Long-term effects of nitrogen (N) additions on boreal forest

The studies presented in this thesis aim to increase our understanding of the long-term effects of anthropogenic nitrogen (N) input via fertilization or atmospheric N deposition on tree growth and forest understory in boreal forest. Firstly, I studied the effects of low annual N addition simulating atmospheric N deposition on tree growth and variables linked to site fertility in two different forest ecosystems, i.e. on one Norway spruce (Picea abies [L.] H. Karst.) dominated site and on one Scots pine (Pinus sylvestris L.) dominated site (Paper I and II). On the P. abies dominated site tree growth and needle N concentration were higher on plots treated with 12.5 and 50 kg N ha⁻¹ year⁻¹ than on control plots, which only received background N deposition (1 to 2 kg N ha⁻¹ year⁻¹). P. abies growth increased linearly with 1.2% for every kg of N added, which corresponded to a net tree carbon (C) sequestration of approximately 19 kg C per kg N (Paper II). On the P. sylvestris dominated site tree growth increased linearly with 1.6% per kg N added. Secondly, I studied carry-over effects of forest fertilization (150 kg N ha⁻¹) between tree generations. Growth of 10 years old trees on sites fertilized during the previous tree generation was measured (Paper III). On sites that were previously fertilized twice (25 and 33 years prior to my study), the young P. abies and P. sylvestris trees were on average 24% taller than trees on sites that had never been fertilized. Furthermore, the soil N mineralization rates were nearly four times higher and the amount of plant available soil-N two times higher on sites that were previously fertilized twice than on the other sites. Thirdly, I studied the interaction effects between N form and N dose on tree growth, mycorrhizal fungi and on forest floor vegetation in a c. 50 year old pine forest (Paper IV). Five years after adding 50 and 150 kg N ha⁻¹ tree growth responded similarly on plots treated with the amino acid arginine (ARG) and on plots treated with ammonium-nitrate (AN). The basal area growth was c. 40% higher on the N addition plots than on control plots. The abundance of sporocarps (fruit bodies) of mycorrhizal fungi was significantly reduced in AN treated plots compared to ARG treated plots and the control. Both ARG and AN affected the forest floor vegetation, there was rapid increase of wavy hair-grass (Avenella flexuosa [L.] Parl.) following N addition, which remained throughout the study period. Also, at the end of the study period there was a decrease in lingonberry shrub (Vaccinium vitis-idaea L.) abundance on N treated plots. In conclusion, I found linear relationships between N addition, up to 50 kg N ha⁻¹ year⁻¹, and P. abies and P. sylvestris growth. Also, I present data suggesting that the mechanisms affecting tree growth, on an ecosystem level, can be changed in the long-term by repeated N fertilization with 150 kg N ha⁻¹ occasion⁻¹. That is, N addition can promote a positive long-term effect on an ecosystem’s nutritional status. In addition, I show that inorganic and organic N additions to a forest ecosystem after five years have rather similar effects on tree growth and understory vegetation, whereas organic N addition has significantly less negative effects on sporocarp production of mycorrhizal fungi than inorganic N addition.