Comparative studies on the identification and assessment of basal stem necrosis in ash trees (Fraxinus excelsior) against the backdrop of the European ash dieback

Abstract Basal stem necrosis is the main cause of ash dieback and uprooting, making early detection crucial. This study examines ash dieback in Germany using data collected by the FraxForFuture research network from 14 intensive monitoring plots. Over three years, the crown and trunk condition of 1572 ash trees was assessed using a nationally standardised ash dieback assessment key. In the Brandenburg plot, we applied and compared three different approaches to assess stem collar necrosis. Based on the assessment key, we selected 28 ash trees for resistograph measurements to cover a wide range of symptom severity. From this subset, 19 tress were further selected for biochemical analysis. Resistography measures wood resistance with a fine needle to detect internal decay, density variations and structural defects in trees. Biochemical analyses included the amino acid spectrum, contents of soluble carbohydrate, starch and various phenolic groups of the wood in the stem base. Our results show significant variation in the incidence and progression of basal stem necrosis across Germany. Comparison of the results achieved using the assessment key and the resistography shows that approximately one-third of all severe necroses are not detected by visual or acoustic scoring alone. Biochemical analysis highlights necrosis-related changes, particularly in soluble carbohydrate and total phenolic content, with soluble amino acids also implicated. Previously, ash condition was mainly assessed by crown health, which underestimated stem base necrosis as a mortality factor. Our analyses show that necrosis can remain undetected for a long time despite thorough careful scoring. As ash dieback progresses, thorough assessment of the stem base is essential to prevent damage and economic loss.