Temperate wet (humid) forests of Australia are floristically and climatically distinct from those of the northern hemisphere (Ovington 1983). These forests were once widespread across Gondwana. Due to the aridification of the Australian continent this biome has contracted to refugia, concentrated in southeastern Australia (White 1986). The low nutrient status of Australia’s soils resulted in the evolution of a
scleromorphic flora that was advantaged further by increasing aridity (Loveless 1961, 1962). Increasing fire frequencies associated with drier climates and the arrival of people also contributed to the expansion and radiation of this flora, especially the genus Eucalyptus sensu lato (Jackson 1999a). It is the mix of sclerophyllous and orthophyllous plants that distinguish Australia’s temperate wet forests from forests elsewhere in the Southern Hemisphere and may make them sensitive to changes in disturbance and climatic regimes (Ovington and Pryor 1983; Mackey et al. 2002; Balmer et al. 2004; Byrne et al. 2011).
Kirkpatrick et al. (1988) define wet eucalypt forest as forests dominated by
Eucalyptus species over an understorey "dominated, either singly or in a mixture, by rainforest trees (sensu Jarman and Brown 1983), broad-leaved shrubs, or ferns (from
Chapter 1 – Introduction which [they] exclude bracken and resurrection plants such as Cheilanthes)".
Incorporated within this definition are the mixed forests (sensu Gilbert 1959), which comprise eucalypt forest with an understorey of rainforest vegetation.
The most widely accepted definition of rainforest in Tasmania is that of Jarman and Brown (1983) who defined it "as forest vegetation (trees greater than 8 m) dominated by species of Nothofagus, Eucryphia, Atherosperma, Athrotaxis, Lagarostrobos, Phyllocladus or Diselma." They qualified their description by embracing within their definition, vegetation dominated by species such as Anodopetalum, which are usually understorey species but occasionally achieve greater importance. However, their definition limited rainforest to communities occurring in cool moist environments that were dominated by plant members of the relict sub-element of Gondwanic flora. Thus it excluded the possibility of classifying vegetation dominated by Eucalyptus species as rainforest.
The wet eucalypt forests represent part of the same ecological sere as cool temperate rainforest, occurring in a mosaic with rainforest, scrub and non-forest communities within regions where rainfall exceeds 1000 mm per annum and 50 mm monthly rainfall (Ashton 1981c; Jackson 1999b). The wet eucalypt forests are associated with fire intervals of between 25 to 350 years (Jackson 1968; Wood et al. 2010). Fire in this forest kills many individual plants, clears much of the above ground biomass, exposes mineral soil, creates a nutrient rich ash-bed, while the associated smoke and heat trigger the germination of many light-demanding species (Ashton 1981b; Bell 1999). Vegetative resprouting and seedling regeneration rapidly follow fire, and canopy closure is usually complete within the first two to three decades (Ashton 1975a; Serong and Lill 2008).
Various models have been developed to describe the successional dynamics of vegetation communities that are relevant to the response of wet eucalypt forest communities to disturbance (e.g. Clements 1936; Egler 1954; Holling 1973; Noble and Slatyer 1980; Wilson and Agnew 1992). The theory of ‘initial floristic
composition’ proposed by Egler (1954) is of particular relevance since many of the species dominant in later successional stages of the community are present in the vegetation immediately post-disturbance, having re-established as biological legacies
Chapter 1 – Introduction (seeds or sprouts) from the pre-disturbance vegetation (Purdie and Slatyer 1976; Connell and Slatyer 1977; Noble and Slatyer 1980, 1981). Nevertheless the 'relay floristic model' proposed by Clements (1936) may explain the absence of some species from one or more of the successional stages – young, intermediate or old growth – and the numerous species which differ markedly in abundance depending on time since disturbance (Hickey 1994; Turner et al. 2011).
For the purposes of this thesis, species are grouped into classes of early and late- successional plants (Huston and Smith 1987). The allocation is based here on the recorded prevalence of these species in young and older forest communities, rather than by how quickly they colonize a site after fire. The term ‘pioneer species’ is applied to those that are most abundant in regrowth forests less than 50 years since fire, most of which are known to require large gaps to germinate and grow to
maturity. In contrast, the term ‘mature forest species’ is applied to those species that are more abundant in wet forest understoreys that have not been disturbed for more than 70 years. Most mature forests species are capable of occupying climax rainforest and are able in some situations (e.g. canopy gaps) to regenerate in these forests in the absence of catastrophic disturbance (Huston and Smith 1987).
