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Gymnosperms of Alpine Tasmania
[edit]Gymnosperms are an ancient plant group originating in the carboniferous period approximately 300 million years ago [1]. During this period Gymnosperms dominated up until the evolution of angiosperms (flowering plants) 125 million years ago. In some areas of the northern hemisphere conifers still dominate, forming extensive forests, not so however in the southern hemisphere where their range has become restricted to smaller areas. The Western region of Tasmania has become a refugia for conifers, supporting a relatively large diversity of plants, a large proportion of which are endemic. Their occurrence gives us an indication to what the former make up of the supercontinent Gondwana might have been like [2]. There are eight genera of gymnosperm which occur in Tasmania coming from the Cupressaceae and Podocarpaceae families, these include Athrotaxis, Callitris, Diselma, Largarostrobos, Microcachrys, Pherosphaera, Phyllocladus and Podocarpus.
Cupressaceae
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Cupressaceae, also known as the Cypres family, are evergreen trees with separate woody male and female cones and scale like leaves [1]. In Tasmania there are four native species and one naturalised species which fit into this group - Athrotaxis cupressoides (Pencil Pine), Athrotaxis selaginoides (King Billy Pine), a hybrid of these two plants Athrotaxis Xlaxifolia, as well as Callitris oblonga (South Esk Pine), Callitris rhomboidea (Oyster Bay Pine) and Diselma archeri (Dwarf pine) [3]. Of these species only the genus Callitris does not occur within the alpine region.
Athrotaxis selaginoides
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Tasmanian has two species of endemic Athrotaxis, Athrotaxis selaginoides (King Billy Pine) and Athrotaxis cupressoides (Pencil Pine), these are both long lived trees reaching ages of over 1200 years old [4]. King Billy Pine can grow to the size of a large tree up to 30 meters tall and 2 meters in diameter[3]. It is found in cool, damp sub-alpine, alpine and rainforest areas of western and central Tasmania. Wildfire is a major threat to this species, for example wildfire has caused a decline of 32% over the past 100 years[5]. As such it has been listed as vulnerable on the IUCN Red List of Threatened Species [5].
Athrotaxis cupressoides
[edit]A. cupressoides (Pencil Pine) is a small tree which grows in the western and central mountains of Tasmania from an altitude of 700-1200 meters, in alpine, subalpine and rainforest communities. It is highly sensitive to fire and does not regenerate following a wildfire [1]. A. cupressoides regenerates by seed when suitable climatic conditions, approximately every five years, triggers the production of large quantities of seed[4]. Whilst this species is almost exclusively found within the protected areas of Tasmania, due to its sensitivity to fire it has been listed as vulnerable on the IUCN Red List of Threatened Species [5].
Athrotaxis Xlaxifolia
[edit]Athrotaxis Xlaxifolia is a hybrid of A. selaginoides and A. cupressoides, however had previously been considered a separate species. It is a rare plant found in areas where King Billy Pine and Pencil Pine are growing nearby. Its foliage is intermediate in appearance between the two species.
Diselma archeri
[edit]Diselma archeri (Dwarf or Cheshunt Pine) is the other endemic cupressaceae found in the alpine regions of Tasmania. It grows to the size of a shrub or small tree, in alpine conifer heath and montane rainforest [3]. The leaves are opposite and scale-like forming a square cross section. It is fire sensitive and slow growing, rarely producing seedlings, more commonly resprouting from the trunk.
Podocarpaceae
[edit]Tasmania has five species from the Podocarpaceae family, four of which are endemic to Tasmania. Like other conifers Podocarpaceae are evergreen trees and shrubs [3]. They have variable foliage and are mostly dioecious. The species which occur in Tasmania include Lagarostrobos franklinii (Huon Pine), Microcachrys tetragona (Creeping Pine), Pherosphaera hookeriana (Drooping pine), Phyllocladus aspleniifolius (Celerytop Pine) and Podocarpus lawrencei (Mountain plumpine). Of these species all but Huon pine and Celerytop pine occur in the alpine zone above 1000 meters in altitude.
Pherosphaera hookeriana
[edit]Pherosphaera hookeriana is a rare species, endemic to Tasmania and is listed under the Threatened Species Protection Act 1995 as vulnerable[6]. The Mt Field area supports the majority of this species with approximately 5,000 mature plants surveyed in 2007 [7]. It grows to be a small shrub of approximately 2 meters in height, it has scale like leaves and is dioecious producing small cones. This species is extremely fire sensitive and is slow growing living for hundreds of years. Recruitment from seed is very rare however can occur with seeds dispersed by wind or birds and germinating in moist, shady locations [7]. The main threat to this species is fire, which is likely to increase with Climate Change and the likelihood of more frequent fire and drought.
Podocarpus lawrencei
[edit]Podocarpus lawrencei (Plumb pine) is a native south-eastern Australian shrub which grows in well watered, mostly alpine and subalpine areas. It grows to a height of 50-100 cm with flattened and spirally arranged adult leaves and bright red female cones[3]. Like the other members of the Podocarpaceae family it is fire sensitive and slow growing.
