We're working to identify local drivers and ultimate causes of snow-gum dieback with the aim of guiding long-term management. Our current research is focussed on resolving a few seemingly basic questions - where is dieback occurring, what insect is responsible, and why is it happening now?
Dating dieback - the unique role of dendrochronology
To understand why dieback is happening now, we need to know exactly when it occurred in the past. Tree rings offer a pathway to answer these questions. Using samples collected over decades, we can date long-dead trees, opening a window to the past. Using an array of tree-ring properties, we aim to reconstruct the history of dieback and identify the external events that lead to outbreaks.
An example of snow-gum tree-ring dating is shown below. The dark blue time series depicts ring width from samples collected on the Rams Head Range in the 1970s. The light blue, ring width recorded by trees sampled in the Perisher Valley in 2019. The consistency in year-to-year variability between series allows precise dating - a precision unmatched by other data sources.
The role of landscape determinants and disturbance
Very few quantitative data exist on the severity of current dieback outbreaks throughout the Australian Alps. In the absence of those data, mapping of the extent, and rate of spread is impossible. Similarly, the lack of data impairs our ability to understand the potential role of factors such as topography and disturbance. The remote terrain on which snow-gum stands occur means the ground-based surveys offer limited value in our dieback studies. To overcome this limitation we are turning to high-resolution remote sensing to identify change in snow-gum cover with time.
Identifying and understanding the culprit
Although the damage that leads to dieback is consistent with longicorn (Phoracantha sp.) beetles, the species-level identity of the insect driver of snow-gum dieback remains unclear. Using an array of trapping techniques and experiments, we aim to identify the insect species involved. While interesting in itself, identification of the species is just the start - we need to know when the insects are out and how this corresponds with the vulnerability of the trees they attack. We also need to identify the key factors that determine selection of host trees and survival of larvae.
In addition to the relentless task of checking traps daily and night-time experiments, the entomological aspects of our work involve nigh-time surveys that, as shown below, offer insights to the diversity of Australia's invertebrate diversity