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SummaryThe increasing frequency of global change-type droughts has created a need for fast, accurate and widely applicable techniques for estimating xylem embolism resistance to improve forecasts of future forest changes.We used data from 12 diffuse-porous temperate tree species covering a wide range of xylem safety to compare the pneumatic and flow-centrifuge method for constructing xylem vulnerability curves. We evaluated the agreement between parameters estimated with both methods and the sensitivity of pneumatic measurements to the measurement duration.The agreement between xylem water potentials at 50% air discharged (PAD) estimated with the Pneumatron and 50% loss of hydraulic conductivity (PLC) estimated with the flow-centrifuge method was high (mean signed deviation: 0.12 MPa, Pearson correlation: 0.96 after 15 sec of gas extraction). However, the relation between the estimated slopes was more variable, resulting in lower agreement in xylem water potential at 12% and 88% PAD/PLC. All parameters were sensitive to the duration of the pneumatic measurement, with highest overall agreement between methods after 16 sec.We conclude that, if applied correctly, the pneumatic method enables fast and inexpensive estimations of embolism resistance for a wide range of temperate, diffuse-porous species, which makes it attractive for predicting plant performance under climate change.

An extreme summer drought caused unprecedented tree dieback across Central Europe in 2018, highlighting the need for a better mechanistic understanding of drought-induced tree mortality. While numerous physiological risk factors have been identified, the principal mechanisms, hydraulic failure and carbon starvation, are still debated. We studied 9,435 trees from 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighbourhood competition influence individual mortality risk. We observed a reduced mortality risk for trees with wider hydraulic safety margins, while a rising sugar fraction of the non-structural carbohydrate pool and bark beetle infestation were associated with higher risk. Taller trees had a lower mortality risk. The sign and magnitude of neighbourhood effects on mortality risk depended on the species-identity of the involved trees, with most species having beneficial and some having deleterious effects on their neighbours. While severe tissue dehydration causing hydraulic failure precedes drought-induced tree mortality, we show that the probability of this event depends on a series of mutually inclusive processes including pest infestation and starch depletion for osmotic adjustment, and is modulated by the size and species identity of a tree and its neighbours.