Testing methods for mitigation of tree dieback in Tasmanian dry eucalypt forests and woodlands

Dieback of eucalypts is a widespread phenomenon in subhumid eastern Australian vegetation remnants. It has been shown to have a multifactorial etiology, including drought, vertebrate and invertebrate predation and stock impacts. At three dieback sites in subhumid Tasmania experimental plots were established in the period 1996-1998 to test the single and interactive effects of possum exclusion, watering and stock exclusion on the health of trees, and to determine the influence of fencing on soil bulk density and eucalypt regeneration. At two sites the exclusion of possums mitigated dieback over the two year measurement period, although at one site the trees continued to deteriorate. At the third site there was no impact of any of the treatments, and changes in health over the experimental period related to tree size, indicating a self-thinning process. Soil penetration resistance decreased in the fenced treatment at all sites, and eucalypt regeneration was encouraged by fencing at two sites. Although the results of the experiment are not consistent with moisture deficiencies as a causal factor in dieback, this may relate either to a lack of severe drought during the measurement period, which followed a decade of such drought, or to the magnitude and/or application method of the watering. The results of the experiment indicate that, at some sites, the exclusion of possums will improve tree health, and that the exclusion of stock can improve tree regeneration and soil conditions.