Microcachrys tetragona
[edit]Microcachrys tetragona (Creeping pine or Strawberry Pine) is the sole species of the genus Microcachrys and is endemic to Western Tasmania. It is a prostrate alpine shrub favouring rocky areas and is sometimes confused with Diselma archeri due to similar scale like leaves. This is a slow growing species which is sensitive to fire. Reproduction from seedlings has never been recorded, with the plant reproducing by resprouting from the trunk [3].
Tasmanian Alpine Vegetation
[edit]The Tasmanian highlands is defined as an area above 1000 metres [1], with the highest peak reaching 1,617 meters at Mt Ossa. Most of the mountains occur in the western half of the state, with the exception being Ben Lomond in the North East. Due to Tasmania's maritime climate the temperatures are relatively mild and snow cover generally only lasts for a few weeks, however glancing storms do occur bringing strong winds and ice. Whilst the alpine zone is usually considered to be above the treeline, in Tasmania coniferous tree and shrub species such as Athrotaxis cupressoides, Athrotaxis selaginoides and Diselma archeri usually occur above 1000 meters in altitude, generally around the margins and in more sheltered locations [1].
In Tasmania's alpine region there are a number of different vegetation communities which are characterised by Gymnosperm species (Table 1).
| Community Name | Name Code |
|---|---|
| Alpine coniferous heathland | HCH |
| Athrotaxis selaginoides subalpine scrub | RKS |
| Athrotaxis cupressoides open woodland | RPW |
| Athrotaxis cupressoides–Nothofagus gunnii short rainforest | RPF |
Table 1 - Gymnosperm dominated alpine and subalpine vegetation communities of Tasmania [8].
Alpine coniferous heathland is a vegetation community which is dominated or co-dominated by heath and dwarf conifer species - Diselma archeri, Microcachrys tetragona, Phaerosphaera hookeriana and Podocarpus lawrencii[8]. It occurs only on long unburnt parts of the eastern and western mountains above 900m. Whilst this is generally a treeless vegetation type there can be occurrences of Eucalyptus species, Athrotaxis species and Northofagus gunnii [8].
Athrotaxis selaginoides subalpine scrub is listed as a Threatened vegetation community under the Nature Conservation Act 2002[9]. It occurs between 650m and 1 050m on peaty soils in unburnt areas of mountains in the south-west of Tasmania [8].
Athrotaxis cupressoides open woodland and Athrotaxis cupressoides / Nothofagus gunnii short rainforest are both listed as Threatened communities under the Nature Conservation Act 2002[9]. Both of these vegetation types occur above 900 meters, Athrotaxis cupressoides open woodland occurs through alpine heathland and along creek margins and does not contain N. gunnii [8]. Athrotaxis cupressoides / Nothofagus gunnii short rainforest occurs in hills and on sheltered moraines and has a dense understorey.
The impact of fire on alpine Gymnosperms
[edit]Throughout history wildfire has been a major factor in influencing the different types of vegetation occurring across the island of Tasmania. On the East Coast of Tasmania fire is a common occurrence, encouraging fire adapted vegetation communities such as dry eucalypt forest and grassy woodlands[8]. In other parts of the Tasmanian landscape fire occurs rarely enabling fire intolerant species such as Gymnosperms to persist. These areas include zones of cool temperate rainforest and alpine and sub alpine areas.
Fire is presently the most serious threat to alpine vegetation in Tasmania, as regeneration of species after a fire is a slow process which can take hundreds if not thousands of years [10]. Areas of the alpine region have been burnt throughout history affecting floral composition and reducing the number of fire intolerant species present [8]. Gymnosperms are particularly vulnerable to fire as they have poor regenerative capacity causing localised extinctions, general decline in species number and population fragmentation. Additionally gymnosperms are slow growing and have a low rate of seedling recruitment, with the majority of species favouring resprouting from the trunk. In certain areas of Tasmania, such as the Mount Field National Park, you can see dead stags of conifers caused by previous fires. In fact since European settlement one-third of King Billy pines have been killed due to fire, much of which was caused by escaped campfires[4].
During extreme climatic events, wildfires in Tasmania can be severe such as the most recent 2013 bushfires as well as the 1967 Tasmanian fires, which caused widespread devastation to both property and the natural environment. Whilst the generally cool damp climates which characterise areas of rainforest and alpine shrubland usually act to inhibit the fires from establishing, when extreme fire events occur these areas can also be affected. With the onset of Climate Change it is predicted that Australia is likely to have an increased frequency, intensity and size of bushfires [11], which could cause serious issues for the long term survival of the gymnosperms. The story of climate change in Tasmania is somewhat more complex due to its variable topography and climate, with current modelling showing that there could be more precipitation in the western part of the state and dryer conditions in the east [12]. Climate change in Tasmania is also believed to increase the rate of lightning strikes in Western Tasmania, contributing to an increase in lightning caused fires[13].
