Spatial and temporal variation in the health of an endemic Tasmanian tree, Eucalyptus gunnii Hook.f.

Abstract

Eucalyptus gunnii Hook.f. is a tree endemic to the subalpine regions of Tasmania. A subspecies, E. gunnii spp. divaricata (McAulay & Brett) B.M. Potts, has recently been listed as endangered, due to extensive dieback over the last few decades. It has been suggested that the dieback may be in part due to increased intensity and frequency of droughts brought about by climate change. Examination of weather records indicated that a prolonged dry period has occurred across in much of the species' range. Mean annual temperatures have also risen. The timing of dieback, observed through a time series of aerial photographs, coincided with the timing of the worst droughts in some regions. However, field surveys of the health of E. gunnii across its range indicated a stronger negative relationship between livestock grazing and tree health than between rainfall and tree health. This may have been partly due to the long term suppression of regeneration in these populations, and therefore the predominantly old age of many of the trees, and their lower resilience to stress. Possum defoliation was also implicated as a major agent of decline. Furthermore, the possibility was raised that herbivorous insects are having a greater impact at higher altitudes than previously, which could be due to warmer winter temperatures. Although it was difficult to unravel the effects of climate and land management changes, the results suggested that we may already be seeing the impacts of climate change in Tasmania, especially in those environments which have been most heavily modified by land use.