Cylindrocarpon dieback of Eucalyptus obliqua regrowth forests in Tasmania

Many of the natural Eucalyptus obliqua and E. regnans regrowth forests throughout Tasmania have recently been affected by crown dieback which frequently results in the death of scattered former dominant and co-dominant trees. The unexplained dieback occurs in some of the best natural eucalypt forests and was of major concern to the future, productivity and stability of these forests. The investigations described in this thesis, were aimed at studying the aetiology of the dieback complex and evaluating the consequences of regrowth dieback in the natural development of these forests. The relationship between dieback and the natural transition of regrowth forests to cool temperate rainforest was examined. These initial studies indicated that the dieback may be a natural factor in the development of these forests that has occurred previously and is unlikely to result in tree deaths beyond those associated with normal forest development. Furthermore the dieback of former dominant trees may also be natural during the transition of regrowth forests to mature forests and be related to relatively higher stress in dominant trees than in the former subdominant trees which survive to form natural oldgrowth forests. While these studies should substantially reduce concern about the unexplained dieback of regrowth forests they also provide a new perspective of forest growth within which factors likely to contribute to regrowth dieback could be investigated and interpreted. A wide range of factors which may have contributed to the drought stress and dieback in the regrowth forests were investigated. Shallow soils with low capacities for holding moisture, periodic lwterloggj ng and drought stress, and the restriction of the fine root systems of regrowth trees relative to those of understorey plants were all associated with dieback. However, these factors could not explain the high level of fine root decay and the progressive dieback of trees even during periods and on sites favourable for plant growth. Therefore the involvement of potential root pathogens was examined. Nematodes and Pythiilln spp. were inconsistently isolated from rhizosphere soil and eucalypt roots and may contribute to a root rot complex. Phytophthora cinnamomi which was isolated infrequently from forest soils could not be isolated from the fine roots of regrowth trees and is unlikely to be a major factor in the dieback. However, a previously undescribed Cylindrocarpon sp. was consistently isolated from diseased fine roots of regrowth trees and was confirmed to be pathogenic on E. obliqua seedlings. Consequently detailed studies were conducted on the role of the Cylindrocar:e_on sp. in the infection and decay of E. obliqua roots and in the causation of the dieback complex. The population and activity of Cylindrocarpon sp. increased in soils from E. obliqua forest from low levels following burning to a maximum in 70-90 year old forests. This closely parallels the intensification of dieback in these forests. The Cylinjrocarpon sp. appeared to be natural in these ~. obliqua forests and was well adapted for growth and survival in the conditions existing in the regrowth forests.