Case Study: Mt Field
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Mount Field is a peak reaching 1434 meters situated in the south-central region of Tasmania [14]. Rainfall is variable from 2500mm to 1500mm with snowfall occurring in winter time above 1100 metres [14]. It has had a history of relatively little disturbance, aboriginals used fire for hunting and there have been periodic wildfires. Areas of Mt Field contains subalpine forests and woodlands which are characterised by endemic conifers including Athrotaxis cupressoides found near the tarn shelf and around the lakes, Athrotaxis selaginoides and several of the dwarf pine species including Podocarpus lawrencii, Microcachrys tetragona, Diselma archeri and Phaerosphaera hookeriana[15].
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Mt Field has been the site of a number of studies looking at the effect of fire on alpine vegetation and gymnosperms. Major bushfires have occurred in the Mt Field National Park in the 1890s, 1934, in 1960 and again in 1966 [15]. Of these bushfires the fire in 1966 had the most significant affect on the coniferous vegetation of the park, with sparks from the fire lighting up the eastern flank of the Tarn Shelf (see Fire Scar image above). A study in 2010[13] explored the resilience of the Tasmanian alpine flora to fire and looked at the after affects of these bushfires on the vegetation of Mount Field. This study showed that several dominant species of Tasmanian vegetation, including the gymnosperms, had very low resilience if a single fire of sufficient intensity to kill the above ground vegetation occurred[13]. Experimentation showed that after a 40-70 year period only one of the burned plots had more than 10% cover of gymnosperms and many had no cover at all. The study found extensive areas that were free of gymnosperms, dominated by Richea scoparia which suggested that in areas where fire eliminated the gymnosperms and they are unable to recolonise from surrounding populations, then R. scoparia tended to dominate. The study did show that recovery could occur if it is within the dispersal ability of neighbouring gymnosperms. For example Athrotaxis seed can disperse approximately 100 meters by wind and Microcachrys tetragona and Podocarpus lawrencei have sweet fleshy reproductive parts which can be dispersed by birds [13]. In 2013 a study by the University of Tasmania showed that within the burnt areas of the Tarn Shelf of Mount Field there was significant seedling regeneration of Athrotaxis cupressoides (see image of seedling regeneration post fire above). This had only occurred within the previous 10 years and suggested that regeneration of this species post fire can occur given the right conditions.
References
[edit]- ^ a b c d e Howells, C. (2012). Tasmania's Natural Flora, Second Edition. Australian Plant Society Tasmania Inc., Hobart.
- ^ Tasmanian Arboretum (2012) Southern Hemisphere Conifers [1]
- ^ a b c d e f University of Tasmania, 2011. Key to Tasmanian Vascular Plants. [2]
- ^ a b c Tasmanian Parks and Wildlife Service (updated July 2008) Native conifers of Tasmania.[3]
- ^ a b c IUCN (updated 2000). IUCN Red List of Threatened Species - Athrotaxis selaginoides and Athrotaxis cupressoides. [4] [5]
- ^ Department of Primary Industries Water and Environment (updated 8/10/2012). Native Plants and Animals, List of Threatened Species.[6]
- ^ a b Threatened Species Section (2009) Listing Statement for Pherosphaera hookeriana (drooping pine), Department of Primary Industries & Water, Tasmania.
- ^ a b c d e f g 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.
- ^ a b Nature Conservation Act 2002, Tasmanian Consolidated Acts [7].
- ^ Kirkpatrick, J.B., Bridle, K.L. and Wild, A.S. (2002). Succession after fire in alpine vegetation on Mount Wellington Tasmania. Australian Journal of Botany, 50, 145–154
- ^ Ellis, S, Kanowski, P & Whelan, R (2004). National Inquiry on Bushfire Mitigation and Management, Commonwealth of Australia, Canberra.
- ^ White CJ, Fox-Hughes P, Grose MR, Corney S, Bennett JC, Holz GK, Gaynor S and Bindoff NL, (2013). Implications for fire danger in bushfire prone areas of Tasmania. Antarctic Climate and Ecosystems Cooperative Research Centre
- ^ a b c d Kirkpatrick, J.B., Bridle, K.L. and Dickson K. J. M. (2010). Decades-scale vegetation change in burned and unburned alpine coniferous heath. Australian Journal of Botany. Vol 58, 453-462.
- ^ a b Minchin, P.R (1989). Montane Vegetation of the Mt. Field Massif, Tasmania: A Test of Some Hypotheses about Properties of Community Patterns. Vegetatio 83: 97-110.
- ^ a b Tasmanian Parks and Wildlife Service (2002). Mount Field National Park, Marriotts Falls State Reserve, & Junee Cave State Reserve Management Plan. Department of Tourism, Parks, Heritage and the Arts. Hobart.[8]