Effects of simulated long-term N deposition on Picea abies and Pinus sylvestris growth in boreal forest

Modelling studies have suggested that atmospheric nitrogen (N) deposition will increase forest carbon sequestration by stimulating tree growth. However, few long-term experiments studying N deposition effects on tree growth in boreal forests have been reported. This study empirically explores the relation between application rates of N, simulating levels of N deposition relevant for Europe, and the growth of Picea abies (L.) Karst. and Pinus sylvestris L. in a low N deposition area (<2 kg N.ha(-1).year(-1)), we manually added 0, 12.5, and 50 kg N.ha(-1).year(-1) to a forest site dominated by P. abies for 19 years and 0, 3, 6, 12.5, and 50 kg N.ha(-1).year(-1) to a forest site dominated by P. sylvestris for 10 years. On both sites, linear regression analyses displayed significant relations between mean annual tree growth and N additions: P. abies relative growth rate increased by 1.2% per kg N added and that of P. sylvestris increased by 1.6% per kg N added. The growth response, however, varied over time, highlighting the necessity for long-term experiments to apprehend effects of eutrophication on forests. Generally, our study confirms the results of previous modelling studies assessing forest growth responses to N deposition over Europe.