Tree growth and ground vegetation 17 years after disc trenching and pre-harvest nitrogen fertilization

Silvicultural practices such as site preparation and nitrogen (N) fertilization are commonly used to increase growth of a stand. These practices may affect the forest ecosystem for several decades, impacting not only the trees, but also the stand structure and ground vegetation. Long-term field experiments are thus necessary to study these effects. In our study, two field experiments, Hagfors and Nissafors, established around 1980, were used to study residual effects of previous N-fertilization and disc trenching (DT) on tree growth and ground vegetation 17 years after final felling. At both sites, DT reduced tree mortality and increased individual tree growth, resulting in higher stem numbers and tree volume per hectare after 17 years. Previous N-fertilization had no significant effect on tree growth. The ground vegetation was affected by DT at both sites. At Hagfors, the most pronounced effects were a reduction in the cover of Cladonia rangiferina, and an increase in Calluna vulgaris after DT. Cladonia rangiferina was also negatively affected by previous N-fertilization. At Nissafors, DT caused a shift in cover of Vaccinium vitis-idaea to Vaccinium myrtillus. The impact of DT on the field layer composition appears to have been caused by the soil disturbance but also the increased shading resulting from the increased tree growth and survival. This study indicates that previous N-fertilization and DT affect the ground vegetation in the longer term. This should be considered when developing guidelines and forestry practices.