ISSN 0394-9125 STUDIA GEOBOTANICA An international journal Voi. 20 2001 EDITORS G. Cristofolini - Bologna Lj. llianic' - Zagreb E. Mayer - Ljubljana P.L. Nimis - Trieste L. Orlòci - London, On. F. Pedrotti - Camerino S. Pignatti - Roma A. Pirola - Pavia L. Poldini - Trieste E. Wikus Pignatti - Trieste MANAGING EDITOR M. Tretiach - Trieste SECRETARY M. Codogno - Trieste Dipartimento di Biologia Sezione di Geobotanica ed Ecologia vegetale Università di Trieste Studia Geobotanica. Voi. 20: 3-16 (2001) MALLEE COMMUNITIES ALONG ROADSIDES IN SOUTH-WESTERN AUSTRALIA Erika PIGNATTI WIKUS ', Giuseppe PIGNATTI 2 and Stephen D. HOPPER' ' Dipartimento di Biologia, Università di Trieste, Via Giorgieri I O, l-34127 Trieste, ltaly. 2 Istituto Sperimentale per la Selvicoltura, Viale S. Margherita 80, l-52100 Arezzo, ltaly. 3 Botanic Gardens and Parks Authority, Kings Park & Botanic Garden, West Perth, 6005 Western Australia. Keywords: conservation, Eucalyptus, mallee vegetation, phytosociology, Western Australia. Abstract: Vegetation with eucalypts of the mallee type along four roadside transects in the South-West of Western Australia has been studied. Mallee communities occur under dry-mediterranean climate conditions. Species composition, diversity and turnover, locai endemism with particular regard to Euca/yptus and Acacia species and habitat conservation are discussed. Introduction Mallee eucalypts are many-branched evergreen A von, Roe and Eyre, and is the major agricultural shrubs with woody stems of about 2-4 m height Wheatbelt of the southwest and part of the rising from prominent underground fire-resistant Goldfields, woodlands and pastora] country beyond lignotubers. In areas with dry-mediten-anean clima­ (i.e. Beard's Southwestem Interzone). Of approxi­ te, where rainfall and soil conditions can only sup­ mately 500 species of Western Australian eucalypts, port small trees in natural vegetation, the mallee about 225 are mallees and the above mentioned habit seems an adaptation to above ground damage, three regions are the richest in taxa, with numbers caused more often by frequent fires in ecosystems declining inland and westward (Hopper 1992). As (Beadle 1981). Alternatively, the lignotuber may discussed by Hnatiuk & Maslin (1988), the TRZ is have arisen originally as a simple way of maximi­ the area of highest species richness for Acacia in sing space for access to soil, water and nutrients. Australia. There are two foci of richness, one cen­ Besides the mallee eucalypts many different species tered on Wongan Hills and the other on Newdegate, of the genus Acacia of the phyllodineous type play with in excess of 100 species recorded for each cen­ an important role in mallee communities (Maslin & tre. As with Eucalyptus, the numbers of Acacia spe­ Hopper 1982). cies decline rapidly towards the arid interior and the According to Beard (1990) in southwestern temperate western areas. Western Australia mallee vegetation occurs mainly At present conservation status of species genera­ in the natural regions (Fig. 1) of the Wheatbelt tes concerns due to the land use in the area, where 2 (extent of mallee vegetation 3,596 km ), Mallee extensive farming has changed most of the native 2 Region (60,680 km ) and Esperance Plains (21,410 vegetation into agricultural land (Hopper 1990). 2 km ), developing three physiognomic types: mallee Burgman (1988), who studied the design of appro­ with sclerophyll understorey, mallee with patches of priate reserves specifically for mallee vegetation in woodland and mallee-heath (kwongan). These Western Australia, determined that reserves should regions, placed between the low-rainfall zone of the be located at least every 15 km for the same broad interior of Western Australia and the high-rainfall formations and soil types if they are to conserve of the western and southem coasts, and therefore representative flora, especially rare species. Rapid called Transitional Rainfall Zone (TRZ, Hopper geographical replacement of taxa occurs in the 1979), may be regarded as species rich in relation to Western Australian mallee, and a close network of the other regions (Marchant 1973). The TRZ (300- reserves is needed as a consequence. 800 mm annua! rainfall) includes the four districts Of particular interest in this context is the road­ of the South-West Botanical Province, i.e. Irwin, side vegetation, which, in a heavily cleared agricul- 3 E. PIGNATTI WIKUS e/ al. 300 ?50 South-West Western Australia 350 400 RAINFALL Annua! average in m,llimelres O Predom1nantly matlee areas D Areas where mallee ,s s,gnificanl bui noi predom,nanl 30' 0 4• O 100 200 300 1200 1000 KILOMETRES ° 112· 11s· 120" 124 128' I Fig. I - Map of the area with natural regions of rnallee cornrnunities in Western Australia (Wheatbelt Region, Mallee Region, Esperance Plains). turai land, plays an important role as a corridor or the regime of winter rainfall, but annua! averages reserve for isolated pockets. Roadside vegetation in vary. In the TRZ, to which the transects of the pre­ Western Australia is protected by rather strict laws sent study belong, precipitations