Extreme drought and heat triggers severe dieback in multiple, dominant canopy species in south-western Australia

Background/question/methods: Ecosystems in Mediterranean climate regions are projected to undergo considerable changes as a result of shifting climate, including from extreme drought and heat events. Ensuing forest dieback and mortality are emerging as global concerns. Although Mediterranean-type forest (MTF) ecosystems are considered to be resilient to drought, we observed sudden forest collapses in two regionally significant MTFs in Western Australia (the Northern Jarrah Forest and Tuart woodland). Remote sensing and field investigations were undertaken to characterise the extent and severity of canopy dieback and tree mortality, as well as the associations between canopy health and potential predisposing site factors. Results/conclusions: In both of these MTFs, canopy dieback was found to be concentrated in distinct areas and was affected between February and March 2011. Tree foliage rapidly discolored and died over this period. A relatively shallow soil profile and extreme drought and heat in 2010-2011 are thought to predispose water-shedding sites to drought triggered canopy dieback during extended periods of dryness. These results suggest MTFs, once thought to be resilient to climate change, are susceptible to sudden and severe forest collapse when key thresholds are reached. Tracking forest health changes in response to severe disturbance is an important key to deciphering past and future vegetation change