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Wet Eucalypt and Woodland Vegetation in Tasmania

Wet eucalypt forests and woodlands are an important feature of the Tasmania landscape and are valued biologically, aesthetically and economically. There are 18 community types classified in Tasmania ^[1] which occupy 993 600ha ^[2]. They are differentiated according to descriptions of canopy and understorey layers ^[3], which vary considerably in structure and floristics. They are defined as being dominated by eucalypts and having an understorey of broad leaved shrubs, rainforest species or ferns. The dominants provide 30 – 70% foliage cover and the dense understorey prevents the continual regeneration of shade intolerant species, including eucalypts ^[4].

These communities occupy a diverse range of niches but have a requirement for high, reliable rainfall exceeding 1000mm per annum with at least 25mm in the driest month and where low temperatures are not limiting to tree growth ^[5]. They occur on soils with good drainage and adequate to moderate fertility from sea level to 1200m and the steep gradients in Tasmania’s topography and geology create a mosaic distribution across the state with the highest occurrence in the south west.

Of particular importance is the often termed ‘mixed’ or ‘old growth’ forests, containing Eucalyptus regnans, the world’s tallest flowering plant, capable of reaching heights greater than 110m. These communities, a transition to the climax rainforest vegetation type, comprise 20% of Tasmania’s eucalypt forests. This dominant often occurs with a cool temperate rainforest understorey and have their greatest extent and diversity in Tasmania ^[6]. They are valued economically for wood production and a source of eucalypt and rainforest species veneer and sawlog^[7].

The Floristics of Wet Eucalypt Forests and Woodlands

Wet eucalypt forests can vary in structure from tall forests exceeding 90m in height with closed rainforest understoreys to short forests in the subalpine zone with open montane rainforest understoreys. They have a distinctive layered structure with variation in both canopy and understorey species ^[8] which are only weakly correlated, being controlled independently by different tolerances and requirements ^[9].

The canopy of wet forest may be dominated by a range of eucalypts. Most swamp gum (Eucalyptus regnans), yellow gum (Eucalyptus johnstonii), and yellow alpine gum (Eucalyptus subcrenulata) forests are wet forests. A number of other eucalypt forests may also be wet forest, including white gum (Eucalyptus viminalis), blue gum (Eucalyptus globulus), stringybark (Eucalyptus obliqua), gum-topped stringybark (Eucalyptus delegatensis), Smithton peppermint (Eucalyptus nitida), and snow gum (Eucalyptus coccifera) ^[10]. A sub stratum may be present as secondary trees such as silver wattle (Acacia dealbata) and Australian blackwood (Acacia melanoxylon). Beneath this is typically an understorey of broad-leaved tall shrubs and small trees such as dogwood (Pomaderris apetala), musk (Olearia argophylla) and blanket leaf (Bedfordia salicina). Mixed forest can have a ground layer in which ferns, excluding bracken, are dominant, or an understorey dominated by temperate rainforest trees, such as myrtle beech (Nothofagus cunninghamii), sassafras (Atherosperma moschatum), and celerytop pine (Phyllocladus aspleniifolius) ^[11]. A ground layer of ferns form the third layer, intermixed with a variety of sedges (including Lepidosperma elatus and Gahnia grandis), herbs and climbers (Billardiera longiflora and Clematis aristata), bryophytes and lichens. ‘Mixed’ forests have many similarities with rainforests both in their vascular and non-vascular flora and in many cases mixed forest understoreys ^[12].

Ecology

The eucalypt species that occupy wet forest vegetation have evolved to have preferences for specific environmental conditions and have strategies to modify their environments by promoting fire. Climate and fire frequency and their interaction with soils have a major role determining the distribution of Eucalyptus ^[13]. The Tasmanian environment The heterogeneity of the Tasmanian environment influences the ecology and distribution of vegetation. A small island at latitude of 40ᵒ to 43ᵒ30’S the climate is more maritime than its Northern hemisphere counterparts and the influence of aspect is more extreme ^[14]. The ancient geomorphologic and glacial history of the island has given rise to a rugged topography and complex geologies. Steep altitudinal gradients are present and differences in geology are found between the west and east. The mountainous regions on the west intercept the Roaring Forties, a warm airstream, creating a marked precipitation gradient from west to east. Complex gradients in multiple abiotic factors create many microclimates and hence mosaic distributions of community and vegetation types, which may be divided by sharp or gradual boundaries ^[15].

Eucalyptus Biology

Habitat preference

Eucalyptus species have evolved specific habitat preferences and replacement series exist across different environmental gradients, creating sharp and gradual changes in vegetation and community types. Wet eucalypt forests develop on sites where water is readily available and where extremes of temperatures do not occur. Replacement series exist within the subgenus for exampleE. regnans or E. obliqua are replace by E. delegatensis with increasing altitude and E. obliqua replaces E. regnans on slightly drier sites with higher fire frequency. Increases in altitude above 700m and a decrease in temperature or increase in rainfall, the micro-environmental effects of soil type, aspect, drainage, frost and water logging become more important in determining distribution ^[16].

