Paul Barber

Environmental changes such as reduced vegetation cover and altered plant species composition can have serious consequences for ecosystems, and can lead to the extinction of animal species. Tree declines are justifiably causing alarm worldwide. Not only are we losing tree species, but also animal species dependant on these trees. The loss of trees and associated fauna result in the loss or compromise of essential ecosystem processes. Fundamental processes that may be influenced include altered food web dynamics and the nitrogen cycle and pollinati on processes. The tuart (Eucalyptus gomphocephala) decline in Western Australia is a great concern. This once dominant and iconic tree species is suffering significant decline and has disappeared from most of the Swan Coastal Plain. Researchers from the Centre of Excellence for Climate Change, Woodland & Forest Health have been researching the possible consequences of tuart decline for fauna. Not only will this research provide us with greater understanding of the implications of tuart decline, but it may also enable nature conservationists and land managers to proactively put management strategies in place to safeguard animal species that may be threatened by tuart decline.

Tuart (Eucalyptus gomphocephala) is a magnificent woodland tree endemic to the Swan Coastal Plain of Western Australia and is one of the few eucalypts that is adapted to calcareous soil profiles (Eldridge et al. 1994). Prior to European settlement there were more than 111,609 ha of tuart woodlands (Hopkins et al. 1996) but this has been reduced to 30,317 ha (W.A. Government 2003). In Yalgorup National Park and private property ca. 100 km south of Perth, tuart is undergoing a severe decline or ‘dieback’.

Pythiacious soil borne micro-organism, encompassing Phytophthora species, include a diverse group of pathogens that have been identified as contributing and inciting significant forest declines throughout the world, including the south-west of Western Australia and the world. Continuing research indicates that Phytophthora species may have significant, yet unclear, roles in forest declines. Improved methods of molecular species identification indicate a greater diversity of species than once evident from strictly morphological identification, and the ongoing evolution and divergence of new species.

The tuart forest and woodlands contain a rich diversity of higher fungi. Because of the known benefit of symbiotic fungi that form associations with roots (known as mycorrhizas), this component of the Tuart Project is focussing on fungi in this group. Examination of the roots of tuart seedlings grown in pots of soil collected from under healthy tuart in the field showed that the most abundant type of mycorrhizas are those formed with higher fungi such as the toadstools, puffballs and truffle-like fungi.

Over the last 10 years, tuart trees in the Yalgorup region of south-western Australia have been undergoing an alarming period of decline and dieback. Research into the cause of the decline indicates the possible involvement of a complex disease syndrome involving a number of unknown factors contributing to the overall reduction in health. Factors affecting nutrient supply and availability have been suggested as playing a role in the decline. Tuart belongs to the largest Eucalyptus subgenus Symphyomyrtus but is taxonomically distinct, having no close relatives and thus forming a monospecific section (Ruthrof et al. 2002). The Symphyomyrtus species has a greater demand for calcium, magnesium, and probably potassium whereas Monocalyptus has a greater demand for magnesium. Such differences must have a physiological basis but as yet no explanation has been offered (Judd et al. 1996). The nutrient status of tuart has been largely unknown, creating a gap in our knowledge in the role of nutrient supply to healthy tuart trees. Therefore, there has been an obvious need to investigate the current nutrient status of both healthy and declining tuart in native plant communities along the Swan Coastal Plain and further determine the role of particular macro and/or micronutrients in the decline.