Seeking the 'point of no return' in the sequence of events leading to mortality of mature trees.

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify this sequence, we used a 2016 tree mortality event in a semi-arid pine forest where dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the surviving trees, eight months 'prior to the visual signs of mortality' (PVSM; e.g., near complete canopy browning). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, six months PVSM. Third, cessation of stem sap flow three months PVSM. Eventual mortality could therefore be detected long before visual signs were observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capacity, and soil water uptake. The results indicated that breakdown of stem radial water flow and phloem function is a critical element in defining the 'point of no return' in the sequence of events leading to mortality of mature trees. This article is protected by copyright. All rights reserved.