The role of stress and factors contributing to the decline of Eucalyptus wandoo (Blakely) in southwestern Australia

Disturbance observed in Eucalyptus wandoo (Wandoo) woodland in a local area of Helena catchment (Talbot) in southwestern Australia during the 1990s was remarkable. Historical evidence suggests the rapid, severe foliage yellowing (flagging) and crown dieback in Talbot was unprecedented. Symptoms were assumed to be linked to growing population decline in endemic woodlands across the landscape since the 1990s. Previous studies suggested cause was climate related but no empirical evidence had been collected. Although a particular wood-boring damage was found exclusively on declining Wandoo trees, the insect had not been identified, nor had specific links with climate been made. No other study had been successful in demonstrating cause and effect, although there had been a number of speculations. This thesis aimed to identify processes driving woodland decline using a number of field experiments in naturally occurring Wandoo stands. The work could not be described as strictly experimental however, the approach was akin to "impartial study" [Whittaker R.H. 1956. Ecol. Monogr. 26, 2-80]. The null hypothesis states: natural balance of Wandoo stands with climate, soil and biota would be replicable across the landscape where decline was not evident. Conversely, stands expressing canopy decline were expected to show divergence from natural balance that was detectable in measureable parameters; indicative of the processes involved. Several parameters, including biotic (e.g. wood-borers, canker fungi, stand health/structure) and abiotic factors (e.g. branch moisture, climate-tree-soil interactions), were measured concurrently on layered spatiotemporal scales to maximize the probability that divergence would emerge in a dataset relevant to the wide space-time domain that canopy decline occurs.