Natural Limits to the Expansion of Subtropical Rainforest at Mt Nebo, Queensland

Abstract

Small patches of rainforest are numerous in subtropical Queensland. Their expansion into adjacent eucalypt forests is limited by fires and other factors. This study was completed in 1980 within a compact site of 200ha in conserved forest at Mt Nebo, Queensland. It describes the separate contributions of many natural site factors to the stability of the boundaries between these forest types. The survey area included areas of eucalypt forest with a grassy understorey and adjoining ecotonal areas. These, in turn, merged into patches of old-growth complex notophyll vine forest (CNVF). Three separate datasets were recorded from 160 plots, clustered around 32 grid points: (1) canopy species; (ii) woody species in the understorey stratum; and (iii) site characteristics, as indicators of the potential for localised progression from eucalypt to CNVF at canopy level. CSIRO Division of Computing Research in Brisbane processed the data, using methods developed to define initial stages of recovery from clearing inside old-growth CNVF at nearby Mt Glorious. Apart from recent fires, the long-term expansion of CNVF across an ecological gradient at Mt Nebo was limited at various stages by factors such as the orientation, aspect and shape of slopes, soil derivation, the type and thickness of litter cover, soil moisture levels, and the presence and persistence of the exotic shrub lantana (Lantana camara). The compact survey area was free of some confounding influences seen in broader regional studies (e.g. variations in elevation, soil derivation, temperature and rainfall, and incomplete records of fire, grazing and clearing). In this study the ranges of separate sets of observations of canopy and understorey trees, and natural site characteristics within each plot, were unusually broad, allowing definition of how natural site factors combine to allow, and limit, succession from eucalypt forest to mature subtropical CNVF in the absence of fire.