Seeking the “point of no return” in the sequence of events leading to mortality of mature trees

Plant, Cell and Environment. 2021. Vol. 44. No. 5. P. 1315–1328.

Preisler Y., Fedor Tatarinov, Grünzweig J., Yakir D.

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, 8 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, 6 months PVSM. Third, cessation of stem sap flow 3 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.

Keywords: drought, semiarid forests, sap flow, tree mortality, tree diameter variation, Aleppo pine, ток пасоки, усыхание деревьев, дендрометры