A questionnaire survey and associated field studies were conducted in rural areas of southern Queensland over the period 1981 to 1983 to obtain information on the extent, severity and possible causes of native tree dieback in the region, the range of tree species affected, and community attitudes to the problem. The survey entailed interviews with 171 landowners, and field inspections, in a total of 70 Shires in Queensland and, for purposes of comparison, four Shires in northern New South Wales.
Dieback was recorded in all Shires surveyed but, in Queensland, was particularly severe in a total of 20 Shires in the Fitzroy, Wide Bay-Burnett, Moreton and Brisbane regions, and on parts of the Darling Downs. It was generally more severe for properties in the coastal strip east of the Great Dividing Range than for properties to the west (i.e. in areas of population concentration and intensive land management). Dieback was first noticed by most landowners only within the last decade, and the rate of decline seems to have accelerated in recent years. Dieback was recorded in a total of 62 tree species in Queensland, and affected all ages of trees. Eucalyptus crebra, E. drepanophylla, E. melanophloia and E. tereticornis were the "indicator" species of dieback in this area.
Dieback tended to be more severe on small properties which, generally, were more intensively managed, had a greater percentage of their total area cleared, and had smaller blocks of remnant vegetation than large properties. Older-established properties usually exhibited the most dieback. Properties with the largest percentage of their area devoted to improved pasture, and where fertilizer had been used on crops and pasture, had the highest dieback ratings. Some salinity in soil and/or water supplies was recorded in every survey district, from a total of 38 Shires, and on 46% of survey properties. Eleven of the Shires with salting were also areas of widespread and severe dieback. Dieback was most severe on flat lands. Survey data were suggestive of a link between climatic water stress of trees, insect grazing and severity of tree dieback.
The results of pattern analysis of the data were consistent with the hypothesis that a multiplicity of factors contribute to dieback severity, and the effects of any individual factor, or combination of factors, vary with locality. There was a high level of awareness and concern about dieback among survey respondents, but a general lack of knowledge of the ecological processes involved in tree decline and land degradation. The findings of the attitudinal assessment highlighted aspects requiring more research and extension emphasis.
A detailed investigation of a serious dieback problem identified in the survey (viz. dieback of Casuarina cunninghamiana, and other tree species, along many streams in the Mary River catchment of south-east Queensland) was conducted over the period 1982 to 1984. Studies were carried out at a total of 220 stream sites in the catchment to determine the extent and severity of tree dieback and to identify the cause (s) of the problem. Of the 142 sites with Casuarina spp., dieback of these trees was evident at 136 sites and was severe to very severe at 54 sites (38%). Dieback of Eucalyptus spp. Was evident at 212 sites but was generally less severe than that recorded for Casuarina.
The major factor contributing directly to the dieback and death of trees at these sites (particularly Casuarina spp.) was severe and repeated defoliation by the leaf-eating chrysomelid beetle Rhyparida limbatipennis Jacoby. However, a link was demonstrated between the extent of tree clearing/land use in different parts of the catchment, levels of streamwater salinity, and the severity of insect-related dieback of trees along streams. Generally, streamwater quality was poorest and tree dieback most severe in areas which had been the most extensively cleared/intensively managed. These relationships are illustrated in a proposed model of dieback aetiology for catchment trees, and options for remedial action are outlined.
A model of the initiation and development of dieback of rural trees in southern Queensland is presented and discussed with reference to the study results and to the literature, and research needs are highlighted. It is concluded that the extent of tree clearing since first settlement, and the inadequacy of tree replacement, may have contributed most to the problems of tree decline and degradation of rural lands. If tree clearing is the pivotal factor, as suggested, then the present impetus towards re-establishing tree cover in critical areas is well directed and offers hope of a solution.