Recent contraction of wet sclerophyll forest in the wet tropics of Queensland due to invasion by rainforest

Abstract

Vegetation maps were prepared from aerial photographs taken in 1943?45 and 1991?92 of three, widely separated areas of sclerophyll forest adjacent to the western edge of rainforest on granitic soils in north Queensland. Nine types of sclerophyll communities could be discerned from aerial photos and characterized by field measurement. Two types of Wet Sclerophyll Forest (WSFa and b) were separated on the species of tree composing the tallest stratum and these were subdivided according to whether the ground layer was dominated by grass or young rainforest. A related type showed large, residual Eucalyptus grandis emergent from mature rainforest. Closed canopy sclerophyll forest with no emergents (SF), sclerophyll woodland and Acaciaforest were also discerned. WSF was defined as having more than 30 per cent of the closed crown cover contributed by trees more than 35 m tall. During the 50-year study period rainforest invaded 70 per cent of WSFa (tallest stratum dominated by E. grandis), which principally occurs as a narrow strip along the rainforest margin, and 57 per cent of the adjacent WSFb (tallest stratum composed of mixed species). Grass would be quickly excluded from invaded areas and thereafter they would only burn under extreme atmospheric conditions. Because sclerophyll trees are unable to regenerate in shade and usually require fire to provide the appropriate conditions, a long-term transition to rainforest may ensue. The final stages of this transition were observed in areas that exhibited full-stature rainforest with large, relictual E. grandis emergents in 1943, but had disappeared by 1992. The initial cause of this vegetation transition is a fire-free period of sufficient length for rainforest tree seedlings to establish and suppress the grass layer. It is not known whether these vegetation changes represent a trend, possibly caused by a change a century ago from fire management by Aboriginal people to management for the cattle industry, or whether it is a temporary phase in the fire-induced, dynamic relationship between rainforest and sclerophyll vegetation. The current loss of WSF probably endangers the survival of a range of genetically endemic biota. Most groups are poorly known but the marsupial Yellowbellied Glider Petaurus australis reginae is totally dependent upon WSF and a number of vertebrates would probably go locally extinct if WSF is replaced by rainforest. WSF is the wettest part of the sclerophyll communities and probably acts as a refuge in times of unusual aridity. To maintain the WSF habitat, fire management is clearly indicated, but the intensity of fire required to reverse the advance of rainforest may be socially unacceptable to instigate or impossible to control if it occurs by accident.