in certain years which means that it should remain untouched and may be extremely reduced or even lacking entirely, preserved as much as possible; farmers are obliged while in others unexpected rainfall may alter the to safeguard and protect these vegetation strips yearly mean values. The criticai climatic factor for along roadsides, around farmland and pastures. vegetation is not the annua! average rainfall but the Even where the present laws are respected, it may length of the dry period and the season of the year be sometimes difficult to interpret what the natural in which it falls (Hnatiuk & Maslin 1988, Beard vegetation might have been Iike, since much may 1992). depend on the surface extension of such areas and The vegetation relevés are located in the extra­ the intensity of human impact in the past. dry mediterranean to dry-medite1i-anean zone with Nevertheless the roadside vegetation pockets are 7-8 dry months and winter precipitation from 300- often the last remains of the former natural vegeta­ 500 mm per year (Fig.1). Vegetation changes in tion, so that it is indeed possible to provide a relia­ composition from W to E (increasing aridity) and ble analysis of the changes in species composition from N to S (increasing humidity). The yearly rain­ in strongly altered areas by human intervention. fall is almost constant from 338 mm (Morawa) and This research aimed to study roadside vegetation 356 mm (Buntine) in the North to 417 mm types and their relation to environmental changes of (Ravensthorpe) in the South. But most striking dif­ mallee communities according to the phytosociolo­ ferences are the number of days with high atmo­ gical method of Braun-Blanquet in the TRZ along spheric humidity (61-70 in the North and 112 in the the direction north-west/south-east. South) and temperatures. Therefore the relevés are Materiai and methods scattered along a climatic gradient from a warmer climate with longer lasting aridity in the North Climate and soils towards cooler and moister conditions in the South (Gentilli 1951). The South-West of Western Australia is under The whole area is characterised by geologica! 4 Studia Geobot. 20 (200 I) - Mallee communities in SW Australia - uniformity with gently undulating surfaces. The cies complexes and hybrids are frequent there landscape is largely flattened and shaped by erosion (Hopper 1992). The natural vegetation of this zone long ago. Only modest orogenic activity has taken is complex due to its history. piace since the Permian. Important rivers are no lon­ By law countryroads in Western Australia have ger active in the area of the transects. to have on both sides a strip of uncleared vegetation The different soil types lie on archaean granites which has to be left rather undisturbed and therefo­ with unfolded metamorphics of the Yilgarn Block. re can represent natural or seminatural conditions. Bedrocks of granites and gneisses are largely cove­ Roadside vegetation may not be the best example to red by sandy and gravelly soils overlying iron- and study plant communities, but in Western Australia, aluminium -rich duricrust, the latter above the pal­ especially in the Wheatbelt area, this often repre­ lid clay horizon. All soil types in the area are sents the only remnants of the natural vegetation. nutrient-poor for plant life. Nevertheless vegetation The possibility of aliens entering such areas can be is interrelated with soil. On the basis of changes in expected, accidental fires may start more easily soil, in generai, catenary sequences of natural vege­ close to roadsides, fire-sensitive species may be tation can be observed i.e. kwongan (scrub-heath) destroyed after frequent fires, seedbanks impoveri­ on sandplains, thickets on ironstone gravels, wood­ shed, and evolution of plant communities and their lands of Eucalyptus loxophleba, Eucalyptus salmo­ normai species composition might be impeded. nophloia on loams, and halophytes on saline soils. Human impact and grazing effects may cause Valley soils usually are red loams. Uplands show a further disturbance. Also extra run-off water and deeply weathered profile and are often covered with dust may affect plant-life along roadsides. Outside sandplain soils. The sand quantity is varying in pro­ roadside strips the natural vegetation has often been portion of ironstone gravels. In generai, woodlands completely destroyed due to agricultural and pasto­ are on valley soils, kwongan is on sandplain, while ra! use or human settlements in the vicinity. To mallee occupies an intermediate position between interpret what the natural vegetation might have valley soils and sandplains (Beard 1976). been like, much depends on the surface extension of Southwest Western Australia has experienced such areas and the intensity of human impact in the no glaciation since the Permian Era. During the late past. Nevertheless often it is indeed possible to pro­ Tertiary and Pleistocene environmental perturba­ vide a reliable interpretation of the natural plant life tions were most pronounced in the TRZ (Bowler based on the actual species composition.