Drivers of ash (Fraxinus excelsior L.) natural regeneration spread into suboptimal sites – Refugee or dead end?

Ash disease caused by ascomycete fungus Hymenoscyphus fraxineus leads to the retreat of ash, particularly in ash-dominated stands characterized by moist, fertile, and neutral soils. Thus, it has a more severe impact on species regeneration and survival in ash optimal sites. We aimed to investigate the joint effects of seed dispersal limitation triggered by the distance from seed source, as well as soil fertility, browsing, and understory vegetation on ash natural regeneration in optimal and neighboring suboptimal sites. We examined ash regeneration within 40 circular plots (25 m2) in Western Poland along soil fertility gradient and increasing distance from seed source (0–100 m). We confirmed the seed dispersal limitation of ash regeneration. We found that the increase of distance from seed source increases ash natural regeneration (both of <0.6 and 0.6–1.3 m height) survival by escaping distance-dependent mortality, referring to the Janzen-Connell effect. Moreover, soil drivers i.e. groundwater table level, CaCO3, and soil C:N influenced ash regeneration damage rate. Furthermore, we indicated that light availability can enhance ash regeneration. We found the highest browsing in sites with the highest ash natural regeneration density. Understory species composition was related to ash regeneration density, browsing, and damage proportion, indicating that species typical of mesic broadleaved forests can be considered as positive indicators of the ash regeneration niche. In conclusion, we state that suboptimal sites can act as refugees for ash, helping this species to persist over ash dieback. Therefore, we recommend broadening the scope of ash conservation and silviculture into suboptimal sites, that can host ash regeneration and allow it to emerge and reproduce. Such sites in the neighborhood of typical ash sites damaged by H. fraxineus can in the future serve as seed sources for forest regeneration and improve the resilience of the forest ecosystem.