Assessing the Role of Mixed Tree Species in Mitigating Ash Dieback: Insights From Greenhouse Experiments

Since its introduction, ash dieback has been a disease affecting ash ( Fraxinus excelsior ) stands across nearly their entire range. As the disease cannot be contained, various options for mitigating its severity have been explored in numerous studies. It has frequently been proposed that ash, particularly in dense pure stands, is more severely impacted by the disease. Additionally, it has been claimed that the presence of mixed tree species might have a species‐specific effect in mitigating disease progression. To investigate the impact of admixed tree species, we conducted greenhouse experiments in which ash trees were grown under varying quantities of infected ash and mixed tree species leaf litter. The objective was to assess the effect of admixed tree species based on pathogen spore production and the assessment of disease symptoms. The mixed tree species used in the experiments were beech ( Fagus sylvatica ), sycamore ( Acer pseudoplatanus ) and winter lime ( Tilia cordata ). Spore production from the infected leaf litter in the greenhouse cabins followed patterns broadly consistent with previous field observations. We demonstrated that spore pressure in the cabins depended on the applied quantity of infected ash leaf litter and that the mixed tree species exerted no further influence. The number of infected shoots on an ash tree correlated with both its total number of shoots and the quantity of infected ash leaf litter. The number of shoots was considered an indicator for leaf area and thus the tree's potential to become infected. For stem necroses, only the quantity of infected ash leaf litter was found to be decisive. We conclude that the milder progression of ash dieback in mixed stands is primarily due to a dilution effect caused by the presence of mixed tree species. No additional effects of the three mixed tree species on spore production or disease progression were observed in our study.