The term rainforest species is distinguished from the term ‘mature forest species’ by comprising those species regularly represented within Tasmanian rainforest
vegetation (sensu Jarman et al. 1983), and not simply adventitious within it. Jarman et al. (1983) defined rainforest as those species which occur within rainforest that are able to regenerate either vegetatively or from seed without the need for catastrophic disturbance. The rainforest species listed by Jarman et al. (1984) include at least a few among their number that are more abundant in regrowth eucalypt forest compared with mature eucalypt forest within the study region, and which are therefore classed as pioneer species for the purpose of this thesis (e.g. Monotoca glauca).
Tng et al. (2012) put the case that Eucalyptus regnans may be better classified as a fire adapted rainforest pioneer. However, traditionally this species has been placed along with other members of the genus Eucalyptussensu lato as a member of the Australian autochthonous flora and not usually classified as a rainforest species. For the purposes of this thesis, E. regnans is considered as a pioneer species and not a
Chapter 1 – Introduction rainforest species. However, in general the analyses focus not on the eucalypts but the associated understorey species. Furthermore, rather than accepting the proposal of Lynch and Nelder (2000) to classify mixed forest as a separate vegetation community and treat it as transitional between wet eucalypt forest and rainforest, this thesis examines wet eucalypt forest as a continuum with mixed forest located on the mature forest end of the spectrum and young recently regenerated stands representing the early successional stage within the fire sere. For further discussion on the proposal by Tng et al. (2012) see Appendix 1.1 in Volume B.
Jackson (1968) developed a comprehensive model, termed 'ecological drift', for western Tasmania. This model emphasized the importance of feedbacks between the climate, topography, soil, vegetation and fire regime, such that steady states in the vegetation develop for particular combinations of factors (Wood et al. 2011; Wood and Bowman 2012). Vegetation structure and relative abundance of pioneer and rainforest plants is strongly determined by disturbance regime (Jackson 1968; Brown and Podger 1982a; Podger et al. 1988). Areas frequently or recently burned usually have an abundance of pioneer species with high light demands for establishment. The life-span of many of these pioneers are short, mostly one century or less, although individual canopy eucalypts may survive for up to 500 years (Wood et al. 2010). The abundance of pioneer species is reduced over time in the wet eucalypt forest
understorey as more shade-tolerant and slower growing rainforest species (sensu
Jarman and Brown 1983) increase in importance, eventually forming mixed forest (Gilbert 1959, Jackson 1968). Without disturbance, the eventual death of the canopy eucalypts leads to the development of cool temperate rainforest (Gilbert 1959, Jackson 1968). In fire-prone landscapes, rainforest is rare and the retention of the rainforest flora may depend on their capacity to colonize and persist in secondary wet eucalypt forests. Jackson's (1968) model is particularly relevant for explaining the distribution of vegetation types across a region and over millennia. Ashton (1981) incorporates Jackson's (1968) model in his review of the dynamics of Australia's tall wet eucalypt forests over a time-scale of decades and centuries.
There is evidence that both underlying environmental variation and the vegetation itself provides feedbacks that may maintain relatively stable boundaries between
Chapter 1 – Introduction Knox and Clarke 2012). Such relative stability has been observed between the forest and non-forest of infertile areas of Tasmania (Brown and Podger 1982a; Brown and Podger 1982b; Bowman et al. 1986; Balmer 1990; Brown et al. 2002; Marsden- Smedley et al. 2010; Wood et al. 2011; Wood and Bowman 2012). The stability between the forests and non-forest in this Tasmanian region is also supported by palynological studies across millennia (Fletcher and Thomas 2007a, b; Fletcher and Thomas 2010). However, palynological and soil profile evidence also demonstrate that major vegetation shifts have occurred in some areas providing strong evidence for Jackson’s alternative stable states model (Podger et al. 1988; di Folco and Kirkpatrick 2013; Fletcher et al. 2014), and against the model proposed by Mount (1979). What remains unclear is whether there is an equivalent level of boundary stability between rainforest or mixed forest vegetation and that of wet eucalypt forest with understoreys dominated by mesophytic and sclerophyllous plants. In a study of the boundaries between warm temperate rainforest and eucalypt forest in northern New South Wales Knox and Clarke (2012) found that the boundary did not move through time, not even after severe fire. They also noted that following fire the plant communities each recovered to those resembling pre-disturbance assemblages, many species re-
establishing by resprouting. Therefore evidence is mounting that disturbance patterns on their own may be insufficient to explain the stability of boundaries (Hoffmann et al. 2009; Knox and Clarke 2012; Fletcher et al. 2014).
