The impact of ash dieback on ash regeneration in the forest reserve Dalby Söderskog

Abstract: Ash dieback is affecting European ash (Fraxinus excelsior) throughout its distribution area, threatening economical forests and associated biodiversity. Studying regeneration is important to foresee the future development of ash. Dalby Söderskog forest reserve in Sweden was selected as a case study to investigate the impact of ash dieback on ash regeneration. Regeneration was sampled in 74 permanent sample plots recording two height classes (<1.3 m and ≥1.3 m, until 10 cm DBH) and five damage classes (healthy, browsed, slightly affected, severely affected, dead). Compared to studies in plantations or managed forests, the regeneration in Dalby natural forest is less damaged (20% of total regeneration, 50% of regeneration ≥1.3 m). The share of healthy small ash (<1.3 m) seems very high (77%), compared to only 5% for taller regeneration (≥1.3 m) indicating a reduced infection pressure for small ash. Canopy cover and damage class are significantly correlated (Chi- Square-Test) and can be described by a log-linear regression model for regeneration ≥1.3 m: An increase in canopy cover percentage results in decreasing density of severely affected ash regeneration. A low correlation coefficient and a low R2 (0.15) suggest more influencing factors on damage to ash regeneration. Further research is needed to understand and predict the effects of ash dieback. In Dalby forest, ash will most likely not persist as a major species with only 400 stems ha-1 of healthy regeneration (≥1.3 m until 9 cm DBH) despite the comparably low damage.