CORE Metadata, citation and similar papers at core.ac.uk Provided by University of Tasmania Open Access Repository 61 VEGETATION OF THE CENTRAL PLATEAU by W.O. Jackson Department of Botany, University of Tasmania. MAJOR VARIABLES As might be expected in an upland region, the dis­ tribution of plant communities is determined largely by low temperatures, in particular by the degree and frequency of frosts below -5°C, glazing storms, and ice-bearing winds. The most significant single index which is readily available is probably the mean summer temperature. A mean temperature of 10°C for the warmest month has been accepted as the ind­ icator of the tree line (Daubenmire 1954). Most of the Central Plateau above 1040 m is thus in this category. There is a marked gradient in precipitation across the plateau on a NW - SE axis. The environs of Lake Mackenzie in the north west receive an annual rainfall approaching 3000 mm (Hydro-Electric Commission Hydrological Report, 1971), whereas the annual precipitation in the Bothwell-Steppes region is under 650 mm. This gradient produces corresponding gradients in the vegetation. For example the precipitation gradient in part accounts for the replacement of coniferous forest and shrubbery by eucalypt woodland. However in general the effect of precipitation is minor compared with the control exerted by temperature. As with so much of the vegetation in Tasmania, local deflecting influences are strong. Fire frequency and edaphic factors often predominate in determining patterns of plant distribution. Over much of the plateau aspect and drainage are important physiographic factors. This is because much of the plateau is close to the limit of tree growth so that even small topographic features have a pronounced effect by produc­ ing gradient variations in cold air drainage, soil drainage and exposure. As a result of even moderate physiographic re­ lief there is a mosaic of plant communities arising from local variations in the micro-environment. Since the temperature exerts a major influence on plant growth and development, the distribution of vegetation can be related to altitude. The plateau has a "stepped" land 62 form due to the development of sub-aerial erosion surfaces following periods of uplift (Davies 1959), and these sur­ faces form a useful reference system. Quite apart from the lapse rate of 1°C per 100 m in altitude, the climate of the plateau deteriorates rapidly at altitudes over 1100 m be­ cause the cloud base descends to this level frequently, even in summer. Davies (lac. cit.) recognised six surfaces in Tas­ mania:- Lower Coastal Surface, 90-275 m; Higher Coastal Surface, 365-455 m; St. Clair Surface, 730-820 m; Lower Plateau Surface, 915-1065 m; Higher Plateau Surface, 1190- 1340 m; and high monadnocks 1340-1614 m. These surfaces are shown in figure 11. 11. Areal distribution of geo~ morphological surfaces of the Central Plateau region Tasmania(after Davies 1959). Key: Major lakes black 1. Lower Coastal Surface 90-275 m 2. Higher Coastal Surface 365-455 m 3. St. Clair Surface 730-820 m 4. Lower Plateau Surface 915-1065m 5. Higher Plateau Surface 1190-1340m 6. High Monadnocks 1340-1614m Vegetation patterns on the surfaces are complicated by changes in drainage, frost intensity and aspect effects produced by minor features of relief. Thus before consider­ ing the vegetation of these surfaces it is advisable to con­ sider the normal unbroken altitude progression in vegetation on well-drained slopes. This may conveniently be demonst­ rated by considering the vegetation of the major scarp face to the plateau on the northern and eastern margins, where it forms the "Western Tiers". Here the vegetation provides a compressed section of the sequences from the lowland vege­ tation of the Midland Graben (400 m) to the high rim of the Plateau (1400 m). 63 VEGETATION OF THE WESTERN TIERS SCARP The vegetation is summarized on an idealised sect­ ion [figure 12). 12. Diagramatic idealized section of the northern scarp of the Western Tiers; precipitation 760-1000 mm; showing vegetation types. At the base of the scarp the eucalypt savannah forests of the Midlands Graben consist of open communities of Eucalyptus pauciflora - E. viminalis or E. pauciflora - E. rubida in drier areas (Jackson 1965). Some savannah of E. ovata or E. rodwayii occur where the drainage is poor in the winter. The ground vegetation of these savannahs is a tussock grassland of Poa hillardieri. Themeda australis and Lomandra longifolia may predominate on clayey soils. Corri­ dor forests of E. viminalis - Acacia melanoxylon occur on stream courses, and dry sclerophyll forests occur on low hills of sandy or lateritic soils. On the lower slopes of the scarp the open savannah is replaced by dry sclerophyll forests of the same tree species. These forests increase in density and height as the rainfall increases. On areas of deep soil and suffic­ ient moisture the ash species E. obliqua may replace the peppermint species E. pauciflora