Frequent coppicing deteriorates the conservation status of black alder forests in the Po plain (northern Italy)

Alluvial forests with black alder are a priority conservation habitat in Europe. In the Po plain, black alder is traditionally managed by coppicing with frequent rotations. This study aims to ascertain whether such management is compatible with habitat conservation, by measuring the effect of time since coppicing on forest structure and plant species composition across different layers. We compared the effects of three treatments, each thrice replicated: recent (10-20 years), medium (20-30 years) and old coppice (>40 years). In all nine stands we measured basal area, tree and regeneration density, mean tree diameter and height, dominance by alder, species richness, Shannon diversity, and the number of ruderal and non-native species. Significant differences in dendrometric variables, species richness, diversity, and percent cover by chorotype were assessed for treatment effects by two-way ANOVA. Frequently coppiced stands had a lower basal area, mean tree size, and volume, a more simplified vertical structure, a lower cover of the herbaceous layer and higher bare soil cover due to harvesting disturbance, a significantly lower cover by typical woodland Fraxinetalia species, and a significantly higher frequency and cover of non-native species. Our study showed that frequent coppicing worsened the conservation status of black alder forests in the study area, simplified stand structure, deteriorated species composition, and increased the spread of non-native and ruderal plant species. Such negative effects persisted even 20–30 years after cutting. We recommend amending the current legislation and introducing mandatory Implications Assessment procedures everywhere alder forests are susceptible to be impacted in a similarly negative way.