The Transitional Rainfall Zone (TRZ) is one of three biogeographic zones into which south-west

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to ...

is divided, the others being the

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. The zones were first defined by Stephen Hopper in his 19 ...

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 Stephen Donald Hopper AC FLS FTSE (born 18 June 1951) is a Western Australian botanist. He graduated in Biology, specialising in conservation biology and vascular plants. Hopper has written eight books, and has over 200 publications to his na ...

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 (15 February 1916 – 17 February 2011) was a British-born forester and ecologist who resided in Australia. Beard studied at the University of Oxford where he completed his doctoral thesis on tropical forestry. While worki ...

'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 District''s, later renamed ''Northern Sandplains Region'', ''Wheat Belt Region'' and ''Mallee Region'' respectively. When the

Interim Biogeographic Regionalisation for Australia The Interim Biogeographic Regionalisation for Australia (IBRA) is a biogeographic regionalisation of Australia developed by the Australian government's Department of Sustainability, Environment, Water, Population, and Communities. It was deve ...

(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 ( Wajarri: ''Jambinu'', Wilunyu: ''Jambinbirri'') is a coastal city in the Mid West region of the Australian state of Western Australia, north of the state capital, Perth. At June 2018, Geraldton had an urban population of 37,648. ...

Avon Wheatbelt The Avon Wheatbelt is a bioregion in Western Australia. It has an area of . It is considered part of the larger Southwest Australia savanna ecoregion. Geography The Avon Wheatbelt bioregion is mostly a gently undulating landscape with low reli ...

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 A heath () is a shrubland habitat found mainly on free-draining infertile, acidic soils and characterised by open, low-growing woody vegetation. Moorland is generally related to high-ground heaths with—especially in Great Britain—a cooler ...

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.

See also

References

Further reading