The Tree Decline Recovery Seesaw; a conceptual model of the decline and recovery of drought stressed plantation trees

• A new tree decline/recovery model for eucalypts was developed.
• Model based on prediction of increased drought periods in regions with eucalypt plantations.
• The model incorporates recovery from the effect of drought and secondary biological stressors.
• Biological stressors include both insect pests and pathogens.
• This model could be expanded to other types of tree plantations and to other stress events.

Worldwide tree plantations face prolonged drought periods that are predicted to increase in duration and intensity under climate change scenarios. Trees stressed by drought are susceptible to secondary stressors including pests and pathogens. Depending on the timing and intensity of these stressors, their impact may be persistent or acute, with varied temporal and spatial responses by different hosts. There are complex processes involved in tree performance and mortality, and often it can be difficult to identify the leading cause of growth decline and death. However, it is important to recognise that often death can be a gradual process with periods of recovery. While previous models have focussed on the decline of the tree in response to stress, there has been little focus on the recovery from stress. Hence, the development here of the Tree Decline Recovery Seesaw model. This conceptual model is based on the premise that drought stressed trees may alternate between different stages of health through time, particularly those facing intermittent drought over extended periods. This approach takes into account that recovery periods allow for the tree to regain health, and potentially create new opportunities for pest and disease outbreaks. The Tree Decline Recovery Seesaw model provides researchers with the understanding of the complexity of drought/pest/pathogen interactions, allowing them to assess the order and severity of the stressors to determine the likely outcome. This paper provides a vital step towards the development of predictive tools for plantation managers. While our model has focussed on eucalypt plantation trees, there is potential to apply the model to other tree species and forms of plant stress.

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