Fire

Fire is an integral component in the ecology of the Tasmanian vegetation. It is required for the persistence of wet eucalypt forests and woodlands which are a disclimax vegetation. Eucalypts have evolved with fire and the genus has adaptions to both promote and survive fire incidence. These include volatile oils in leaves, prolific bark production leading to high fuel accumulation, growth from epicormic buds and vegetative growth from lignotubers (excluding E. regnans and E. delegatensis) and crown stored seed capsules ^[17].

Succession

Wet eucalypt forests are adapted to fire intervals between 20 and 350 years and the frequency of fires ^[18] and intensity is a defining influence in forests, with different types of fire affecting different parts of the forest ^[19]. This phenomenon is induced by the complex Tasmanian landscape in which complex topographies and terrain characteristics have a fundamental influence on the behavior of the fire ^[20], which affects the structure and floristics of the forest.

Infrequent high intensity fires resulting in a total reduction of the forest to an ash bed, followed by mass seedbed germination, is a regime to which many wet forest types are adapted ^[21]. For example Wood et al. (2010) showed that E. regnans stands in the Styx Valley are comprised of an even aged cohort as a result of a severe disturbance over 500 years ago. Following intense fire, mature trees experience high mortality ^[22] because they not have the ability to resist fire damage and or recover by epicormic and lignotuberous shoots (Ashton 1981). Crown help capsules release seeds onto a nutrient rich, competition free environment. Seedling regeneration is intense, thinning is rapid ^[23] and after 300 years there may be as few as two trees per ha¯¹ ^[24]. These types of forest reach their tallest in the south-west, where in the absence of disturbance, species would be replaced by rainforest. This type of forest however appears to be the exception, rather than the majority ^[25].

Low fire intensity or low residence time is likely to be responsible for the creation of a multi-cohort forest (Turner et al 2009) and may result in simplification of the understorey ^[26] and asynchrony in understorey and over story elements ^[27]. Turner et al. (2009) found that for pure E. regnans stands with trees assumed to be >110 years old, only 15% were comprised of a single cohort, and inferred that the average time since stand-replacing fire for these stands was approximately 230 years. Other studies in south west Tasmania of wet E. obliqua forest confirm a varied fire history and multi-aged structure with up to seven different cohorts in a single stand ^[28]. These structural features are important attributes that assist in the maintenance of biodiversity ^[29].

Fires also affect the vegetation through indirect feedbacks with both vegetation ^[30] and topography ^[31]. Frequent fires cause losses in soil nutrients that lead to characteristic vegetation types ^[32]. These vegetation types affect the intensity and frequency of fires leading to mosaics of fire prone and fire sensitive vegetation ^[33]. Ashton (1981) observed that fires occurring in old forests of E. regnans are less likely to be a complete stand-replacing event than fires occurring in young forests. The interaction between fire occurrence and a range of topographic variables suggests that topography plays an important role ^[34].

Forestry

Forest harvesting is a major use of wet forest communities ^[35]. The most heavily cleared and modified wet forest communities are swamp gum (E. regnans) forest and blue gum (E. globulus) forest ^[36]. A prescribed practice in lowland wet forests is clear fell, burn and sow ^[37] and 56 000 hectares were cleared between 1996 and 2003 ^[38]. There is also a major timber industry based on native forest outside reserves, and plantations of eucalypt on public and private land ^[39]. While one-third of the mixed forest is formally reserved, much of the remainder is subject to logging. The latter community has lost a substantial part of its range in Tasmania large areas have been modified by logging and considerable areas will be lost to plantations in the future ^[40].

Lowland mixed forest is maintained by infrequent wildfires at intervals of 100-350 years (Jackson 1968), which allow the re-establishment of eucalypt and wet sclerophyll understorey species. If fires occur at intervals exceeding 400 years the eucalypts die without being replaced and the forest becomes rainforest. Jackson (1968) indicates that fires which occur at intervals less than 100 years will eliminate rainforest species and result in a wet sclerophyll forest. These predicted effects have led to concern from conservationists, special timber users, apiarists as well as scientists that many of the values of mixed forests, particularly those associated with rainforest species, were being compromised by their management for eucalypt sawlogs on rotations of 80-100 years ^[41].

A long-term ecological research (LTER) site with links to existing networks of national and international LTER sites, was established at Warra in the Southern Forests of Tasmania in 1995. The 15 900 ha site is comprised mostly of previously forested wet Eucalyptus obliqua forests. It is a multi disciplinary research initiative with the goal of achieving sustainable forest management through a better understanding of fundamental ecological processes. Included in their research are studies of biodiversity and investigating alternatives to clearfell, burn and sow techniques. Congruence between natural and human made disturbance may minimise the effects of logging on biodiversity ^[42].

