, University of Melbourne). We thank Michael Comfort from the Department of Primary
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