These datasets were used in our study of the effects of adding biochar and nitrogen-enriched biochar on the growth of two middle-aged pine forest sites in SE Norway. In replicated field experiments, we applied four treatments consisting of an unfertilized control, and addition of biochar (2.5 t ha-1), nitrogen fertilizer (150 kg N ha-1), or biochar loaded with nitrogen (2.5 t biochar + 150 kg N ha-1). Height and diameter of trees were measured before and five growing seasons after treatment, and increment cores were taken at the end of the study period. The dataset “Annual_rings_10years” contains data for annual rings from each tree five years before treatment (2015-2019) and five years after treatment (2020-2024). Units are 1/100 mm. The dataset “Plot_data” contains the plot wise data on average diameter (cm), height (m), trees per ha, basal area (BA, m2 ha-1) and standing volume (VOL, m3 ha-1) for autumn 2019 (before treatment) and autumn 2024 (five years after treatment).