^ (Harris & Kitchner 2005)
^ (Forestry Commission 1990)
^ (Harris & Kitchner 2005)
^ (Reid et a.l 2005)
^ (Reid et al. 2005)
^ (Hickey 1994)
^ s and ecologically as a refuge for rainforest species (Hickey 1994) and a habitat for endangered are rare animals (Lindenmayer 2000)
^ (Reid et al. 2005)
^ (Ogden & Powell 1979)
^ (DPIPWE)
^ (DPIPWE)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Reid et al. 2005)
^ (Wells 1991)
^ (Wood et al 2011b)
^ (Wells 1991)
^ (Turner et al. 2009)
^ (Ashton 1981)
^ (Jackson 1968)
^ (Lindenmayer 2000)
^ (Wells 1991)
^ (Lindenmayer 2000)
^ (Alcorn 2001)
^ (Lindenmayer & McCarthy 2002)
^ (Wood 2011a)
^ (Wood 2011b)
^ (Wood 2011a)
^ (Wood 2011a)
^ (Wood et al. 2011b)
^ (Hickey 1994)
^ (DPIPWE)
^ (Hickey et al. 2001)
^ (Forest Practices Board Annual Report 2002-2003)
^ (Brown et al. 2001)
^ (DPIPWE)
^ (Hickey 1994)
^ (Lindenmayer & McCarthy 2002)

References

Alcorn, P.J., Dingle, J.K. and Hickey J.E. Age and stand structure in a multiaged wet eucalypt forest at the Warra silvicultural systems trial. Tasforests. 13: 45 - 259

Ashton, D.H. (1981) Fire in tall open forests (wet sclerophyll). In ‘Fire and the Australian Biota.’ (Eds A.M. Gill, R.H. Groves and I.R. Noble.) pp 339 – 366. (Australian Academy of Science: Canberra)

Brown, M.J., Elliott, H.J. and Hickey, J.E. (2001) An overview of the Warra Long-Term Ecological Research Site. Tasforests 13: 1 - 8

Forest Practices Board Annual Report (2002-2003) Hobart. Tasmania

Harris, S. and Kitchener, A (2005). From Forest to Fjaeldmark: Descriptions of Tasmania's Vegetation. Department of Primary Industries, Parks, Water and Environment, Printing Authority of Tasmania. Hobart.

Hickey, J.E. (1994) A Floristic Comparison of Vascular Species in Tasmanian Oldgrowth Mixed Forest with Regeneration Resulting from Logging and Wildfire. Aust. J. Bot 42: 383-404

Hickey, J., Neyland, M.G, and Bassett, O.D. (2001) Rationale and Design for the Warra Silvicultural Systems Trial in Wet Eucalyptus obliqua Forests in Tasmania. Tasforests. 13(2). Forestry Tasmania.

Jackson, W.D. (1968) Fire, air, water and earth—an elemental ecology of Tasmania. Proc Ecol Soc Aust 3:9–16

Lindenmayer, D.B., Cunninghamb, C.F., Donnelly, J.F. (2000) Structural features of old-growth Australian montane ash forests. Franklin Forest Ecology and Management 134: 189 – 204 Lindenmayer, D.B. and McCarthy, M.A. (2002) Congruence between natural and human forest disturbance: a case study from Australian montane ash forests Forest Ecology and Management 155: 319–335

Ogden, J. and Powell, J.A. (1979) Auquantitative description of the forest vegetation on an altitudinal gradient in the Mt Field National Park Tasmania, and a discussion of its history and dynamics. Australian Journal of Ecology 4: 293 - 325 Reid, J.B., Hill, R.S., Brown, M.J. and Hovenden, M.J. (eds) (2005). Vegetation of Tasmania. Flora of Australia supplementary series number 8. Australian Biological Resources Study, Environment Australia, Department of Environment and Heritage, Canberra. Turner, P.A.M., Balmer, J., Kirkpatrick, J.B., (2009) Stand-replacing wildfires? The incidence of multi-cohort and single-cohort Eucalyptus regnans and E. obliqua forests in southern Tasmania. Forest Ecology and Management 258, 366– 375. Wells, P.M. (1991) Wet Forests. In: Tasmanian Native Bush; A Management Handbook (Ed Kirkpatrick, J.B.), pp. 35–53, Tasmanian Environment Centre Inc., Hobart.

Wood, S.W., Huab, Q., Allena. I., Bowman, D.M. (2010) Age and growth of a fire prone Tasmanian temperate old-growth forest stand dominated by Eucalyptus regnans, the world’s tallest angiosperm. Forest Ecology and Management 43: 8 - 447

Wood, S.W, A,C, Q. Hua, A.C.Q, Bowman, M.S.J. (2011a) A Fire-patterned vegetation and the development of organic soils in the lowland vegetation mosaics of south-west Tasmania Australian Journal of Botany 59, 126–136

Wood, S.W., Murphy, B.P., Bowman, D.M. (2011b) Firescape ecology: how topography determines the contrasting distribution of fire and rain forest in the south west of the Tasmanian Wilderness World Heritage Area. J Biogeogr. 38:1807–1820

http://www.dpipwe.tas.gov.au/inter.nsf/WebPages/SSKA-6G845E?