Drought stress physiology of sclerophyllous broad-leaved trees assessed with the non-invasive magnetic turgor pressure probe

Background/question/methods: Banksia woodlands in south-west Australia are currently facing multiple threats, including fragmentation and drought- or temperature-induced mortality due to the region’s changing climate. We examined, in unprecedented detail, water relations in Banksia menziesii R. Br. trees using a novel magnetic leaf patch clamp pressure probe (ZIM-probe) that allows continuous, real-time monitoring of leaf water status. Several ZIM-probes were installed across the crown and complemented by sap flow and traditional ecophysiological spot measurements. Results/conclusions: Drought stress manifested as increasing daily signal amplitudes. Drought stress symptoms occurred more often locally than across the entire crown. Diurnal recordings during summer showed that stomatal down-regulation of transpiration commenced in the morning when vapour pressure deficit (VPD) reached 2-2.5 kPa. This early reduction in stomatal conductance prevented midday balancing pressures from exceeding 2.5 MPa. During peak summer, water status in the east and north responded more strongly to VPD than in south and west. Sap flow response to VPD reached a saturation plateau at ca. 2.5 kPa coincident with the onset of stomatal down-regulation. Unexpectedly high spring temperatures preceded the sudden death of several B. menziesii trees, suggesting a temperature- or VPD-related tipping point causing rapid hydraulic failure as evidenced by collapsing ZIM-probe readings recorded on an affected tree. The ZIM-probe’s high sensitivity renders it an ideal tool for drought stress monitoring and evaluating adaptive forest and woodland management strategies. In a warmer and drier future, local mortality events in B. menziesii populations may occur more frequently with strong impacts on community structure and ecosystem functioning