Synergistic negative effects of ash dieback and Armillaria root rot on health and stability of mature ash trees

Abstract

Since the early 1990s, the invasive pathogen Hymenoscyphus fraxineus has been spreading in Europe causing severe dieback of common ash (Fraxinus excelsior) and narrow-leaved ash (Fraxinus angustifolia). H. fraxineus also causes necrotic lesions at the stem base and on roots of ash trees, which frequently serve as an entry point of secondary wood decay fungi, like Armillaria spp. Rot of the stem base and roots leads to structural weaknesses of ash trees, which makes them prone to uprooting or stem fracturing during storm events. To prevent fatalities and damage to infrastructure it is crucial to timely identify and remove potentially hazardous ash trees. Here, we investigated the synergistic effects of H. fraxineus and Armillaria spp. on the health status (crown defoliation, presence, and extent of basal stem necroses) of mature trees of common ash in ten mixed forest stands in Switzerland over a four-year period (2018–2022). In addition, we conducted non-destructive static load tests on a set of 30 ash trees to assess their breaking and tipping stability so that stability weakness at the stem base could be related to tree health data. The health of the monitored ash trees declined rapidly during the monitoring period, indicating that also mature ash trees in mixed forests may be heavily impacted by ash dieback after prolonged exposure to H. fraxineus (here 12–13 years) and subsequent colonization by root rot pathogens. At the end of the monitoring, only 4.1 % of ash trees with a healthy crown (defoliation ≤ 25 %) remained and 75.4 % of ash trees showed basal stem necroses, which were, with a few exceptions, all colonized by Armillaria. Although the results from the static load tests indicated that predicting tree stability based on crown defoliation level and stem base damage level is not straightforward, i.e. also trees with advanced crown defoliation and stem necrosis can still be stable, our study shows that ash trees with necroses affecting at least 20 % of the basal stem circumference and trees with more than 75 % crown defoliation are likely to suffer from a weakness at the stem base. Building on the new findings and previous research, guidelines for the management of mature ash trees affected by ash dieback are suggested.