Shrublands and Heathlands of the Wet Tropics Bioregion
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SHRUBLANDS AND HEATHLANDS OF THE WET TROPICS BIOREGION METCALFE © DAN Shrublands and heathlands (vegetation codes 67a-67k) Shrublands and heathlands encompass a diverse range Aeolian dune fields are very uncommon in the Wet Tropics, of floristic associations. The extent and distribution of being limited to a few occurrences on the Cairns, many of these is highly restricted due to specific Helensvale and Hinchinbrook Island map sheets. Although environmental requirements. Sclerophyll shrubs coastal heath communities are more typical of the Cape dominate the formation, although rainforest tree species York Peninsular Bioregion, relatively extensive areas are may occasionally be present depending upon the found on both the Cairns and Helensvale map sheets. Field prevailing environmental conditions. Many of the observation suggests that the aeolian process was much constituent species are shrubland specialists, although a more active in the past but over time a gradual reduction in significant number are common canopy constituents of sand deposition has enabled the incremental stabilisation well developed rainforest and woodland communities. A of the dune fields by myrtaceous shrubs. large number of shrubland communities have also been For example, on the Cairns map sheet dune fields may be incorporated as alliances in other formations based upon occupied by a Thryptomene oligandra heathland (67c) while their floristic association. the Archer Point (Helensvale map sheet) dune field has In this classification a shrub is a woody plant less than 8 been colonised by a more diverse mix of shrubs metres tall, frequently with multiple stems arising at or which include: Asteromyrtus lysicephala, Asteromyrtus near the base (after Specht 1970), while the term heath angustifolia, Thryptomene oligandra, Acacia crassicarpa, refers to shrubland communities where crown cover Jacksonia thesioides and Leucopogon yorkensis (67k). In a exceeds 80%. bioregional context, these shrublands are unique. The rugged, elevated topography of Hinchinbrook Island hosts a range of shrubland communities. Type 67a is an Younger prograding dune systems in the Department of unusual shrubland association, occurring on Mount Defence training area at Cowley Beach host a unique Straloch at altitudes above 600m at the southern end of shrubland complex (67j) controlled largely by seasonal Hinchinbrook Island. The community, characterised by fluctuations in the ground water table. The landscape is Acacia celsa, Podocarpus grayae and Callitris endlicheri, characterised by regularly alternating dune ridges and blankets extensive areas of exposed hillslope on highly perennially wet swales, with the dune ridges rarely infertile acid volcanic detritus. Notophyll rainforest elevated more than a metre above the high point of the occupies adjacent sheltered locations where it attenuates swale. The suppressed topography has not facilitated the along incised gullies and crevasses. Acacia flavescens development of soil that is sufficiently drained at any point dominated shrubland (67b) is prominent on rocky in the landscape to allow development of anything but a headlands where it merges upslope with areas of depauperate shrubland dominated by Melaleuca viridiflora. Allocasuarina littoralis dominated shrubland (67f), which is habitat for the endemic Banksia plagiocarpa. Unlike association 67a, this community is directly influenced by Lophostemon is a prominent genus in a number of fire which is an important habitat regulator and controlling shrubland associations. Lophostemon confertus dominated factor. Observations suggest that hot fires destroy Banksia shrublands (67h) has an extensive distribution on Walsh's plagiocarpa shrubs and that their subsequent regeneration Pyramid (Gordonvale map sheet) and is also a significant is suppressed by the aggressive invasion of Allocasuarina component of the broader rock pavement complex spp. formation. The association consistently occupies the more favourable sites in sheltered enclaves amongst the broader rock pavement formation. The structural and Allocasuarina spp. are particularly well-adapted at floristic uniformity, coupled with the areal extent of these colonising harsh environments (67e). In a very wet area to shrublands warrants their recognition as a separate entity. the south of Innisfail there is a lens of deeply weathered Lophostemon suaveolens is a common shrub species of ultramafic rock forming a low linear ridgeline which has rock pavement complexes, often forming near pure stands. been colonised by an Allocasuarina littoralis dominated These shrublands have, however, been mapped as a shrubland. It is apparent that there is some degree of soil component of a broader rock pavement complex formation toxicity associated with the unusual geology which severely (example 66c) due to their limited areal extent and their limits most vegetation development. It is very unusual to inconsistent expression across the broader landscape. find such a depauperate sclerophyll community in a high Association 67i does, however, recognise the floristic rainfall belt, on what appears to be a soil of reasonable dominance of Lophostemon suaveolens in a mixed tall open structure and depth, in an area that has not been subject shrubland community in the Windin Falls area (Bartle to any severe disturbance event. Frere map sheet). This association is delineated on © DAN METCALFE © DAN ridgelines with extreme drainage and skeletal soils, Geography whereas the more extensive neighbouring syncarpia dominant formation is found down-slope in more This formation is largely confined to areas on the coastal favourable environmental conditions. escarpment or coastal plain. The most extensive shrubland areas are associated with the steep terrain of Hinchinbrook Island and adjacent mainland areas. Association 67g is an unusual shrubland community which Although the formation occurs on a range of geologies, the is confined to an extensive flat on the north-west side of important controlling factors are highly infertile, poorly Curacoa Island (Palm Island Group). Its structure is one of drained or skeletal soil types. widely separated groves of vine thickets, grasslands, scattered Casuarina equisetifolia shrubland interspersed with Corymbia tessellaris and Acacia crassicarpa Impacts and changes shrublands and woodlands. This unusual vegetation Edaphic controls limit the degree and rate of floristic or complex can be attributed to both the recent nature of, structural change in the majority of shrubland and and the lack of substrate homogeneity in the occupied heathland alliances. The exception is those characterised landform. by Allocasuarina littoralis where prolific regeneration of the Although the landform is composed predominantly of species may occur after a hot fire event, or a more gradual alluvial outwash, groves of Casuarina equisetifolia tend to thickening may occur in the absence of fire. Poorly occupy a number of extremely narrow stranded shorelines managed fire regimes are likely to result in severe (beach ridges). Rock rubble mounds have been exposed in reduction of floristic diversity in these communities. depressions on the alluvial surface and these areas are occupied by vine thicket communities. Although classified as a Corymbia tessellaris dominant alliance, the structural Key values complexity of the community should be recognised. It has • Form extensive wilderness areas on Hinchinbrook Island most likely been influenced in part by regular fire NP with considerable scenic value incursions with the groves of vine thicket surviving due to the protection provided by the boulder mounds. • Habitat of the endemic Banksia plagiocarpa. Facts and figures Threatening processes Vegetation alliances Acacia celsa shrublands • Loss of floristic diversity associated with inappropriate Acacia flavescens shrublands fire regimes. Asteromyrtus lysicephala shrublands Tenure Allocasuarina littoralis shrublands Extensive areas conserved on the Hinchinbrook Island NP, Fitzroy Island NP, Paluma Range NP, Tully Gorge NP. Corymbia tessellaris shrublands Lophostemon confertus shrublands Management considerations Lophostemon suaveolens shrublands • Appropriate fire regimes important to maintenance of floristic diversity in Allocasuarina littoralis alliances. Melaleuca viridiflora shrublands Current extent in the bioregion 9,783ha Area protected 7,817ha (80%) wettropics.gov.au.