Tng et al. (2013) sampled representative species from wet eucalypt forests, rainforest and savannah woodlands from both cool temperate Tasmania and tropical Queensland and found that data for plant traits from these species demonstrated that the wet eucalypt forests were both ecologically and functionally convergent between climatic zones and with rainforest. They argued that within the framework of alternative stable states the wet eucalypt forests occur "within the basin of attraction of rainforest" (Tng et al. 2013). If this is the case then the species representing either end of the
continuum within the rainforest basin maybe more vulnerable to changes in
disturbance frequency because there may be fewer feedbacks to ensure the continued occupancy of any particular part of the sere. However, in an apparent contradiction to the view point of Tng et al. (2013), both Wood and Bowman (2012) and Fletcher et al. (2014) posit that weaker feedbacks may exist which serve to hold the distribution
Chapter 1 – Introduction of wet eucalypt forests in a relatively stable state, separate to that of rainforest. The latter two studies described the dynamics within oligotrophic systems, while Tng et al. (2013) applied their hypothesis to the ecological domain of giant trees, which are not located within oligotrophic systems.
Traits such as breeding systems and dispersal distances as well as resprouting capacity are likely to influence the long-term viability of species within a fragmented
landscape (McKinney 1997; Clark et al. 1999; Murray et al. 2002; Broadhurst and Young 2007; Clarke et al. 2013). Many primitive and endemic taxa are maladapted to fire and drought or have poor dispersal mechanisms but may resprout following low intensity fire (Barker 1991). These traits may make them vulnerable to population decline in response to changes in disturbance regime (frequency and intensity of logging and/or fire), habitat fragmentation and climate change (Hickey 1982; Brown et al. 1988; Barker 1992; Hickey 1994; Tng et al. 2013). Tasmanian Angiospermae and Gymnospermae species with very restricted dispersal mechanisms and which are killed by single fire events (e.g., Athrotaxis species) have shown substantial
population declines since European settlement (Brown 1988; Cullen and Kirkpatrick 1988; Robertson and Duncan 1991). In contrast, mature eucalypts, especially thick- barked Eucalyptus obliqua, often survive fire and resprout from epicormic buds (Hickey et al. 1999; Turner et al. 2009). E. regnans has a higher mortality rate than
E. obliqua and may be excluded if fire frequency is high (Turner et al. 2009). Ashton (1981b) provides a review of many traits which assist eucalypts and other pioneer species re-establish after fire, including seed that either survives fire in woody capsules or the soil-stored seed-bank, or which has adaptations to long-distance dispersal (e.g. the plumose seeds of Asteraceae, which are dispersed by wind).
Tasmanian wet eucalypt forests are of international conservation concern in their own right. These forests, and the present study area, include: the tallest flowering tree known on earth; immense carbon stores; and provide habitat for primitive and endemic plant species (Jarman and Brown 1983; Taylor et al. 1993; Balmer et al. 2004; Tng et al. 2012; Tng et al. 2014). Mature eucalypt trees are also critical for the perpetuation of hollow dependent fauna (Lindenmayer and Wood 2010). Recent genetic studies show the importance of Tasmanian wet forest and rainforest for
Chapter 1 – Introduction forests during the Pleistocene in glacial refugia such as southeastern Tasmania and parts of the present study area (Kirkpatrick and Fowler 1998; McKinnon et al. 2004), suggests there was an opportunity for regional differentiation in genetic variation. Distinctive genetic and morphological variation between northwestern and
southeastern populations of several rainforest species have already been described (Barnes et al. 2000; Clark 2006). Worth et al. (2009) reports that Western Tasmania has the highest genetic diversity for Nothofagus cunninghamii, while Nevill et al. (2010) show similar distinctiveness in diversity at the state and regional level for
Eucalyptus regnans.
Recent modelling has predicted climatically suitable habitat for Nothofagus cunninghamii will reduce by more than 90% in Victoria by 2070, making it and associated rainforest and wet forest species more dependent on Tasmania for their long term perpetuation (Worth et al. 2015).