Cyclones as an ecological factor in tropical lowland rainforest, North Queensland


L Webb

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Australian Journal of Botany


The influence of cyclones on the structural and floristm composition of tropical lowland and foothill rain-forest in north Queensland is briefly described. Local topographic effects, as well as the general frequency and intensity of cyclones, are important. The local intensification of wind velocities, probably exceeding 100 m.p.h., which occur regularly in parts of the coastal corridor south of Cairns, produces "cyclone scrubs". Because of extensive windthrows, these have a low uneven canopy with scattered emergents densely draped by vines. In more sheltered areas, with cyclonic winds averaging 60–80 m.p.h. not locally accelerated by turbulence, upper canopy defoliation and occasional windthrows have resulted in a dense understorey of the shade-intolerant lawyer vine (Calamus australis (Mart.) Beccari) under a relatively even canopy averaging 90–110 ft high. On exposed spurs of the rugged coastal ranges, Acacia aulacocarpa A. Cunn. is a common emergent of vine forests. Fire following "dry" cyclone damage may further modify vine forests adjacent to fire-paths in sclerophyll or grassy forest. The principal effects of fire are the absence of the fire-sensitive Calamus spp., and the presence of numerous sclerophyllous species in addition to A. aulacocarpa. The reactions of Queensland tropical rain-forest to cyclones are compared with those described for west Africa, Malaya, and the West Indies. The catastrophic effect of cyclones on rain-forests overrides the usual ecological factors, and in such areas, even without human interference, a stable forest climax is not attained. The frequency of cyclonic damage means that exposed sites of the tropical lowlands and adjacent foothills of north Queensland have an uncertain silvicultural future.