The role of pests in drought mortality, a modelling perspective

Background/Question/Methods

Trees are frequently subjected to abiotic and biotic stresses. Among these, insect defoliation and low water availability are common causes of crown dieback, mortality and loss of productivity.  These stresses often co-occur (Kirshbaum et al 2007 Ag For Met 145, 48 - 68).  How such multiple stresses interact to amplify or moderate the consequences of drought and affect tree survival has received little attention.

The conceptual model of McDowell et al (2008, New Phytologist 178, 719 – 739) proposes that the primary mechanisms of climate-driven tree mortality are hydraulic failure, experienced under conditions of high intensity drought, and carbon (C) starvation, experienced with longer duration droughts.  In the context of McDowell’s framework, key issues to resolve for understanding the role of defoliation in drought mortality revolve around whether defoliation alters drought intensity/duration through changes in water use and hence the pathways/mechanisms of drought mortality.

Using a series of field and glasshouse experiments performed with Eucalyptus globulus we identify key physiological responses to drought, defoliation or their interaction.   We look at how such processes can be represented in stand-level process-based models to integrate physiological responses and examine the consequences of defoliation for drought mortality.  We explore the implications of the physiological findings and the modelling for the understanding of forest resilience and propose future research priorities.

Results/Conclusions

E. globulus responds to defoliation through increasing C uptake rather than depletion of carbohydrate reserves.  The photosynthetic response to defoliation is however dampened by whole plant sink limitation associated with drought.  A range of physiological responses to defoliation under drought conditions suggest that defoliation amplifies drought. While a primary mechanism of drought mortality in E. globulus under relatively short-term terminal drought is hydraulic failure, our modelling suggests that drought conditions that still allow trees to maintain a narrow margin of hydraulic safety, combined with repeated defoliation, will increase the importance of C starvation as a mortality mechanism, leading to accelerated self thinning and scatter pattern mortality in E. globulus stands.  We propose that defoliation will reduce resilience to drought during the period of crown recovery and perhaps beyond, and discuss this in the context of a resilience framework.