Susan Burrows
Dieback of Eucalyptus botryoides in urban forest remnants in the Sydney
Basin
M.Sc. (Env) 1999
Dieback is a complex phenomenon likely to be caused by environmental factors operating over large geographical and long temporal scales. As a consequence studies of dieback are often correlative as these factors cannot be manipulated to investigate these possible causes. Dieback of Eucalyptus spp. and Angophora costata in Sydney Harbour National Park has been observed for several years with many trees already dead and many suffering severe dieback from which recovery is unlikely. As a highly visible environmental problem, it is of great concern to NSW National Parks and Wildlife Service as well as community groups and local residents. The review of literature confirms that there are a wide range of recorded causes, and the causes of dieback in natural forests, rural, and urban areas are unlikely to be comparable.
The aim of this study was to investigate factors implicated with causing dieback to ascertain if they were associated with dieback in Sydney Harbour National Park, and to provide a foundation and direction for further research. Eucalyptus botryoides was chosen as the study tree because this species was most affected by dieback in the Park and is a common tree along the coastline. The factors investigated were; insect herbivory, insect assemblages, Phytophthora cinnamomi, soil levels of available phosphorous, salinity, and acidity. I selected five dieback and three healthy sites. The dieback sites were at Botany Bay, Bradleys Head, Bundeena, Collins Beach, and Store Beach. The healthy sites were at Maianbar, Pearl Beach, and Resolute Beach. From each site I selected 5 trees in winter and 5 trees in spring, giving a total of 80 trees sampled for the study.
I found that trees suffering dieback were subjected to higher levels of insect herbivory than healthy trees, and exhibited spatial and temporal variation in levels of different types of insect damage within all the sites, except Bradleys Head which is suffering from an outbreak of psyllids. This indicates that several insect taxa are involved in inflicting damage. Anecdotal evidence suggests that trees began dying around 1994 coinciding with a period of drought in Sydney which may have predisposed the trees to insect attack. I also found elevated soil levels of available phosphorus within dieback sites which suggests that urban runoff may be contributing to dieback. My results suggest that dieback is not associated with Phytophthora cinnamomi, soil salinity or acidity. Recommendations for management and further research, focusing on understanding causes of dieback at local scales, are discussed.