Transitional Rainfall Zone
Encyclopedia
The Transitional Rainfall Zone (TRZ) is one of three biogeographic zones into which south west Western Australia
is divided, the others being the High Rainfall Zone
and the Low Rainfall Zone
. The TRZ is recognised as having a much higher diversity of rare and endemic plant species than the other Zones.
The Zones were first defined by Stephen Hopper
in his 1979 paper Biogeographical aspects of speciation in the southwest Australian flora. Initially, they were defined in terms of rainfall, with the TRZ being that part of the South West with annual rainfall of between 300 and 800 millimetres (12–31 in). However, following the publication of John Stanley Beard
's phytogeographic
regionalisation of Western Australia in 1980, it was recognised that Hopper's zones could be defined as aggregates of Beard's botanic districts. The Transitional Rainfall Zone was subsequently re-defined as equivalent to Beard's Roe, Avon and Irwin Botanical Districts, later renamed Northern Sandplains Region, Wheat Belt Region and Mallee Region respectively. When the Interim Biogeographic Regionalisation for Australia
(IBRA) was published in the 1990s, Beard's regionalisation was adopted as the baseline for Western Australia. All three of the TRZ regions were accepted as IBRA regions, albeit with slightly different titles. Thus the TRZ is now defined as the aggregate of the Geraldton Sandplains
, Avon Wheatbelt
and Mallee
IBRA regions.
The TRZ is widely recognised as having very high species diversity and endemism compared to the other zones. Some botanists have claimed that this is due to the species richness of the near-coastal heath
lands at the north-western and south-eastern extremes of the zone, suggesting that the intervening Wheatbelt region is relatively species poor. Other botanists have refuted this, however, claiming that the entire TRZ is species rich compared to the other zones.
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
is divided, the others being the High Rainfall Zone
High Rainfall Zone
The High Rainfall Zone is one of three biogeographic zones into which south west Western Australia is divided, the others being the Transitional Rainfall Zone and the Low Rainfall Zone....
and the Low Rainfall Zone
Low Rainfall Zone
The Low Rainfall Zone is one of three biogeographic zones into which south west Western Australia is divided, the others being the High Rainfall Zone and the Transitional Rainfall Zone...
. The TRZ is recognised as having a much higher diversity of rare and endemic plant species than the other Zones.
The Zones were first defined by Stephen Hopper
Stephen Hopper
Stephen Donald Hopper is a Western Australian botanist, specialising in conservation biology and vascular plants. He has written eight books, and has over 200 publications to his name. He was Director of Kings Park in Perth for seven years, and CEO of the Botanic Gardens and Parks Authority for five...
in his 1979 paper Biogeographical aspects of speciation in the southwest Australian flora. Initially, they were defined in terms of rainfall, with the TRZ being that part of the South West with annual rainfall of between 300 and 800 millimetres (12–31 in). However, following the publication of John Stanley Beard
John Stanley Beard
John Stanley Beard was a British-born forester and ecologist who resided in Australia. While working with the Forestry Division in Trinidad and Tobago during the 1940s, Beard developed a system of forest classification for Tropical America and described the forests of Trinidad, Tobago, and the...
's phytogeographic
Phytogeography
Phytogeography , also called geobotany, is the branch of biogeography that is concerned with the geographic distribution of plant species...
regionalisation of Western Australia in 1980, it was recognised that Hopper's zones could be defined as aggregates of Beard's botanic districts. The Transitional Rainfall Zone was subsequently re-defined as equivalent to Beard's Roe, Avon and Irwin Botanical Districts, later renamed Northern Sandplains Region, Wheat Belt Region and Mallee Region respectively. When the Interim Biogeographic Regionalisation for Australia
Interim Biogeographic Regionalisation for Australia
The Interim Biogeographic Regionalisation for Australia is a biogeographic regionalisation of Australia developed by the Australian Government's Department of the Environment, Water, Heritage and the Arts...
(IBRA) was published in the 1990s, Beard's regionalisation was adopted as the baseline for Western Australia. All three of the TRZ regions were accepted as IBRA regions, albeit with slightly different titles. Thus the TRZ is now defined as the aggregate of the Geraldton Sandplains
Geraldton Sandplains
Geraldton Sandplains is an Interim Biogeographic Regionalisation for Australia region in Western Australia and part of the larger Southwest Australia savanna ecoregion.It has two sub regions: -* Geraldton Hills sub region* Lesuer sub region...
, Avon Wheatbelt
Avon Wheatbelt
Avon Wheatbelt is an Interim Biogeographic Regionalisation for Australia region in Western Australia and part of the larger Southwest Australia savanna ecoregion.-Further reading:...
and Mallee
Mallee (biogeographic region)
Mallee, also known as Roe Botanical District, is a biogeographic region in southern Western Australia. Located between the Esperance Plains, Avon Wheatbelt and Coolgardie regions, it has a low, gently undulating topography, a semi-arid mediterranean climate, and extensive Eucalyptus mallee...
IBRA regions.
The TRZ is widely recognised as having very high species diversity and endemism compared to the other zones. Some botanists have claimed that this is due to the species richness of the near-coastal heath
Heath (habitat)
A heath or heathland is a dwarf-shrub habitat found on mainly low quality acidic soils, characterised by open, low growing woody vegetation, often dominated by plants of the Ericaceae. There are some clear differences between heath and moorland...
lands at the north-western and south-eastern extremes of the zone, suggesting that the intervening Wheatbelt region is relatively species poor. Other botanists have refuted this, however, claiming that the entire TRZ is species rich compared to the other zones.
Further reading
- Thackway, R and I D Cresswell (1995) An interim biogeographic regionalisation for Australia : a framework for setting priorities in the National Reserves System Cooperative Program Version 4.0 Canberra : Australian Nature Conservation Agency, Reserve Systems Unit, 1995. ISBN 0642213712