Climate change or tree disease: challenges for diagnosing causes of forest die-off

Context Forest ecosystems experience compositional and structural changes as species’ environmental envelopes shift with climate change. Extreme climate events and pests/pathogens are driving these ecosystem changes. Determining which of the two potential drivers is causing a particular forest die-off can be challenging. In south-western Australia, widespread forest die-off in 2011 coincided with extremely hot and dry conditions. It occurred in a forest ecosystem that has historically experienced Phytophthora cinnamomi root disease (Phytophthora dieback).Aims To determine whether the causal agent of Phytophthora dieback, P. cinnamomi, was associated with forest die-off in the Northern Jarrah Forest.Methods A combination of direct (isolation of pathogen) and indirect (survey of susceptible indicator plant species) measurements were taken inside and outside patches of forest experiencing the die-off.Key results There was no consistent association between die-off patches and the presence of P. cinnamomi. P. cinnamomi was isolated from 3 of 33 control plots and 3 of 33 die-off plots. Although several plant species susceptible to P. cinnamomi were absent from die-off plots, the findings were inconsistent across species. This may be explained by plant tolerance to high temperatures and drought.Conclusions P. cinnamomi was not the proximate cause of the observed die-off in the Northern Jarrah Forest in 2011.Implications Novel disturbance caused by extreme climate events can mimic damage caused by certain pests/pathogens. More research is needed to determine the tolerances of plants to extreme temperature and drought conditions to disentangle abiotic and biotic drivers of tree die-off.