or E. amygdalina partially or completely as the dominant. The macrantherous species E. viminalis~ E. ruhida or E. ovata remain as associate spec­ ies. Above 300m altitude these species are replaced by E. dalrympleana as the associate with E. obliqua. Due to in­ creasing rainfall there is a structural transition from dry sclerophyll forest, with low and medium shrub layers of Epa­ cridaceae and Leguminosae, to wet sclerophyll, with well &4 developed tall shrub layers of OZearia argophyZZa and Poma­ der>r>is apetaZa. On the wetter northern and western slopes where the annual rainfall exceeds 1250 mm, the climax is a rainforest of Nothofagus cunninghamii - Ather>osper>ma moschatum. This climax is attained only in areas topographically protected from high fire incidence (Jackson 1968). Because of the fairly regular disturbance by fire, most areas carry "Mixed" forest (Gilbert 1958), with a stratum of eucalypts over a substratum of rainforest and wet sclerophyll shrub species such as Acacia deaZbata~ Prostanther>a Zasianthos and OZear>ia argophyZZa. Near the upper lip of the scarp face (c.1000 m) water availability is greatly increased by the "fog drip" or "cloud stripping" effect of the vegetation on the cloud base so prevalent around the plateau rim. Under these conditions a dwarf "elfin" rainforest or thicket of Nothofagus occurs. Nothofagus cunninghamii extends around the rim of the West­ ern Tiers, but the deciduous N. gunnii is restricted to the areas of higher rainfall on the western margins. Above 450 m the sub-alpine species become increas­ ingly evident. E. obZiqua is replaced by E. deZegatensis. The E. deZegatensis - E. daZr>ympZeanea forest extends up the slope to altitudes of about 1000 m or slightly higher in sheltered situations. The association is the characteristic forest of the lower slopes and the St. Clair Surface, and extends onto the Lower Plateau Surface. Over much of its range E. deZegatensis is growing on a relict solifluction sorted mantle. This surface is covered in many places with large dolerite boulders, though there is commonly a deep yellow-brown soil on solifluction deposits (cf. alpine humus soil) beneath, with a shallow water table. The smaller shrubs have difficulty reaching this reliable supply of water in the summer and the shrub layer is scattered and more xero­ morphic on the upper slopes. Bedfor>dia saZicina~ OZear>ia viscosa~ 0. phZogopappa and Cyathodes paPVifoZia predominate. With increasing and more reliable precipitation near the cloud base the sub-alpine shrub belt increases in density. Around 910 m dense shrubberies of Hakea Zissosperma~ Or>ites diversi­ folia~ Lomatia poZymorpha and TeZopea tPUncata occur as a fire-determined deflection state of the climax Nothofagus thicket (Plate 2). Where fire is frequent enough to maintain eucalypts, an open woodland overstorey of E. coccifera - E. gunnii or E. archerii occurs. At altitudes exceeding 100 m exposure to glazing 65 winds and snow limits tree and shrub forms and only the spec­ ialised alpine communities are found. Well-drained areas are occupied by a proteaceous - epacridaceous closed heath or shrubland; while those areas receiving a high precipitation but obtaining some topographic protection from the wind, carry dwarf coniferous forest or coniferous shrubbery. Poorly drained areas carry a complex mosaic of herbfield, bog and bolster moor communities. Cold air drainage "basins" carry tussock grassland, sedgeland or bog communities. These alpine communities are described in the section dealing with the Higher Plateau Surface. VEGETATION OF THE ST. CLAIR SURFACE The areal distribution of the St. Clair Surface (730m - 820m) is shown in fig. 11 and a schematic represent­ ation of its vegetation types as figures 13 and 14. Forest Types. The dominant vegetation on the Surface is the E. delegatensis - E. dalrympleana forest. The soils on most of the Surface are derived from basalt or dolerite and range from kraznozems to alpine humus soils. They will support tall forests if temperature and rainfall permit. Thus well­ drained land not in a frost hollow will tend to the climax Nothofagus cunninghamii - Atherosperma moschatum rainforest where the annual precipitation is in excess of 1250 mm, or to E. delegatensis - E. dalrympleana wet sclerophyll forest in lower rainfall areas. A wide ecotone of "mixed forest" occurs as a result of disturbance by fire; for example the forests around Tarraleah. Towards the drier eastern side of the plateau the E. delegatensis - E. dalrympleana forest changes gradually to dry sclerophyll with lower tree heights and an open understorey of Bedfordia salicina3 Olearia vis­ cosa3 Cyathodes parvifolia and Lomatia tinctoria. On the junction scarp with the Higher Coastal Surface, on the Shan­ non Tier, for example, E. delegatensis may be replaced by E. amygdalina. E. pauciflora - E. dalrympleana may displace the E. delegatensis forest where frost becomes important; for example the base of the Shannon and Bakers Tiers.