Incidence and severity of Phoracantha-induced decline within high-elevation eucalypt woodlands are strongly associated with elevation and land management

Observations suggest that sudden canopy decline and death of widely dispersed individual trees, associated with wood-borer infestations, has recently expanded and intensified within high-elevation Eucalyptus pauciflora and E. lacrimans stands on the Kosciuszko massif, southeast Australia. Despite reports of insect infestations and associated tree decline over four decades, the phenomenon has been poorly understood and the identity of the associated wood-borer taxon only anecdotally resolved. We conducted a systematic study of sub-alpine forests in Kosciuszko National Park, NSW, with the intent of building a knowledge base of the phenomenon’s aetiology and guiding ongoing research priorities. Using 10-metre radius plots positioned on elevational transects in areas under shared management for snow-based recreation and conservation and areas managed for conservation-only, we found a strong association between increasing severity of wood-borer feeding galleries and decline of canopy condition. The presence of even the first indicators of insect infestation was associated with a three-fold increase in declining canopy condition. Two-way contingency analysis revealed that increasing severity of feeding galleries corresponded with a highly significant decline in canopy-condition. Indicators of wood-borer infestation held greater explanatory power for deteriorating crown condition, than the converse, indicating that wood-borer infestation more likely precede, than follow, canopy decline and death. Based on insect detections over three years and consistency between observed and previously described wood-borer damage, it is unambiguously clear that the native cerambycid species Phoracantha mastersi is principally responsible for insect galleries in affected trees. Detections were associated with warm daily temperature maxima. Modelling of the incidence and severity of infestations indicated elevation and land-management status were significant predictors for the probability of wood-borer infestation at stand and tree levels. The same predictors were significant for predicting the proportion of stand basal area affected. Combined with temporal patterns in insect detections, our modelling results suggest that low temperatures play a key role in limiting upward migration of infestations. While we suspect that fragmentation of forest cover within shared-management areas favours short-distance dispersal and local intensification, an increase in forest-edges may also account for both greater frequency and severity of stand-level infestations in shared-management areas. Mindful of an apparent lower limit to P. mastersi infestations at the interface of affected/unaffected eucalypt taxa, we advocate for greater understanding of physiological traits affecting tree-level vulnerability to infestation. Further, reflecting their roles in Phoracantha infestations elsewhere, we regard resolving the combined roles of temperature and drought stress in initiating outbreaks as particularly important. Similarly, consistent with the effect of land-management status on P. mastersi infestations, improved knowledge of the spatial and temporal attributes of Phoracantha-induced dieback in snow-gum forests is needed.